Extended interactions between metal ions in transition metal complexes: A symposium sponsored by the Division of Inorganic Chemistry at the 167th meeting ... April 3-5, 1974 (ACS symposium series ; 5)

Extended Interactions between Metal Ions in Transition Metal Complexes LeonardV.Interrante,Editor

A symposiu the D i v i s i o n of Inorganic Chemistry at the 167th Meeting of the American Chemical Society, Los Angeles, Calif., April 3 - 5 , 1974.

ACS

AMERICAN

SYMPOSIUM

CHEMICAL

SERIES

SOCIETY

WASHINGTON, D. C. 1974

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

5

L i b r a r y o f Congress CIP D a t a Extended interactions between metal complexes. ( ACS symposium series; 5 )

metal

ions

in

transition

Includes bibliographical references and index. 1. T r a n s i t i o n metal compounds—Congresses. 2. Com­ plex compounds—Congresses. 3. Solid state c h e m i s t r y — Congresses. I. Interrante, Leonard V . , 1 9 3 9 ed. II. American C h e m i c a l Society. D i v i s i o n of Inorganic Chemistry. III. Series: A m e r i c a n C h e m i c a l Society. ACS symposium series; 5. QD172.T6E49 I S B N 0-8412-0224-9

Copyright ©

546'.6 ACSMC8

5

1-408

74-30455 (1974)

1974

American Chemical Society All Rights Reserved PRINTED IN THE UNITED STATES OF AMERICA

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

ACS Symposium Series Robert F. Gould, Series Editor

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

FOREWOR The

ACS

S Y M P O S I U M SERIES was

founded in 1974

to provide

a medium for publishing symposia quickly in book form. The format of the SERIES parallels that of its predecessor, ADVANCES IN C H E M I S T R Y SERIES, except that in order to save time the papers are not typeset but are reproduced as they are submitted by the authors in camera-ready

form.

As a further

means of saving time, the papers are not edited or reviewed except by the symposium chairman, who

becomes editor of

the book. Papers published in the A C S

S Y M P O S I U M SERIES

are original contributions not published elsewhere in whole or major part and include reports of research as well as reviews since symposia may embrace both types of presentation.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

PREFACE he papers that comprise this volume span two major areas of research — s o l i d state chemistry and physics, and the study of transition metal complexes. The former has been concerned largely with relatively simple ionically or covalently bound inorganic solids such as the metal oxides, halides, and sulfides, where extended interactions are the rule rather than the exception. On

the other hand, until quite recently, the study of

transition metal complexes has been almost entirely a study of molecular systems, where the interaction

i

th

solid stat

generall

f th

weak van der Waals typ closely those of the constituent molecular units in solution. In the last few years it has become clear that there are a number of transition metal complexes which do not fit this description very well. In particular, these materials usually exhibit some of the characteristics typical of molecular systems but display certain optical, magnetic, or electrical properties which evidence "extended

interactions" of appre-

ciable magnitude in the solid state. The study of such complexes constitutes the main subject of this volume. Many of these systems show quite anisotropic and even pseudo-"onedimensional" solid state behavior, reflecting strong intermolecular interactions of a highly directional character in the crystal. Such one-dimensional systems, based on both metal complexes and organic charge-transfer compounds, are currently of considerable interest within the solid state physics community and are under active study in laboratories here and abroad (I, 2).

The intense interest in these materials can be attributed,

in part, to suggestions regarding the possibility of superconductivity in such systems (3, 4).

The actual likelihood of superconductivity here is

a matter of some controversy; however, the wealth of new

concepts and

the physical understanding that has already resulted from the study of these systems, not to mention the high conductivities and highly anisotropic optical, magnetic, and electronic properties that many of them exhibit, promise continued growth of interest. One

important conclusion from the work on these and other types

of transition metal complex solids is that interactions between the metal ions in the crystal, propagated

either directly or through a bridging

group, are often of major importance in determining the solid state properties. The nature of these metal ion interactions and the manner in ix

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

which they relate to the observed properties is the principal focus of the papers in this volume. The first three chapters, which are concerned with transition metal oxides and sulfides, introduce the topic of extended interactions in transi­ tion metal compounds from a solid state viewpoint and provide an account of current work on the simple inorganic, non-molecular systems. The discussion of transition metal complexes begins with Chapter 4, which deals with exchange interactions in, and pathways for exchange in some weakly

associated molecular systems.

The next two papers

describe the application and interpretation of electron spin resonance measurements for studying exchange interactions in such systems. Chapters 7 through 11 deal with systems in which the exchange interactions are localized largely within dimeric or higher polymeric units and provide some fundamenta metal ion, the bridging group

geometry

type and magnitude of the exchange process. Chapters 10 and 11 also introduce the topic of extended exchange interactions in one-dimensional, ligand-bridged systems—a topic which is developed further in Chapters 12-17. Beginning in Chapter 17, which provides a general survey of current work on one-dimensional metal complex systems, the discussion shifts to direct metal-metal interactions in solids containing stacked planar metal complexes.

The remaining chapters continue this discussion, concluding

with some recent work on the mixed valence platinum salts, which have lately been of much interest for their one-dimensional metallic char­ acteristics. The authors include both academic and industrial research scientists from the United States and Europe and represent virtually every phase of the current work on transition metal complex solids. The symposium on which this volume is based was supported, in part, by grants and travel assistance provided by the United States Air Force, through its European Office for Aerospace Research and Develop­ ment, General Electric Corporate Research and Development, and the Inorganic Division of the American Chemical Society. This support made it possible for several European authors to travel to the symposium and contributed greatly to the international flavor and overall success of the program. Literature

Cited

1. Zeller, H. R., Advan.

Solid

State

Phys.

2. Garito, A. F., Heeger, A. J., Acc. Chem.

(1973) 13, 31.

Res. (1974) 7, 232.

χ

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

3. Little, W. Α., Ed., Proc. Intern. Superconductors, J. Polymer

Symp. Phys. Chem. Problems Sci. C (1969), No. 29.

Possible

Org.

4. Coleman, L. B., Cohen, M. J., Sandman, D. J., Yamagishi, F. G., Garito, A. F., Heeger, A. J., Solid

State

Comm.

(1973) 12, 1125. L. V. I N T E R R A N T E

Schenectady, Ν. Y. November 6, 1974

xi

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

1 Extended Interactions in Transition Metal Oxides and Chalcogenides AARON WOLD D i v i s i o n of E n g i n e e r i n g , B r o w n U n i v e r s i t y , Providence, R.I. 02912

As e a r l y as 1937 i n t r a n s i t i o n metal o x i d e s , in which the d-bands o f the t r a n s i ­ t i o n metal ions were partially filled, the p o t e n t i a l energy b a r r i e r between atoms was high enough t o reduce the c o n d u c t i ­ vity by an enormous amount. This was probably the first i n d i ­ c a t i o n that the Block-Wilson band theory of s o l i d s (1,2) could not d e s c r i b e i n a realistic way the t r a n s p o r t p r o p e r t i e s o f t r a n s i t i o n metal compounds. Indeed, the classical band theory had p r e d i c t e d that these compounds with partially filled d-bands would show high electrical c o n d u c t i v i t y (σ = n e μ); the num­ ber o f c a r r i e r s n would show temperature independence and the m o b i l i t y μ would decrease as the temperature Τ increased. The electrical c o n d u c t i v i t y observed f o r these compounds would be de­ r i v e d from these latter temperature dependencies. In 1957 (3) Morin r e p o r t e d on the electrical p r o p e r t i e s of s e v e r a l vanadium and t i t a n i u m o x i d e s , namely VO, V O , VO and Ti 0 . These oxides c o n t a i n 3,2,1 and 1-d e l e c t r o n s per t r a n s i ­ t i o n metal c a t i o n r e s p e c t i v e l y . These compounds show m e t a l l i c behavior a t high temperatures and then become semiconducting when cooled through a critical temperature Τ , (114°K, 153°K, 340°K and 450°K). For most o f these compounSs, the electrical resistivity was observed t o drop by s e v e r a l orders o f magnitude over a s m a l l temperature range. The oxides of vanadium show a r e d u c t i o n i n symmetry a s s o c i a t e d with the resistivity changes. The lattice parameters o f T i O vary q u i t e r a p i d l y i n the vici­ nity of the t r a n s i t i o n temperature ( T ) but there is no change i n symmetry (4). Figure 1 summarizes these f i n d i n g s . I t should a l s o be i n d i c a t e d that f o r pure s t o i c h i o m e t r i c s i n g l e c r y s t a l s , the temperature range over which the t r a n s i t i o n occurs is g r e a t l y reduced and the magnitude o f the d i s c o n t i n u i t y i n electrical con­ d u c t i v i t y i s increased by a f a c t o r o f 1 0 . The 3d t r a n s i t i o n metal oxides may t h e r e f o r e be e i t h e r me­ tallic at a l l temperatures, semiconducting at all temperatures or undergo a semiconductor <-- metal phase t r a n s i t i o n on heating through a critical temperature T . The l i s t of compounds 2

2

3

3

c

2

3

t

3

t

1

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2

2

E X T E N D E D INTERACTIONS

B E T W E E N M E T A L IONS

belonging to these three c l a s s e s may be enlarged t o i n c l u d e *+d and 5 d - t r a n s i t i o n metal oxides as w e l l as 3 d - t r a n s i t i o n metal s u l f i d e s . T h i s i s shown i n Table 1 (5). It i s not the purpose o f t h i s p r e s e n t a t i o n t o d i s c u s s the v a r i o u s t h e o r e t i c a l models used t o c o r r e l a t e the e l e c t r o n i c and s t r u c t u r a l p r o p e r t i e s o f these compounds. One or two o f the models, u s e f u l to the chemist, w i l l be r e f e r r e d t o l a t e r i n the d i s c u s s i o n o f s p e c i f i c compounds. Neither i s i t p o s s i b l e t o t r e a t , i n the a v a i l a b l e space, more than s e v e r a l compositions that have been studied a t Brown U n i v e r s i t y . However, i t would be a p p r o p r i a t e t o mention before proceeding, an important property of many t r a n s i t i o n metal compounds i . e . , that f o r both the semi­ conducting and m e t a l l i c phases, the e l e c t r o n s r e s p o n s i b l e f o r the observed e l e c t r o n i c behavio d-bands. As a r e s u l t theory break down. From an experimental, o r s p e c i f i c a l l y s y n t h e t i c , p o i n t o f view the crux o f the problem i n studying these compounds i s t o prepare pure s t o i c h i o m e t r i c s i n g l e c r y s t a l s . T h i s has i n v o l v e d the development o f new techniques both f o r t h e i r p r e p a r a t i o n and c h a r a c t e r i z a t i o n . An understanding o f the e l e c t r o n i c p r o p e r t i e s of these m a t e r i a l s has had t o depend upon the a v a i l a b i l i t y o f such c r y s t a l s . A number o f t y p i c a l t r a n s i t i o n metal compounds s e l e c t e d from each o f the c l a s s e s p r e v i o u s l y d e s c r i b e d w i l l now be d i s c u s s e d i n some d e t a i l . They w i l l i n c l u d e examples o f b i n a r y t r a n s i t i o n metal oxides and o x y f l u o r i d e s , p e r o v s k i t e "bronzes" and t r a n ­ s i t i o n metal chalcogenides. No attempt w i l l be made t o d i s c u s s compounds other than those which have been prepared a t Brown U n i v e r s i t y . A r e l a t i v e l y simple o n e - e l e c t r o n model proposed by Goodenough (6,7) w i l l be used t o c o r r e l a t e the v a r i o u s s t r u c t u r ­ a l and c r y s t a l l o g r a p h i c p r o p e r t i e s observed f o r these m a t e r i a l s . I. Tungsten (VI) Oxide and the Cubic Tungsten Bronzes. The ReO- type s t r u c t u r e c o n s i s t s o f ReO^ r e g u l a r octahedra j o i n e d t o ­ gether by s h a r i n g corners t o form a three-dimensional l a t t i c e shown i n F i g u r e I I . W0 does not c r y s t a l l i z e a c c o r d i n g t o t h i s scheme. Braekken (8) has i n d i c a t e d that t h i s compound has a s t r u c t u r e o f low symmetry, c o n s i s t i n g o f deformed WCL octahedra j o i n e d i n the same way as the r e g u l a r octahedra o f tne ReO^ s t r u c t u r e . The c e l l i s m o n o c l i n i c (pseudo-orthorhombic) with a = 7.2858, b = 7.517 A, c = 3.835X, α = γ = 90°, 3 = 90.90°. The c u b i c tungsten bronzes c r y s t a l l i z e i n the p e r o v s k i t e s t r u c t u r e , ABOg. This s t r u c t u r e , l i k e ReO^, i s formed by the corner s h a r i n g o f BO^ octahedra. However, i n the case o f the p e r o v s k i t e s t r u c t u r e a l l o f the l a r g e A s i t e s a r e occupied. For the c u b i c bronzes, M W0«, the a l k a l i metal c a t i o n s are s t a t i s χ 3 t i c a l l y d i s t r i b u t e d over the A - s i t e s . The e l e c t r i c a l p r o p e r t i e s o f ReO«, the s t r u c t u r a l l y r e l a t e d 3

0

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

1.

WOLD

Oxides

and

Table

1.

Chalcogenides

3

Classification of T r a n s i t i o n M e t a l

Oxides

a c c o r d i n g to T h e i r E l e c t r i c a l B e h a v i o r

-CLASS I M e t a l l i c Compounds

3d-compounds:

TiO

Md-compounds :

NbO, Ru0 «

5d-compound s :

·

2

-CLASS I I Semiconducting Compounds

3d-compounds :

F e

NiO, CoO, MnO, FeO,

2

Cl>

°3>

2°3

MnS, MnS , FeS « 2

2

-CLASS I I I T r a n s i t i o n a l Compounds

3d-compounds:

Vo, V Og, V 0

T i

2°3>

NiS, Ud-compound s :

T i

3°5*

V ^ , ν^0 , ν 0

2 $

?

F

e

β

2 V

CrS, FeS.

NbO . 9

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

E X T E N D E D INTERACTIONS

4

V Q '.(3d) 2

V0 ;(3d)'

2

TLCUSd)

2

3

B E T W E E N M E T A L IONS

1

log ο* IOV ^. I Ο* Electrical Conductivity! I O ΙΟ σ (σ ohm"'cm") - i o 4 _ _ J L i o IO -ΙΟ" T ^450°K Τ T ^34Q°K τ Τ,*Μ50*Κ Τ (Z>i?J-?J),(2S)*C29-30)J(7),(35-37) P^O 6?i 17-21Y22 - 23V References (22),(3j-33)*,(34) 0

1

J

J

t

t

X X IO*

XX ΙΟ*

XX ΙΟ*

Magnetic 16501 TOO Susceptibility], 1050 X 200 J (x in eg*, ΙΟΟ ΙΟΟ mole"' ) "673Ύ°Κ io iSolooT^ 33 ΙΟΟ 3 4 0 450Τ°Κ J(I2)(42-43)(26)^I)/221)(I3-I4)/3J)*(45) Γ(12)(3β)*.(3?-4Ι) References Κ4ΪΧ(44) Specific Heat) 81 (C in cals, mole** deg"')| p

^Lo700colt/»o»«

Τ

Figure

Ι.

*M.o750ce»»/eol*

20

15

28 20 Τ Κ

Τ Κ

κ

Electrical conductivity, cific heat vs. temperature

magnetic susceptibility, and spe­ for V>0, , VO>, and Ti 0. {

Figure

IL

:

Structure

s

of ReO 2

30 "

Re * 6

[2>[6] σ ·

j[2][2][2]

ζ-JT 1

(E

E

m,

FERML LEVEL

*[2][21Κ

Ε - E, )

}

1

*cf

/

[2>[3]

Figure

III.

Electron

energy diagram for ReO.,

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

1.

WOLD

Oxides

and

5

Chalcogenides

cubic bronze, Ma WO^, and the ordered p e r o v s k i t e Sr^M ReO have been i n v e s t i g a t e d by Goodenough and co-workers (9)· The one e l e c t r o n energy diagram f o r ReO« i s shown i n Figure III. Because o f the o c t a h e d r a l c r y s t a l f i e l d the d - e l e c t r o n energy s t a t e s o f the rhenium are s p l i t i n t o a l e s s s t a b l e , doubly degenerate s t a t e (e ) and a more s t a b l e t r i p l y degenerate ( t ^ )· The outer e l e c t r o n Inergy l e v e l s of the anion are s i m i l a r l y s flown on the r i g h t s i d e o f the f i g u r e . The overlap o f c a t i o n t and anion p^ o r b i t a l s r e s u l t s i n the formation o f bonding and Intibonding bands extending throughout the c r y s t a l (10^. The Fermi l e v e l may be l o c a t e d by l o o k i n g a t the number o f outer elec^roçs per molecule. Consider the ReO^ u n i t f o r rhenium 5d 6s . Ther 2s 2p c o n t r i b u t e [6] e l e c t r o n s per molecule. Upon f i l l i n g the a v a i l a b l e energy l e v e l s , the l a s t e l e c t r o n occupies the ir*band. This band i s p a r t i a l l y f i l l e d and causes the observed m e t a l l i c behavior. The o r i g i n o f the m e t a l l i c c o n d u c t i v i t y o f cubic Na^WO^ has been p o s t u l a t e d by s e v e r a l models: 1. The d i r e c t overlap of sodium 3p o r b i t a l s (11) 2. The d i r e c t overlap o f tungsten t o r b i t a l s l e a d i n g t o tungsten-tungsten bonds (12) 3. The covalent mixing o f tungsten t o r b i t a l s and oxygen ρ orbitals t o form p a r t i a l l y f i l l e d b a n d s ( 1 3 ) . Goodenough and h i s co-workers (9_) performed a unique ex­ periment which i n d i c a t e d t h a t the t h i r d p o s s i b i l i t y was indeed the most probable mechanism f o r the observed m e t a l l i c con­ d u c t i o n i n these m a t e r i a l s . The compound Sr M c crystallizes i n the ordered p e r o v s k i t e s t r u c t u r e (see F i g u r l IV;. The B - s i t e c a t i o n s , Mg and Re, order such that each rhenium has only magne­ sium nearest c a t i o n neighbors and v i c e versa ( 9 ) . Such an arrangement would s t i l l allow f o r the overlap of rhenium t ~ or­ b i t a l s across a cube f a c e . Consequently, m e t a l l i c behavior would be expected i f Model 2 a p p l i e d . However, i f the e l e c t r i c a l pro­ p e r t i e s are a r e s u l t of c a t i o n t and oxygen interactions (Model 3 ) , Sr MgReOg should be a iemi-conductor, s i n c e Mg does not possess t e l e c t r o n s . The observed semi-conductive be­ havior (E = 0.§leV) and temperature dependent paramagnetism con­ f i r m t h a t the e l e c t r o n s are l o c a l i z e d r a t h e r than occupying c o l l e c t i v e energy bands ( 9 ) . Thus, model 3 i s c o n s i s t e n t with the i n t e r a c t i o n s present i n p e r o v s k i t e - l i k e m a t e r i a l s and best e x p l a i n s the observed p r o p e r t i e s . g

g

2

δ

g

π

R e 0

2

2

2

2

II. Vanadium (IV) Oxide and S u b s t i t u t e d Compounds. The high temperature, m e t a l l i c VO phase has a t e t r a g o n a l r u t i l e l i k e (1*0 s t r u c t u r e (space group p4 /mnm). As can be seen from Figure V the s t r u c t u r e may be d e s c r i b e d as s t r i n g s o f edge-shared octahedra j o i n e d by corners extending i n the c d i r e c t i o n . The V-V d i s t a n c e s are equivalent w i t h i n the s t r i n g s (2.87 8 ). 2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

6

E X T E N D E D INTERACTIONS

B E T W E E N M E T A L IONS

• Re O o Figure

IV.

Structure

of

Sr MgReO« 2

V 0 (tetragonal) 2

• Vanadium Ο Oxygen Figure V.

Rutile

structure

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

1.

WOLD

Oxides

and

Chalcogenides

7

Below 340°K, VO^ has been reported to have a d i s t o r t e d r u ­ t i l e s t r u c t u r e with monoclinic symmetry (P2^/c. This d i s t o r t i o n (see F i g u r e VI) causes the vanadium atoms l o c a t e d i n the s t r i n g s of VOg octahedra t o occur as doublets ( s i m i l a r t o MoO^). The V-V d i s t a n c e s are no longer equal but are 2.658 and 3.12^ (1^0· As pointed out by Heckingbottom and L i n e t t (15), i n the low temper­ ature semiconducting phase the c a x i s i s t i l t e d causing the vana­ dium atoms to be d i s p l a c e d from the center o f i t s octahedra. The band s t r u c t u r e f o r t e t r a g o n a l V 0 has a l s o been examined by Goodenough (14) and i s shown i n Figure V I I . Because of edge sharing of VO^ octahedra i n the t e t r a g o n a l phase, an o r t h o r hombic component o f the c r y s t a l f i e l d removes the d-state de­ generacy. The two e o r b i t a l s , normally o c c u r i n g i n an o c t a ­ h e d r a l f i e l d , are s p f i o r b i t a l s s p l i t i n t o tw ρ o r b i t a l and a d n o r b i ! a l d i r e c t e d along the c r u t i l e a x i s . When the 17 outer e l e c t r o n s per molecule i n V0- are placed i n t o t h i s energy scheme the f i n a l e l e c t r o n enters the over­ lapping π* and d i i bands. These o v e r l a p p i n g , non-degenerate bands, being o n l y ' p a r t i a l l y f i l l e d , r e s u l t i n m e t a l l i c conductiv­ i t y i n the t e t r a g o n a l phase (14). Below the t r a n s i t i o n temperature VO^ has a m o n o c l i n i c a l l y d i s t o r t e d r u t i l e s t r u c t u r e c h a r a c t e r i z e d by the formation o f V-V p a i r s along the a monoclinic a x i s ( «i ) * a consequent d o u b l i n g g f the c r y s t a l o g r a p h i c u n i t c e l l ' ( 1 6 ) . This displacement o f the V ion from i t s center of symmetry i s b e l i e v e d to be caused by a f e r r o e l e c t r i c - type d i s t o r t i o n (14). The e f f e c t o f t h i s d i s t o r t i o n on the band s t r u c t u r e o f VO^ i s shown i n Figure V I I I . The d j i band i s s p l i t i n two and the Fermi energy i s lowered below'the bottom of the π» band. As a consequence o f doubling the c e l l , the lower X band i s f i l l e d completely and semi-conducting behavior r e s u l t l (14). 2

c

a x : L S

r u t

a n c

e

+

I I I . O x y f l u o r i d e s . At low l e v e l s of f l u o r i n e , the oxyf l u o r i d e s u s u a l l y possess s t r u c t u r e s q u i t e s i m i l a r to the parent oxide. The c e l l parameters (orthorhombic indexing) a = 7.356 A, o °*> °- r e p o r t e d f o r WO F are q u i t e s i m i l a r to those observed by S l e i g h t (17) f o r monoclinic W0 a = 7.301 X b = 7.538 8, c = 3.844 ^7 3 = 90.89°. S l e i g h t d i d not r u l e out the p o s s i b i l i t y that WO^ gg^Q ^ could be monoclinic with the d e v i a t i o n o f the monoclinic angïe from 90° being too s m a l l to d e t e c t . Higher s u b s t i t u t i o n s of f l u o r i n e i n the WO^ F^ systems s t a b i l i z e s the c u b i c Re0 s t r u c t u r e (17, 18).~¥his s t r u c t u r e i s analogous t o the p e r o v s k i t e type a l k a l i metal bronzes with a l l A s i t e s vacant. In the tungsten o x y f l u o r i d e system the cubic phase extended from χ = 0.17 to 0.66. A one-electron energy diagram would be much simpler f o r 3 x x ^ corresponding cubic tungsten bronzes Na^WO^. I t Is not necessary t o consider i n t e r a c t i o n s i n v o l v i n g A s i t e c a t i o n s s i n c e these p o s i t i o n s are empty·

b

=

7 , 1 + 6 9

c

=

3 , 8 1 4 6

0

g 6

Q

Q l +

3

3

W 0

F

t

n

a

n

o r

t

n

e

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

8

E X T E N D E D INTERACTIONS

£

S

/ y

i

B E T W E E N M E T A L IONS

= 2c

yf

—

/A

rutile axes Figure

V

Figure

7=

monoclinic axes

VI. Monoclinic structure temperature) for VO.

I

V0_

(low

(

VII. Electron energy for tetragonal VO

diagram

:

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

1.

WOLD

Oxides

and

Chalcogenides

9

The s u b s t i t u t i o n o f f l u o r i n e ajlso r e s u l t s i n a d d i t i o n a l elec­ trons because o f the formation o f W i o n s . Thus, the compounds WOg F have χ a d d i t i o n a l e l e c t r o n s per molecule. Although i t mighi £e expected that the more e l e c t r o n e g a t i v e f l u o r i d e ion would have a l o c a l i z i n g e f f e c t on the a d d i t i o n a l e l e c t r o n s , me­ t a l l i c c o n d u c t i v i t y has been observed f o r the c u b i c o x y f l u o r i d e bronzes. By comparison with the WO. F system, i n which l a r g e f l u o r i n e s u b s t i t u t i o n s s t a b i l i z e a high symmetry c u b i c phase, there may a l s o be an analogous decrease i n the semiconductor metal t r a n s i t i o n with i n c r e a s i n g f l u o r i n e s u b s t i t u t i o n i n 2 - x . For V 0 , the monoclinic t o t e t r a g o n a l t r a n s i t i o n temper­ ature decreases with i n c r e a s i n g anion s u b s t i t u t i o n i . e . the high temperature t e t r a g o n a l phase becomes more s t a b l e (19). For the system V 0 from the r e s i s t i v i t y dat i n Table II f o r d i f f e r e n t values of χ i n V0F . I t can be seen 2—χ χ from these curves t h a t a m e t a l l i c t o semiconductor t r a n s i t i o n occurs a t a temperature which decreases with i n c r e a s i n g values of x. T h i s change i n the e l e c t r i c a l p r o p e r t i e s o f V 0 ^F^ can be explained by the corresponding t r a n s i t i o n from the monoclinic phase t o the t e t r a g o n a l phase which has been observed by means of low-temperature X-ray a n a l y s i s . A l i n e a r r e l a t i o n s h i p e x i s t s be­ tween the value of χ and the t r a n s i t i o n temperature (T ) shown i n F i g u r e X; t h i s e x t r a p o l a t e s t o the c o r r e c t t r a n s i t i o n temperature f o r pure V0 . For the higher f l u o r i n e compounds, the t r a n s i t i o n r e g i o n i s c o n s i d e r a b l y broadened and the t r a n s i t i o n p o i n t was chosen as the f i r s t d e v i a t i o n from l o g - l i n e a r behavior. A s i m i ­ l a r l i n e a r r e l a t i o n s h i p e x i s t s between the volume of the t e t r a ­ gonal c e l l and the value o f x, again e x t r a p o l a t i n g t o the value of the pure VC> phase a t χ = 0 (see F i g u r e X I ) . The same behavior has been observed i n compounds corresponding t o the formula V W 0 (0$x^0.067) by Nygren and I s r a e l s s o n (19). x-x x . i s seen, t h e r e f o r e , that the a d d i t i o n o f f l u o r i n e tends to s t a b i l i z e the high temperature, higher symmetry, r u t i l e phase. The compositions c o n t a i n i n g l a r g e r amounts of s u b s t i t u t e d f l u o r i n e shows p r i m a r i l y m e t a l l i c behavior. However, f o r a l l compositions s t u d i e d there s t i l l appears t o be a d i s c o n t i n u i t y i n the r e s i s t i ­ v i t y a t the expected t r a n s i t i o n temperature ( T ) . The m e t a l l i c behavior observed i n these m a t e r i a l s may be ex­ p l a i n e d on the model presented by Goodenough (13) and Rogers (20). In t h e i r model the band formed between the overlap o f the t σ o r b i t a l s p a r a l l e l t o the c r y s t a l l o g r a p h i c c d i r e c t i o n s p l i t s 'into a more s t a b l e , p a i r - l o c a l i z e d , bonding V-V s t a t e and a higher l e s s s t a b l e σ* s t a t e . The lower l y i n g V-V l e v e l i f f i l l e d with one e l e c t r o n per vanadium and hence the semiconducting p r o p e r t i e s of the monoclinic V 0 may be e x p l a i n e d . The s u b s t i t u t i o n of f l u o r i n e f o r oxygen i n V 0 r e s u l t s i n the c r e a t i o n o f a d d i t i o n a l unpaired d - e l e c t r o n s . Despite the tendency f o r the more e l e c t r o ­ negative anion t o l o c a l i z e d - e l e c t r o n s , i t i s apparent t h a t f o r V 0

P

2

2

2

2

2

t

t

2

2

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

10

E X T E N D E D INTERACTIONS

BETWEEN

M

monoclinic

tetragonal

V

2°4

Figure

/

VO F 2-X X ι /'

L0Gp|

X-0-208

0

2 b

L 10

20

x. SO

7\

X0

I

-2

2

J

I

Figure

I

6

4

IX.

8

Log

P

I

10

vs. W/Ί

L

12

Ιθ/

Γ <

for VO>. ,F,

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

M E T A L IONS

WOLD

1.

Oxides

and

11

Chalcogenides

T a b l e 2.

C e l l Parameters of VO_» F C o m p o u n d s V

C e l l Parameters (A)

X

A c t i v a t i o n Energy (eV) Τ (°K)

Compound 4.530 - 4

V0„

V0

1.97 0.03

V0

1.96 0.04

2 %9 - 3 e

340

0.5 j u s t below Τ

F

4.552 -

F

4.554 ί 4

2.854 ί 3

0.06

282

VO

F 1.86 0.14

+ 4.562 - 4

+ 2.876 - 5

< 0.01

155

VO

F 1.79 0.21

+ 4.569 - 4

+ 2.886 - 3

< 0.01

65

Figure

X.

Transition

temperature

vs. composition

for

VO>.F.

r

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

12

EXTENDED

INTERACTIONS B E T W E E N M E T A L

IONS

the amount o f f l u o r i n e which has been s u b s t i t u t e d , the conduction paths have not been s u b s t a n t i a l l y a l t e r e d . IV. T r a n s i t i o n Metal Chalcogenides. Binary and ternary t r a n s i t i o n metal chalcogenides e x h i b i t a v a r i e t y o f d i f f e r e n t s t r u c t u r e s which a r e o f t e n q u i t e complex. This has been a t t r i b u ­ ted i n p a r t t o the considerable covalent nature o f the metals u l f u r bonds. Many o f these compounds a l s o e x h i b i t semimetallic or m e t a l l i c p r o p e r t i e s and t h i s i n d i c a t e s that not a l l o f the bonding e l e c t r o n s behave as i n simple covalent c r y s t a l s . A c h a r a c t e r i s t i c o f s u l f i d e minerals i s t h a t p a r t i a l or complete replacement o f e i t h e r the c a t i o n s or anions i s p o s s i b l e without a change i n the c r y s t a l s t r u c t u r e o c c u r i n g . Among the many chalco­ genides studied a t Brow cogenides, with the p y r i t a t t r a c t i v e s i n c e t h e i r e l e c t r o n i c p r o p e r t i e s can be r a d i c a l l y changed by s u b s t i t u t i n g f o r e i t h e r the c a t i o n or anion i n the host compound. This group o f compounds combine a simple s t r u c t u r e (Figure XII) with a wide v a r i e t y o f magnetic and e l e c t r i c a l properties(21). Of p a r t i c u l a r i n t e r e s t i s the e f f e c t o f c a t i o n and anion sub­ s t i t u t i o n on the ferromagnetic compound CoS^. A one-electron energy scheme f o r CoS has been described by B i t h e r e t a l (22) and i s shown i n F i g u r e ^ X I J I . ^ In t h i s model σ-d sp o r b i t a l s on the metal atom and sp o r ­ b i t a l s on the anions a r e assumed. S u l f u r has s i x valence e l e c ­ trons that a r e shared among four t e t r a h e d r a l bonds. Each s u l f u r c o n t r i b u t e s one e l e c t r o n t o the S-S bond and the remaining f i v e t o the three M-S bonds. Since a l l the M-S bonds are e q u i v a l e n t , each s u l f u r c o n t r i b u t e s 1 2/3 e l e c t r o n s t o each o f them. Cobalt i s coordinated t o s i x s u l f u r atoms a t the apices o f an octahedron and these s u l f u r atoms thus c o n t r i b u t e 6 x 1 2/3 = 1 0 e l e c t r o n s t o the bonding i n each octahedron. These ten e l e c t r o n s plus the two 4 s e l e c t r o n s o f the t r a n s i t i o n metal j u s t f i l l the ground s t a t e a(s-p) and σ e manifold o f s t a t e s . The remaining d - e l e c trons o f the metal occupy the next-lowest a v a i l a b l e l e v e l s . Since c o b a l t possesses seven d - e l e c t r o n s , the one unpaired e l e c t r o n i n d i c a t e d by the magnetic moment o f CoS^ suggests that the t l e v e l s a r e f i l l e d with s i x e l e c t r o n s ( s p i n - p a i r e d ) . The u n p a i r i d e l e c t r o n t h e r e f o r e occupies the σ* e s t a t e . In the presence o f a s u f f i c i e n t l y strong covalent i n f e r a c t i o n the σ* e l e v e l w i l l broaden i n t o a band o f c r y s t a l l i n e s t a t e s ; the e l e c trons a r e no longer bound to s p e c i f i c s i t e s but a r e f r e e t o move through the c r y s t a l under the i n f l u e n c e o f an e l e c t r i c f i e l d . This concept already has been discussed by Goodenough (23,24). t o e x p l a i n the occurrence o f m e t a l l i c c o n d u c t i v i t y i n oxides and sulfides. For both compounds CoS and CoSe , as w e l l as members o f the 2

2

g

0

0

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

1.

WOLD

Oxides

and

Chalcogenides

13

Figure Large

CoS

Co

s-s

6

]^s,p)

8 y P3

XII. Pyrite structure. halls are sulfur; small halls, cobalt.

•,\σΚβς)

^

2

f

3a*(S-S)

\\ÉrfCoS2> / e g

"χ

»

\ \b — \V"

Figure

XIII.

À

^ u

Inn* il * 9 \\ 2

Electron

energy diagram for CoS,

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

14

E X T E N D E D INTERACTIONS B E T W E E N

METAL

IONS

system CoS-_ Se , the only p a r t i a l l y f i l l e d s t a t e s are the o*e s t a t e s . This implies that i n these m a t e r i a l s the o*e states c o n s t i t u t e a band; the degree of covalence i s l a r g e . % h e occur­ rence o f an energy d i f f e r e n c e between the t ^ and a*e s t a t e s that i s l a r g e r than the intraatomic exchange enerfy ( l o w - l p i n con­ f i g u r a t i o n s i n l i g a n d f i e l d parlance) f o r the s o l i d s o l u t i o n s and CoS^ i s c o n s i s t e n t with the presence o f strong covalent i n t e r ­ actions. The stronger the bonding between the c a t i o n i c e or­ b i t a l s and the a n i o n i c o r b i t a l s , the l a r g e r w i l l be the s p l i t t i n g between the t ^ and the o*e s t a t e s . Since the l e v e l s are e s s e n t i a l l y noibonding they^are l i t t l e a f f e c t e d in^energy; hence, the energy d i f f e r e n c e between the t ^ and the o*e s t a t e s (which corresponds to the l i g a n d f i e l d s p l i f t i n g , or 10D§) becomes l a r g e r with i n c r e a s i n covalence CoAsS has been reporte conductor where the low s p i n s t a t e c o b a l t (d ) i s a l s o present i n an octahedral f i e l d . S u b s t i t u t i o n of a r s e n i c f o r s u l f u r i n CoS> r e s u l t s i n a continued depopulation o f e l e c t r o n s from the σ* a n t i bonding band ( 2 6 ) u n t i l f o r the end member CoAsS the σ* band i s empty. The p o s s i b i l i t y c^f r e p o p u l a t i n g t h i s band by p r o g r e s s i v e s u b s t i t u t i o n of n i c k e l d f o r c o b a l t d has r e c e n t l y been studied (27). The s u b s t i t u t i o n of a small amount o f n i c k e l (x = 0.05) f o r c o b a l t i n the system Co^ ^Ni^AsS changed the e l e c t r i c a l pro­ p e r t i e s from semiconducting to m e t a l l i c . H a l l - e f f e c t measure­ ments a l s o showed that the number o f c a r r i e r s ( e l e c t r o n s ) was p r o p o r t i o n a l to the n i c k e l concentration. These r e s u l t s are con­ s i s t e n t with the i n t r o d u c t i o n of e l e c t r o n s i n t o the σ* band as n i c k e l i s s u b s t i t u t e d i n t o CoAsS. g

This work was supported by the U.S. Army Research O f f i c e , Durham, the National Science Foundation and the M a t e r i a l s Research Laboratory Program at Brown U n i v e r s i t y .

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

1. WOLD

Oxides

and

Chalcogenides

15

Literature Cited 1 de Boer, J. Η., and Verwey, E. J. W., Proc. Phys. Soc. London (1937) A49, 59 2 Block, F., Z e i t s , f., Phys. (1928) 52, 555 3 Morin, F. J., Phys. Rev. L e t t e r s (1959) 3, 34 4 Pearson, A. D., J. Phys. Chem S o l i d s (1958) 5, 316 5 A d l e r , D., Rev. Mod. Phys. (1968) 40, 714 6 Goodenough, J . Β., Mat. Res. B u l l (1967) 2, 37 7 Goodenough, J . G., Mat. Res. B u l l (1967) 2, 165 8 Braeken, Z. Krist., (1931) 78, 484 9 F e r r e t t i , Α., Rogers, D., Goodenough, J . B., J. Phys. Chem. S o l i d s (1965) 26, 2007 10 Banks, Ε., and Wold, Α., "Preparative Inorganic Reactions", 4, 237 I n t e r s c i e n c e P u b l i s h e r s , New York, 1968 11 MacKintosh, A. R., 12 Sienko, M. J., "Pape 39, 224 Adv. Chem., Amer. Chem. Soc., Wash., D. C. 1963 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

Goodenough, J. B., B u l l Soc. Chim. F r . , (1965) 1200 Goodenough, J. B., S o l i d State Chem., (1971) 3, 490 Heckingbottom, L i n e t t , J. V., Nature (London) (1962) 194, 678 Hagg, G., Z., Physik Chem. (1935) B29 , 192 S l e i g h t , A. W., Inorg. Chem., (1969),8, 1764 P i e r c e , J . W., McKinzie, H. L., V l a s s e , Μ., Wold, Α., J. S o l i d State Chem., (1969) 1, 332 Nygren, Μ., I s r a e l s s o n , Mat. Res. Bull., (1969) 4, 881 Rogers, D. B., Shannon, R. D., S l e i g h t , A. W., G i l l d o n , J . L., Inorg. Chem., (1969) 8, 841 H u l l i n g e r , F., J. Phys. Chem. S o l i d s (1965) 26, 639 B i t h e r , Τ. Α., Bouchard, R. J., Cloud, W. Η., Donohue, P. A. and Siemond, W. J., Inorg. Chem. (1968) 7, 2208 Goodenough, J . B., "Magnetism and the Chemical Bond", I n t e r science P u b l i s h e r s , Inc., New York, (1963) Goodenough, J . B., Proc. Colloque I n t . Orsay 1965 (1967) C.N.R.S. No. 157, 263 Nahigian, H., Steger, J., McKinzie H., A r n o t t , R. J., Wold, Α., to be published Goodenough, J. B., J . S o l i d State Chem. (1971) 3, 26 Steger, J. J., Nahigian,Η., A r n o t t , R. J., Wold, Α., t o be published

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2 VO

H i g h Temperature Crystal Chemistry of

2

the Metal-Insulator Transition and of (Cr . 0

through

3

V

01

0.99

)2Ο ,

an Insulator WILLIAM

R. R O B I N S O N

P u r d u e University, W e s t Lafayette, Ind. 47907

A variety of interestin are observed upon heating the chromium doped V 2 0 3 system (Cr Vl-x) O . At 155-170°K, those systems with χ ≤ 0.018 undergo an antiferromagnetic i n s u l a t o r to metal t r a n s i t i o n (1). This t r a n s i t i o n i s accompanied by a change from a monoclinic s t r u c t u r e to a rhombohedral s t r u c t u r e isomorphous with α-corundum. Those systems with x > 0.018 e x h i b i t an antiferromagnetic i n s u l a t o r to i n s u l a t o r t r a n s i t i o n at about 170°K and maintain t h e i r negative thermal c o e f f i c i e n t s o f resistivity at elevated temperatures. The electrical behavior o f systems containing l e s s chromium (x < 0.018) i s q u i t e d i f f e r e n t at higher temperatures. Pure V O e x h i b i t s a continuous electrical t r a n s i t i o n i n the range 225-325°C which r e s u l t s in an increase i n resistivity of about one order o f magnitude ( 1 , 2 , 3 , 4 ) . The doped systems with x < 0.018 e x h i b i t an e l e c t r i c a l e f f e c t which was o r i g i n a l l y ascribed to a Mott t r a n s i t i o n (1) But which more r e c e n t l y has been described as an e x t r i n s i c e f f e c t r e s u l t i n g from the c o e x i s t ­ ence o f two phases (4). Above the t r a n s i t i o n a t 155-170°K, a l l of the various elec­ trical m o d i f i c a t i o n s of (Cr Vl ) 0 are rhombohedral and i s o ­ morphous with corundum. However, the s i z e o f the rhombohedral cell v a r i e s between two extreme v a l u e s . V0 at room temperature and the low temperature form o f (Cro. V ) 0 (the α form) have u n i t cell parameters with a approximately 4.95A and c approximately 14.00A i n the hexagonal indexing of the rhombo­ hedral system. V 0 at high temperature, the high temperature form o f (Cr V ) 0 (the β form), and (Cr V ) 0 have a approximately 5.00Α and c approximately 13.94A. The change i n the V 0 c e l l parameters i s continuous with temperature while the primary change i n the (Cr0 V ) 0 c e l l parameters occurs with the α-β s t r u c t u r a l t r a n s i t i o n . In view o f the s t r u c t u r a l changes accompanying the semicon­ ductor to metal t r a n s i t i o n upon heating Ti 0 (5,6) or doping with vanadium to give (V Ti ) 0 where x ≤ 0.1 (7,8), we thought i t i n t e r e s t i n g to follow the s t r u c t u r a l changes accompanying the e l e c t r i c a l t r a n s i t i o n i n V 0 in the 225°-325°C range. Since x

2

3

2

3

x

-x

2

3

2

o1

2

0.o1

2

0.99

o.99

3

2

3

3

2

0.o4

3

0.96 2

3

3

.01

0.99

2

3

2

x

l-x

2

3

3

2

3

16

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

3

2.

ROBINSON

Crystal

Chemistry

of

17

V 0, À

{

6 - ( C r . 0 ^ 0 . 9 9 ) 2 0 3 does not change much i n t h i s range, i t was s e l e c t e d as a reference m a t e r i a l . 0

Experimental. Samples of V2O3 and ( C r 0 . 0 ^ 0 . 9 9 ) 2 0 3 (4) were provided by Professor J . M. Honig, Department o f Chemistry, Purdue U n i v e r s i t y . A small s i n g l e c r y s t a l o f each material was cut from the boule and mounted along the 210 d i r e c t i o n i n evacuated s i l i c a c a p i l ­ l a r i e s using the procedure described by Brown, Sueno, and Prewitt (9). These c r y s t a l s were used i n a l l subsequent s t u d i e s . The u n i t c e l l dimensions reported i n Table I and Figure 1 were determined by l e a s t - s q u a r e s refinement of the 2Θ values of 15-20 r e f l e c t i o n s , p r i n c i p a l l y o f the type h k l , l y i n g i n the region 50° < 2Θ < 5 5 ° . The 2Θ values were 3ëtermined on a P i c k e r d i f f r a c t o m e t e r equippe average peak p o s i t i o n at p o s i t i v e and negative 2Θ was determined using the quarter height technique and graphite monochromated MoKa r a d i a t i o n . At each temperature i n d i c a t e d i n Table I, about 360 i n t e n ­ s i t i e s l y i n g i n a quadrant o f r e c i p r o c a l space with 5° < 2Θ < 62° were c o l l e c t e d . The d i f f r a c t o m e t e r was operated i n a Θ - 2Θ mode using a graphite monochromator and MoΚα r a d i a t i o n . Following a p p l i c a t i o n o f absorption c o r r e c t i o n s and the Lp c o r r e c t i o n , e q u i v a l e n t r e f l e c t i o n s were averaged g i v i n g about 100 independent r e f l e c t i o n s a t each temperature. Both i s o t r o p i c and a n i s o t r o p i c refinements were c a r r i e d out f o r each s e t of i n t e n s i t y data i n space group RJc using the V and 0 p o s i t i o n s reported by Newnham and deHaan (10) as s t a r t i n g param­ eters. Three c y c l e s o f refinement i n each case r e s u l t e d i n con­ vergence and gave f i n a l R values o f 0 . 0 2 - 0 . 0 4 . Selected s t r u c ­ t u r a l parameters are reported i n Table I and Figures 2 and 3. Results The c r y s t a l s t r u c t u r e o f V 0 at 23° was found to be i d e n t i c a l w i t h i n experimental e r r o r to that reported by Dernier (11) and Newnham and deHaan (10). The s t r u c t u r e of V 0 at 600°C ancT the s t r u c t u r e o f B - ( C r . o T V o . 9 9 ) 2 0 3 were found to be i d e n t i c a l to t h a t of (Cro.otCo.sehOaUl). The s t r u c t u r e s of a l l o f T h e s e systems, l i k e that of c o r u n ­ dum, c o n s i s t s of an approximately hexagonal c l o s e s t packed array of oxide ions with metal ions i n two-thirds of the octahedral holes (Figure 4 ) . Each metal ion has four near metal neighbors; one sharing an octahedral face of the c o o r d i n a t i o n polyhedron (Mi-M i n Figure 4) and three sharing edges of the octahedron 2

3

2

3

0

2

(M1-M3 i n Figure 4 ) .

The nonlinear v a r i a t i o n o f the u n i t c e l l parameters o f V 0 upon heating (Table 1 and Figure 1) i s s i m i l a r to t h a t p r e v i o u s l y reported (l_ 12). The l i n e a r increase with temperature of the c e l l parameters of B - ( C r . 0 ^ 0 . 9 9 ) 2 0 3 p a r a l l e l s that of 2

t

0

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

3

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2

s

2

0

2.697(1 2.880(1 2.051(1 1.968(1 2.676(3 2.804(1 2.889(1 2.952(1

2

*

2.747(1) 2.917(1) 2.061(1) 1.976(1) 2.661(2) 2.792(1) 2.897(1) 3.004(1)

23° 4.9974(1) 13.9260(6)

ο A l l e n t r i e s i n A.

2

3

Μχ-Μ M!-M Μχ-Oj Μχ-00i-0 Οχ-Ο* Οχ-Os 0.,-Os

7*iex ^hex

0

2.746(1) 2.919(1) 2.061(1) 1.977 1 2.663(2) 2.793(1) 2.898(1) 3.006(1)

113° 5.0018(1) 13.9238(7)

2.705(1) 2.888(1) 2.053(1) 1.970(1) 2.675(3) 2.802(1) 2.891(2) 2.962(2)

115° 4.9605(1) 13.9857(6) 2.734(1) 2.916(1) 2.064(1) 1.977(2) 2.680(3) 2.799(2) 2.897(1) 2.997(2)

2.728(1) 2.910(1) 2.061(1) 1.975(2) 2.676(3) 2.799(2) 2.895(2) 2.990(1)

2.718(1) 2.900(1) 2.057(2) 1.973(1) 2.675(4) 2.800(1) 2.893(1) 2.978(1)

2.739(1) 2.920(1) 2.062(2) 1.979 1) 2.670(2) 2.796(1) 2.899(1) 3.006(1)

310° 5.0052(1) 13.9318(6)

400° 5.0013(1) 13.9416(5)

0

and ( C r . 0 ^ 0 . 9 9 ) 2 0 3

300° 4.9912(2) 13.9489(6)

3

212° 4.9776(2) 13.9647(7)

2

C r y s t a l l o g r a p h i c Data f o r V 0

23° 4.9492(2) 13.998(1)

(Cr .oiV .99)203

0,-0s 0„-0

x

0 -0>

Mt-0, Μχ-0,, 0j-0

M1-M3

Μχ-Μ

V2O3

Table I:

2.738(1) 2.924(1) 2.066(1) 1.981(1) 2.681(3) 2.800(1) 2.901(1) 3.007(1)

600° 5.0140(2) 13.9427(7)

2.

ROBixsox

Crystal

Chemistry

of V,0.

19

{

A I4.0C%,

-V 0 2

3

- l%Cr-V 03 2

13.9813.96-

-hex

13.94 13.92 A* 5.00 -.. 4.98 4.964.94100 Figure

1.

300 T(°C)

Variation of unit cell with temperature

500 parameters

A 2.92

MO)- M(3)

2.91 2.90 2.89 238jh 2.74

M(D-M(2)

2.73 2.72 2.71 2.701 200

Figure

2. Variation tances with

400 T(°C) of metal-metal temperature

600

dis-

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

20

E X T E N D E D INTERACTIONS B E T W E E N

METAL

ο 2 07 h

1.96 i

0

.

i

200

.

i

.

400

1

600

T(°C) Figure

3.

Figure 4.

Variation with

Projection

of metal-oxygen temperature

of the corundum

distances

structure

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

ROBINSON

2.

Crystal

Chemistry

of

21

VjO.

{

(Cr .o»»Vo 96)203 (1) although the c e l l dimensions d i f f e r s l i g h t l y . It i s apparent from the data that the u n i t c e l l dimensions of the pure and doped systems converge at elevated temperatures. The coordinates o f the metal and oxygen atoms of V 0 and S - ( C r . o i V o 9 9 ) 2 0 3 a l s o converge with i n c r e a s i n g temperature. The changes i n m e t a l - m e t a l , metal-oxygen, and oxygen-oxygen distances which r e s u l t are given i n Table 1. P l o t s of metal-metal and metal-oxygen distances are presented i n Figures 2 and 3. Both the M!-M2 and M i - M 3 e q u i l i b r i u m distances increase with temperature i n V 0 . The M i - M 2 change from 2.697(1)A at 23° to 2.738(1)A a t 600° represents a change o f about 40 e . s . d . ' s as does the increase from 2.880(1)A a t 23° to 2.924(1)A a t 600° i n the M i - M 3 d i s t a n c e . Over the temperature range 23° to 310°C the M1-M3 e q u i l i b r i u m distance o f 6 - ( C r 01V0 9 9 ) 2 0 3 increases s l i g h t l y from 2.917(1)A to 2.920(1)A*whilé the distance decreases from 2.747(1) even though the c a x i s which i s p a r a l l e l to the Μ χ - Μ 2 d i r e c t i o n is increasing. No s i g n i f i c a n t changes i n the metal-oxygen d i s t a n c e s i n the doped system are observed upon h e a t i n g . The maximum change i s three e . s . d . ' s . Small changes i n the metal-oxygen d i s t a n c e s i n V 0 are observed. The M1-O1 d i s t a n c e v a r i e s by 0.015A from 2.051(1)A to 2.066(1)A while M i - 0 - v a r i e s from 1.968(1)A to 1.981(1)A, a change of 0.013A. The only s i g n i f i c a n t change i n oxygen-oxygen distances i s i n the 0i»-0 d i s t a n c e i n V 0 . This d i s t a n c e changes from 2.952(1 )A a t 23° to 3.007(1)A at 600°C. The other changes are 0.009A or less. In g e n e r a l , the s t r u c t u r a l changes i n V 0 upon heating involve a s l i g h t reduction of the spacing between the a p p r o x i ­ mately hexagonally c l o s e s t packed planes of oxygen atoms with a concommitant increase i n the average oxygen-oxygen separation i n the planes. One can d i s t i n g u i s h two types of oxygen-oxygen d i s t a n c e s w i t h i n a given p l a n e ; distances o f the type 0 i - 0 along the edge o f a shared octahedral face and distances o f the type Ο1-Ο3 or 0i*-0s along the edges o f an unshared face. It i s expansion of the oxygen-oxygen distances along the edges of the unshared face which i s r e f l e c t e d in the increase i n the a a x i s . P a r a l l e l i n g the increase i n the a axis i s the M i - M 3 s e p a r a t i o n . The M i - M 2 d i s t a n c e increases at the same rate as the M1-M3 d i s t a n c e . The net r e s u l t o f these changes i s that a t high temperatures V 0 , which i s i s o s t r u c t u r a l with a - ( C r . 0 ^ 0 . 9 9 ) 2 0 3 (13) a t room temperature, becomes i s o s t r u c t u r a l with 0

E

2

0

3

e

2

3

0

2

3

5

2

3

2

3

2

2

3

0

B^TCro.oiVo.99)203 and (Cro.owVo.96)203.

Acknow!edgements. T h i s work was done i n the Department o f Earth and Space S c i e n c e s , SUNY, Stony Brook, N.Y. We wish to thank Professor C. T . P r e w i t t f o r h i s a s s i s t a n c e with t h i s p r o j e c t and the

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

22

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

National Science Foundation (Grant No. GP-17554) and Purdue U n i v e r s i t y f o r support of the work. Literature Cited. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

McWhan, D. B. and Remeika, J. P . , Phys. Rev. B, (1970), 2, 3734. Foex, Μ., Compt. Rend., (1946), 223, 1126. M o r i n , F. J., Phys. Rev. Lett., (1959), 3, 34. Honig, J. M . , Chandrashekhar, G. V . , Sinha, A. P. B . , Phys. Rev. Lett., (1974), 32, 13. Honig, J. Μ., and Reed, T . Β . , Phys. R e v . , (1968), 174, 1020. Simonyi, E., and Raccah, P. Μ., B u l l . Am. Phys. Soc., (1973), 11, 339. Chandrashekhar, G. V . , Won C h o i , Q . , Moyo, J., and Honig, J. Μ., Mat. Res. Bull., Robinson, W. R . , J. S o l i d State Chem., (1974), 9 , 255. Brown, G. E., Sueno, S . , and P r e w i t t , C. T., Amer. M i n e r a l . , (1973), 58, 698. Newnham, R. E . and deHaan, Y. M . , Z. Kristallog., (1962), 117, 235. D e r n i e r , P. D . , J. Phys. Chem. S o l i d s , (1970), 31, 2569. E c k e r t , L . J. and B r a d t , R. C . , J. A p p l . P h y s . , (l973), 44, 8. Robinson, W. R . , Unreported Observations.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

3 Synthesis and Properties of Some N e w

Organic

Intercalation Complexes of Tantalum Sulfide R. L .

HARTLESS

and A . M .

TROZZOLO

B e l l Laboratories, M u r r a y Hill, N.J. 07974

Intercalation of layered compounds by metals has been p r e v i o u s l y r e p o r t e d ( 1 - 5 ) . A number o f i n o r g a n i c and biological m o l e c u l e s have been found to i n t e r c a l a t e c l a y s and g r a p h i t e ( 6 , 7 ) . The first intercalation of layered transition m e t a l c h a l c o g e n i d e s by o r g a n i c m o l e c u l e s was r e p o r t e d by Weiss and Rudhardt ( 8 ) . They intercalated TiS w i t h h y d r a z i n e and aliphatic amides and o b s e r v e d i n c r e a s e s o f s e v e r a l angstroms in the interlayer s p a c i n g o f the c h a l c o g e n i d e . More r e c e n t s t u d i e s show t h a t a v a r i e t y o f o r g a n i c m o l e c u l e s intercalate layered transition m e t a l c h a l c o g e n i d e s (9-11). The o r g a n i c m o l e c u l e s p e n e t r a t e the i n t e r l a y e r p l a n e s and form a p e r i o d i c crystalline structure. In a d d i t i o n to i n c r e a s e d i n t e r p l a n a r s p a c i n g , the i n t e r c a l a t e d complexes showed enhancement o f the critical superconducting temperature ( Tc) above t h a t o b s e r v e d f o r the unintercalated chalcogenide. Tantalum sulfide crystallizes i n several polytypes which are c h a r a c t e r i z e d by layer-like structures i n which the m e t a l atoms are l o c a t e d between a l t e r n a t e sulfur layers. The bonds forming the i n t r a p l a n a r l a y e r s are s t r o n g ( p r i m a r i l y c o v a l e n t ) bonds. Each s u l f u r - t a n t a l u m - s u l f u r l a y e r ( i n t e r p l a n a r ) i s bonded to a d j a c e n t l a y e r s by weak van der Waals attraction. Intercalation ( i n s e r t i o n ) o f atoms o r m o l e c u l e s between the l a y e r s o c c u r s by c l e a v a g e a l o n g the p l a n e s which have the low energy i n t e r a c t i o n . F i g u r e 1 g i v e s a schematic d e s c r i p t i o n o f the s t r u c t u r e o f the l a y e r e d transition metal chalcogenide prior to intercalation where M i s the transition m e t a l atom and X i s the c h a l c o g e n atom. Shown are t h r e e i n t r a p l a n a r l a y e r s s e p a r a t e d by the van der Waals gap. The intraplanar l a y e r s are a p p r o x i m a t e l y 6 Å t h i c k and the i n t e r l a y e r distance between transition m e t a l atoms i s a p p r o x i mately 6Å. Upon intercalation of the organic molecule, 2

o

23

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

24

EXTENDED

INTERACTIONS B E T W E E N

METAL

IONS

the van der Waals gap expands as s c h e m a t i c a l l y r e p r e ­ sented i n F i g u r e 2. T a b l e 1 shows the s y n t h e s i s con­ d i t i o n s and parameters f o r some p r e v i o u s l y i n t e r c a l a t e d complexes ( 9 ) . The i n t e r l a y e r d i s t a n c e f o r the p y r i d i n e - T a S g c o m p l e x i n c r e a s e s ~Joh t o g i v e a m e t a l m e t a l d i s t a n c e o f ^12A. In the case o f the o c t a decylamine-TaS^ complex, the enhanced i n t e r l a y e r d i s t a n c e was Λ,^ΟΑ which was a t t r i b u t e d t o two l a y e r s o f the amine o r i e n t e d end-to-end and p e r p e n d i c u l a r to the TaS2 p l a n e s ( F i g u r e 3 ) . In g e n e r a l , o r g a n i c m o l e c u l e s capable o f i n t e r c a l a t i n g T a S a r e Lewis bases c o n t a i n ­ i n g an sp3 o r s p h y b r i d i z e d n i t r o g e n . T h i s study i n v o l v e s i n t e r c a l a t i o n o f the 2Hp o l y t y p e o f tantalum s u l f i d e ( T a S ) w i t h o r g a n i c molecules o f variou increased i n t e r l a y e s u p e r c o n d u c t i n g temperatures. The e x t e n t o f i n t e r p l a n a r l a y e r expansion and T enhancement v a r i e s w i t h the p a r t i c u l a r i n t e r c a l a t e , thus a l l o w i n g p o s s i b l e chemical c o n t r o l of T . 0

2

2

2

c

c

Experimental The powdered tantalum s u l f i d e was p l a c e d i n a 4mm p y r e x tube w i t h the l i q u i d i n t e r c a l a t e o r a benzene s o l u t i o n o f the s o l i d i n t e r c a l a t e . The tube was evacuated and s e a l e d and the r e a c t i o n s c a r r i e d out i n i s o t h e r m a l baths a t 25, 100, 150 and 200°. Inter­ c a l a t i o n was e v i d e n t when the volume o f the chalcogenide increased. A f t e r i n t e r c a l a t i o n , the excess o r g a n i c r e a c t a n t s were removed by washing w i t h benzene f o l l o w e d by d r y i n g a t reduced p r e s s u r e . S u p p o r t i n g evidence f o r i n t e r c a l a t i o n was o b t a i n e d by powder x-ray d i f f r a c t i o n w i t h a G u i n i e r camera u s i n g CUKQ r a d i a t i o n . The c r i t i c a l s u p e r c o n d u c t i n g tem­ p e r a t u r e s were determined on powdered complexes by AC s u s c e p t i b i l i t y measurements. R e s u l t s and D i s c u s s i o n S e v e r a l mono- and d i s u b s t i t u t e d amines form complexes w i t h T a S ( T a b l e 2 ) . In g e n e r a l the amines which i n t e r c a l a t e d had p K > 9 . D i e t h y l a m i n e inter­ c a l a t e s i n two hours a t ambient temperatures to y i e l d a comçlex w i t h an i n t e r l a y e r d i s t a n c e i n c r e a s e (σ) o f 3.69 A and an onset o f s u p e r c o n d u c t i v i t y ( T ) o f 3.0°K. The d i b e n z y l amine complex was s u p e r c o n d u c t i n g at 2 7 ° K and the i n t e r l a y e r s p a c i n g was i n c r e a s e d by 6.39A. P i p e r i d i n e i n t e r c a l a t e s T a S and i n c r e a s e s the i n t e r l a y e r d i s t a n c e by 4.20A, but the complex i s not 2

a

Q

&

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

3.

HARTLESS A N D TROZZOLO

X

X

25

M

X

X

χ

VAN DER

X

χ

WAALS BOND

χ

Μ

Μ

X

X

VAN DER WAALS BOND

X

χ

"χ

Χ

Μ

Figure

Sulfide

X

M

X

Tantalum

Μ X

X

1.

Schematic

of structure

of TaS >

X

Χ

Χ

ο 6Α

Μ

M X

Χ

II

Χ

1

X

Χ

Χ Μ

M Χ

Χ

Χ

Figure 2. Schematic of structure of TaS> substituted pyridine complexes

TABLE I SYNTHESIS OF T a S

TaS

2

CH -(CH ), -NH 5

2

7

2

2

COMPLEXES

TEMP. (°C)

TIME (DAYS)

8(A)

T (»K>

—

—

6.05

0.8

200

1

40

30

0

5 81

3.5

50

3.0

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

26

E X T E N D E D INTERACTIONS B E T W E E N

T 6A i

METAL

IONS

x~^( χ M M X X X

0

25 A

Figure

50A

3. Schematic of structure TaS> (octade calamine

of

X X X M M X X X

TABLE 2 TdS - AMINE COMPLEXES 2

SYNTHESIS INTERCALATE

TEMP. C O

T I M E (DAYS)

B(l)

T (*K)

25

3.6a

3.0

150

4.02

150

6.39

2.7

25

6.15

2.4

3

4.20

NS

8

3.65

1.8

0

CH,CHo CH3CH2 CHJCH2CH2CH2

>

>-H

CH3CH2CH2CH2

;N-H

0-CH

0 - C H

2

2

C M N~N

- <

I50

200

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

3.

HARTLESS AND TROZZOLO

Tantalum

27

Sulfide

s u p e r c o n d u c t i n g down t o 1°K. 2-Aminoanthracene failed to form a complex even a t 200° f o r 30 days. T h i s contrasts with the i n t e r c a l a t i o n of a n i l i n e reported earlier. The i n a b i l i t y o f 2-aminoanthracene t o i n t e r ­ c a l a t e may be r e l a t e d t o a l e s s e r c a p a b i l i t y t o donate e l e c t r o n s t o t h e u n f i l l e d m e t a l l i c bands. S u b s t i t u t e d hydrazines r e a d i l y i n t e r c a l a t e the 2 H - p o l y t y p e o f T a S t o form complexes ( T a b l e 3 ) . The e l e c t r o n donor a b i l i t y o f t h e h y d r a z i n e s v a r i e s s u f f i c i e n t l y such t h a t c o r r e l a t i o n s w i t h t h e c r i t i c a l s u p e r c o n d u c t i n g t e m p e r a t u r e can be made. The charge d e n s i t y p o t e n t i a l l y a v a i l a b l e f o r t r a n s f e r t o the u n f i l l e d m e t a l l i c bands i s r e l a t e d t o t h e asymmetric charge d i s t r i b u t i o n r e p r e s e n t e d by t h e q u a d r u p o l e moment o f nucleus Gamble and o t h e r o f e l e c t r o n donors layered-transition m e t a l c h a l c o g e n i d e s i n v o l v e s some charge t r a n s f e r from the i n t e r c a l a t e d m o l e c u l e t o t h e i n c o m p l e t e l y f i l l e d bands o f t h e c h a l c o g e n i d e ( 9 , 1 0 ) . E h r e n f r e u n d , G o s s a r d and Gamble have o b s e r v e d an a l t e r a t i o n i n t h e t r a n s i t i o n - m e t a l K n i g h t s h i f t and e l e c t r i c f i e l d g r a d i e n t w h i c h i n d i c a t e s t h a t occupancy o f t h e con­ d u c t i o n band i s changed upon i n t e r c a l a t i o n ( 1 2 ) . T r a n s m i s s i o n s p e c t r a ( 0 . 4 - 5 . 0 eV) o f l a y e r e d t r a n s i t i o n m e t a l c h a l c o g e n i d e s show s h i f t s and b r o a d e n i n g s upon i n t e r c a l a t i o n w h i c h have been i n t e r p r e t e d i n terms o f i n c r e a s e d o c c u p a t i o n o f d bands ( 1 3 ) . A c c o r d i n g t o t h e Bardeen-Cooper-Schrieffer theory o f superconductivity, the c r i t i c a l s u p e r c o n d u c t i n g t e m p e r a t u r e can be i n ­ c r e a s e d by e l e c t r o n d e n s i t y enhancement a t t h e F e r m i l e v e l ( 1 4 ) . The f o r m a t i o n o f i n t e r c a l a t e d complexes from i n t e r c a l a t e s o f v a r y i n g e l e c t r o n donor a b i l i t y appears t o o f f e r a method o f e n h a n c i n g t h e e l e c t r o n d e n s i t y a t the Fermi l e v e l thereby i n c r e a s i n g the temperature f o r the onset o f s u p e r c o n d u c t i v i t y . There appears t o be a d i r e c t c o r r e l a t i o n between t h e enhancement o f t h e s u p e r c o n d u c t i n g t e m p e r a t u r e o f t h e h y d r a z i n e - T a S complexes and t h e a t o m i c - o r b i t a l o c c u p a t i o n numbers o f t h e u n s u b s t i t u t e d amino n i t r o g e n atoms ( T a b l e 4 ) . The o c c u p a t i o n numbers a r e from Sauer and B r a y and were d e t e r m i n e d p r i o r t o i n t e r ­ c a l a t i o n ( 1 5 ) . The o c c u p a t i o n number (σ) was d e r i v e d from n u c l e a r q u a d r u p o l e r e s o n a n c e d a t a and i s e x p r e s s e d as t h e d i f f e r e n c e between t h e σ-charge d e n s i t i e s o f t h e n i t r o g e n - h y d r o g e n and n i t r o g e n - n i t r o g e n bonds. As t h e o c c u p a t i o n numbers i n c r e a s e from 0.3429 t o 0.3795, t h e temperature f o r the onset o f s u p e r c o n d u c t i v i t y s p e c i e s i n c r e a s e s from 2 . 0 t o 4.9°K. I n t e r e s t i n g l y , t h e r e appears t o be no c o r r e l a t i o n between T and p K o f t h e 2

2

Q

a

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

28

EXTENDED INTERACTIONS B E T W E E N

METAL

IONS

TABLE 3 T 0 S - HYDRAZINE COMPLEXES 2

SYNTHESIS INTERCALATE

8(A)

T I M E (DAYS)

T fK) 0

2 5

2 HOURS

3.15

4.9

2

2 5

2 HOURS

3.53

3.3

2

25

2 HOURS

3.53

4.0

2

25

2 HOURS

3.14

2.0

H N-NH 2

CH

T E M R CO

2

3

NJ-NH H CH /

3

N-NH

CH

3

>J-NH

H

O

s

N—NH CH

Qj-NH

0

2

3

2

25

14

13.00,6.26

100

5

6.26,4.08

2.9,2.4

2.9

25

10

3.14,1.66

2.4,1.8

150

5

3.48,2.09

2.5,1.6

0

H-C-NH-NH-C-H

TABLE 4 CORRELATION OF PKa .CHARGE DENSITY (ACT) AND THE TEMPERATURE FOR THE ONSET OF SUPERCONDUCTIVITY (To) INTERCALATE H N-NH 2

CH

pKq

Δσ

T CK)

2

7.93

.3795

4.9

2

7.21

.3608

4.0

2

7.87

.3485

3.3

5.2

.3429

2.0

Q

3

^N-NH CH 3

CH X

3

N~NH

N-NH,

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

3.

ιιARTLESS A N D T R O Z Z O L O

Tantalum

29

Sulfide

intercalate. The c r i t i c a l s u p e r c o n d u c t i n g t e m p e r a t u r e s f o r t h e h y d r a z i n e , 1 , 1 - d i m e t h y l h y d r a z i n e and m e t h y l h y d r a z i n e complexes a r e 4 . 9 * 3-3°K, whereas t h e p K s a r e 7.93* 7.21 and 7.87 r e s p e c t i v e l y . T h i s has been o b s e r v e d by D i S a l v o f o r amine-TaS2 complexes i n w h i c h an i n c r e a s e i n p K may produce an i n c r e a s e o r d e c r e a s e i n T . T h i s i s c o n f i r m e d by n u c l e a r q u a d r u p o l e r e s o n a n c e d a t a where t h e e l e c t r o n d e n s i t y i n t h e nonbonding n i t r o g e n o r b i t a l s o f h y d r a z o n e s does n o t c o r r e l a t e w i t h T o f t h e i n t e r c a l a t e d complexes. Bray and Sauer have o b s e r v e d t h e same e f f e c t f o r p y r i d i n e - T a S complexes ( 1 6 ) · The c o r r e l a t i o n o f n u c l e a r q u a d r u p o l e resonance d e r i v e d charge d e n s i t i e s w i t h To i s c o m p l i c a t e d by t h e f o r m a t i o n o f two phas two d i s t i n c t i n t e r l a y e complex formed between T a S and 1 - p h e n y l - l methylhydrazine has i n t e r l a y e r distance increases o f 13.00 and 6.26 A and t h e Sauer and Bray o c c u p a t i o n number f o r t h e h y d r a z i n e i s Ο.388. The t e m p e r a t u r e f o r t h e o n s e t o f s u p e r c o n d u c t i v i t y f o r t h e complex s h o u l d be > 4.9°K, b u t t h e t r a n s i t i o n o c c u r s a t 2.9°K. The s y n t h e s i s c o n d i t i o n , i n t e r l a y e r d i s t a n c e i n c r e a s e , and t h e t e m p e r a t u r e f o r t h e o n s e t o f s u p e r ­ c o n d u c t i v i t y o f some Ta.S amide complexes a r e i n c l u d e d i n T a b l e 5. The i n t e r c a l a t i o n o f N-methylformamide under v a r i o u s r e a c t i o n c o n d i t i o n s p r o d u c e s d i f f e r e n t complexes. The complex formed a t ajnbient t e m p e r a t u r e s had an i n t e r l a y e r s p a c i n g o f 3.65 A and T = 4.2°K. The complex formed a t 100° had an i n t e r l a y e r s p a c i n g o f 3.00 A and was s u p e r c o n d u c t i n g a t 2.4°K. I f t h e r e a c t i o n i s c a r r i e d o u t a t 90° w i t h u l t r a s o n i c m i x i n g of the reactants, the i n t e r l a y e r distance increases to ^50 Κ and t h e complex i s s u p e r c o n d u c t i n g a t 2.7°K. This evidence supports the previous o b s e r v a t i o n that t h e r e i s no c o r r e l a t i o n between t h e i n t e r l a y e r d i s t a n c e and t h e t e m p e r a t u r e f o r t h e o n s e t o f s u p e r c o n d u c t i v i t y (9). D i - n - b u t y l formamide, N - m e t h y l f o r m a n i l i d e , and d i f o r m y l h y d r a z i n e a l s o f o r m complexes w i t h T a S . D i f o r m y l h y d r a z i n e formed a two phase complex w i t h i n t e r l a y e r s p a c i n g i n c r e a s e s o f 3.48 and 2 . 0 9 Κ and showed two t r a n s i t i o n s f o r t h e o n s e t o f s u p e r ­ c o n d u c t i v i t y a t 2.5 and 1.6°K. I n t e r p r e t a t i o n o f t h e r e s u l t s o f t h e amide complexes i s c o m p l i c a t e d by t h e t e n d e n c y o f t h e s e m o l e c u l e s t o decompose under t h e r e a c t i o n c o n d i t i o n s . Elemental a n a l y s i s i n d i c a t e s the p o s s i b i l i t y o f t h e i n t e r c a l a t i o n o f more than one s p e c i e f o r t h e N-methylformamide complex. The t e n d e n c y o f amide i n t e r c a l a t e s t o decompose has p r e v i o u s l y been o b s e r v e d by D i S a l v o . a

n

d

a

a

0

Q

2

2

0

2

Q

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

30

E X T E N D E D INTERACTIONS B E T W E E N

METAL

IONS

TABLE 5 T 0 S - AMIDE COMPLEXES 2

SYNTHESIS INTERCALATE Η 0 I II CHj-N—C—H

CH CH CH CH2 3

2

2

TEMP. C O

TIME (DAYS)

25 100 90

30MINS. 14 14

8(A)

3.65 3.00 ~50

T ÇK) 0

4.2 2.4 2.7

<j}

CH CH CH 3

2

0 II

0 II

H-C-NH-NH-C-H CH 0 ® - N —C-H

150

5

3.48,2.09

200

30

6.05

3

TABLE 6 TaS

2

COMLEXES EXHIBITING TWO PHASES

SYNTHESIS TEMP. CO

INTERCALATE

N—NH

2

CH, H

v

/ C

H

8(2)

25

14

13.00,6.26

100

5

6.26,4.08

ToCK)

2.9 2.9,2.4

3

Ν

100

30

4.85

2.7

I

150

30

4.85,3.95

2.7,1.5

150

60

3.95

1.5

100

30

13.63,7.58

3.5

CH CH 0-P=0 3

TIME (DAYS)

2

0

in

2

CHx CH CH 3

CH

s?

3

I

> - P = O

CH

3

N

/ \ CH CH 3

3

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2.5,1.6

3.

HARTLESS AND TROZZOLO

Tantalum

Sulfich

31

The p r e c e d i n g p a r a g r a p h r e f e r r e d t o the f o r m a t i o n o f a two-phase complex between T a S and d i f o r m y l ­ h y d r a z i n e i n w h i c h the complex had two i n t e r l a y e r s p a c i n g s and two t e m p e r a t u r e s f o r the o n s e t o f s u p e r ­ c o n d u c t i v i t y . The f o r m a t i o n o f two phase complexes appears t o be f a i r l y g e n e r a l and t h e r e are i n t e r e s t i n g r e l a t i o n s h i p s between σ, To, and r e a c t i o n c o n d i t i o n s (Table 6 ) . Reaction of 1-methyl-l-phenylhydrazine w i t h T a S a t ambient t e m p e r a t u r e s y i e l d e d a two-phase complgx w i t h i n t e r l a y e r d i s t a n c e i n c r e a s e s o f 13*00 and 6.26 A w h i c h was s u p e r c o n d u c t i n g a t 2.9°K. R e a c t i o n at h i g h e r t e m p e r a t u r e s y i e l d e d a complex i n w h i c h the 13.00 A d i s t a n c e d i s a p p e a r e d and a new d i s t a n c e (4.08 K) appeared. The 6.26 A i n t e r l a y e r d i s t a n c e i n c r e a s e was p r e s e n t i b o t h complexes Th appearance and d i s a p p e a r a n c i n the f o r m a t i o n o d i e t h y methylamidophosphat complexes. The complex formed a t 100° had one i n t e r ­ l a y e r d i s t a n c e o f 4.85 $ and one t e m p e r a t u r e f o r the o n s e t o f s u p e r c o n d u c t i v i t y . The complex formed a t 150° had 3.95 A and 1.5°K f o r the i n t e r l a y e r d i s t a n c e i n c r e a s e and s u p e r c o n d u c t i n g t r a n s i t i o n , r e s p e c t i v e l y . At i n t e r m e d i a t e r e a c t i o n c o n d i t i o n s , the complex formed had b o t h i n t e r l a y e r d i s t a n c e i n c r e a s e s and b o t h superconducting t r a n s i t i o n s . From p r e v i o u s r e s u l t s t h e d e c r e a s e i n the t r a n s i t i o n t e m p e r a t u r e w i t h a d e c r e a s e i n i n t e r l a y e r s p a c i n g cannot be a t t r i b u t e d s o l e l y t o the i n t e r l a y e r d i s t a n c e change. I t seems r e a s o n a b l e t h a t the i n c r e a s e d tempera­ t u r e s and r e a c t i o n t i m e s a l l o w the i n t e r c a l a t e t o r e a c h an e q u i l i b r i u m o r i e n t a t i o n where e l e c t r o n d o n a t i o n t o the u n f i l l e d m e t a l l i c bands i s l e s s e f f i c i e n t , t h e r e b y c a u s i n g a d e c r e a s e i n the t r a n s i t i o n temperature. 2

2

Summary A number o f o r g a n i c i n t e r c a l a t e d complexes o f the 2H-phase o f T a S have been p r e p a r e d w h i c h have i n ­ c r e a s e d i n t e r l a y e r s p a c i n g s and enhanced c r i t i c a l superconducting temperatures. There i s a d i r e c t c o r r e l a t i o n between the n u c l e a r q u a d r u p o l e resonance d e r i v e d charge d e n s i t i e s o f the i n t e r c a l a t e w i t h the enhanced s u p e r c o n d u c t i n g t e m p e r a t u r e s o f h y d r a z i n e complexes. Some o r g a n i c i n t e r c a l a t e s form two phase complexes w h i c h may have one o r two t e m p e r a t u r e s f o r the o n s e t o f s u p e r c o n d u c t i v i t y . 2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

32

EXTENDED INTERACTIONS BETWEEN METAL IONS Literature

Cited

1. Voorhoeve, J . M. and Robbins, M., J. Solid State Chem., (1970), 1, 134. 2. Huisman, R., Kadijk, F., and Jellinek, F., J. Less-Common Metals, (1970), 21, 187. 3.

Omloo, W. P. F. Α. Μ., and Jellinek, F., J. LessCommon Metals, (1970), 20, 121.

4. Rudorff, W., Chimia (1965), 19, 489. 5. Rudorff, W. and Sich H H. Angew Chem. (1959) 71, 724. 6.

Freeman, A. G. and Johnston, J. H., Carbon (Oxford), (1971), 9, 667.

7. Grim, R. E.,"ClayMinerology,"McGraw-Hill, New York, N.Y., 1968. 8. Weiss, A. and Rudhardt, R., Z. Naturforsch., Β (1969), 24, 265, 355, 1066. 9. Gamble, F. R., DiSalvo, F. J., Klemm, R. Α., and Geballe, T. H., Science (1970), 168, 568. 10.

Gamble, F. R., Osiecki, J. H., Cais, M., Pisharody, R., DiSalvo, F. J., and Geballe, T. H., Science, (1971), 174, 493.

11.

DiSalvo, F. J., Schwall, R., Geballe, T. H., Gamble, F. R., and Osiecki, J. Η., Phys. Rev. Lett., (1971), 27, 310.

12.

Ehrenfreund, E., Gossard, A. C., and Gamble, F. R., J. Phys. Rev. Β (1972) 5, 1708.

13.

Beal, A. R. and Liang, W. Y., Phil. Mag. (1973), 27, 1397.

14.

Bardeen, J., Cooper, L. N. and Schrieffer, J. R., Phys. Rev., (1957), 108, 1175.

15.

Sauer, Enrique G. and Bray, P. J., J. Chem. Phys., (1972), 56, 820.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

3.

16.

HARTLESS

AND

TROZZOLO

Tantalum

Sulfide

B r a y , P. J. and S a u e r , E n r i q u e G., Communications, (1972), 11, 1239.

33

Solid

State

Acknowledgment The a u t h o r s acknowledge helpful d i s c u s s i o n s w i t h F. J. D i S a l v o , Jr., G. W. Hull, T. H. G e b a l l e , D. W. Murphy and F. J. Padden, Jr. We particularly thank G. W. Hull f o r t h e superconductivity data.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

4 Intermolecular Magnetic Exchange Interactions in Molecular Crystals J. M .

GROW

and H .

H.

WICKMAN

O r e g o n State U n i v e r s i t y , C o r v a l l i s , O r e . 97331

Abstract Intermolecular exchange interactions have been studied in a representative, homologous series of metal complexes: iron(III) bis (dithiocarbamates) . Several of these molecular crystals exhibit cooperative m a g n e t i c order at t e m p e r a t u r e s b e l o w approximately 4 °K. Exchange paths include weak interactions between formally non-bonded molecular units. The m a g n e t i c phenomena a r e sensitive to zero-field splittings within the orbital-singlet, spin-quartet ground term o f iron(III). Discussion Molecules containing a single paramagnetic ion s u r r o u n d e d by b u l k y , organic ligands often form magnetically dilute molecular crystals. For metalm e t a l d i s t a n c e s g r e a t e r t h a n a b o u t 7 A, t h e d i p o l a r f i e l d s a r e s i g n i f i c a n t a s an o r i g i n o f m a g n e t i c o r d e r o n l y a t t e m p e r a t u r e s b e l o w -0.01 °K. Hence, most such m e t a l complexes remain s i m p l e paramagnets t o q u i t e low temperatures. However, s e v e r a l homologues o f t h e iron(III) halo-bis-dithiocarbamatesorder magnetically a t t e m p e r a t u r e s i n t h e r a n g e 1-4 K (_l-_3) . We summarize h e r e c e r t a i n s t r u c t u r a l o r c r y s t a l l o g r a p h i c f e a t u r e s o f t h e s e c o m p l e x e s w h i c h a p p e a r t o be i m p o r t a n t d e t e r m i n a n t s f o r exchange paths i n these molecular crystals. I n a d d i t i o n , we n o t e t h a t v a r i a b l e ground term c r y s t a l f i e l d interactions may enhance o r depress the c o o p e r a t i v e t r a n s i t i o n s f o r a f i x e d c r y s t a l s t r u c t u r e o r s e t o f exchange p a t h s . The s t r u c t u r a l f o r m u l a f o r an i r o n ( I I I ) b i s - d t c i s g i v e n i n F i g . 1. Much o f t h e c r y s t a l l o g r a p h i c i n f o r m a t i o n on b i s - d t c s has b e e n r e p o r t e d by H o s k i n s , e

34

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

complex of a halobisdithiocarbamatoiron(IH) formula Structural 1. Figure

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

36

EXTENDED

INTERACTIONS B E T W E E N

METAL

IONS

M a r t i n , W h i t e , a n d c o - w o r k e r s (4_-6_) . F o r example, the dimensions g i v e n i n the F i g u r e are f o r the d i e t h y l chloro derivative = e t h y l , i = l , 4 ; X = CI) (_5) . The i r o n ( I I I ) i s s i t u a t e d n e a r t h e c e n t r o i d o f a r e c t a n g u l a r p y r a m i d w h o s e a p e x may be o c c u p i e d b y a m o n o v a l e n t a n i o n ( h a l i d e , SCN", e t c . ) and whose b a s e c o n s i s t s o f 2 p a i r o f s u l f u r atoms f r o m t h e d t c ligands. T h i s s y m m e t r y l e a d s t o an o r b i t a l - s i n g l e t , s p i n - q u a r t e t g r o u n d t e r m (£) . The f o u r - f o l d s p i n d e g e n e r a c y i s removed by s p i n - o r b i t e f f e c t s , t o g e t h e r w i t h m e t a l - l i g a n d i n t e r a c t i o n s (7_-8_) , a n d two K r a m e r s d o u b l e t s , s e p a r a t e d b y ~2 t o 20 cm" , a r e commonly observed (9). The f o r e g o i n g l e v e l s t r u c t u r e i s a t t r a c t i v e f o r magnetism s t u d i e s First th low-symmetr environ ment i s f e l t w i t h i rank c r y s t a l f i e l d p o t e n t i a l , y y spin-Hamiltonian c o e f f i c i e n t s . Second, the magnetic e x c h a n g e i s r e p r e s e n t e d by an i s o t r o p i c Heisenberg i n t e r a c t i o n , owing t o the s m a l l o r b i t a l c h a r a c t e r o f the ground term. These i n t e r a c t i o n s o f t e n t u r n out t o be s m a l l c o m p a r e d w i t h t h e c r y s t a l f i e l d s p l i t t i n g s ; t h e r e s u l t i s t h a t t h e m a g n e t i c p r o p e r t i e s may be d i s c u s s e d w i t h i n a s i n g l e ground Kramers d o u b l e t . The well-known Hamiltonian f o r the s p i n - q u a r t e t m a n i f o l d is 1

The the

p a r a m e t e r s D and Ε d e s c r i b e t h e c r y s t a l f i e l d and remaining term r e p r e s e n t s magnetic exchange. B o t h f e r r o m a g n e t i c , c h l o r o - b i s - d i e t h y l d t c (1) , and a n t i f e r r o m a g n e t i c , i o d o - b i s - d i e t h y l d t c ( 3 ) , h o m o l o g u e s a r e known. The t r a n s i t i o n t e m p e r a t u r e s a r e 2.48 °K a n d 1.9 °K, r e s p e c t i v e l y . M o r e r e c e n t l y we have o b s e r v e d c o o p e r a t i v e t r a n s i t i o n s i n the f o l l o w i n g complexes: Fe(morphylyldtc) I, Fe(morphylyldtc)2Br, Fe(morphylyldtc) C1, Fe(pyrrolidyldtc) C1, F e ( p y r i o l i d y l d t c ) I , and F e ( p y r i d y l d t c ) C 1 . In these complexes, t h e m a g n e t i c o r d e r has been o b s e r v e d by Mossbauer spectroscopy (1^,2^) . A l l t r a n s i t i o n s occur b e l o w 4 °K. C r y s t a l s t r u c t u r e d a t a p r o v i d e s a means f o r i d e n t i f y i n g exchange p a t h s between atoms i n a d j a c e n t molecules. O n l y a few s t r u c t u r e s h a v e b e e n r e p o r t e d , b u t i n c l u d e d a r e t h e f e r r o m a g n e t i c and a n t i f e r r o ­ m a g n e t i c d e r i v a t i v e s m e n t i o n e d a b o v e (5^-6^) . A d e t a i l e d d i s c u s s i o n of the o r i e n t a t i o n s o f the m o l e c u l e s i n c o n n e c t i o n w i t h exchange paths i s 2

2

2

2

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

4.

GROW A N D

/ntcrmolccular

WICK Μ A Ν

37

Exchange

h

a

v

e

a v a i l a b l e e l s e w h e r e O'^'É.) · We recently obtained s t r u c t u r a l i n f o r m a t i o n on t h e SCN-bis-diethyldtc, b r o m o - b i s - d i e t h y l d t c , and isodo-bis-morpholyldtctoluene adduct. The l a t t e r e x h i b i t s a m a g n e t i c t r a n s i t i o n at approximately 2 °K. The f o l l o w i n g p i c t u r e e m e r g e s f r o m a s t u d y o f t h e structural information. As e x p e c t e d , i r o n - i r o n d i s t a n c e s i n a l l complexes are i n e x c e s s o f 7 Â. In t h o s e c o m p l e x e s w h o s e s t r u c t u r e i s known a n d w h i c h show a m a g n e t i c t r a n s i t i o n , o r e x c h a n g e b r o a d e n e d e p r spectra, there e x i s t s a j u x t a p o s i t i o n of molecules s u c h t h a t t h e r e i s an i n t e r m o l e c u l a r S » - S l i n k a g e . T h a t i s , we may w r i t e t h e i n t e r m o l e c u l a r i n t e r a c t i o n a s F e - S » S - F e ^ w i t h t h e Fe-S u n i t s b e l o n g i n g t o d i s t i n c t molecules. The S***S 4.5 A. The e x c h a n g l a t t i c e i n rather complicated paths: however, simple d i m e r i n t e r a c t i o n s may also occur. Because of the packing c h a r a c t e r i s t i c s of the p a r t i c u l a r l a t t i c e d e t e r m i n e d t h u s f a r , we h a v e n o t o b s e r v e d m o l e c u l e s a r r a n g e d so t h a t the p o t e n t i a l s t r o n g exchange p a t h Fe-X-^Fe-X i s favored. We h a v e a l s o n o t o b s e r v e d p a c k i n g w h i c h p r o d u c e s s a n d w i c h e d d t c l i g a n d s , s u c h as overlapping aromatic r i n g s t r u c t u r e s . O u r pmr shifts (3) show a d e r e a l i z a t i o n o f t h e i r o n s p i n d e n s i t y t o the d t c p e r i p h e r y , but the major d e r e a l i z a t i o n i s t o the immediate i r o n l i g a n d s . This i s of course cons i s t e n t w i t h the Fe-S- »S-Fe type exchange paths. I t i s i n t e r e s t i n g t h a t the e x i s t e n c e o f subs t a n t i a l exchange i n t e r a c t i o n s i s a necessary but not s u f f i c i e n t c o n d i t i o n f o r magnetic o r d e r i n the t e m p e r a t u r e r a n g e o f o u r e x p e r i m e n t s , Τ > 1°K. This may be s e e n b y c o n s i d e r i n g t h e e f f e c t o f g r o u n d K r a m e r s l e v e l a n i s o t r o p i e s on the c o l l e c t i v e m a g n e t i c p r o p e r t i e s of the complexes. T h a t i s , we c o n s i d e r a s i t u a t i o n where a f i x e d i n t e r m o l e c u l a r exchange e x i s t s b e t w e e n m o l e c u l e s i n two i s o m o r p h o n s l a t t i c e s . The m o l e c u l a r l a t t i c e s a r e assumed t o d i f f e r o n l y i n t h e type of s p l i t t i n g t h a t occurs i n the ground S = 3/2 manifold. I n Eq. ( 1 ) , i f D i s l a r g e i n m a g n i t u d e and d o m i n a t e s t h e r h o m b i c c r y s t a l f i e l d and t h e exchange t e r m , we may r e w r i t e the exchange i n t e r a c t i o n w i t h i n t h e g r o u n d K r a m e r s d o u b l e t u s i n g an e f f e c t i v e s p i n S = 1/2 (low t e m p e r a t u r e a p p r o x i m a t i o n ) . This i l l u s t r a t e s the magnetic a n i s o t r o p y o f the o r i g i n a l IS = 3/2, M = ± 3/2> Kramers d o u b l e t . The r e s u l t i s e

e e

e

g

TC

I

=

-2

I

9J

S S i j iz iz

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

(2)

38

E X T E N D E D INTERACTIONS

B E T W E E N M E T A L IONS

This i s the usual Ising Hamiltonian (E i s n e g l e c t e d ) . U p o n c h a n g i n g t h e s i g n o f D, b u t m a i n t a i n i n g t h e r e l a t i v e magnitudes o f the i n t e r a c t i o n s , the a l t e r n a t i v e ground doublet y i e l d s the a n i s o t r o p i c Heisenberg interaction "K* = -2

Τ

J..[S.

S. + 4 ( S . S. +S.

S. ) ] .

(3)

I t i s e a s i l y shown (10) t h a t i n t h e m o l e c u l a r f i e l d approximation the r a t i o o f magnetic t r a n s i t i o n temperatures w i t h and w i t h o u t t h e c r y s t a l f i e l d D term (exchange f o r c e s c o n s t a n t ) , i s g i v e n by t h e expression Τ

(D)/T

I n E q . (4) , <S >IJ» i s the i o n i c expectation value S a t t h e magfietië t r a n s i t i o n t e m p e r a t u r e . For D-*+<» a n d o u r l o w t e m p e r a t u r e a p p r o x i m a t i o n w i t h H p a r a l l e l t o the z-axis of Eqs. ( l ) - ( 3 ) , the r a t i o is 2

of

2

2

T (D)/T (0) c

c

= 1/5.

.

(5)

T h i s r e p r e s e n t s a d e p r e s s i o n o f t h e o r d e r i n g tempera t u r e due t o c r y s t a l f i e l d e f f e c t s . C o n v e r s e l y , as we f i n d T (D)/T (0) c

c

= 9/5.

(6)

Thus t h e t r a n s i t i o n t e m p e r a t u r e i s enhanced. In e i t h e r c a s e , however, t h e c r y s t a l f i e l d induced anisotropies are important f a c t o r s i n the occurence of magnetic order i n a b i s - d t c . T h i s p o i n t becomes i m p o r t a n t when i t i s n e c e s s a r y t o a s s e s s a c c u r a t e l y t h e m a g n i t u d e o f e x c h a n g e c o n s t a n t s i n E q . (1) w h i l e working w i t h systems i n which o n l y a s i n g l e Kramers l e v e l i s o c c u p i e d {3) · I d e a l l y , i t i s d e s i r a b l e t o know i n some d e t a i l the s p a t i a l d i s t r i b u t i o n o f s p i n d e n s i t y i n the p e r i p h e r y o f a m o l e c u l e , s i n c e t h i s i n l a r g e measure dominates i n t e r m o l e c u l a r exchange. A single, static o r i e n t a t i o n o f molecules i n a c r y s t a l l a t t i c e does not provide t h i s i n f o r m a t i o n . Fortunately, molecular c r y s t a l s o f t e n e x h i b i t polymorphism i n t h e i r c r y s t a l h a b i t s , o r t h e y may f o r m s e v e r a l s o l v e n t a d d u c t structures. We h a v e r e c e n t l y f o u n d t h a t s u c h i s t h e case i n F e ( m o r p h y l y l d t c ) I . A t l e a s t f i v e polymorphs and/or s o l v e n t adducts have been i s o l a t e d ; each 2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

4.

GROW A X D WICKMAN

Intermolecular

39

Exchange

e x h i b i t s somewhat d i f f e r e n t m a g n e t i c p r o p e r t i e s . Determination o f the c r y s t a l s t r u c t u r e s i s i n progress. As t h e r e l a t i v e o r i e n t a t i o n s o f m o l e c u l e s a r e c o r r e l a t e d w i t h m a g n e t i c p r o p e r t i e s , we e x p e c t a further extension of our understanding o f factors i m p o r t a n t f o r m a g n e t i c exchange i n Van d e r Waal s lattices. 1

Literature Cited 1.

Wickman, Η. Η . , Trozzolo, Α. Μ., Williams, H. J., Hull, G. W., a n d Merritt, F . R., P h y s . R e v . (1967) 155, 563. 2. Kostikas, Α., Petridis, D., S i m o p o u l o s , Α . , a n d Pasternak, Μ., Solid Stat Comm (1973) 1 3 1 6 6 1 3. C h a p p s , G. E . S h e r w o o d , R. C., J. Phys (1974) , 4. H o s k i n s , B. F . , Martin, R. L . , a n d W h i t e , A. H., N a t u r e (1966) 2 1 1 , 6 2 7 . 5. H o s k i n s , B. F . , a n d W h i t e , A . H., J. Chem. S o c . (A) ( 1 9 7 0 ) , 1 6 6 8 . 6. H e a l y , P. C., W h i t e , Α . Η . , a n d Hoskins, B. F . , J. Chem. S o c . (A) ( 1 9 7 2 ) , 1 3 6 9 . 7. A k e , R. L . , a n d H a r r i s - L o e w , G. M., J. Chem. P h y s . (1970) 5 2 , 1 0 9 8 . 8. de Vries, J. L. K. E., Keijzers, C. P., a n d de B o e r , Ε . , Inorg. Chem. (1972) 1 1 , 1 3 4 3 . 9. Brackett, G. C., Richards, P. L . , a n d C a u g h y , W. S. Jr., J. Chem. P h y s . (1971) 54, 4 3 8 3 . 10. Lines, Μ . Ε . , P h y s . R e v . (1967) 1 5 6 , 5 3 4 .

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

5 Computation of Field-Swept E P R Spectra for Systems with Large Interelectronic Interactions R. L .

BELFORD,

P.

H.

DAVIS, G.

G.

BELFORD,

and T. M .

LENHARDT

Center for A d v a n c e d C o m p u t a t i o n , University of Illinois, U r b a n a - C h a m p a i g n ,

Ill.

61801

Abstract. The c o m p u t a t i o physical model often is a particular p r o b l e m for m e t a l s y s t e m s having large interelectronic interactions, because o f t h e l a r g e field range c o v e r e d . T h i s contribution p o i n t s out t h a t theoret­ ical f o r m u l a t i o n s c a n be d e v i s e d specifically f o r this p u r p o s e and s k e t c h e s t h e development and u s e o f two s u c h methods - one e x a c t and one a p p r o x i m a t e . 1.

Introduction

As s e v e r a l o t h e r contributions to this volume a t t e s t , e l e c ­ t r o n p a r a m a g n e t i c r e s o n a n c e i s among t h e most i m p o r t a n t t o o l s f o r s t u d y i n g e l e c t r o n i c i n t e r a c t i o n s between m e t a l c e n t e r s . Large z e r o - f i e l d s p l i t t i n g s , leading t o epr t r a n s i t i o n s over a wide range o f magnetic f i e l d i n t h e t y p i c a l f i x e d - f r e q u e n c y s p e c t r u m , c h a r a c t e r i z e many h i g h - s p i n m u l t i c e n t e r s y s t e m s . In t h e s e c a s e s , computation o f p r e d i c t e d epr s p e c t r a from t r i a l H a m i l t o n i a n p a r a m e t e r s i s c o m p l i c a t e d by t h e f a c t t h a t t h e H a m i l t o n i a n i s a f u n c t i o n o f t h e s p e c t r o s c o p i c sweep v a r i a b l e , the m a g n e t i c f i e l d . Suppose a l l t h e H a m i l t o n i a n p a r a m e t e r s which c h a r a c t e r i z e a system are given. Then t h e p r o b l e m i s t o f i n d a l l v a l u e s o f t h e m a g n e t i c f i e l d , x, s u c h t h a t two e i g e n ­ v a l u e s o f t h e H a m i l t o n i a n d i f f e r by t h e s p e c t r o m e t e r e n e r g y , w. In t h e u s u a l c a s e o f l i n e a r Zeeman t e r m s , a s y s t e m o f η b a s i s s t a t e s may g i v e r i s e t o as many as n ( n - l ) / 2 d i f f e r e n t t r a n s i t i o n fields. A l t h o u g h t h e two e n e r g y e i g e n v e c t o r s a s s o c i a t e d w i t h e a c h t r a n s i t i o n f i e l d a r e o r t h o g o n a l , n e i t h e r need be o r t h o g o n a l t o any o f t h e o t h e r n -n-2 e i g e n v e c t o r s i n v o l v e d i n t r a n s i t i o n s at t h e o t h e r f i e l d s . V a r i o u s methods t o t r e a t t h i s p r o b l e m a r e commonly employed ( ] ) . One example i s c o n s t r u c t i o n o f a map o f e n e r g y l e v e l s as f u n c t i o n s o f f i e l d by e i g e n s y s t e m s o l u t i o n on a f i n e magnetic f i e l d g r i d (2). A n o t h e r i s t h e c o n v e n t i o n a l p e r ­ t u r b a t i o n treatment i n which the magnetic f i e l d ( o r f i e l d s h i f t ) i s t h e p e r t u r b a t i o n p a r a m e t e r and t h e e n e r g y l e v e l s and t h u s f r e ­ quency a r e d e v e l o p e d as power s e r i e s i n t h e f i e l d ( s h i f t ) ; t h e 2

40

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

5.

BELFORD E T A L .

Field-Swept

EPR

41

Spectra

f r e q u e n c y s e r i e s must be t e r m i n a t e d and t h e c o r r e c t r o o t o f t h e p o l y n o m i a l w( x ) - w e x t r a c t e d (.3,4). W h i l e a l l t h e s e methods c a n be made t o work, and some do v e r y w e l l i n some c a s e s , t h e y a r e g e n e r a l l y adapted w i t h minimal m o d i f i c a t i o n from t e c h n i q u e s ap­ p r o p r i a t e t o c a l c u l a t i o n o f energy l e v e l s f o r systems c h a r a c ­ t e r i z e d by f i x e d Hami1 t o n i a n s . As s u c h , one m i g h t e x p e c t them t o be g e n e r a l l y l e s s e f f i c i e n t o r l e s s p r e c i s e and d i r e c t t h a n methods e s p e c i a l l y d e v e l o p e d t o h a n d l e t h e f i x e d - f r e q u e n c y f i e l d swept p r o b l e m . We have begun t o l o o k i n t o t h e p o s s i b i l i t y o f d e ­ v i s i n g s u c h methods, and h e r e we o u t l i n e two o f them. The f i r s t , the e i g e n f i e l d formulation ( J j , displays a l l the t r a n s i t i o n f i e l d s a s t h e r e a l e i g e n v a l u e s , x, o f a g e n e r a l i z e d e i g e n v a l u e e q u a t i o n AZ=xBZ. S e c t i o n 2 d i s c u s s e s t h e e i g e n f i e l d t e c h n i q u e b r i e f l y ; i t has been d e s c r i b e d i n some d e t a i l r e c e n t l y (V) a s has i t s a p p l i c a t i o n t o time-dependen p r o b l e m s ( 5 ) . The e i g e n f i e l w a r d b u t s l o w f o r p r o b l e m s w i t h many b a s i s s t a t e s . The s e c o n d method, d e v e l o p e d i n S e c t i o n 3, i s a p e r t u r b a t i o n f o r m u l a t i o n d e ­ vised to predict transition fields. 2.

Eigenfield

Equations f o r Exact

Calculation

A r e c e n t p a p e r ( ] ) d e s c r i b e s a new ( g e n e r a l i z e d e i g e n s y s t e m ) f o r m u l a t i o n f o r e x a c t c a l c u l a t i o n o f r e s o n a n c e f i e l d s and i n t e n ­ s i t i e s f o r a m o l e c u l e o r l a t t i c e s i t e d e s c r i b e d by a t i m e independent H a m i l t o n i a n which i s a polynomial f u n c t i o n o f f i e l d ; i t a l s o d e s c r i b e s some p r o p e r t i e s o f t h i s f o r m u l a t i o n , w h i c h we c a l l t h e e i g e n f i e l d e q u a t i o n . The e i g e n f i e l d c o n c e p t c a n a l s o be used t o f a c i l i t a t e s o l u t i o n o f r e l a x a t i o n master e q u a t i o n s , which p r o v i d e i n t e n s i t y as a f u n c t i o n o f f i e l d f o r m o l e c u l e s i n slow m o t i o n o r f o r e x c i t o n h o p p i n g among l a t t i c e s i t e s i n a c r y s t a l (5). H e r e , we r e s t r i c t o u r d i s c u s s i o n t o a s i m p l i f i e d d e v e l o p ­ ment o f t h e s t a t i c e i g e n f i e l d e q u a t i o n f o r s y s t e m s w i t h l i n e a r Zeeman t e r m s a n d t o a p r a c t i c a l a p p l i c a t i o n t o a s y s t e m o f i n t e r ­ a c t i n g t r a n s i t i o n metal ions. A s we s h a l l s e e , t h e e i g e n f i e l d approach i s very a t t r a c t i v e because i t i s a d i r e c t , s t r a i g h t f o r ­ w a r d , e x a c t method, b u t a t p r e s e n t i t has t h e d r a w b a c k t h a t com­ p u t a t i o n t i m e i n c r e a s e s much t o o r a p i d l y w i t h t h e number o f b a s i s states. A c c o r d i n g l y , u n t i l b e t t e r numerical t e c h n i q u e s a r e de­ v e l o p e d f o r computer s o l u t i o n o f t h e e i g e n f i e l d e q u a t i o n s , t h e method i s o f p r a c t i c a l u s e o n l y f o r s y s t e m s h a v i n g j u s t a few (~10) b a s i s s t a t e s . I t does p r o v i d e r e s o n a n c e f i e l d s and t r a n s i ­ t i o n v e c t o r s o f h i g h a c c u r a c y w h i c h we have employed a s t e s t s t a n d a r d s o f e x c e l l e n c e f o r a p p r o x i m a t e methods. 2.1 Development o f E i g e n f i e l d E q u a t i o n . F o r c o n v e n i e n c e , we u s e t h e m a t r i x r e p r e s e n t a t i o n t h r o u g h o u t . M a t r i x o p e r a t o r s , v e c t o r s , and s c a l a r s a r e d i s t i n g u i s h e d by c o n t e x t , n o t by s p e c i a l symbols. The H a m i l t o n i a n , H, f o r t h e p r o b l e m i s t h e sum o f two p a r t s — a z e r o - f i e l d t e r m , F, ( e x c h a n g e c o u p l i n g , d i p o l a r and

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

42

E X T E N D E D INTERACTIONS B E T W E E N

METAL

IONS

p s e u d o d i p o l a r c o u p l i n g , h y p e r f i n e c o u p l i n g , e t c . ) and a Zeeman t e r m , xG ( e l e c t r o n i c and n u c l e a r Zeeman e n e r g i e s ) . Here χ d e ­ notes t h e magnitude o f t h e magnetic f i e l d . F o r example, t h e c o u p l i n g o f two Cr i o n s ( S=3/2) may be d e s c r i b e d bv a s j j i n H a m i l t o n i ^ n ^ H = F + x G i n w h i c h F=D( S , + S - V 2 ) + 3 , · ^ S 4 J S ,·Τ and G={3e.g.S, where e d e n o t e s a u n i t v e c t o r i n t h e d i r e c t i o n o f the magnetic f i e l d . Now f o r a f i x e d s p e c t r o m e t e r e n e r g y , w, r e s o n a n c e l i n e s w i l l o n l y o c c u r between two s t a t e s , u-»v, when t h e f o l l o w i n g two c o n d i ­ tions are s a t i s f i e d together: 2

2

z

2 Z

ζ

2 Z

2

( F+xG-e)u=0 , Γ2-11 L

1

( F+xG-e-w)v=0

That i s , i f e i s t h e energy o f t h e upper s t a t e c o n v e r t e d i n t o a s i n g l e one w h i c h c o n t a i n s o n l y one e i g e n v a l u e , x, by means o f o u t e r - p r o d u c t a l g e b r a . The o u t e r p r o d u c t , A 0 B, o f a n mXn m a t r i x A and an m'Xn m a t r i x Β i s a n mm'Xnn m a t r i x : ( ®B) , . , =(A©B) . . . ^ B . The o u t e r 1

1

A

i

i

; k

k l

( n l i

( n l +

l i n l k

n l + k l

i k

i l k l

p r o d u c t o p e r a t i o n has t h e f o l l o w i n g u s e f u l p r o p e r t i e s : ( I ) i t i s a s s o c i a t i v e ; (2) Α 0 Β i s H e r m i t i a n i f and o n l y i f A a n d Β a r e b o t h H e r m i t i a n ; and (3) ( A 0 B)( C 0 D)=( AC) 0 ( BD) ( 6 ) . The two e q u a t i o n s [2-1] c a n be o u t e r - m u l t i p l i e d on t h e r i g h t and l e f t , r e s p e c t i v e l y , by I v and l u ( I d e n o t e s a u n i t m a t r i x ) , c o n v e r t e d by u s e o f p r o p e r t y (3) o f t h e o u t e r p r o d u c t , and s u b t r a c t e d t o y i e l d the eigenfield equation: [2-2]

(F0I-I0F+wI0I)(u0v)

= x( I 0 G-G 0

I ) ( u 0 v)

2.2 P r o p e r t i e s o f E i g e n f i e l d Equation. Here we summarize some o f t h e more i m p o r t a n t p r o p e r t i e s o f [2-2] and i t s s o l u t i o n s ; a more d e t a i l e d d e s c r i p t i o n i s found i n r e f . J_. E q u a t i o n [2-2] i s a g e n e r a l i z e d e i g e n v a l u e e q u a t i o n o f t h e f o r m AZ=xBZ, w h e r e t h e o p e r a t o r s A and Β a r e H e r m i t i a n m o t r i c e s o f o r d e r n ( f o r H o f o r d e r n) and t h e e i g e n v e c t o r s , Z, a r e t r a n s i t i o n v e c t o r s u 0 v of dimension n . A c c o r d i n g l y , t h e r e a r e n e i g e n f i e l d values. The f o r m o f Β i n s u r e s t h a t i t i s a t l e a s t η-fold s i n g u l a r ; t h e r e ­ fore, η o f the eigenvalues are i n f i n i t e . They c o r r e s p o n d t o u n ­ r e a l i z a b l e t r a n s i t i o n s f r o m an e n e r g y l e v e l t o i t s e l f . In gen­ e r a l , i f t h e e i g e n v a l u e s o f t h e Zeeman o p e r a t o r , G, a r e g r o u p e d i n t o d e g e n e r a t e s e t s , Β wi11 have s i n g u l a r i t y e q u a l t o t h e sum o f s q u a r e s o f t h o s e d e g e n e r a c i e s ( E m ) ; t h e r e f o r e t h e r e w i l l be (n -Em ) n o n i n f i n i t e e i g e n f i e l d s ( J J . Normally, these w i l l occur i n p a i r s o f equal magnitude but o p p o s i t e s i g n , r e f l e c t i n g the p h y s i c a l f a c t that r e v e r s a l o f t h e magnetic f i e l d d i r e c t i o n does n o t change t h e r e s o n a n c e s p e c t r u m ( c f . r e f . J_ c o n c e r n i n g t h e t i m e - r e v e r s a l symmetry o f F and G ) . Thus, t h e r e a r e 2

2

2

2

2

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

BELFORD E T

5.

2

2

2

2

Field-Swept

AL.

EPR

Spectra

43

(n -Em )/2 physically d i s t i n c t transitions. Moreover, A i s p o s i ­ t i v e - d e f i n i t e ( i . e . , a l l o f i t s e i g e n v a l u e s a r e g r e a t e r t h a n 0) i f and o n l y i f t h e s p e c t r o m e t e r e n e r g y i s g r e a t e r t h a n t h e t o t a l span o f t h e z e r o - f i e l d energy l e v e l s . Only i n t h i s c a s e a r e a l l e i g e n f i e l d v a l u e s g u a r a n t e e d t o be r e a l . O b v i o u s l y , complex e i g e n f i e l d s o l u t i o n s , l i k e the i n f i n i t e ones, are p h y s i c a l l y s p u r i o u s ; t h u s t h e r e a c t u a l l y may be f e w e r t r a n s i t i o n s t h a n (n -Em )/2. I n t e n s i t i e s a r e r e a d i l y o b t a i n e d from e i g e n f i e l d t r a n s i t i o n vectors ( J j . 2.3 Example o f E i g e n f i e l d E q u a t i o n . The e i g e n f i e l d method y i e l d s a l l t r a n s i t i o n f i e l d s and i n t e n s i t i e s w i t h no s u b s t a n t i a l d i f f i c u l t i e s ; i t i s e f f e c t i v e l y a u t o m a t i c , and we have been u s i n g i t r o u t i n e l y f o r small problems ( e . g . , t r i p l e t s t a t e s , S=l; coupled doublets — i.e. q u a r t e t s , S=3/2) b o t h and f o r s i m u l a t i o n o f powder s p e c t r a . S i n c e t h e w o r k and s t o r a g e requirements i n c r e a s e very r a p i d l y w i t h s i z e of b a s i s s e t , the l a r g e s t p r o b l e m w h i c h we have y e t h a n d l e d by d i r e c t s o l u t i o n o f the e i g e n f i e l d equations i s that of a coupled Cr " -Cr" " " p a i r , f o r w h i c h F and G a r e 16x16 m a t r i c e s and A and Β c o n s e q u e n t l y a r e 256x256 m a t r i c e s . That w i l l be t h e e x a m p l e d e s c r i b e d h e r e . We t a k e two i d e n t i c a l S=3/2 i o n s on t h e ζ a x i s w i t h i s o t r o p i c Zeeman t e r m s and a x i a l i n t e r - and i n t r a - i o n z e r o - f i e l d t e r m s . The p a i r b a s i s s e t c a n be c o n s t r u c t e d as t h e o u t e r p r o d u c t o f b a s i s s e t s f o r t h e two S=3/2 p a r t i c l e s , and t h e 16x16 p a i r H a m i l t o n i a n as t h e o u t e r p r o d u c t o f 4x4 s i n g l e - p a r t i c l e f a c t o r s . T h e r e f o r e , we can s e t up t h e e n t i r e p r o b l e m i n t e r m s o f 4x4 m a t r i c e s . Our e x ­ ample i s as f o l l o w s : +

f+

f

H

[2-3] F=D( S [2-4]

2

©

z

2

I+I ©

S - 2 . 51 0 z

G=g0( c o s 9 [ S

0

z

I+I ©

I)+d( 2 S

z

0

S »S © z

S -S ©

x

S j +s i n G [ S ©

x

y

I+I ©

x

S) y

)

The m a g n e t i c f i e l d i s i n t h e xz p l a n e a t an a n g l e θ t o t h e ζ axis. Here I i s t h e 4x4 u n i t m a t r i x , and S i s a diagonal z

m a t r i x w i t h v a l u e s (-3/2,-1/2,+1/2,+3/2), and follows:

[2-5]

^0 S

χ

=1/2

v

S

and

S are ^

X

0

/3

0

\ 0

0

"/3

0

2

0

/3

0

-2

0

/3

0

0

"/3

0

/3

0

0^

/3

0

2

0

2

0

0

0 /3

0y 0

P a r a m e t e r s f o r t h e t h r e e t e s t s a t 0=1.5°, 51

and

as

88.5° were

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

44

EXTENDED INTERACTIONS B E T W E E N

METAL

IONS

w/3=6802.6523 g a u s s , g=1.9759, η/β=1006.25 g a u s s , and d/$= -107.74 g a u s s . Now F and G a r e r e a d i l y ( a n d i n a computer program, q u i t e e a s i l y ) c o n s t r u c t e d , as a r e A and B. For example, t h e f i r s t t e r m o f F l e a d s t o a d i a g o n a l m a t r i x c o n t r i b u t i o n t o A, D ( S (χ) I ® I © I - I ® I ® S ® I ) , i t s diagonal elements being (D/4) (64 9's, 128 I ' s , 64 9 ' s j - (D/4) ( 1 6 r e p e a t s o f t h e p a t ­ t e r n 4 9's, 8 l ' s , 4 9 ' s ) . N o t e h e r e t h a t G has 2 n o n d e g e n e r a t e e i g e n v a l u e s (:ί^β), 2 t w o f o l d (+2gβ), 2 t h r e e f o l d ( + 0 ) , and 1 f o u r f o l d ( 0 ) d e g e n e r a t e sets. Thus t h e r e must be e x a c t l y 2+8+18+16=44 i n f i n i t e f i e l d s . The computer program y i e l d e d 44 f i e l d s between 3 x l 0 and 3 x l 0 i n absolute value. I n no c a s e was t h e computed i n t e n s i t y f o r one o f these ' i n f i n i t e - f i e l d t r a n s i t i o n s as g r e a t as 1 0 " (strongest line=l). Also, since fo s m a l l compared w i t h w (w>|9D/2 d e f i n i t e , t h e r e s h o u l d be e x a c t l y ( 2 5 6 - 4 4 ) / 2 = l 0 6 r e a p o s i t i v e e i g e n f i e l d s and 106 n e g a t i v e images. The computer r e s u l t s a l s o showed t h i s f e a t u r e , w i t h t h e p l u s - m i n u s p a i r s g e n e r a l l y a g r e e ­ i n g t o 14 d i g i t s . At 0=51° t h e 106 e i g e n f i e l d s ranged f r o m 404 t o 3716 g a u s s . A u s e f u l f e a t u r e o f t h e e i g e n f i e l d program i s t h a t i t p r o ­ d u c e s f i e l d v a l u e s and i n t e n s i t i e s f o r a l l l i n e s a t once. Here, i t i s i n t e r e s t i n g t o see t h a t s e v e r a l l i n e s a r e p r e d i c t e d i n t h e 400-900 g a u s s r e g i o n w i t h i n t e n s i t i e s ~. 01 t o .03$ o f t h o s e o f t h e most s t r o n g l y a l l o w e d l i n e s and ~1-3$ o f t h o s e o f t h e w e a k e r l i n e s i n t h e 1000 g a u s s r e g i o n . These a r e e s s e n t i a l l y d o u b l e s p i n - f l i p t r a n s i t i o n s i n v o l v i n g e x c i t a t i o n o f one i o n and dee x c i t a t i o n o f i t s p a r t n e r ; u n d e r some c i r c u m s t a n c e s t h e y c o u l d be o b s e r v e d e x p e r i m e n t a l l y and w o u l d be s e n s i t i v e t o t h e i n t e r m o l e c ­ ular coupling. Most a p p r o x i m a t i o n methods w o u l d p r o v i d e o n l y t h e ' n o r m a l l i n e s , w i t h t h e r e s u l t t h a t e x p e r i m e n t a l i s t s might i g n o r e t h e abnormal ones a l t o g e t h e r . The computer program u s e d t o s o l v e t h i s g e n e r a l i z e d e i g e n ­ v a l u e p r o b l e m was w r i t t e n by A. Sameh and C. Chang o f t h e U n i ­ v e r s i t y o f I l l i n o i s ' Center f o r Advanced Computation. Their method ( SQ,Z") i s a v e r s i o n o f t h e Q.Z a l g o r i t h m o f M o l e r and S t e w a r t ( 7 ) , w h i c h we have u s e d ( J j f o r s m a l l e r p r o b l e m s . The SQZ a l g o r i t h m ( 8 ) r e q u i r e s t h a t A and Β b o t h be r e a l , s y m m e t r i c m a t r i c e s ( w i t h one o f them p o s i t i v e d e f i n i t e ) and t h e n t a k e s a d ­ v a n t a g e o f symmetry t o g e n e r a t e s o l u t i o n s more r a p i d l y ( a n d o f t e n more a c c u r a t e l y ) t h a n Q.I. Even t h e n , t h e example d i s c u s s e d h e r e r e q u i r e d a b o u t t e n m i n u t e s o f p r o c e s s i n g t i m e on an IBM 360/91 c o m p u t e r , w h e r e a s a s y s t e m o f two c o u p l e d i o n s o f S=l/2 e a c h would r e q u i r e o n l y a small f r a c t i o n o f a second. The s p a c e r e ­ q u i r e m e n t s w e r e a l s o f o r m i d a b l e , t h e example g i v e n r e q u i r i n g more t h a n one m i l l i o n b y t e s . The r e a s o n f o r t h e t i m e and s p a c e d i f ­ f i c u l t i e s i s t h a t the c u r r e n t s t a t e o f numerical a n a l y s i s o f gen­ e r a l i z e d e i g e n v a l u e p r o b l e m s i s as y e t q u i t e p r i m i t i v e ; n e i t h e r Q.Z nor SQZ t a k e s any a d v a n t a g e o f t h e s p a r s e n e s s o r s p e c i a l f o r m 2

2

Z

2

1 7

1

2 8

1

M

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

, 9

5.

Field-Swept

BELFORD E T A L .

EPR

45

Spectra

o f A a n d B. A d v e n t o f a n u m e r i c a l method w h i c h does s o w i l l make t h e e i g e n f i e l d f o r m u l a t i o n w i d e l y p r a c t i c a l f o r r e a s o n a b l y l a r g e systems. A measure o f t h e g o o d n e s s o f t h e e i g e n s y s t e m i s t h e r e s i d u ­ al a s s o c i a t e d w i t h e a c h e i g e n f i e l d x^. The p r o g r a m c a l c u max.[(A-x B)Z ] I a t e d f o r e a c h f i e l d a r e s i d u a l R. — T T T T T i 1 — u n i t , where k

k

IMI

+

k

B

K l ll ll

I I A| J ^max^Spl A^p|, and t h e n o r m a l i z a t i o n o f i s chosen so that 0
5.

Frequency-Shift

Perturbation

5.1 A p p r o a c h . The H a m i l t o n i a n , H( χ ) , i s a p r e s c r i b e d f u n c t i o n o f t h e f i e l d , x. The s p e c t r o m e t e r f r e q u e n c y , w, i s given. We c h o o s e a s t a r t i n g a p p r o x i m a t i o n x° and f i n d t h e e i g e n s y s t e m o f H(x°) t o o b t a i n z e r o ' t h o r d e r s t a t e s o f o r i g i n and t e r m i n a t i o n , u° and v ° , e n e r g i e s , μ and v , and f r e q u e n c i e s , w°=v-^. Then we r e g a r d t h e f r e q u e n c y s h i f t s , 6 w ^ - w ° , a s p e r ­ turbations. That i s , t o any i n i t i a l l y c a l c u l a t e d f r e q u e n c y we add a p e r t u r b a t i o n Xôw w h e r e λ i s a p e r t u r b a t i o n p a r a m e t e r and develop a perturbation s e r i e s f o r the f i e l d : χ=χ°+λχ'+λ χ"+ As i n t h e s t a n d a r d v e r s i o n s o f p e r t u r b a t i o n t h e o r y , i n i t i a l d e ­ g e n e r a c i e s c a u s e c o m p l i c a t i o n s and must be h a n d l e d c a r e f u l l y . The a p p r o a c h d e s c r i b e d h e r e a l l o w s p e r t u r b a t i o n s i n o t h e r p a r t s o f t h e H a m i l t o n i a n — f o r example, n u c l e a r h y p e r f i n e , q u a d r u p o l e , and Zeeman t e r m s — t o be t r e a t e d a l o n g w i t h t h e f r e q u e n c y s h i f t . 2

5.2 N o n d e g e n e r a t e Case. As b e f o r e , l e t H=F4xG, where F i s t h e z e r o - f i e l d o p e r a t o r and G t h e Zeeman o p e r a t o r . Let the n'th p r i m e on any symbol d e n o t e i t s n ' t h o r d e r c o n t r i b u t i o n . Then t h e f o l l o w i n g r e l a t i o n s hold. η

Η=Σλ Η [ 3

-

1 ]

°

( η )

0

0

0

; H ° = F + x G ° , H ^ F ' + x ^ '+χ'ΰ ,

H Î ^ W ^ G » ,

n;>2

0

N o t i c e t h a t we have assumed Ρ=Ρ +λΡ' and G=G°+XG'; h i g h e r terms may be i n c l u d e d i f d e s i r e d . [3-2] [3-3]

Hu=e u and Hv=e v; e -e =w-(l-λ) U V V u ' υ=Συ η

( η )

η

λ , ν=Σ

ν

( η )

λ

Π

η

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

order

46

E X T E N D E D INTERACTIONS B E T W E E N

e =μ+λμ·+ Σ

[3-4]

u

METAL

IONS

X J ) n

n

n>l

e =(p 4w°)+X(^ + ew)+ Σ X e ^ ^

[3-5]

,

v

n

n

i

n>1

The d i f f e r e n c e between t h e u s u a l s o r t o f p e r t u r b a t i o n t r e a t m e n t and t h i s one i s s i m p l y t h a t h e r e we l e t χ have c o r r e c t i o n s t o a l l o r d e r s a s f o r c e d by t h e c o n s t r a i n t t h a t w has o n l y a f i r s t o r d e r c o r r e c t i o n , w h i l e n o r m a l l y one i n t r o d u c e s a l l t h e c o r r e c ­ t i o n s t o χ a s a f i r s t - o r d e r t e r m , t h e r e b y c a u s i n g w t o have c o n ­ t r i b u t i o n s i n a l l orders. Now e x p a n s i o n o f [3-2] and i s o l a t i o n o f t h e c o e f f i c i e n t o f λ f o r each η leads t o t h e f o l l o w i n g .

[ 3

"

6 ]

( H ° ^ - w ° ) ν -Κ Η ' -μ»-ôw) v°=0 ,,

,

,

,

,,

0

(H°-^)u +(H -^ )u +(H -e")u =0 [ 3

"

7 ]

1

1

( H°-u,-w°) v"+( H -μ -6w) v»+{ H"-^) v°=0

In g e n e r a l , f o r n>2, we have 0

η

(η

,,

(Η -μ)ϋ< >-ΚΗ'-μ')υ - + Σ ( H j

[3-8] (Η°-μ-*#°)ν

(η,

ίη

=

4

(j )

(n

J > )u "

j

U

2

1,

-ΚΗ'-μ'-β«)ν " + Σ (H J =2

( j )

-e(j))v

( n

"

j )

=0

Each o r d e r i s r e p r e s e n t e d by a c o u p l e d p a i r o f e q u a t i o n s . Taking t h e inner product o f t h e f i r s t equation i n each p a i r w i t h u° and t h e s e c o n d w i t h v°, s u b t r a c t i n g , and r e a r r a n g i n g w i t h t h e aid o f [3-1], provide equations f o r the successive c o r r e c t i o n s t o the f i e l d . [3-9]

J

n

For convenience,

n

we l e t (u^ ^,u°)=( v^ ^,v°) = 6 no 1

x'=[&y-KF' -F' )+x°(G' -G )]/(G° -G° ) ^ uu vv' uu vv' vv uu' v

v

[3-10] ,

l

0

0

G

„ - G v„ v. ^ ' uu „ r ' vvvv ) ,,_ F' u'u -F'v ' v ^ u ' u v- 'G v„ ' ) + x ( G u'u < (G -G° ) vv uu' \/\/ llll' l v

l l |

l

u

x

etc.

,

H e r e we u s e t h e c o n d e n s e d n o t a t i o n (u ,Gu°)=G

, , etc. u u The c o m p l e t e p r e s c r i p t i o n f o r a p e r t u r b a t i o n s o l u t i o n i s now clear. One f i r s t c a l c u l a t e s ( u °, v°, M-,w°, bu) by e x a c t d i a g o n a l i z a t i o n o f H°. Then one f i n d s x by [ 3 - 9 ] , μ' f r o m one o f t h e e q u a t i o n s w h i c h i s a p r e c u r s o r t o [ 3 - 9 ] , and u and ν' f r o m [3-6], The p r o c e s s c a n c o n t i n u e t o any o r d e r . I t i s a l s o pos­ s i b l e , as i n t h e s t a n d a r d p e r t u r b a t i o n developments, t o d e r i v e 1

1

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

5.

BELFORD E T A L .

Field-Swept

EPR

47

Spectra

t h e f i r s t 2n+l c o r r e c t i o n s t o t h e e i g e n f i e l d f r o m t h e f i r s t η corrections t o the eigenvectors. A good s t r a t e g y o f c a l c u l a t i o n i s t o w o r k i n t h e b a s i s i n w h i c h H° i s d i a g o n a l ; t h e n many o f t h e r e l a t i o n s h i p s t a k e a s i m ­ p l e form. I n o u r f i r s t programs u s i n g f r e q u e n c y - s h i f t p e r t u r b a ­ t i o n s , we s t a r t by a p p l y i n g t o a l l m a t r i c e s i n t h e p r o b l e m t h e s i m i l a r i t y t r a n s f o r m a t i o n w h i c h d i a g o n a l i z e s H°. From t h i s p o i n t u n t i l t h e end (where e i g e n v e c t o r s a r e transformed back i n t o t h e o r i g i n a l b a s i s f o r o u t p u t ) we w o r k i n t h i s b a s i s a n d f i n d t h a t t h e programs t h e r e b y c a n be made v e r y e c o n o m i c a l . We have programmed t h e method up t o f o u r t h o r d e r i n v a l u e s a n d t h i r d order i n vectors. 3.3 Example o f F r e q u e n c y - S h i f t P e r t u r b a t i o n a n d C o m p a r i ­ son w i t h E i g e n f i e l d R e s u l t s We have u s e d a ^ r e q u e n c y - s h i f t p e r t u r b a t i o n program t s c r i b e d i n t h e next pape e x a m p l e s d e s c r i b e d i n S e c t i o n 2.3 p r o v i d e d s t a n d a r d s o f c o m p a r i ­ son f o r t h e f r e q u e n c y - s h i f t p e r t u r b a t i o n program. For t h e s e s t u d i e s , o n l y t h e f r e q u e n c y s h i f t was t a k e n a s t h e p e r t u r b a ­ tion — i.e., F'^^O. No a c t u a l d e g e n e r a c i e s o f H° c a u s e d trouble. The d i a g o n a l i z a t i o n s o f H° u s e d t h e w i d e l y a v a i l a b l e EISPAC m a t r i x e i g e n v a l u e package. G e n e r a l l y , t h e p e r t u r b a t i o n program, up t h r o u g h f o u r t h - o r d e r f i e l d c o r r e c t i o n s , r e q u i r e s o n l y a b o u t 1$ o f t h e p r o c e s s i n g t i m e expended by t h e e i g e n f i e l d program, and i t c a n be made more e f f i c i e n t . It also requires o n l y a f r a c t i o n (~10#) o f t h e s t o r a g e s p a c e u s e d by t h e e i g e n ­ f i e l d program. For t h e most p a r t , agreement between t h e e x a c t ( e i g e n f i e l d ) and t h e p e r t u r b a t i o n f i e l d s was good t o 6, 7, o r 8 s i g n i f i c a n t d i g i t s ( i . e . , t h e d i f f e r e n c e s were ~ 1 0 ~ gauss o r l e s s ) . A t y p i c a l r e s u l t i s g i v e n b e l o w ; θ=51°: 3

M,

,m

x°=1022.94, χ'=59.5029, x"=-0. 1604, x = 0 . 0 0 6 6 , x = - 0 . 0 0 0 3 x=l082.288879. (Compare e x a c t = l 0 8 2 . 2 8 8 8 9 1 ) Intensity: 0.1013 (Compare exact=0.1013) F i n a l v e c t o r s ( a l l i n agreement w i t h t h e e x a c t the four places p r i n t e d ) :

result t o

u=(.0720, .0672, .1350, .2371, . 0672,-.0938,-.1747,-.4974, . 1350, -.1747, . 1826, . 3233, . 2371,-. 4974, . 3233,-. 1728) v=(.0026, . 0 0 8 6 , . 0 2 5 6 , . 0 6 1 2 , . 0 0 8 6 , .0259, .0707, .1571, .0256, .0707, .1812, .3827, .0612, .1571, .3827, .7780) I n a l m o s t a l l c a s e s , t h e r e s u l t s c o u l d be i m p r o v e d by a very simple, r a p i d e x t r a p o l a t i o n o f t h e t h i r d and f o u r t h order f i e l d c o r r e c t i o n s toward i n f i n i t e order. The i d e a b e h i n d t h e e x t r a p o l a t i o n i s t h a t t h e s u c c e s s i v e p e r t u r b a t i o n s may a p p r o x i ­ mate a g e o m e t r i c s e q u e n c e , i . e . , x ^ ^ = r x ^ where r i s n e a r l y J

+

American Chemical Society Library 1155 13th between St. N.Metal W. Ions; Interrante, L.; In Extended Interactions ACS Symposium Series; American Society: Washington, DC, 1974. Washington, D.Chemical C. 20036

48

EXTENDED INTERACTIONS B E T W E E N

constant f o r j>3.

By t a k i n g x 5

6

t u r b a t i o n s e r i e s x^ ^+x^ ) + . . . examples

,,,,

/x

111

t o be t h i s

sums t o ( x

l l l l

2

ratio,

METAL

IONS

the per­

,

) /(x" -x"").

Some

follow:

Σ ο χ ^ ) =1022.94+173.0747-2.0890+0.7314-0.2216=1194.4355 x( e x t r a p o l a ted) = 1194.4870 x( e x a c t ) = 1 1 9 4 . 4 8 5 7 . Σ

4 ( J) Χ

.94+97.7430-0.6986+0.0915-0.0157=1120.0602 x(extrapolated)=l120.0625 x(exact)=1120.0626.

=1022

EQX^ ^=1022.940+407.033+3.635-15.536+10.061=1428. 133 x(extrapolated)=l424.178 x(exact)=1425.149. J

The l a s t c a s e i n t h e p r e v i o u s p a r a g r a p h i l l u s t r a t e s a s l o w c o n v e r g e n c e p r o b l e m w h i c h o c c u r r e d o c c a s i o n a l l y and which was i n a few c a s e s m a n i f e s t e d b These c a s e s o c c u r r e d whe s t a t e v e c t o r s i n v o l v e d i n a t r a n s i t i o n was c h a n g i n g v e r y r a p i d l y w i t h f i e l d , s o t h a t t h e range o f c o n v e r g e n c e o f the p e r t u r b a t i o n method was p a t h o l o g i c a l l y s m a l l — i . e . , o n l y a s m a l l 6w c o u l d be t o l e r a t e d . T h i s t e n d s t o happen c l o s e t o t h e c r o s s i n g p o i n t o f two i n t e r a c t i n g s t a t e s . We s u f f e r no s e v e r e d i f f i c u l t i e s f r o m t h i s p r o b l e m , b e c a u s e i t i s e a s y t o r e c o g n i z e such c a s e s and t o c o r r e c t them by t r y i n g a new x°. However, f o r a p r a c t i ­ c a l s e l f - c o n t a i n e d program, one would need t o d e v i s e an a l g o ­ r i t h m f o r a u t o m a t i c r e c o g n i t i o n and a d j u s t m e n t . Incidentally, in the p a t h o l o g i c a l c a s e s t h e v e c t o r s and i n t e n s i t i e s a r e f a r w o r s e than the f i e l d s . As e x p e c t e d , h e r e t h e a p p r o x i m a t e method, though f a s t e r , i s much l e s s f o o l p r o o f t h a n t h e e x a c t ( e i g e n ­ f i e l d ) method. In some c a s e s , t h e same t r a n s i t i o n was computed s t a r t i n g from two o r t h r e e r a t h e r d i s t a n t v a l u e s o f x°. I n most c a s e s , the agreement was r e a s o n a b l e . Of c o u r s e , l a r g e f i e l d s h i f t s from x° produced l e s s a c c u r a t e r e s u l t s t h a n s m a l l s h i f t s , the p a t h o ­ l o g i c a l c a s e s m e n t i o n e d above e x c e p t e d . Some c o m p a r i s o n s f o l l o w : F o r θ=88.5°, x ( e x a c t ) = 2 8 5 8 . 5 4 3 0 , întensity=l.0007. T h r e e p e r t u r ­ b a t i o n t r i a l s f r o m d i f f e r e n t s t a r t i n g f i e l d s produced J

Σοχ^ * ^ =2200+655. 8418+3. 7346- 1.3842+0. 4590=2858. 6512 x ( e x t r a p o l a ted)=2858.5269 ; I n t e n s i t y = 0 . 9 9 8 5 J

Σοχ^ * ) =2900-41.4622+0.0051+0.0001+0.0000=2858. 5430 x( e x t r a p o l a ted)=2858.5430 ; I n t e n s i t y = l . 0 0 0 0 J

Σοχ^ * ^ =3600-742.3981+0.6875+0. 1904+0.0483=2858. 5281 x(extrapolated)=2858.5445; Intensity=0.9990 A l l t h e t h i r d - o r d e r v e c t o r s a g r e e d t o w i t h i n a few p a r t s i n t h e f o u r t h d e c i m a l p l a c e f o r e a c h v e c t o r component; i n e a c h c a s e t h e t r a n s i t i o n a r o s e f r o m the 5th-»10th l e v e l s o f t h e s t a r t i n g Hami1tonian.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

5.

BELFORD

Field-Swept

E T AL.

EPR

49

Spectra

3.4 D e g e n e r a t e S t a t e s . Suppose t h a t a f i e l d p o s i t i o n must be c o m p u t e d f r o m a n i n i t i a l l y R e g e n e r a t e s e t o f t r a n s i t i o n s . C o n s i d e r { U|,..., u }-*[ V|,... ,v }, where t h e s e t { u°) i s a p - f o l d Q

0

degenerate

s e t o f e i g e n s t a t e s o f H° and { v } a q - f o l d s e t . 0

[3-11]

H u°=yj,u!; Η^°=(μτΗ*°)ν]

T h e r e a r e s e v e r a l ways t o p r o c e e d . Normally, a degenerate per­ t u r b a t i o n t r e a t m e n t w o u l d c a l l f o r d i a g o n a l i z a t i o n o f H' w i t h i n d e g e n e r a t e b l o c k s o f H°. However, t h a t i s n o t a l w a y s p o s s i b l e > a p r i o r i , because H i s F'+x°G'+χ'ΰ , i n w h i c h x i s not y e t known. The d e s i r e d s e p a r a t i o n c a n be o b t a i n e d by d i a g o n a l i z i n g b o t h t h e f u } and t h e { v } s u b - b l o c k s o f F + x G ' and t h e n diagonalîzing G w i t h i remainin degenerat blocks If degeneracies p e r s i s t , the An a l t e r n a t i v e f o d e g e n e r a t s m a l l e i g e n f i e l d problem. B e g i n n i n g w i t h [ 3 - 1 1 ] , one g e t s t h e f o l l o w i n g p a i r o f f i r s t - o r d e r equations analogous t o those f o r the nondegenerate case. n e r e

1

0

0

1

0

,

,

,

0

l

E.b.(H -u, )u?+(H°-M )u =0 i

[3-12] (H -M, -6W)V .+(H -^-W°)V =0

Y_.c.

,

i

Taking the inner product left,

i

0

j

and t h e second

o f the f i r s t

e q u a t i o n w i t h u° on t h e

by v£, and d e n o t i n g t h e s u b m a t r i c e s

0

0

w i t h i n t h e { u } and { v } b l o c k s a s h

U

and h

V

of H

1

and t h e v e c t o r s o f

c o e f f i c i e n t s b. and c. a s b and c r e s p e c t i v e l y , we g e t t h e f o l ­ lowing matrix

equations. u

[3-13] (Here

v

(h%"I )b=0; I

U

V

( h -(μ,'+ôw) I ) c=0

d e n o t e s t h e u n i t m a t r i x h a v i n g t h e same o r d e r a s h V

U

—

U

i . e . , p.) A p p l y i n g 0 I c t o t h e f i r s t and I b © t o t h e second o f t h e s e e q u a t i o n s and s u b t r a c t i n g y i e l d s [3-14] . (h 0 U

[3-14]

V

I -I

U

V

©

U

h +6wI ©

U

V

I ) (b © U

c) =0 U

The f o r m o f h i s now f ^ x ' g " , where f and g a r e s u b m a t r i c e s w i t h i n t h e { u } b l o c k o f F + x ° G and G ° , r e s p e c t i v e l y . Thus [3-14] c a n be r e a r r a n g e d t o [ 3 - 1 5 ] . 0

,

l

[3-15] U

( f ©

V

I -I

U

©

V

U

f +ôwI ©

V

I ) (b©

U

c) =x « ( I ©

V

U

g -g ©

V

I ) (b©

c)

T h i s i s a s m a l l e i g e n f i e l d e q u a t i o n based on t h e t r a n s i t i o n s from the { u } t o f v } b l o c k s ; t h e e i g e n f i e l d s a r e t h e f i r s t - o r d e r 0

0

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

50

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

t r a n s i t i o n f i e l d s f o r t h e s e t r a n s i t i o n s , and t h e e i g e n v e c t o r s are the c o r r e c t z e r o ' t h order t r a n s i t i o n v e c t o r s . Acknowledqements. T h i s r e s e a r c h was s u p p o r t e d by t h e P e t r o l e u m R e s e a r c h t h e N a t i o n a l S c i e n c e F o u n d a t i o n , and t h e Advanced R e s e a r c h j e c t s Agency. Literature 1. 2. 3. 4. 5. 6.

7. 8. 9.

Fund, Pro­

Cited.

B e l f o r d , G. G., Belford, R. L., and Burkhalter, J. F., J . Magn. Resonance (1973) 11, 251. Hempel, J . C., Morgan, L. O., and L e w i s , W. B., I n o r g . Chem. ( 1 9 7 0 ) 9, 2064. B y f l e e t , C. R., Chong C. Α., J . Magn. Resonance ( 1 9 7 0 ) 2, 69. Abragam, A. and B l e a n e y , Β., "Electron P a r a m a g n e t i c Reso­ nance o f Transition I o n s , " C l a r e n d o n P r e s s , O x f o r d , 1970. B e l f o r d , R. L. and Belford, G. G., J . Chem. Phys. ( 1 9 7 3 ) 59, 853. K o r n , G. A. and K o r n , Τ. Μ., "Mathematical Handbook f o r Scientists and Engineers," § 13.2-10, M c G r a w - H i l l , New Y o r k , 1961. M o l e r , C. B. and S t e w a r t , G. W., SIAM J. Numer. Anal. (1973) 1O, 241. Chang, C.-C., M.S. T h e s i s i n Computer S c i e n c e , U n i v . o f Illinois a t Urbana-Champaign, 1974. D a v i s , P. H. and Belford, R. L., this volume.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

6 Multisite Magnetic Interactions in HexaureaMetal(III) Halide Lattices PHILLIP

H.

DAVIS

and

R. L .

BELFORD

U n i v e r s i t y of Illinois, U r b a n a , Ill. 61801

Introduction X-ray crystallographic studies carried out i n our l a b o r a t o r y and o t h e r s on a number of salts o f t h e t y p e M(urea) X3 , where M = Al, Ti, V, Cr, o r Fe and X = halide o r perchlorate, have shown them to be isomorphous (1-7). Furthermore, where complete x - r a y structure d e t e r m i n a t i o n s w e r e done, t h e m o l e c u l a r g e o m e t r i e s o f the v a r i o u s M(urea) 6 3 + c o m p l e x i o n s w e r e f o u n d to be virtually identical (2-7). The crystal structure is s u c h as t o s u g g e s t t h e possibility o f o b s e r v i n g weak o n e - d i m e n s i o n a l interactions be­ tween t h e m e t a l i o n s w h i c h o c c u r in infinite linear chains paral­ lel t o t h e crystallographic c-axis. The existence of the iso­ morphous series p r o v i d e s an opportunity f o r a variety o f compara­ tive studies. For example, by v a r y i n g t h e concentration of the p a r a m a g n e t i c s p e c i e s i n binary s y s t e m s involving one o f t h e para­ magnetic i o n s doped into one o f t h e d i a m a g n e t i c a l u m i n u m salts, it w o u l d be possible to v a r y t h e effective length o f the chain along which interactions might t a k e p l a c e . A l s o of interest a r e studies of interactions among different paramagnetic ions i n binary and t e r n a r y s y s t e m s (e.q. Cr:Ti(urea) X and Cr:Ti :Al(urea) X ). 6

6

6

Structural

3

3

Background

The above s a l t s c r y s t a l l i z e i n t h e t r i g o n a l ( rhombohedral ) s p a c e g r o u p R3C w i t h a p p r o x i m a t e u n i t c e l l p a r a m e t e r s a = 17.5A and c = 14.OA. T a b l e I p r o v i d e s a c o m p a r i s o n o f u n i t c e l l parameters f o r the v a r i o u s s a l t s . The b a s i c s t r u c t u r e o f t h e c o m p l e x i o n i s shown i n F i g u r e 1. Each t r i p l y p r i m i t i v e u n i t c e l l c o n t a i n s s i x f o r m u l a u n i t s . The m e t a l atoms o c c u p y t h e s i x - f o l d s e t o f s p e c i a l p o s i t i o n s a o f 32 symmetry. W h i l e t h e c o m p l e x i s t h u s c r y s t a l l o g r a p h i c a l l y c o n ­ s t r a i n e d t o p o s s e s s D symmetry, t h e c o o r d i n a t i o n p o l y h e d r o n i s v e r y n e a r l y a r e g u l a r o c t a h e d r o n . Two t y p e s o f d i s t o r t i o n o f an 3

51

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

EXTENDED

INTERACTIONS B E T W E E N

METAL

IONS

Table I Comparative Unit

C e l l P a r a m e t e r s f o r S a l t s M(urea) ό Χ 3

Compound

8

·(*)

£ (A°)

Ref.

Al(urea) I

3

17.539( 3)

13. 855( 8)

6

Ti(urea) I

3

17.67(2)

14. 15(2)

3

6

6

Ti(urea) (C10 )

( 300°K)

18. 132(5)

14. 149(5)

4

Ti(urea) (C!0 )

( 90°K)

17.57

13. 84

5

17.53

13.87

6

17.49(2)

14.38( 3)

7

6

4

6

4

Cr(urea) I 6

3

3

3

V(urea) I

3

(300°K)

V(urea) I

3

(90°K)

6

6

Where known, e s t i m a t e d s t a n d a r d d e v i a t i o n i n l a s t cant d i g i t given i n parentheses.

signifi­

o c t a h e d r o n a r e p o s s i b l e w h i c h m a i n t a i n a t l e a s t D symmetry. The f i r s t i n v o l v e s the compression or e l o n g a t i o n o f t h e octahedron along a t h r e e - f o l d a x i s and i s m a n i f e s t e d i n a d e v i a t i o n o f t h e a n g l e (commonly d e n o t e d Θ) between an M-0 bond and t h e t h r e e - f o l d a x i s from t h e o c t a h e d r a l v a l u e o f 5 4 . 7 4 ° . The s e c o n d d i s t o r t i o n i n v o l v e s a t w i s t i n g about t h e t h r e e - f o l d a x i s o f o p p o s i t e f a c e s o f the octahedron r e s u l t i n g i n a d e v i a t i o n from 6 0 ° o f the a n g l e ( u s u a l l y d e n o t e d <£) between p r o j e c t i o n s o n t o t h e p l a n e o f t h e m e t a l atom o f s u c c e s s i v e M-0 bonds. D i s t o r t i o n s o f t h e former t y p e a r e v i r t u a l l y a b s e n t i n t h e s e c o m p l e x e s ( n o t e , however, t h a t t h e symmetry o f t h e e l e c t r o n d i s t r i b u t i o n about t h e metal atom may d i f f e r s i g n i f i c a n t l y f r o m t h a t o f t h e p o s i t i o n s o f t h e a t o m i c c e n t e r s o f t h e c o o r d i n a t i n g atoms), w h i l e those o f the l a t t e r type a r e small but s i g n i f i c a n t . The c h i e f o b s e r v a b l e d i f f e r e n c e i n m o l e c u l a r s t r u c t u r e among t h e v a r i o u s M ( u r e a ) ions l i e s i n t h e M-0 bond l e n g t h , w h i c h r a n g e s f r o m 1.88A i n t h e A l c o m p l e x t o 2.04Â* i n t h e T i c o m p l e x ( p e r c h l o r a t e s a l t ) . T a b l e I I l i s t s com­ p a r a t i v e v a l u e s o f t h e M-0 bond l e n g t h and d i s t o r t i o n s f r o m o c t a ­ h e d r a l geometry f o r t h e s e complexes. 3

3 +

6

In t h e c r y s t a l t h e c o m p l e x i o n s f o r m i n f i n i t e l i n e a r c h a i n s along the t h r e e - f o l d a x i s ( c r y s t a l l o g r a p h i c c - a x i s ) . Successive i o n s a r e r e l a t e d by a n i n v e r s i o n c e n t e r and t h e s e p a r a t i o n between metal atoms i s e q u a l t o o n e - h a l f o f t h e c - a x i a l l e n g t h . Each s u c h c h a i n i s ' i n s u l a t e d ' f r o m i t s n e i g h b o r s by s i x c h a i n s o f a n i o n s . The i o n s f o r m i n g e a c h o f t h e l a t t e r c h a i n s a r e r e l a t e d by a t h r e e ­ f o l d s c r e w o p e r a t i o n ( h e n c e t h e v e r t i c a l s e p a r a t i o n between i o n s i s e q u a l t o o n e - t h i r d o f t h e £-axial r e p e a t ) . The c h a i n s a r e v e r y n e a r l y l i n e a r , t h e ions being l o c a t e d very c l o s e t o t h e a x i s o f rotation. The e n t i r e s t r u c t u r e i s h e l d t o g e t h e r by a t h r e e d i m e n s i o n a l n e t w o r k o f h y d r o g e n bonds i n v o l v i n g s h a r e d h a l i d e i o n s , but t h e r e a r e no d i r e c t h y d r o g e n bonds between s u c c e s s i v e

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

6.

DAVIS

AND

BELFORD

Multisite

53

Interactions

ο

$ ο »o

to C to co

i. CO

Ο

ο

ο "α Η 1 .δ" Ο

—

^ to » .g»

s i to •δ «ο to ν»

£Γ ν. 5J

ο ν. Ο

3 ν.

to "ο" «ο

.to "S ο

»Ή

to ν.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

54

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

Table I I Comparison

o f M o l e c u l a r Geometr i e s

f o r S a l t s M(urea)5X3

a

θ Compound

M-0 ( A )

( degrees)*

5

(degrees)

Ref.

c

Al(urea) I

3

1.883(4)

55. 1(3)

6.4

6

Ti(urea) I

3

2.014(5)

55. 5( 3)

5.5

3

T i ( u r e a ) ( C 1 0 ) (300°K)

2.04( 1)

55.4

5.0

4

Ti(urea) (C10 )

2.00

55. 1

5.0

5

1.98(2)

55. 3( 7)

7.0

7

6

6

6

4

6

4

3

(90°K)

V(urea) I

3

(300°K)

V(urea) I

3

(90°K)

6

6

Where known, e s t i m a t e d s t a n d a r d d e v i a t i o n d i g i t given i n parentheses. O c t a h e d r a l geometry C

requires

in last

significant

= 54.74°. =

D e v i a t i o n p l u s and minus f r o m o c t a h e d r a l eg.

60°.

3+

M(urea)ô u n i t s o r between t h e c h a i n s o f t h e s e i o n s . Figure 2 i l l u s t r a t e s t h e manner i n w h i c h t h e i o n s a r e p a c k e d i n t h e crystal. The c r y s t a l s t r u c t u r e o f V(urea)ôl3 has been d e t e r m i n e d a t b o t h room t e m p e r a t u r e and a t a b o u t 90°K ( 7 j . On g o i n g t o t h e lower t e m p e r a t u r e , t h e u n i t c e l l undergoes a small a n i s o t r o p i c c o n t r a c t i o n ( s e e T a b l e I ) . T h i s i s a c c o m p a n i e d by smal1 changes i n t h e c o o r d i n a t i o n g e o m e t r y , n o t a b l y a d e c r e a s e o f 0.05M i n t h e V-0 bond l e n g t h a n d an e l o n g a t i o n o f t h e c o o r d i n a t i o n p o l y h e d r o n a l o n g t h e t h r e e - f o l d a x i s , d e c r e a s i n g Θ. f r o m 55.3° t o 53.5°. The s t r u c t u r e o f T i ( u r e a ) ( C l 0 ) l i k e w i s e has been d e t e r m i n e d a t h i g h and l o w t e m p e r a t u r e ( 4 , 5 ) . The changes i n s t r u c t u r e w e r e p a r a l l e l i n n a t u r e t o t h o s e f o r V ( u r e a ) I ( s e e T a b l e s I and I I ) , a l t h o u g h m a t t e r s w e r e c o m p l i c a t e d by an o b s e r v e d d i s o r d e r i n g o f the p e r c h l o r a t e ions. 6

4

3

6

Interactions

3

i n Cr(urea)6X3

The f a i l u r e t o r e c o g n i z e t h e p r e s e n c e o f i n t e r i o n i c i n t e r a c ­ t i o n s a p p e a r s t o have been r e s p o n s i b l e f o r t h e i n c o r r e c t i n t e r ­ p r e t a t i o n o f t h e e l e c t r o n paramagnetic resonance ( e p r ) spectrum of C r ( u r e a ) C l i n a p r e v i o u s paper ( 8 ) . For a C r i o n i n an a x i a l l y symmetric environment, t h e epr spectrum i s u s u a l l y f i t t e d t o a s p i n H a m i l t o n i a n o f t h e form 3 +

6

3

* = 9| [044 evaluated

+ g ^ C H ^ + H y S y ) + Dfs^-jis+O/s]

f o r S_ = 3/2.

Q)

From s u c h a H a m i l t o n i a n one p r e d i c t s

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

6.

DAVIS

Mttltisite

AND BELFORD

t h r e e a l l o w e d ( AM

55

Interactions

=+1) t r a n s i t i o n s occurring at f i e l d s

g

hv/g$

and (hv+2J))/g|3 when t h e m a g n e t i c f i e l d i s i n t h e z_ d i r e c t i o n and at approximately hv/gp and (hv+Q)/gp when t h e m a g n e t i c f i e l d i s anywhere i n t h e xy_ p l a n e . F i g u r e 3 shows t h e e p r s p e c t r u m o f C r ( u r e a ) C l powder a s w e l l a s s p e c t r a o f a s i n g l e c r y s t a l o f t h i s compound a t s e v e r a l o r i e n t a t i o n s . C l e a r l y t h e s i n g l e c r y s ­ t a l s p e c t r a a r e n o t c o n s i s t e n t w i t h t h e above H a m i l t o n i a n , y e t when a v e r a g e d o v e r a l l p o s s i b l e o r i e n t a t i o n s t h e y g i v e a powder spectrum which i s s i m i l a r i n o v e r a l l appearance t o that which w o u l d be e x p e c t e d f o r H a m i l t o n i a n ( ]_) ( a l b e i t p o o r l y r e s o l v e d ) . I n t e r p r e t i n g t h e powder s p e c t r u m o n ~ t h i s b a s i s and a s s u m i n g t h a t t h e s h o u l d e r s e x t e n d t o t h e e x t r e m a (dH/d0 = 0 ) a s s o c i a t e d w i t h t h e p a r a l l e l f i e l d p o s i t i o n , one c a l c u l a t e s v a l u e s o f g = 1.98 and £ = 277xl0~ cm" i n agreement w i t h t h o s e p r e v i o u s l y r e p o r t e d ( 8 ) . When C r ( u r e a ) i the A l ( u r e a ) I l a t t i c e t r u m , i n c l u d i n g n o t o n l y t h e a n g u l a r dependence o f t h e r e s o n a n c e f i e l d p o s i t i o n s o f t h e allowed t r a n s i t i o n s but a l s o that o f t h e f o r m a l l y f o r b i d d e n ΔΜ = +2,+3 t r a n s i t i o n s w h i c h a r e p r e d i c t e d t o have o b s e r v a b l e i n t e n s i t y when t h e m a g n e t i c f i e l d i s a p p r o x i ­ m a t e l y mid-way between t h e z_-axis and t h e xy_ p l a n e , a r e a d e ­ q u a t e l y d e s c r i b e d by t h e H a m i l t o n i a n ( i ) w i t h gj | = = 1.9752 6

3

A

1

3 +

6

6

3

and D_ = 4 8 3 . 4 x 1 0 " cm" (we a l s o o b s e r v e h y p e r f i n e c o u p l i n g t o t h e 9 $ abundant C r i s o t o p e [ I = 3/2] w i t h a n e a r l y i s o t r o p i c c o u p l i n g constant A ~ 18.7x10" cm" ). However, e v e n h e r e a d d i ­ t i o n a l f e a t u r e s , whose i n t e n s i t y i n c r e a s e d w i t h i n c r e a s e d C r c o n c e n t r a t i o n , w e r e o b s e r v e d ( s e e F i g u r e 4 ) . We a t t r i b u t e t h e m a j o r i t y o f t h e s e l i n e s t o p a i r w i s e i n t e r a c t i o n s between C r ions occupying a d j a c e n t s i t e s along t h e c - a x i s o f t h e host l a t t i c e . The g e n e r a l s p i n H a m i l t o n i a n f o r a c o u p l e d p a i r o f C r ions may be w r i t t e n ( e x c e p t f o r a u s u a l l y s m a l l b i q u a d r a t i c e x c h a n g e term, j ( S , - S ) ) 4

1

5 3

T

1

3 +

3 +

3 +

2

2

e(iiz +i2 2

2 Z

- f) +d[2S

l 2

S

2 z

-(S

l x

S

2 x

+ S , S y ) ] +JS,-S y

2

2

(2)

I n a l l o f o u r c a l c u l a t i o n s o n t h i s s y s t e m we have assumed t h a t _g and JD have t h e same v a l u e s i n t h e i n t e r a c t i n g p a i r a s i n t h e i s o ­ lated ion, although e s p e c i a l l y , w h i c h i s a measure o f t h e d i s ­ t o r t i o n o f t h e c o m p l e x i o n f r o m o c t a h e d r a l g e o m e t r y , might w e l l be d i f f e r e n t f o r t h e p a i r . E m p l o y i n g t h e above H a m i l t o n i a n , c a l ­ c u l a t i o n s o f t h e type d e s c r i b e d i n t h e p r e c e d i n g paper p r e d i c t a s p e c t r u m composed o f e i g h t e e n p r e d o m i n a n t l i n e s - t h r e e g r o u p s o f s i x , e a c h g r o u p r o u g h l y c e n t e r e d about one o f t h e t h r e e a l l o w e d l i n e s o f t h e i s o l a t e d i o n spectrum (which f i e l d s form convenient z e r o ' t h order approximations f o r t h e p e r t u r b a t i o n c a l c u l a t i o n s ) . B e c a u s e t h e l i n e s a r e n a r r o w e r and we c a n measure t h e i r p o s i t i o n s w i t h g r e a t e r c e r t a i n t y , we have c o n c e n t r a t e d o u r e f f o r t s on t r y i n g

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

Figure

3. X-hand epr spectrum of Cr(urea), Cl. powder and of single crystal at several orientations. For reference, arrow indicates position of dpph resonance. :

:

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

6.

DAVIS

AND

Multisite

BELFORD

Interactions

57

(a)

H

5 0 0 gouss

Figure 4. Concentration dependence of X-band epr spectrum of Cr(urea) ': (a) ~ 1% Cr in Al(urea)J,„ (b) ~ 2 5 % Cr in Al(urea)J,, and (c) Cr(urea) ,I,. For all spectra the magnetic field is directed along the z-axis. Sharp spike near the center of each spectrum is due to dpph used as an internal standard. 3 e

r

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

58

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

t o f i t t h e o b s e r v e d p o s i t i o n s and s p a c i n g s o f t h e m i d d l e s e t o f s i x l i n e s w h i c h o c c u r i n t h e n e i g h b o r h o o d o f 3300 g a u s s a t χ-band (9.1 GHz). L i k e w i s e , b e c a u s e t h e v a r i o u s s p e c t r a l f e a t u r e s , b o t h t h o s e due t o i s o l a t e d i o n s and t h o s e due t o c o u p l e d i o n s , a r e b e t t e r s e p a r a t e d t h e r e , t h e i n i t i a l f i t t i n g was done f o r c a s e s i n w h i c h t h e m a g n e t i c f i e l d d i r e c t i o n was c l o s e t o t h e z_-axis. The s i m p l e s t r e a s o n a b l e model f o r t h e c o u p l i n g between two i o n s i s t h a t o f two m a g n e t i c d i p o l e s i n t e r a c t i n g t h r o u g h s p a c e (f* ^ °» 1 (|))· *f z - d i r e c t i o n i s t a k e n t o be a l o n g t h e i n t e r i o n i c v e c t o r £, e x p a n s i o n o f t h e c l a s s i c a l s p i n H a m i l ­ t o n i a n f o r a t h r o u g h - s p a c e dîpole-dîpole c o u p l i n g =

ν

0

ΐ η

t

2

= g e

2

< ^

h

e

> [-s,-s

5

+

2

U r ^ ( ^ ]

(3)

and c o m p a r i s o n w i t h t h the i d e n t i f i c a t i o n d = value o f r t h e c r y s t a l l o g r a p h i c a l l y observed metal-metal separa t i o n , we c a l c u l a t e d = - 5 0 . 7 5 x 1 0 " cm" . We n o t e a t t h i s p o i n t t h a t t h e above p h y s i c a l r e a s o n i n g im­ p l i e s t h a t d i s n e g a t i v e . The i n t r a i o n i c z e r o f i e l d s p l i t t i n g p a r a m e t e r , J), i s a l s o a s i g n e d q u a n t i t y , but t h e o r d i n a r y e p r e x ­ p e r i m e n t p e r m i t s o n l y t h e d e t e r m i n a t i o n o f i t s magnitude. Be­ cause o f the i n t e r a c t i o n o f t h e v a r i o u s z e r o f i e l d s p l i t t i n g t e r m s , t h e p o s i t i o n s and s p a c i n g s o f l i n e s i n t h e s p e c t r u m o f t h e c o u p l e d i o n s depend on b o t h t h e m a g n i t u d e s and r e l a t i v e s i g n s o f t h e i n t e r i o n i c i n t e r a c t i o n p a r a m e t e r s d^ and and t h e i n t r a i o n i c i n t e r a c t i o n p a r a m e t e r , JD. Knowledge o f t h e s i g n o f d^ s h o u l d t h u s e n a b l e us t o d e t e r m i n e t h e s i g n s o f £ and J. A comparison o f observed s p e c t r a l features w i t h those c a l ­ c u l a t e d f o r a p u r e l y d i p o l a r i n t e r a c t i o n model and f o r e a c h o f t h e p o s s i b l e s i g n s f o r j) i s f o u n d i n T a b l e I I I . Considering the s i m p l i c i t y o f t h e model and t h e f a c t t h a t t h e v a l u e o f d was c a l ­ c u l a t e d from f i r s t p r i n c i p l e s r a t h e r than being f i t t e d t o t h e ob­ s e r v a t i o n s , t h e agreement between t h e o b s e r v e d and c a l c u l a t e d s p e c t r a ( e s p e c i a l l y f o r t h e c a s e when d and £ a r e o f o p p o s i t e s i g n ) i s good. Examination o f the vectors f o r the states involved i n the t r a n s i t i o n s d e n o t e d 1 and 6 shows them t o be e s s e n t i a l l y ( i n t h e |M ,,M > b a s i s ) ι J r i I 3 ι 3 1 . _L π ι I Ί / J l"2'"? " l"2 "2 " /2 2 '2 " \ ~ 2*2?* 4

S

1

S2

3

, :

A.

i

1

fl

>

3

1

,

ι

1

3

\

> j

1

l |

fl

3

1

,

3

1

>

I

1

3

ι

W i t h i n each p a i r t h e e f f e c t o f t h e i s o t r o p i c exchange t e r m i n ( 2 ) i s t o s h i f t t h e e n e r g y o f e a c h s t a t e by t h e same amount and i n t h e same d i r e c t i o n . The p o s i t i o n s o f and s p a c i n g between t h e s e two l i n e s w i l l t h u s be d e t e r m i n e d a l m o s t s o l e l y by t h e m a g n i t u d e o f t h e d i p o l a r term i n ( | ) . That t h e s e two l i n e s a r e f i t t e d

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

6.

DAVIS

AND

BELFORD

Multisite

Interactions

59

Table I I I C o m p a r i s o n o f Observed and C a l c u l a t e d C o u p l e d Cr( u r e a )

3 + 6

-Cr( u r e a ) Calculated

Transi t ion

Observed Field (gauss)

EPR S p e c t r a f o r 3 +

Pairs

6

Field*

5

3

(gauss)

D = -483.4 d = - 50.75 0.0 J

D = 483.4 d = -50.75 J = 0.0

D = -483.4 d = - 50.75 J = 45. 11

1

3047 ( 2 )

3047 ( 4 )

3047 ( 4 )

3047 ( 4 )

2

3107 ( 1 0 )

3117 ( 7 )

3145 ( 9 )

3107 ( 10)

3

3169 ( 1 )

3231

3168 ( 2 )

4

3420 ( 1)

5

3482 ( 10)

3472 ( 7 )

3444 ( 9 )

3482 ( 10)

6

3540 ( 2 )

3542 ( 4 )

3542 ( 4 )

3542 ( 4 )

(3)

S p e c t r a here l i m i t e d t o d i a g n o s t i c group o f s i x t r a n s i t i o n s c e n t e r e d a r o u n d 3300 g a u s s ( a t 9.1 GHz) when m a g n e t i c f i e l d d i r e c t i o n i s p a r a l l e l t o z_-axis. R e l a t i v e i n t e n s i t y i n paren­ theses f o l l o w i n g f i e l d . 'For a l l c a l c u l a t i o n s g = 1.9752. R e m a i n i n g s p i n p a r a m e t e r s e x p r e s s e d i n u n i t s o f 1 0 " cm"'.

Hamiltonian

4

q u i t e w e l l and t h e r e m a i n i n g f o u r l i n e s l e s s w e l l by t h e p u r e l y d i p o l a r i n t e r a c t i o n model i n d i c a t e s 1) t h a t t h e m a g n i t u d e o f t h e d i p o l a r c o u p l i n g c a l c u l a t e d from t h e c r y s t a l s t r u c t u r e i s ap­ p r o p r i a t e and 2) t h a t t h e r e i s a s i g n i f i c a n t amount o f e x c h a n g e coupiing present. I f t h e other parameters o f t h e s p i n Hamiltonian a r e held constant, the f i e l d s at which the s i x d i a g n o s t i c t r a n s i t i o n s oc­ cur a r e each l i n e a r i n t h e exchange parameter over a small range (\A\ < \A\)> f a c i l i t a t i n g t h e p r e c i s e d e t e r m i n a t i o n o f J_. W i t h t r i e i n c l u s i o n o f e x c h a n g e a f i t between t h e o b s e r v e d and c a l ­ c u l a t e d s p e c t r a i n t h e d i a g n o s t i c r e g i o n was o b t a i n e d w h i c h was e x a c t , w i t h i n e x p e r i m e n t a l u n c e r t a i n t y ( s e e T a b l e I I I ) . The parameters o f best f i t , which c o n s t i t u t e a unique s e t , a r e g = 1.9752, D = - 4 8 3 . 4 x 1 0 " cm" , d = - 5 0 . 7 5 x 1 0 " cm" and J = 45. l l x l O " cm" . I t i s worth noting t h a t w h i l e i n t h e absence o f e x c h a n g e t h e b e s t agreement between o b s e r v e d and c a l c u l a t e d f i e l d s f o r t h e d i a g n o s t i c t r a n s i t i o n s was o b t a i n e d when t h e p a r a m e t e r s J) and d w e r e o f o p p o s i t e s i g n , t h e b e h a v i o r o f t h e t r a n s i t i o n f i e l d s w i t h t h e i n c l u s i o n o f exchange i s c o n s i s t e n t o n l y w i t h t h e s e p a r a m e t e r s h a v i n g t h e same s i g n . The p o s i t i v e v a l u e o b s e r v e d f o r J c o r r e s p o n d s t o an a n t i f e r r o m a g n e t i c c o u p l i n g . 4

4

1

4

1

1

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

60

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BETWEEN

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IONS

T a b l e I I I compares o b s e r v e d and c a l c u l a t e d s p e c t r a i n t h e d i a g n o s t i c region f o r several combinations of spin Hamiltonian p a r a m e t e r s , w h i l e F i g u r e 5 i l l u s t r a t e s t h e agreement between t h e c a l c u l a t i o n e m p l o y i n g t h e p a r a m e t e r s o f b e s t f i t and t h e e n t i r e s p e c t r u m o b s e r v e d when t h e m a g n e t i c f i e l d i s p a r a l l e l t o z_. In T a b l e IV o b s e r v e d t r a n s i t i o n f i e l d s a t t h e Q-band f r e q u e n c y ( 3 5 . 1 GHz) and t h o s e c a l c u l a t e d f r o m t h e p a r a m e t e r s o f b e s t f i t ( o b t a i n e d a t 9.1 GHz) a r e compared f o r s e v e r a l o r i e n t a t i o n s . The computer p r o g r a m u s e d f o r a l l s p e c t r a l c a l c u l a t i o n s i n t h i s paper was w r i t t e n by Mr. T. M. L e n h a r d t o f t h e U n i v e r s i t y o f I l l i n o i s and i s based on t h e f r e q u e n c y s h i f t p e r t u r b a t i o n t e c h ­ nique of reference ( 9 ) . I f t h e c r y s t a l s o f C r : A 1 ( u r e a ) 6 I 3 a r e s u b j e c t t o tempera­ t u r e dependent changes i n c r y s t a l and m o l e c u l a r s t r u c t u r e o f t h e type observed i n the cas p e r a t u r e o f most o f t h pected. A c o n t r a c t i o n o f t h e l a t t i c e ( a t l e a s t i n t h e cod i r e c t i o n ) w o u l d r e d u c e t h e m e t a l - m e t a l s e p a r a t i o n and have t h e e f f e c t o f i n c r e a s i n g b o t h d^ and i n a b s o l u t e v a l u e . The param­ e t e r d w i l l , o f c o u r s e , v a r y as l / r , w h i l e t h e r a t e o f change o f w o u l d depend on t h e m e c h a n i s m ( s ) o f t h e exchange, but i n any c a s e m i g h t r e a s o n a b l y be e x p e c t e d t o be somewhat g r e a t e r t h a n t h a t f o r d. A change i n t h e c o o r d i n a t i o n g e o m e t r y about t h e Cr a t o m w o u l d r e s u l t i n a c h a n g e i n J), but i t i s d i f f i c u l t t o make a priori p r e d i c t i o n o f the d i r e c t i o n or magnitude o f the change. E x p e r i m e n t a l l y , D, d, and J. a l l i n c r e a s e i n m a g n i t u d e w i t h decreasing temperature. A t a b o u t 90°K t h e p a r a m e t e r s g i v i n g t h e b e s t agreement between o b s e r v e d and c a l c u l a t e d s p e c t r a ( t h e f i t was a t l e a s t as good as t h a t a t room t e m p e r a t u r e ) w e r e g = 1.9752, D = - 5 U . 8 x l O - cm" , d = - 5 4 . 3 5 x l 0 " cm"', and J = 50. 1 3 x 1 0 " cm" . The v a l u e o f d, w h i c h was o b t a i n e d f r o m t h e s p a c i n g between d i a g n o s t i c l i n e s ]_ and 6 ( v i d e s u p r a ) , c o r r e ­ sponds t o an a p p a r e n t c o n t r a c t i o n o f t h e l a t t i c e i n t h e co­ d i r e c t i o n , or at l e a s t a r e d u c t i o n o f the Cr-Cr p a i r d i s t a n c e , by a b o u t 2$. T h i s i s about t w i c e t h e l a t t i c e c o n t r a c t i o n ob­ s e r v e d i n t h e v a n a d i u m c o m p l e x , but i s not a p h y s i c a l l y u n r e a ­ s o n a b l e amount. The f r a c t i o n a l i n c r e a s e i n i s about 1.6 t i m e s t h a t o b s e r v e d f o r id; t h a t i s , ,J a p p e a r s t o be v a r y i n g a p p r o x i ­ m a t e l y as l / r . W h i l e a pathway f o r d i r e c t e x c h a n g e e x i s t s and the l / r dependence does not seem u n r e a s o n a b l e f o r t h i s mecha­ nism at the observed i n t e r n u c l e a r d i s t a n c e , a super-exchange mechanism i s not c o n c l u s i v e l y r u l e d o u t . When t h e m a g n e t i c f i e l d d i r e c t i o n i s a p p r e c i a b l y away f r o m t h e z _ - a x i s , most o f t h e p a i r t r a n s i t i o n s w h i c h a r e c a l c u l a t e d t o have an o b s e r v a b l e i n t e n s i t y o c c u r a t f i e l d s s u c h t h a t t h e y a r e masked by t h e much more i n t e n s e i s o l a t e d - i o n l i n e s , and t h e agreement between t h e c a l c u l a t e d t r a n s i t i o n f i e l d s and t h o s e l i n e s w h i c h a r e o b s e r v e d i s not as good as f o r n e a r l y p a r a l l e l o r i e n t a t i o n s ( s e e T a b l e I V ) . I n a d d i t i o n , f o r t h e most d i l u t e 3

a n

4

4

1

4

1

5

5

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974. 3 G

6

3

Figure 5. Comparison of observed and calculated X-band epr spectra for coupled Cr(urea) -Cr(urea) * pairs. Heights of lines in the calculated stick spectrum represent rehtive intensities. The two outermost lines in the calculated spectrum are each actually two transitions calculated to occur at fields differing by less than 3 gauss.

62

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

T a b l e IV Observed

and C a l c u l a t e d Q-Band EPR S p e c t r a a t S e v e r a l

Orientations

f o r C o u p l e d Cr( u r e a )

θ = 0° (H // z) Calculated Field (gauss) 113651 11368 J

Observed Field (gauss)

Calculated Field (gauss) 114721 11483J

11367

13570 13635 13649

139601 13963 J

3

+

6

Pairs

θ = 30°

Observed C a l c u l a t e d Field Field (gauss) (gauss)

11480

Observed Field (gauss)

I 1767 11809

11500

11692 ( s h ) 11765 11837 12415 12476 12538 12790 12849 12910 13485 13538 13570 ( ? ) 13633 13660 ( s h ) 13793 13876

- C r ( urea)

θ = 15°

11447 11537 116791 11694 J 11758 11840 12417 12476 12538 12790 12852 12912 13488

3 +

6

a

11777J 11841 11924 12447 12501 12539 12805 12845 12892 13394 13477 135391 13540J 13815 13846

11765 11803 11843

12073J

11838 11930 12446 12501 12540 12802 12842 12888 13395 13460 Y

b

12155 12255 125251 12532J 12562 12817 12834 1 12840 J 13140

X

13225Ί

13805

13245 f 13287J 13429

12524 12563 12815 12831 13000 13120

13420

13958

S h o u l d e r s a r e o b s e r v e d on t h e i n t e n s e at a p p r o x i m a t e l y t h e s e f i e l d s .

isolated-ion transitions

P a i r t r a n s i t i o n s a t t h e s e f i e l d s w o u l d be o b s c u r e d by t h e i n ­ t e n s e i s o l a t e d - i o n t r a n s i t i o n i n t h e same r e g i o n .

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

6.

DAVIS

AND

Muitisite

BELFORD

Interactions

63

s a m p l e s weak l i n e s beyond t h o s e p r e d i c t e d by o u r model a r e ob­ served i n the r e g i o n o f the outermost l i n e s a s s o c i a t e d w i t h the i s o l a t e d - i o n , e v e n a t t h e p a r a l l e l o r i e n t a t i o n ( s e e F i g u r e s 4a and 5 ) . These d i s c r e p a n c i e s , w h i c h we do not f e e l i n d i c a t e any s e r i o u s d e f i c i e n c y i n t h e model, may a r i s e f r o m t h e terms w h i c h we have l e f t o u t o f o u r s p i n H a m i l t o n i a n . Another p o s s i b l e o r i ­ g i n f o r t h e ' e x t r a l i n e s w o u l d be t h e e x i s t e n c e o f two ( o r more) d i s t i n c t Cr( u r e a ) - C r ( u r e a ) s i t e s , c h a r a c t e r i z e d by d i f f e r ­ e n t I) v a l u e s . The e f f e c t o f s l i g h t l y d i f f e r e n t j) v a l u e s w o u l d be m i n i m a l on t h e d i a g n o s t i c p o r t i o n o f t h e s p e c t r u m , but i n t h e two o u t e r r e g i o n s w o u l d r e s u l t i n s e t s o f l i n e s d i s p l a c e d by 2 Δ ρ ^ β . As t h e c o n c e n t r a t i o n o f C r i s increased, the i n t e n s i t y o f the i s o l a t e d - i o n l i n e s r e l a t i v e t o the remaining f e a t u r e s o f t h e s p e c t r u m d e c r e a s e s , and a l l o f t h e l i n e s a r e b r o a d e n e d , r e ­ ducing r e s o l u t i o n (se that the 'extra' l i n e peared i n the spectru Figur spectru pur C r ( u r e a ) I ( F i g u r e 4c) i s d i f f e r e n t y e t , a l t h o u g h some g e n e r a l s i m i l a r i t i e s between i t and t h o s e o f t h e d i l u t e d s a m p l e s a r e evident. U n d o u b t e d l y t h e same i n t e r a c t i o n s a r e a t w o r k h e r e , but a t t h e l e a s t we c a n e x p e c t i t t o be n e c e s s a r y t o c o n s i d e r i n t e r a c t i o n s between e a c h C r i o n and b o t h o f i t s a x i a l n e a r e s t neighbors. 1

3 +

3 +

6

6

3 +

6

3

3 +

Additional

Studies of Interactions in Κ urea)5X3 L a t t i c e s

The v a r i o u s a d d i t i o n a l i n v e s t i g a t i o n s o f m a g n e t i c i n t e r a c ­ t i o n s i n hexaurea-metal ( I I I ) h a l i d e l a t t i c e s contemplated or c u r r e n t l y underway i n t h i s l a b o r a t o r y a r e c o n v e n i e n t l y d i v i d e d i n t o t h r e e c a t e g o r i e s by t h e c o m p o s i t i o n o f t h e m a t e r i a l s b e i n g studied. The f i r s t g r o u p c o n s i s t s o f s y s t e m s i n v o l v i n g a s i n g l e p a r a m a g n e t i c s p e c i e s , e i t h e r as a s u b s t i t u t i o n a l i m p u r i t y i n one o f t h e a l u m i n u m s a l t s o r as t h e p u r e M ( u r e a ) X s a l t . Here we w i s h t o d e t e r m i n e w h e t h e r c o u p l i n g between p a r a m a g n e t i c s i t e s o c c u r s and, i f s o , w h e t h e r a mechanism o f t h e t y p e e s t a b l i s h e d f o r t h e chromium s y s t e m a p p e a r s a p p r o p r i a t e . In t h e t i t a n i u m s y s t e m , t h e o n l y o t h e r one f o r w h i c h e x p e r i m e n t a l information is c u r r e n t l y a v a i l a b l e , t h e l a c k o f dependence o f t h e e p r s p e c t r u m on T i concentration suggests that i n t e r i o n i c coupling is m i n i ­ mal. A n a l y s i s i s c o m p l i c a t e d by t h e f a c t t h a t , e v e n f o r d i l u t e samples, the spectrum o f T i ( u r e a ) i s not t h a t a n t i c i p a t e d f o r a s i m p l e S, = 1/2 ( d ) i o n and has not been a d e q u a t e l y inter­ preted. I n t h e s e c o n d g r o u p a r e s y s t e m s i n w h i c h one o f t h e p a r a ­ m a g n e t i c i o n s i s doped i n t o t h e l a t t i c e o f a d i s s i m i l a r paramag­ n e t i c i o n . At t h e p r e s e n t o u r a t t e n t i o n has c e n t e r e d on C r : T i ( u r e a ) 0 X 3 and Cr:V( u r e a ) X . Because o f l i n e broadening due t o r a p i d r e l a x a t i o n , n e i t h e r T i ( u r e a ) ô n o r V ( u r e a ) 6 gives an o b s e r v a b l e e p r s p e c t r u m a t room t e m p e r a t u r e . For o u r 6

3

3

+

3 +

6

1

6

3

3+

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

3 +

64

EXTENDED

INTERACTIONS

BETWEEN

METAL

3+

IONS

p u r p o s e s , s i t e s o c c u p i e d by C r ( u r e a ) ô i o n s as g u e s t s i n one o f t h e o t h e r l a t t i c e s may be c l a s s i f i e d a c c o r d i n g t o t h e a x i a l neighbors of a given C r ion. I f the c o n c e n t r a t i o n of C r is low, most w i l l have d i s s i m i l a r i o n s i n b o t h n e i g h b o r i n g a x i a l p o s i t i o n s , a few w i l l have one s i m i l a r and one d i s s i m i l a r n e i g h ­ b o r , w h i l e t h e number w i t h two s i m i l a r n e i g h b o r i n g i o n s w i l l be negligible. Depending on t h e s t r e n g t h o f t h e i n t e r i o n i c c o u ­ p l i n g , i f any, s p e c t r a l f e a t u r e s due t o i o n s i n t h e f i r s t t y p e o f s i t e p o t e n t i a l l y w o u l d p r o v i d e i n f o r m a t i o n on t h e p a r a m a g n e t i c ion o f the host l a t t i c e , s i n c e the Cr(urea)ô system i s a l r e a d y w e l l c h a r a c t e r i z e d . (Note, however, t h a t d i f f e r e n c e s i n l a t t i c e d i m e n s i o n s may r e s u l t i n d i f f e r i n g d e g r e e s o f d i s t o r t i o n o f t h e Cr(urea) i o n s o t h a t changes i n t h e s p e c t r u m f r o m t h e Cr:A1( u r e a ) 5 X 3 r e f e r e n c e w i l l p r o b a b l y be a c o m b i n a t i o n o f l a t ­ t i c e and s p e c i f i c i o n - c o u p l i n o f samples o f C r : T i ( u r e a ) f r o m Cr : A l ( u r e a ) I . The s t r o n g e s t f e a t u r e s o n c e a g a i n a r e c o n s i s t e n t w i t h the Hamiltonian (!). W h i l e t h e r e i s no d e t e c t a b l e change i n £, j) i s l a r g e r i n m a g n i t u d e t h a n o b s e r v e d i n t h e a l u ­ minum l a t t i c e . There are s e v e r a l p o s s i b l e e x p l a n a t i o n s o f the f a i l u r e to observe s i g n i f i c a n t T i - C r c o u p l i n g , but t h e most a t t r a c t i v e w o u l d seem t o be t h e a s s u m p t i o n t h a t t h e T i is re­ l a x i n g so r a p i d l y t h a t the C r i s not c o u p l e d t o i t , but s e e s o n l y an a v e r a g e d e n v i r o n m e n t w h i c h w o u l d p r o v i d e a v e r y s m a l l paramagnetic s h i f t . S i m i l a r l y , t h e s e c o n d a r y f e a t u r e s c a n be i n t e r p r e t e d on t h e b a s i s o f c o u p l e d C r - C r pairs. The u s e o f d i f f e r e n t i s o m o r p h o u s l a t t i c e s c o u p l e d w i t h measurements a t low temperature p r o v i d e s a range o f Cr-Cr s e p a r a t i o n s which s h o u l d h e l p t o f u r t h e r e l u c i d a t e t h e p r e c i s e n a t u r e o f t h e c o u p l i n g be­ tween t h e C r ( u r e a ) i o n s and, i n p a r t i c u l a r , t h e e x c h a n g e mechani sm. As y e t we have not p r e p a r e d any c r y s t a l s o f t h e t h i r d t y p e i n w h i c h one o f t h e Al(urea)ôX s a l t s i s doped s i m u l t a n e o u s l y w i t h two d i s s i m i l a r p a r a m a g n e t i c M( u r e a ) ions. The primary a i m h e r e w i l l be t o s t u d y c o u p l i n g i n d i s c r e t e M( u r e a ) W(urea) + pairs. I t i s a n t i c i p a t e d t h a t a l a r g e p a r t o f any s u c h i n v e s t i g a t i o n s w i l l have t o be c a r r i e d out a t l i q u i d h e l i u m temperature. I n summary, t h e i s o m o r p h o u s , i s o s t r u c t u r a l M ( u r e a ) X salts p r o v i d e an o p p o r t u n i t y f o r a v a r i e t y o f i n v e s t i g a t i o n s o f weak c o u p l i n g between p a r a m a g n e t i c i o n s i n t h e s o l i d s t a t e . Epr i n ­ v e s t i g a t i o n s o f t h e s e s y s t e m s not o n l y c a n p r o v i d e i n f o r m a t i o n on t h e c o u p l i n g i t s e l f , but i n d i r e c t l y , t h r o u g h t h e c o u p l i n g , i n f o r ­ m a t i o n about t h e i n d i v i d u a l i o n s i n v o l v e d c a n be o b t a i n e d w h i c h w o u l d not n o r m a l l y be a v a i l a b l e . 3 +

3 +

3+

3 +

6

6

3

3 +

3 +

3

+

3 +

3 +

3 +

3 +

6

3

3 +

6

3 +

6

6

5

6

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

3

6.

DAVIS

AND BELFORD

Multisite

Interactions

65

Acknowledgements T h i s r e s e a r c h was s u p p o r t e d by t h e P e t r o l e u m R e s e a r c h t h e N a t i o n a l S c i e n c e F o u n d a t i o n , and t h e A d v a n c e d R e s e a r c h j e c t s Agency.

Fund, Pro­

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9.

D i n g l e , R., J . Chem. Phys. ( 1 9 6 9 ) 50, 545. L i n e k , Α., Siskova, J., a n d J e n s o v s k y , L., Acta Crystallogr. (1969) A25, 155S. D a v i s , P. H. a n d Wood, J . S., I n o r g . Chem. ( 1 9 7 0 ) 9, 1111. Figgis, Β. N., W a d l e y , L. G. Β., and Graham, J., Acta Crystallogr. ( 1 9 7 2 ) B28, 187. Figgis, Β. N. a n d 25, 2233. D a v i s , P. Η., Ph.D. Thesis, University of Illinois (1972), pp. 7-18. Figgis, Β. N. a n d W a d l e y , L. G. Β., J . Chem. Soc. Dalton (1973), 2182. S a n t a n g e l o , Μ., Atti a c a d . sci. lettere ed arti P a l e r m o , P a r t I ( 1 9 5 7 - 5 8 ) 1 8 , 123. Belford, R. L., D a v i s , P. Η., Belford, G. G., and L e n h a r d t , Τ. Μ., this volume.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

7 Polymeric, Mixed-Valence Transition Metal Compounds GILBERT

M.

THOMAS

J.

BROWN, MEYER,

ROBERT

and

W.

TOM

CALLAHAN,

RAY

EUGENE

C.

JOHNSON,

WEAVER

U n i v e r s i t y o f N o r t h C a r o l i n a , C h a p e l Hill, N . C . 27514

In our work we hav i n which metal atoms or ions are held i n c l o s e proximity by chemical l i n k a g e s . One o f our goals has been to e s t a b l i s h the chemical and p h y s i c a l p r o p e r t i e s which a r i s e from metal-metal i n t e r a c t i o n s i n such systems and to modify the p r o p e r t i e s i n a c o n t r o l l e d way by d i r e c t e d chemical s y n t h e s i s . Three d i f f e r e n t kinds o f systems have been studied which d i f f e r with regard to the nature o f the metal-metal i n t e r a c t i o n : 1. Compounds i n which there i s s t r o n g , d i r e c t metal-metal bonding. 2. Cases where there are strong metal-metal i n t e r a c t i o n s through b r i d g i n g ligands. 3. Cases where there are weak metal-metal i n t e r a c t i o n s through bridging l i g a n d s . In compounds c o n t a i n i n g strong metal-metal bonds, the e f f e c t o f the metal-metal bond(s) i s t o modify s t r o n g l y the chemical and e l e c t r o n i c p r o p e r t i e s o f the compounds when compared to the component monomers.(1) One o f the i n t r i g u i n g p r o p e r t i e s of such systems i s that if c e r t a i n bonding and/or s t r u c t u r a l features are p r e s e n t , they can e x i s t i n a v a r i e t y o f molecular oxidation states, e.g., [(π-C H )Fe(CO)] (2), {[(π-C H ) 5

Fe(CO)] (Ph P(CH ) PPh )} / / 2

2

2 3

[(π-C H )Fe(CO)]+4 5

5

2

2+

and

+

2+/+/0/4

5

5

5

(Ph is phenyl) (3). Ions l i k e

0

{[(π-C H )Fe(C0)] (Ph P(CH ) PPh )} 5

5

2

2

2 3

2

+

are

formally mixed-valence c a s e s , but evidence i s now being obtained which i n d i c a t e s that the metal atoms are s t r o n g l y coupled and that o x i d a t i o n - r e d u c t i o n processes involve d e l o c a l i z e d molecular orbitals. S t r o n g , C h e m i c a l l y - S i g n i f i c a n t I n t e r a c t i o n s Through a_ Bridging Ligand. μ-οχο-Bridged Complexes o f Ruthenium ( I I I ) . We have prepared several μ-οχο-bridged complexes o f ruthenium (III): [(AA)XRu-0-RuX(AA) ] *(AA i s 2 , 2 » - b i p y r i d i n e (bipy) o r 2

1,10-phenanthroline

2

(phen); X i s CI o r N 0 ) . The complexes 9

66

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

7.

BROWX

Polymeric

E TAL.

67

Compounds

have been c h a r a c t e r i z e d by elemental a n a l y s i s (as hexaflurophosphate s a l t s ) , s o l u t i o n c o n d u c t i v i t y measurements, e l e c t r o ­ chemical measurements, and spectrophotometric t i t r a t i o n s using Cr as reductant. Although x - r a y c r y s t a l l o g r a p h i c data i s not y e t a v a i l a b l e , i n the complexes the X groups are almost c e r t a i n l y c i s to the b r i d g i n g oxide ion(4·) and molecular models i n d i c a t e that d i r e c t , through-space Ru-Ru i n t e r a c t i o n s are probably not p o s s i b l e . The p r o p e r t i e s o f the μ-οχο-bridged dimers are unusual when compared to c l o s e l y r e l a t e d b i s ( 2 , 2 ' - b i p y r i d i n e ) complexes of ruthenium (II) and ruthenium ( I I I ) . From electrochemical studies i n a c e t o n i t r i l e , the system L(bipy) ClRu-0-RuCl(bipy) ] 2 +

2

2

+

2

a l s o e x i s t s as mixed-valence +3 (Ru(III)-Ru(IV)) and +1 ( R u ( I I ) Ru(111)) i o n s . The +1 i o n i s chemically unstable on time s c a l e s longer than th The e l e c t r o n i c spectr include h i g h l y intense bands i n the v i s i b l e : for [ ( b i p y ) 2

ClRu-0-RuCl(bipy) ] ,

x 668nm(e 17,000); f o r [ ( b i p y ) C ! R u <5, c max c 0-RuCl(bipy) ] , A 470nm ( ε 17,000). Magnetic s u s c e p t i b i l i t y data has been obtained on the s a l t [(bipy) (N02)Ru-0-Ru(N0 ) 9

2

2 +

m a v

9

m a x

2

2

( b i p y ) ] ( P F ) i n the temperature range 77-275°K (5). An e x c e l l e n t f i t o f the data to the Bleaney-Bowers equation ( £ } has been obtained which i n d i c a t e s t h a t the +2 i o n has a s i n g l e t ground s t a t e with a l o w - l y i n g t r i p l e t s t a t e (2J = -173 c m " with g = 2.48). The unusual chemical and p h y s i c a l p r o p e r t i e s o f the μ-οχο-bridged dimers appear to a r i s e because o f a s t r o n g , chemically meaningful i n t e r a c t i o n between the ruthenium i o n s . Many o f the p r o p e r t i e s o f the dimers can be explained using the q u a l i t a t i v e molecular o r b i t a l model given i n Figure 1. The model i n F i g . 1 i s s l i g h t l y modified from the model used by 0rgel(7)for the l i n e a r i o n ClgRu-0-RuCl^" i n order to account f o r 2

6

2

1

the lower symmetry and the p o s s i b i l i t y t h a t the Ru-0-Ru linkage may be s l i g h t l y bent. Ru-O-Ru

Ru

Figure 1. Qualitative molecukir orbital scheme for Ubipy),XRu-0-RuX' (bipy) ] '. The ζ axis for each ruthenium ion is taken to be along the Ru-O axis. 2

Ο

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

EXTENDED

68

INTERACTIONS

BETWEEN

METAL

IONS

Using the molecular o r b i t a l scheme i n F i g . 1, the valence molecular o r b i t a l s are the s l i g h t l y separated p a i r π ι * and π * which c o n s i s t o f antibonding combinations o f ruthenium 4d and oxygen 2p o r b i t a l s . The 1201 value measured i n the magnetic study can be i n t e r p r e t e d i n terms o f the energy separation between the s i n g l e t and t r i p l e t states (-n\* ] and ( τ τ * π * ) , although other i n t e r p r e t a t i o n s can be g i v e n . From the molecular o r b i t a l model simple redox processes o f the μ-οχο-bridged ions n e c e s s a r i l y involve the gain or l o s s o f e l e c t r o n s i n net a n t i bonding molecular o r b i t a l s which explains the r e l a t i v e s t a b i l i t y of the +3 i o n [ ( b i p y ) C l R u - O - R u C l ( b i p y ) ] and the somewhat s u r p r i s i n g i n s t a b i l i t y o f the +1 ion [ ( B i p y ) C l R u - O - R u C l ( b i p y ) J . By comparing reduction p o t e n t i a l data f o r the μ-οχο-bridged dimers with data f o r r e l a t e d Ru(III)-Ru(II) c o u p l e s , i t can be estimated that the extent o f d é s t a b i l i s a t i o n of the π* o r b i t a l s may be severa chemical m o d i f i c a t i o n Systems, l i k e the*μ-οχο-bridged i o n s , i n which there are strong metal-metal i n t e r a c t i o n s across a bridging l i g a n d should be o f considerable i n t e r e s t i n the f u t u r e . In s t r o n g l y coupled systems, as i n systems containing d i r e c t metal-metal bonds, the strong coupling leads to s i g n i f i c a n t l y changed chemical and e l e c t r o n i c p r o p e r t i e s when compared to the component monomers. U l t i m a t e l y , such compounds may c o n s t i t u t e a new c l a s s of materials having d i s t i n c t and s y n t h e t i c a l l y c o n t r o l l a b l e p r o p e r t i e s of t h e i r own. 2

l

3

2

2

1

2

3 +

+

2

Weak Metal-Metal Complexes.

Interactions

i n Ligand-Bridged Ruthenium

We have prepared a s e r i e s o f complexes i n which ruthenium ions i n d i f f e r e n t l i g a n d environments are l i n k e d by orqanic bridging ligands. The complexes are o f the type [(NH ) R u ( L ) R u C l ( b i p y ) j * i n which L i s a d i b a s i c , N - h e t e r o c y c l i c l i g a n d , for example, p y r a z i n e . From the r e s u l t s o f spectrophotometric t i t r a t i o n s ( B r i n a c e t o n i t r i l e ) , and chemical and e l e c t r o ­ chemical i s o l a t i o n s t u d i e s , the complexes a l s o e x i s t as the o x i d i z e d +4 and +5 i o n s . From s p e c t r a l and reduction p o t e n t i a l d a t a , i n the mixedvalence +4 i o n s , the s i t e of o x i d a t i o n i s l o c a l i z e d l a r g e l y on the ( N H ) R u - end g i v i n g the o x i d a t i o n s t a t e c o n f i g u r a t i o n 5

2

34

2

3

5

[(NH ) Ru(III)(L)Ru(II)Cl(bipy) ] 3

5

2

4 +

.

This i s an expected

r e s u l t since the monomeric complexes R u ( b i p y ) L C l are more d i f f i c u l t to o x i d i z e by ^0.4 ν than are the complexes R u ( N H ) L In the system 2

+

3

[(NH ) RuN^)^H CH H@NRuCl(bipy) ] 3

5

2

2

2

5 + / 4 + / 3 +

,

where b i s ( 4 - p y r i d y l )ethane (BPA) i s the bridging l i g a n d , the

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

s

2

.

7.

BROWN

E T AL.

Polymeric

69

Compounds

e l e c t r o n i c and redox p r o p e r t i e s of the +3 and +4 ions are e s s e n t i a l l y the superimposed p r o p e r t i e s o f the monomeric ions [Ru(NH ) py] 3

5

3 + / 2 +

(py i s p y r i d i n e )

and R u ( b i p y ) ( p y ) C l 2

2 + / +

.

No evidence has been obtained f o r Ru-Ru i n t e r a c t i o n s , presumably, because the two π-systems o f the bridging l i g a n d are separated by a saturated (-ChU-Crfe) l i n k a g e . Ru-Ru i n t e r a c t i o n s can be "turned on" by using bridging ligands with extended, unbroken π-svstems. For the dimers with the b r i d g i n g ligands

^Oj\

@

(pyrazine)

{^^<^(θ^

Η

( θ ) *

(4,4'-bipyridine)

(trans-

data i n s o l u t i o n ( a c e t o n i t r i l e ) i n d i c a t e that i n t e r a c t i o n s do e x i s t between metal c e n t e r s . The i n t e r a c t i o n s are weak but t h e i r presence i s c l e a r l y seen i n the s p e c t r a l p r o p e r t i e s of the +3 and mixed-valence, +4 i o n s . For the +3 i o n s , [ ( N H ) R u ( L ) R u C l ( b i p y ) r , qualitative 3

5

2

band assignments can be made for the t r a n s i t i o n s d -> T T * ( L ) , d -> 7T*(bipy), and π + i r * ( b i p y ) . In comparing monomer and dimer spectra the only bands which are s i g n i f i c a n t l y s h i f t e d i n the dimeric complexes are the d π * ( ί ) charge t r a n s f e r bands f o r which the t r a n s i t i o n moment n e c e s s a r i l y l i e s along the Ru-Ru a x i s . For the mixed-valence, +4 i o n s , Intervalence T r a n s f e r (IT) bands appear i n the v i s i b l e and n e a r - i n f r a r e d s p e c t r a l regions. In IT a b s o r p t i o n , l i g h t - i n d u c e d e l e c t r o n t r a n s f e r occurs between the metal c e n t e r s , [(NH ) Ru(III)(L)Ru(II)Cl(bipy) ] 3

5

2

-^[(ΝΗ )^υ(ΙΙ)(ί)-

4 +

3

Ru(III)Cl(bipy) ] * 2

4 +

g i v i n g the product ion i n n o n - e q u i l i b r i u m v i b r a t i o n a l and rotational states. The IT x ^ values f o r a s e r i e s o f complexes are given i n Table I. Where i t has been p o s s i b l e to t e s t , we f i n d that the IT bands have the solvent dependence and approximate band widths p r e d i c t e d by Hush.(7) The energies o f the bands for the dimers i n Table I are c o n s i d e r a b l y higher than the energy of the IT band reported by Creutz and Taube (8) f o r the ion [ ( H ) R N g ^ R u ( N H ) ] . The bands are m

N

3

5

U

x

3

5

expected to appear at higher energies

5 +

f o r the unsymmetrical

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

5

2

2

4+

4+

3

2

a

5+

5

R

u

r

® ^ ( £ ) Η

R

" C l B _

2

5

4

+

2

8+

ο

4+

5

2

4+ 2

2

4 +

14.7 14./

14.4

13.9

13.3

12.7

10.4

5

2

4 +

680 (sh) o8u ^ s n ;

695 (sh)

720

750 (sh)

790

960

nm

-ÎÎ^->[A Ru(II)(L)Ru(III)ClB

A i s ammonia and Β i s 2 , 2 ' - b i p y r i d i n e .

2

ARUN(O)-CH— CH-^Ô)JRUCIB

5

ARUNQK=C-{O))RUCI B.

A

5

A Ruf(Ô)
5

A RUNQ]RU(CH CN)B

5

2

Ion

A Rur(0)|Ru(N0 )B

A RUNQ)RUC1B

5

A RU(III)(L)RU(II)C1B

Intervalence T r a n s f e r Bands i n A c e t o n i t r i l e

Table I

]* a

<300 <JUU

<300

920

530

r §

>

g

4

7.

BROWN

Polymeric

E T AL.

71

Compounds

dimers since the product o f l i g h t - i n d u c e d e l e c t r o n [(NH ) Ru(II)(L)Ru(III)X(bipy) ] , 3

5

i s a high energy o x i d a t i o n

4+

2

transfer,

s t a t e isomer i n which the o x i d a t i o n s t a t e c o n f i g u r a t i o n i s reversed from the c o n f i g u r a t i o n of the ground s t a t e i o n . We have drawn two important conclusions from t h i s work. The f i r s t i s that i f the b r i d g i n g l i g a n d includes an u n i n t e r ­ rupted ττ-system, the e f f e c t s o f metal-metal i n t e r a c t i o n s can be seen i n the p h y s i c a l p r o p e r t i e s o f the i o n s . Secondly, cooperative e l e c t r o n i c i n t e r a c t i o n s between metal c e n t e r s , as viewed s p e c t r a l l y , and rates o f intramolecular e l e c t r o n t r a n s f e r , as estimated from the energies o f IT bands(7)can be v a r i e d by a s e r i e s o f r e l a t i v e l y simple chemical m o d i f i c a t i o n s . The E f f e c t s o f Weak Metal-Meta Ligand-Bridged Compounds Several

ligand-bridged,

polymeric complexes o f ruthenium(II) - +2n+2 (n=0, 1, 2, [(b1py) ClRu(pyz)[Ru(b1py) pyz] RuCl(b1py) ] 2

2

n

2

3, 4 ; bipy i s 2 , 2 * - b i p y r i d i n e ; pyz i s pyrazine) ClRu(L)[Ru(bipy) L] RuCl(bipy) ] 2

0

n

{ϋ^°

Γ

2

Η

Ζ'

0

Η

2 n + 2

and [ ( b i p y ) 2

( n = 0 , 1, 2 ; L i s

ζ(θ^)

—

h

a

v

e

b

e

e

n

P

r e

P

a r e d

and i s o l a t e d as P F " o r C10 ~ s a l t s . ( 9 ) I n the preparations a 6

4

s e r i e s o f s e q u e n t i a l , stepwise r e a c t i o n s are used based on the r e a c t i v i t y o f the N 0 and NO?" groups when bound to b i s ( 2 , 2 - b i p y r i d i n e ) r u t h e n i u r n ( 1 1 ). We have i n v e s t i g a t e d the o x i d a t i o n s t a t e p r o p e r t i e s o f the polymeric ruthenium complexes and o f the s e r i e s o f l , ! - p o l y ferrocene compounds: +

1

1

Fe

η = 0 ; biferrocene

Fe

Fe

(Fc-Fc)

η = 1; 1 , Γ - t e r f e r r o c e n e η = 2; 1,1'-quatreferrocene

(Fc-Fc-Fc) (Fc-Fc-Fc-Fc)

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

72

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

In the p o l y f e r r o c e n e s , i r o n i s present i n the +2 formal oxidation state. Iron (III) i s a l s o an a c c e s s i b l e o x i d a t i o n s t a t e i n the ferrocene c o o r d i n a t i o n environment,and the mjxedvalence biferrocenium i o n , [(CgHgîFeiCgH^-CgH^FeiCgHg)] , has been well characterized.(10) From chemical and electrochemical s t u d i e s , both the ruthenium(II) and ferrocene polymers undergo a s e r i e s o f d i s ­ t i n c t , chemically r e v e r s i b l e , o n e - e l e c t r o n o x i d a t i o n s . By o x i d i z i n g the polymers a t c o n t r o l l e d p o t e n t i a l s , or by using c o n t r o l l e d amounts o f a chemical o x i d a n t , s o l u t i o n s containing mixed-valence or f u l l y o x i d i z e d ions can be obtained, e . g . , [(bipy) ClRu(pyz)[Ru(bipy) pyz] RuCl(bipy) ] 2

2

2

2

1 0 + / 9 + / 8 + / 6 +

[(C H )Fe(C H ^ 5

5

5

and

The

4

polymeric, mixed-valence ions are of i n t e r e s t when compared to r e l a t e d dimeric systems. Cooperative i n t e r a c t i o n s between more than two metal i o n s i t e s may lead to molecular p r o p e r t i e s more normally a s s o c i a t e d with s o l i d s t a t e m a t e r i a l s . For the ruthenium polymers, spectrophotometry t i t r a t i o n s , using Ce(IV) i n a c i d i c aqueous s o l u t i o n , have shown that each ruthenium(II) s i t e i n the polymeric chains can be o x i d i z e d to ruthenium(III), (Ru ) n

n

+ η Ce(IV)

>(Ru

i n

)

n

+ η Ce(III)

A s e r i e s of both f u l l y o x i d i z e d and p a r t l y o x i d i z e d (mixedvalence) ions have been i s o l a t e d as C10,~ s a l t s . Recently, work has begun on the low temperature magnetic p r o p e r t i e s o f the s a l t s i n c o l l a b o r a t i o n with Professor W. E . H a t f i e l d o f the U n i v e r s i t y of North C a r o l i n a . Work has a l s o begun on the s o l i d s t a t e e l e c t r i c a l c o n d u c t i v i t y p r o p e r t i e s o f the s a l t s . It may prove p o s s i b l e , to some e x t e n t , to vary s y s t e m a t i c a l l y the c o n d u c t i v i t y p r o p e r t i e s o f a s e r i e s of polymeric ions in the s o l i d s t a t e . The s y n t h e t i c chemistry involved i s extremely v e r s a t i l e and the polymeric ions can be modified i n a c o n t r o l l e d way with regard to such features a s : 1. The number of u n i t s i n the polymeric c h a i n . 2. The r a t i o o f Ru(II) to Ru(III) s i t e s . 3. The pattern of bridging l i g a n d s . 4. The non-bridging l i g a n d s . 5. The i n t r o d u c t i o n of metal ions d i f f e r e n t from ruthenium. For the 1 , 1 ' - p o l y f e r r o c e n e compounds, electrochemical measurements have shown that each ferrocenyl group (-Fc) can be o x i d i z e d to ferrocenium ( - F c ) . For example, f o r 1 , 1 ' - t e r f e r r o c e n e , there are three voltammetric waves a t w e l l separated p o t e n t i a l s . The voltammetric waves correspond to the electrode r e a c t i o n s : +

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

7.

BROWN

Polymeric

E T AL.

Compounds

73

E

(Fc-FoFc)

+

e

>(Fc-Fc-Fc)

(Fc-Fc-Fc)

2 +

+

e

»(Fc-Fc-Fc)

(FoFc-Fc)

3 +

+

e

>(Fc-Fc-Fc)

+

^ >

V

0.22 0.44

+

2 +

0.82

The EL. values are half-wave p o t e n t i a l s v s . the saturated sodiurfi c h l o r i d e calomel e l e c t r o d e i n 1:1 v/v dichloromethanea c e t o n i t r i l e a t 25+2°C. S o l u t i o n s c o n t a i n i n g mixed-valence ions such as ( F c - F c - F c ) and (Fc-Fc-Fc)2+ can be prepared by c o n t r o l l e d p o t e n t i a l electrolysis. For the mixed-valence biferrocenium i o n (Fc-Fc) , i t has been concluded that d i s c r e t e F e ( I I ) and F e ( I I I ) s i t e s e x i s t and that e l e c t r o n i c d e r e a l i z a t i o n between the two s i t e s i s small.(lO)Because o propertiesHBetween the biferrocenium i o n and the mixed-valence polyferrocene i o n s , i t appears t h a t the mixed-valence p o l y ferrocene ions a l s o contain weakly i n t e r a c t i n g but d i s c r e t e Fe(II) ana! F e ( I I I ) s i t e s . For the 1 , 1 ' - p o l y f e r r o c e n e compounds there are c h e m i c a l l y d i f f e r e n t s i t e s ( F c - and - F c - ) i n the polymeric c h a i n s . Upon o x i d a t i o n to the mixed-valence ions more than one o x i d a t i o n s t a t e isomer can e x i s t . The o x i d a t i o n s t a t e isomers d i f f e r with regard to the s i t e ( s ) o f o x i d a t i o n . For example, f o r the ion ( F c - F c - F c ) there are two e n e r g e t i c a l l y equivalent isomers — F c - F c - F c and F c - F c - F c — and one e n e r g e t i c a l l y nonequivalent isomer - - F c - F c - F c . From the e f f e c t of the f e r r o c e n y l group as a s u b s t i t u e n t , i t can be estimated that the isomers F c * - F c - F c and F c - F c - F c are of s i m i l a r energy. However, f o r other mixed-valence ions the d i f f e r e n c e i n f r e e energy between isomers can be s i g n i f i c a n t . From E% data and the f e r r o c e n y l s u b s t i t u e n t e f f e c t i t i s p o s s i b l e to estimate that f o r the r e a c t i o n +

+

+

+

+

+

+

Fc -Fc-Fc +

+

>

Fc -Fc -Fc +

+

AG * 2.8 kcal/mole. The p r o p e r t i e s of Intervalence T r a n s f e r (IT) bands are influenced by o x i d a t i o n s t a t e isomerism. IT bands f o r several of the mixed-valence ions are given i n Table I I . For some o f the ions the IT x i s s h i f t e d s i g n i f i c a n t l y from Xm^ f o r Fc -Fc. The band s h i f t s are expected when i t i s r e a l i z e d that the products o f l i g h t - i n d u c e d e l e c t r o n t r a n s f e r are not symmetric f o r the mixed-valence polyferrocene i o n s . In some cases the products of l i g h t - i n d u c e d e l e c t r o n t r a n s f e r are e n e r g e t i c a l l y unfavorable o x i d a t i o n s t a t e isomers, f o r example, m a x

x

+

F c

+

-Fc-Fc

+

—~—>

(Fcî F c - F c ) * +

As described by Hush(7)the IT t r a n s i t i o n energy i n such a case

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

a

I n 1:1

+

2

2

hv

hv

hv

hv

->

->

3

Transition

+

+

+

(Fc -Fc -Fc-Fc)*

+

(Fc -Fc -Fc)*

+

(Fc -Fc-Fc)*

+

(Fc-Fc )*

4

( v / v ) C H C l - C H C N , 0.1M i n (n-Bu) NPFg.

+

Fc -Fc

Fc-Fc

+

Fc -Fc

Fc -Fc

+

II

1790

1990

1900

nm

5590

max

1670

x

5990

5020

5260

kK

1720

1080

1560

760

ε

Intervalence T r a n s f e r Bands f o r Mixed-Valence 1 , 1 ' - P o l y f e r r o c e n e

Table Ions

7.

BROW

x

Polymeric

E T AL.

75

Compounds

w i l l i n c l u d e the d i f f e r e n c e i n ground s t a t e energies between the isomers F c - F c - F c and F c - F c - F c i n a d d i t i o n to the usual Franck-Condon energy b a r r i e r . An equation has been derived by Hush which r e l a t e s the energy of IT absorption to the rate of i n t r a m o l e c u l a r e l e c t r o n t r a n s f e r i n mixed-valence compounds.(7jThe equation allows estimates to be made for processes l i i c e : +

Fc -Fc +

+

>Fc-Fc

+

+

k * 3x10

+

Fc -Fc-Fc—>Fc-Fc -Fc+ +

k * IxlO

+

10

1 0

sec"

1

sec"

1

The rate of e l e c t r o n hopping between f e r r o c e n y l and f e r r i cenium groups i s slower i n F c - F c - F c than i n F c - F c , mainly because o f the e n e r g e t i c a l l y d i f f e r e n t chemical s i t e s i n Fc -Fc-Fc . The r a t e d i f f e r e n c e s involved are s m a l l , but s i g n i f i c a n t , since the complexes, rates of i n t r a m o l e c u l a r e l e c t r o n t r a n s f e r can be v a r i e d s y s t e m a t i c a l l y by changing the chemical environments of the c o n s t i t u e n t i o n s . +

+

+

+

+

Acknowledgements. Acknowledgements are made to the M a t e r i a l s Research the U n i v e r s i t y of North C a r o l i n a under grant number with DARPA and to the Army Research O f f i c e - Durham grant number DA-AR0-D-31-124-73-G104 f o r support of research.

Center of GH33632 under this

Literature Cited 1. 2. 3. 4. 5.

6. 7. 8. 9. 10.

Meyer, T . J., P r o g r . I n o r g . Chem., i n p r e s s . Ferguson, J. A. and Meyer, T . J., J. Amer. Chem. Soc., (1972), 94, 3409. Ferguson, J. Α . , and Meyer, T . J., Chem. Commun., (1971), 1544. Godwin, J. B. and Meyer, T . J., Inorg. Chem., (1971). 10, 471. The magnetic data was obtained i n c o l l a b o r a t i o n with Professor W. E . H a t f i e l d and h i s research group at the U n i v e r s i t y of North C a r o l i n a . Bleaney, B. and Bowers, K . , Proc. Royal Soc. (London), (1952), 214A, 451. Hush, N. S., Progr. Inorg. Chem., (1967), 8, 391. C r e u t z , C. and Taube, H . , J. Amer. Chem. Soc., (1973), 95, 1006. Adeyemi, S. Α . ; Johnson, E. C.; Miller, F. J.; and Meyer, T . J.; Inorg. Chem. (1973), 12, 2371. Cowan, D. O.; Le Vanda, C.; Park, J.; and Kaufman, F.; Accounts Chem. R e s . , (1973), 6, 1.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

8 Exchange Interactions i n Nickel(II), Copper(II), and Cobalt(II) Dimers Bridged by Small Anions DAVID N. HENDRICKSON and D. MICHAEL DUGGAN University of Illinois, Urbana, Ill. 61801

Introduction In this paper some very recent work on "magnetic" exchange interactions in certain nickel(II) and copper(II) dimers will be summarized and some init­ ial findings on analogous cobalt(II) and manganese(II) compounds will be presented. The compounds to be discussed include the following: [M (tren) X ] (BPh )2 where, 2

2

2

4

M = Ni (II), Cu(II), Co(II), Mn(II) X = Ν ¯, OCN¯, SCN¯, SeCN¯, CN¯(Cu only) and, tren = 2,2,2"-triaminotriethylamine 3

One objective at the outset of this work was to keep the non-bridging ligand tren and the counterion tetraphenylborate constant while observing the effects on the exchange interaction of changing either the metal or the bridging ligand. The anticipation of obtaining similar dimer structures upon inter­ changing metals was not to be realized; however, interesting variations developed. It will be noticed that extended bridging groups have been selected to eliminate the consideration of direct metal-metal exchange interactions. In addition, the tetraphenyl­ borate counterion should provide some degree of shielding between dimer units. The phenomenon of magnetic exchange i s , of course, electronic in origin. A particular distinction is made in our work between the parameterization of the observed phenomenon (as per some effective spin Hamiltonian) and the interpretation of the effective 76 In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

8.

HENDRICKSON

AND

DUGGAN

Ni,

Cu, Co

77

Dimers

s p i n H a m i l t o n i a n p a r a m e t e r s f r o m t h e e l e c t r o n i c wavef u n c t i o n s of the system i n q u e s t i o n . Tnis l a t t e r pur­ s u i t i s the most i m p o r t a n t ; a t p r e s e n t i t i s o n l y p o s ­ s i b l e to provide q u a l i t a t i v e or pernaps semi-quantita­ t i v e i n t e r p r e t a t i o n s of observed trends i n parameters. nickel(II)Dimers T h e f o u r n i c k e l s y s t e m s a r e d i m e r i c w i t h two X~ a n i o n s e n d - t o - e n d b r i d g i n g s u c h t n a t t h e n i c k e l atoms are o c t a h e d r a l l y coordinated. Initially this was d e d u c e d f r o m c a r e f u l i n f r a r e d and e l e c t r o n i c absorp­ t i o n s p e c t r o s c o p y and X - r a y powder p a t t e r n work. For example, t h e powder p a t t e r n s o f [ N i - ( t r e n ) - N C O ) Λ (BPh ) and [Ni (tren) s i m i l a r and s i n c e t h e s t a b l i s h e d (1) t o b r i d g e i n a b i s - b i d e n t a t e f a s h i o n b e t w e e n two m e t a l c e n t e r s , t h e e n d - t o - e n d d i - b r i d g i n g n a t u r e o f t h e a z i d e c o m p o u n d i s i n d i c a t e d . A s we ex­ amined tue magnetic p r o p e r t i e s of t h i s s e r i e s o f n i c k e l d i m e r s we b e c a m e c o n c e r n e d w i t n t h e d e t a i l s o f the molecular s t r u c t u r e . The s i n g l e - c r y s t a l X - r a y s t r u c t u r e o f [Ni (tren) (NCOkl ( B P h J was d e t e r m i n e d (2) . Discrete cationic [Ni Ttren) NCO) J a n d a n i o n i c BPh " u n i t s a r e found, w h e r e t h e m e t a l s a r e b r i d g e d b y two c y a n a t e g r o u p s b o n d i n g i n a n e n d - t o - e n d mode. The structural c h a r a c t e r i s t i c s o f t h e c a t i o n a r e d e p i c t e d i n F i g u r e 1. A s a s i d e - p r o d u c t o f o u r e x c h a n g e w o r k we h a v e t h u s e s t a b l i s h e d t h e f i r s t c a s e o f an a u t h e n t i c a t e d o x y g e n bonded cyanate metal complex. E a c h n i c k e l atom i n the c a t i o n i s s i x c o o r d i n a t e with t r e n occupying four s i t e s (Ni-N=2.047(7), 2 . 0 5 4 ( 5 ) , 2 . 0 9 5 ( 7 ) , and 2.130(7) A) a n d t h e o t h e r two s i t e s o f t h e d i s t o r t e d octahedron a r e o c c u p i e d by t h e Ν and 0 atoms o f t h e b r i d g i n g c y a n a t e i o n s (Ni-N-2.018(7) and N i - 0 = 2 . 3 3 6 ( 5 ) A ) . The two c y a n a t e b r i d g e s a r e e s s e n t i a l l y c o - p l a n a r w h i l e t h e n i c k e l a t o m s l i e a b o v e a n d b e l o w t h e p l a n e b y ± 0.25A. I n r e s p e c t t o t h e e x c h a n g e work t h e r e a r e t h r e g germane points. F i r s t , the N i - N i d i s t a n c e of 5.385(1)A p r e ­ c l u d e s a d i r e c t N i - N i exchange i n t e r a c t i o n . Second, p a c k i n g d i a g r a m s show t h a t t h e l a r g e BPh-"" a n i o n s p r o ­ v i d e e f f e c t i v e s n i e l d i n g between d i m e r i c c a t i o n s . T h i r d , and m o s t i m p o r t a n t t o o u r d i s c u s s i o n , i t c a n be s e e n t h a t e a c h NCO i n the c a t i o n i s e s s e n t i a l l y b o n d i n g end-on a t t h e n i t r o g e n end ( $ N i N C = 1 5 5 . 0 ° ) and a l m o s t a t r i g h t a n g l e s a t the oxygen end ( £NiOC 117.1°). 4

2

2

2

2

2

2

2

2

β

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

78

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

lé

1

£ c v. to

to S S «

toS ·~

0

88

§

II 11 ω

Ο υ

1 "•3 £3 i

f

3

C

Ci

to

to ^ ε:

υ^έ

ο *ε 3 ^ « c to ? ^ α Ιτ £ § cf ^

ν

1—1

f

it to to £ 2 to 3D S

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

8.

HENDRICKSON

AND

Ni,

DUGGAN

Cu,

Co

79

Dimers

B e c a u s e t h e r e was a v a i l a b l e a s t r u c t u r e o f a r e ­ lated end-to-end bridged di-p-thiocyanate n i c k e l dimer, i . e . , [ N i ( e n ) - ( N C S ) ~ ] I ~ (_3) t a n d o u r w o r k (4) on [Ni (trenf2(WCS) ](BPh-f had c o n v i n c e d us of their s i m i l a r i t y of bridge s t r u c t u r e , i n our o p i n i o n i t only remained to determine the s t r u c t u r e of [Ni (tren) ( W ^ ) 1 ( B P h - ) , w h i c h was s o l v e d v e r y r e c e n t l y by P i e r p o n t e t a l (_5) . Tn§ b a s i c s t r u c t u r a l f e a t u r e s o f tne [ N i ( t r e n ) ( N ) J c a t i o n are i l l u s t r a t e d i n F i g u r e z. The two e n d - t o - e n d b r i d g e d a z i d e g r o u p s a r e p a r a l l e l and t h e n i c k e l atoms a r e l o c a t e d ± 0.52A f r o m tiie a z i d e plane. Each a z i d e b r i d g e i s somewhat asym­ m e t r i c w i t h N i - N - N a n g l e s o f 1^5.3(7)° a n d lg3.3(6)° and Ni-N d i s t a n c e s o f 2.069(8)A and 2.195(7)A. The c o o r d i n a t i o n geometr mately octahedral We h a v e m e a s u r e d t h e m a g n e t i c s u s c e p t i b i l i t y o f t n e f o u r n i c k e l d i m e r s i n t h e r a n g e o f 4.2-283°K. In eacn case tne (molar) paramagnetic s u s c e p t i b i l i t y data were l e a s t - s q u a r e s f i t t o an e q u a t i o n s e t o u t by G i n s b e r g e t a l (β) : 2

2

2

2

2

2

2

2

2

2

3

2

ν χ

= (2NgV/3k) ίψζ^ΓΤΎΙ

Α

+

l-Îz'j'F*

1

+

N

a

In Not as pli fie ter

t n i s e q u a t i o n , Ν , β, a n d k n a v e t n e i r u s u a l m e a n i n g , i s tne temperature-independent paramagnetism (taken -200x10 c g s / m o l o f a i m e r ) , and a n d F" a r e com­ c a t e d functions of temperature, s i n g l e - i o n zerol d s p l i t t i n g D, a n d t n e i n t r a d i m e r e x c n a n g e p a r a m e ­ J. Tne e f f e c t i v e i n t e r d i m e r e x c n a n g e i s Ζ'J'. A s we r e p o r t e d e a r l i e r ( 7 ) , t h e c o m p o u n d [ N i (tren) (N^) J(BPn^) shows a r e l a t i v e l y s t r o n g antiferromagnetic exchange i n t e r a c t i o n w i t n a s u s c e p t i b i l i t y maximum a t ^10u°K. I n F i g u r e 3 we n a v e r e p r o d u c e d t n e s u s c e p t i o i l i t y ana e f f e c t i v e m o m e n t c u r v e s for [i>ii ( t r e n ) N C O ) J (BPn^) . In t n i s c a s e tae s u s c e p t i D i l i t y i n c r e a s e s with d e c r e a s i n g temperature u n t i l a maximum i s r e a c n e d a t 14°K, waereu^on χ decreases r a p i d l y to 4.2°K. Tne r a p i d decrease i s i n d i c a t i v e of a sample wnich i s r e l a t i v e l y f r e e of paramagnetic impurities. Tne s o l i d l i n e s i n F i g a r o 3 are leastsquares t n e o r e t i c a l l i n e s , f i t to tne auove equation. T h u s , q u a l i t a t i v e l y we s e e t n a t , a l t h o u g h t h e cyanate and a z i d e n i c k e l d i m e r s t r u c t u r e s a r e g r o s s l y s i m i l a r , t n e r e i s an a p p r e c i a b l e a t t e n u a t i o n i n a n t i f e r r o m a g n e t i c e x c n a n g e i n t e r a c t i o n i n g o i n g f r o m tne azide t o tne c y a n a t e c o m p o u n d . 2

2

2

2

2

2

2

2

An e v e n more s t r i k i n g n i c k e l b r i d g i n g groups are

cnange o c c u r s wnen tne c n a n g e d t o e i t h e r tne

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

80

EXTENDED

Figure 2. geometrical

INTERACTIONS

ORTEP parameters; and carbon and hydrogen

RETWEEN

atoms are not shown.

Figure 3. Experimental and calculated magnetic susceptibility data for [Ni (tren).(NCO) ](BPh, )j. Lines are least-squares fit theoretical lines; for fitting parameters, see text. :

J

l

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

METAL

IONS

8.

HENDRICKSON

AND

Cu, Co

Ni,

DUGGAN

Dimers

81

t n i o c y a n a t e (NCS ) o r s e l e n o c y a n a t e (NCSe ) g r o u p s . T u e e f f e c t i v e moment c u r v e s f o r t n e n i c K e l t n i o c y a n a t e a n d s e l e n o c y a n a t e d i m e r s a r e snown i n F i g u r e 4. liotn compounds Have f e r r o m a g n e t i c i n t e r a c t i o n s o f a p p r o x i ­ m a t e l y t i i e same m a g n i t u d e . For comparison purposes, t h e e f f e c t i v e moment c u r v e o f o n e a n a l o g o u s , s t r u c t u r ­ a l l y cnaracterized(£) , c i s - d i s u b s t i t u t e d t r e n n i c k e l c o m p l e x , i . e . , N i ( t r e n ) ( N C S ) , i s a l s o snown. In t h i s l a s t c a s e t h e r e i s a v e r y s m a l l d e c r e a s e i n t h e moment a t t h e l o w e s t t e m p e r a t u r e s and t n i s i s u n d o u o t e d l y due to s i n g l e - i o n z e r o - f i e l d s p l i t t i n g and p e r h a p s a l i t t l e intermolecular interaction. P a r a m e t e r i z a t i o n of the s u s c e p t i b i l i t y data f o r the four n i c k e l dimers g i v e s the f o l l o w i n g v a l u e s : 2

Compound NCO NCS*" Ncse"

J , cm" -35 -4.4 +2.4 +1.6

1

0.71 -0.16 -0.07 -0.02

-10.1 -0.45 -0.49 -0.76

2.282 2.255 3.347 2.181

I t i s i m p o r t a n t t o m e n t i o n t n a t G i n s b e r g e t a l (β) re­ p o r t e d a f e r r o m a g n e t i c exchange f o r [ N i ( e n ) ( N C S ) Λ I w h e r e t h e y f o u n d J=+4.5 cm" . W i t h t h e m a g n e t i c and s t r u c t u r a l d a t a i n hand i t i s now a p p r o p r i a t e t o a t t e m p t t o e x p l a i n t h e c h a n g e i n s i g n and magnitude o f the exchange p a r a m e t e r f o r t h e s e four n i c k e l dimers. I n o u r o p i n i o n i t i s t h e symmetry of the b r i d g i n g u n i t s t h a t i s the most i m p o r t a n t factor i n d e t e r m i n i n g t h e s i g n and t o a c e r t a i n d e g r e e tne magnitude o f the exchange parameter J . Assistance i n understanding t h e f o l l o w i n g e x p l a n a t i o n c a n be nad by r e f e r r i n g t o F i g u r e 5 w h e r e somewhat i d e a l i z e d (i.e., tne a z i d e system i s r e p r e s e n t e d as symmetric, b e c a u s e t h e f i g u r e was p r e p a r e d b e f o r e t h e s t r u c t u r a l d e t a i l s w e r e known) p l a n e p r o j e c t i o n s o f t h r e e o f t h e n i c k e l dimer b r i d g i n g u n i t s are given. The g e o m e t r y r e p r e ­ s e n t e d f o r the t h i o c y a n a t e c a s e i s t n a t drawn from the s t r u c t u r e f o r [ N i ( e n ) ( N C S ) ] I ( 3 ) , w h i c h we a r e c o n ­ v i n c e d a c c u r a t e l y approximates to t h a t p r e s e n t i n our compound. In the c a s e of the most symmetric system, the a z i d e dimer, the metals i o n s are bonding i n t o the same b r i d g e m o l e c u l a r o r b i t a l s . This leads to a p a i r ­ ing of e l e c t r o n s , that i s a net antiferromagnetic ex­ change. Changing the b r i d g e from a z i d e t o cyanate g i v e s a b r i d g i n g u n i t w h i c h i s no l o n g e r s y m m e t r i c and now t h e two n i c k e l a t o m s b o n d i n t o b r i d g e o r b i t a l s t h a t are not of n e c e s s i t y of equal c o n s t r u c t i o n with r e s p e c t t o t h e two m e t a l c e n t e r s . As a r e s u l t t h e r e 2

2

4

2

4

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2

82

EXTENDED

INTERACTIONS

BETWEEN

METAL

4.00

3.50 > ( t r e n ) 6eCN} ](HR^ ^ 2

2

9

2 Niitrer)6CND

3.00

2

3.50 i>4 (tren) (SCN) ](BPh 5 2

2

2

4

2

3.00

300

100 200 TEMPERATURE (°K)

Figure 4. Experimental and calculated magnetic susceptibility data for [Ni (tren),(NCSe),](BPh,,)2 (top, left), Ni(tren)(NCS) (top, right), and [Ni^tren^NCS^fBPh^ (bottom). The lines are least-squares fit theoretical lines. 2

2

Nil

Nil

Ν

Ν

Figure 5. Diagramatic structures of di^-azide,

Ό

representation of di-fi-cyanate, and systems

the bridging di-fi-thiocyanate

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

8.

HENDRICKSON

AND

DUGGAN

N l , Cu,

Co

Dimers

83

a r e b o t h a n t i f e r r o m a g n e t i c and f e r r o m a g n e t i c pathways p o s s i b l e i n the d i - y - c y a n a t e system. The n e t e x c h a n g e i s weakly a n t i f e r r o m a g n e t i c . I t must be r e a l i z e d t h a t i t i s not p o s s i b l e to p r e d i c t from the X-ray s t r u c t u r e that [Ni (tren)^(NCO) ](BPh ) w o u l d be w e a k l y a n t i ­ f e r r o m a g n e t i c , however, i t appears p o s s i b l e t o r a t i o n ­ a l i z e t h e e x c h a n g e i n t h i s compound r e l a t i v e t o t h a t i n t h e a z i d e compound. I f we c o n t i n u e t h e a n a l y s i s , t n e n , i n t h e c a s e o f t n e t h i o c y a n a t e - and s e l e n o c y a n a t e - b r i d g e d d i m e r s t h e t r a n s i t i o n f r o m s t r o n g symmetry t o s t r o n g a n t i s y m m e t r y has r e a c n e d the p o i n t wnere the n i c k e l - b r i d g e b o n d i n g a t the s u l f u r o r s e l e n i u m atom i s c l o s e l y approacning 90°. B e c a u s e t h e b o n d i n g a t t h e n i t r o g e n atom i s s t i l l c l o s e to 180° and t h e n e t i n t e r a c t i o netic, vie d o n o t r e a l l y Know t h e d e t a i l s o f t n e geom­ e t r y o f t n e s e l e n o c y a n a t e o r i d g i n g u n i e , JJUC i t i s r e a s o n a b l e t o assume t n a t tne geometry i s c l o s e s t t o t n a t for tne t n i o c / a n a t e case. B e f o r e a c c e p t i n g t n i s a n g u l a r i t y o r syinmetry ex­ p l a n a t i o n of tne observed exchange e f f e c t s , t h e r e are t h r e e o t h e r c o n s i d e r a t i o n s w n i c n must be d i s c u s s e d : ' 1. Is the exchange propagated through a σ-bonding o r ττ-jjonding p a t h w a y ? , 2. D o e s t n e n o n - p l a n a r i t y o f t n e b r i d g i n g s y s t e m a f f e c t t h e symmetry a r g u m e n t s ? and, 3. vVnat r o l e m i g h t t n e isii-X (X=0 ,i\f ,S) d i s t a n c e p l a y ? Tne t n i r d q u e s t i o n i n e f f e c t a m o u n t s t o w n e t n e r o r not tne Ni-X o v e r l a p i n t e g r a l s v a r y g r e a t l y f o r tne d i s t a n c e s o f t\ii-u=2.34, i i i - a = 2 . o l , a n a n"i-W=2.02A. Tnese d i s t a n c e s are a r e s u l t of m i n i m i z a t i o n o f the energy of eacn system witn r e s p e c t t o o v e r l a p , o r o i t a l e n e r g y , and n u c l e a r r e p u l s i o n , anu as s u c h tne d i s t a n c e s o b t a i n e d a r e a t l e a s t p a r t i a l l y a d j u s t e d on t h e b a s i s of o v e r l a p . T h e o v e r l a p i n t e g r a l s c e r t a i n l y do n o t v a r y a s mucn a s t n e d i s t a n c e s w o u l d t e n d t o i n d i c a t e . I t c a n n o t be d e n i e d t h a t , i n c o m p a r i s o n t o t n e tfi-N Dond i n t n e a z i d e , t n e l o n g e r N i - 0 b o n d i n t n e cyanate may c o n t r i b u t e t o a l e s s e n i n g o f t h e e x c n a n g e i n t e r a c ­ tion. C e r t a i n l y t n e c n a n g e o f s i g n o f J down t n e s e r i e s i s not a r e f l e c t i o n of d i f f e r e n c e s i n Ni-X dis­ tances . Tne e f f e c t o f d i f f e r e n c e s i n p l a n a r i t y , as guaged b y t h e d i h e d r a l a n g l e φ b e t w e e n t h e iNîi-ΧΥΖ a n d X Y Z - N i ' p l a n e s , i s d i f f i c u l t t o judge. For tne t h i o c y a n a t e and c y a n a t e s y s t e m s φ i s c l o s e t o 0 ° , w h e r e a s , f o r t h e azide ψ i s 38.4°. T h e s e c n a n g e s seem t o p a r a l l e l a n expected i n c r e a s e i n " a l l e n e - l i k e " c h a r a c t e r o f the bridge unit. In r e f e r e n c e to p o s s i b l e σ-overlaps o f tne m e t a l d 2 _ 2 o r b i t a l s w i t n π-symmetry b r i d g e 2

2

v

4

2

v

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

84

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

o r b i t a l s , i t seems r e a s o n a b l e t h a t f o r Ο ° < φ < 9 0 ° , m o l e c ­ u l a r o r b i t a l s i n c o r p o r a t i n g i n a continuous bonding way b o t h m e t a l c h a r a c t e r a n d P and Ρ type π-orbitals on the b r i d g e would e x i s t . The 38.4° d i h e d r a l a n g l e of tne a z i d e dimer c o u l d , i n p a r t , l e a d t o a l a r g e r a n t i f e r r o m a g n e t i c exchange. An a n a l y s i s o f t n e d i h e ­ d r a l angle e f f e c t i s presented i n greater d e t a i l f o r two n i c k e l - a z i d e compounds i n a r e c e n t p a p e r (.5) a n d i t i s argued t h a t tne d i h e d r a l angle d i f f e r e n c e s are not i n the p r e s e n t case a major geometric factor. C o n s i d e r a t i o n of tne f i r s t q u e s t i o n r e v o l v e s around whether the excnange i n t e r a c t i o n i s o f a f i r s t o r d e r σ-exchange type o r a h i g n e r - o r d e r "promoted" e l e c t r o n π-exchange. The u n p a i r e d " n i c k e l d - e l e c t r o n s " are i n ά2 2 and d in construction. I m e t a l d - o r b i t a l s and t h e a p p r o p r i a t e s i g m a b r i d g e o r b i t a l s , then p o t e n t i a l l y v i a b l e σ-exchange pathways a r e p r e s e n t , a n d i t i s s u c h a m e c h a n i s m t h a t we b e l i e v e is present. In the p r o m o t i o n π-exchange mechanism one o f t h e u n p a i r e d n i c k e l e l e c t r o n s i s p r o m o t e d ( c o n ­ f i g u r a t i o n i n t e r a c t i o n w i t h an e x c i t e d s t a t e ) i n t o a π-type d - o r b i t a l (eg. , d ) and t h i s a l l o w s an o v e r l a p ( i e . , i n t e r a c t i o n ) v i a tne π-system o f the b r i d g e . The nature of say the ^ "P ( b r i d g e ) i n t e r a c t i o n w o u l d n o t be e x p e c t e d t o change s i g n i f i c a n t l y as a f u n c t i o n o f N i - o r i d g e a n g l e , and c e r t a i n l y n o t such as t o a f f e c t the s i g n o f the exchange i n t e r a c t i o n . Moreover, i n a v a l e n c e bond d e s c r i p t i o n (eg., sp h y b r i d s on t h e Ο and Ν a t o m s o f OCN" and sp h y b r i d i z a t i o n a t t h e c a r b o n ) , tne π-system o f the b r i d g e w o u l d i n v o l v e an orthogon­ a l i t y a t t h e c a r b o n atom. To t h e e x t e n t t h a t t h i s d e s c r i p t i o n i s a c c u r a t e , t h e e x c i t e d s t a t e π-exchange pathway i s i n e f f e c t i v e . To a c c o u n t q u a n t i t a t i v e l y f o r a l l f a c t o r s c o n ­ t r i b u t i n g t o tne observed exchange i n t e r a c t i o n s i s a t t h i s t i m e i m p o s s i b l e i n t h a t t o do s o w o u l d r e q u i r e a tnorougn u n d e r s t a n d i n g of the bonding i n v o l v e d . v

χ

Copper(II)

Dimers

We h a v e f o u n d t h a t t h e c o p p e r compounds p r e p a r e d under i d e n t i c a l c o n d i t i o n s are not i n n e r - s p h e r e bridged dimers. T h e c o p p e r d i m e r s a r e w h a t we shall c a l l "outer-sphere" dimers. The s i n g l e - c r y s t a l X - r a y s t r u c t u r e s o f t h e c y a n a t e and c y a n i d e c o p p e r compounds nave been d e t e r m i n e d . F i g u r e 6 g i v e s an ΟRTEΡ d r a w i n g of tne ççpper-cyanate dimer, i e . , [ C u ( t r e n ) ~ (NCO) ] . The r e m a r k a o l e f e a t u r e o f t h i s d i m e r i s t n a t t n e two h a l v e s o f t n e c y a n a t e d i m e r a r e b r i d g e d 2

2

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

8.

HENDRICKSOX

AND

DUGGAN

NU Cu, Co

85

Dimers

s o l e l y b y means o f two Ν - Η · ··ϋ n y d r o g e n - D o n d i n g c o n ­ t a c t s b e t w e e n a c y a n a t e o x y g e n and a t r e n n i t r o g e n on the second copper c e n t e r . The b r i d g i n g h y d r o g e n c o u l d n o t be s e e n , b u t i t s p o s i t i o n c a n b e c a l c u l a t e d t o be ^0.2Â f r o m t h e N-O v e c t o r . Eacn copper atom i n tne cation i s trigonal bi^yramidally coordinated witn tren o c c u p y i n g f o y r s i t e s [Cu-N=2.076(5), 2.090(5), 2.119(5) and 2.083(5)A] and w i t n an a x i a l c a r b o n - b o n d e d c y a n i d e [Cu-CN=1.967(7) a n d C - N - l . 1 2 7 ( 9 ) A ] . The t r i g o n a l b i p y r a m i d s a r e d i s t o r t e d f r o m p e r f e c t t h r e e - f o l d symmetry. F o l l o w i n g tne e l u c i d a t i o n o f the s t r u c t u r a l chara c t e r i s t i c s of these copper outer-spnere dimers, we nave s e t about the t a s k o f c o n s t r u c t i n g an extended s e r i e s o f sucn coppe c o n s i d e r t o be a s o l i o u t e r - s p h e r e e l e c t r o n t r a n s f e r as a s s i s t e d by v a r i o u s anions. P r z y s t a s a n d S u t i n (9) have s t u d i e d s e v e r a l o u t e r - s p h e r e redox r e a c t i o n s as tney a r e i n f l u e n c e d i n s o l u t i o n by added a n i o n s . Perhaps tne n y d r o g e n - b o n d i n g c o n t a c t t h a t i s p r e s e n t i n , f o r example, [ C u ( t r e n ) 2 ( N C O ) 1 ( B P n ) , i s s u f f i c i e n t to est a b l i s h an e l e c t r o n t r a n s f e r between the c o p p e r c e n t e r s . I f t h i s i s t h e c a s e a n d i f we c a n m e a s u r e t h e e x c h a n g e p a r a m e t e r J , t h e n we c o u l d s t u d y _ t h e e f f e c t s o f c h a n g i n g t h e b r i d g e ( i e . , CN~ f o r OCrt") o n t h e e x c h a n g e p a r a m e t e r J and t h e r e f o r e on the o u t e r - s p h e r e e l e c t r o n transfer rate. We h a v e v e r i f i e d t n a t t h i s same h y d r o g e n - b o n d i n g a s s o c i a t i o n i s f o u n d f o r a s e r i e s o f c o p p e r compounds. F i g u r e 7 shows a n ORTEP d r a w i n g o f t h e o u t e r - s p h e r e dimeric cation i n [Cu (tren) (CN) 1(BPn ) (10). A g a i n we h a v e e s s e n t i a l l y t r i g o n a l o i p y r a m i d a T copper c e n t e r s i n an o u t e r - s p h e r e a s s o c i a t i o n . Inspection of tiie d e t a i l s o f p a c k i n g t h e s e o u t e r - s p h e r e dimers w i t h BPh-~ a n i o n s shows t h a t t h e d i m e r s a r e r e a s o n a b l y i s o l a t e d by t h e v e r y l a r g e c o u n t e r i o n s . As m i g h t b e e x p e c t e d , e x c h a n g e i n t e r a c t i o n s a r e r e l a t i v e l y weak f o r these outer-sphere systems. F i g u r e 8 i l l u s t r a t e s the s u s c e p t i b i l i t y a n d e f f e c t i v e moment c u r v e s f o r t h e copper-cyanide dimer. A s c a n b e s e e n t h e r e i s no s i g n o f a n i n t e r a c t i o n u n t i l v e r y low t e m p e r a t u r e s . Howe v e r , i t i s q u i t e c l e a r t h a t t h e r e i s an i n t e r a c t i o n v i a t h e o u t e r - s p h e r e a s s o c i a t i o n i n t h i s compound. F i t t i n g the s u s c e p t i u i l i t y d a t a t o the usual^exchange e q u a t i o n f o r a c o p p e r d i m e r g i v e s J=-1.8 cm Because t h e c y a n a t e , t h i o c y a n a t e and s e l e n o c y a n a t e a n i o n s a r e m o r e e x t e n d e d , i t was a n t i c i p a t e d t h a t J would be s m a l l e r t h a n t n a t f o r t h e c y a n i d e . As s u c h i t b e c a m e c l e a r t o us t h a t we n e e d e d t o u s e e s r t o 2

2

2

4

2

2

2

4

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

86

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

Figure 6. Molecufor structure of the "dimer" cation in form I of [Cu/tre^ifNCO)^ (BPh )t after refinement of atomic positions with isotropic thermal parameters. Hydrogen bonding h

Figure

7.

ORTEP

plotting

of [Cu,(tren),(CN)<] \ cated by a dashed 2

-boo

50

100 150 200 TEMPERATURE (°K)

250

Hydrogen line.

bonding

contact

is indi-

Figure 8. Experimental and calculated magnetic susceptibility data for [Cu/tren),(CNMfBPhi),. Lines are least-squares fit theoretical lines.

300

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

8.

HENDRiCKSON

AND

DUGGAN

Ni, Cu, Co

Dimers

87

determine J . P r e v i o u s t o t h i s work, s i n g l e t - t r i p l e t e s r t r a n s i t i o n s have been r e p o r t e d i n o n l y f o u r c a s e s : v a n a d y l t a r t r a t e dimer (11), n e a r e s t - n e i g h b o r exchange c o u p l e d Cu i o n s doped I n t o a p o t a s s i u m z i n c s u l f a t e l a t t i c e ( 1 2 ) , f o r a Gd s y s t e m ( 1 3 ) , a n d f o r o n e d i s ­ c r e t e copper dimer (14). F i g u r e 9 shows a n i d e a l i z e d e s r s p e c t r u m f o r t h e c a s e where J i s o f a magnitude t h a t i s comparable t o t h e m i c r o w a v e e n e r g i e s (^0.3 cm i n X-band) u s e d i n an esr experiment. The z e r o - f i e l d s p l i t t i n g i n our c o p p e r dimers i s s m a l l i n comparison w i t h the exchange p a r a ­ meter J . By f a r t h e d o m i n a n t f e a t u r e i n a n X - b a n d s p e c t r u m o f s u c h a s y s t e m i s t h e Διι =1 t r a n s i t i o n a t ^3200 Gauss, r e p r e s e n t e d i n F i g u r e ? as a s i n g l e f e a ­ tureless derivative of the proper magnitude a t h a l f the f i e l d v a l u e o f the AM =l t r a n s i t i o n ; the p r e s e n c e o f t h i s f e a t u r e has been noted f o r s e v e r a l copper dimers. T n e two s i n g l e t - t r i p l e t t r a n s i t i o n s a r e i n d i c a t e d i n F i g u r e 9 a s two t r a n s i t i o n s w h o s e f i e l d p o s i t i o n s a r e ± 2 J / g 3 f r o m t n e i n t e n s e ΔΙ1 =1 t r a n s i t i o n . I t i s thus p o s s i b l e to determine J v e r y a c c u r a t e l y using e s r , i f the J v a l u e i s i n the e s r range. As an a s i d e i t s h o u l d be n o t e d t h a t t n e i n t e n s i t y c j t h e s i n g l e t - t r i p l e t t r a n s i t i o n d e p e n d s o n (A / 4 J ) x (Mj-M-p ) where A i s the c o p p e r n u c l e a r hyper f i n e i n t e r a c t i o n and t n e % . v a l u e s a r e n u c l e a r s p i n p r o ­ j e c t i o n s f o r t n e two c o p p e r c e n t e r s . Tnus, t n e r e i s a s l i g h t a d m i x t u r e o f s i n g l e t and t r i p l e t f u n c t i o n s as a result of electron-nuclear hyperfine. A r e p r o d u c t i o n o f t n e X-band s p e c t r u m o f tne o u t e r - s p h e r e c o o p e r c y a n a t e d i m e r i s g i v e n i n F i g u r e 10. I n t n e 3 4 5 ° K s p e c t r u m ( t o p t r a c i n g ) t h e two s i n g l e t t r i p l e t t r a n s i t i o n s a r e r e a d i l y s e e n , one i s c l o s e t o t h e ΔιΙ =2 t r a n s i t i o n . T n e two s i n g l e t - t r i £ > l e t transi­ t i o n s a r e e q u a l l y spaced from the Δ ^ = 1 t r a n s i t i o n , as e x p e c t e d - and t h e i r p o s i t i o n s g i v e a | j | v a l u e o f 0.09 cm"" . When t h e t e m p e r a t u r e o f [ C u ( t r e n ) (NCO) ] ( B P h . ) i s l o w e r e d t o 9 5 ° K , t a e two s i n g l e t - t r i p l e t f e a t u r e s move t o n i g n e r a n d l o w e r f i e l d p o s i t i o n s , r e s p e c t i v e l y , and at,95°K t h e v a l u e o f | j | i s c a l c u ­ l a t e d t o b e 0.16 cm" . vie n a v e , i n f a c t , m e a s u r e d t n e t e m p e r a t u r e d e p e n d e n c e o f J f o r t h i s compound a n d w i l l r e p o r t on t h i s i n a l a t e r paper (4). T h u s , w i t h a c o m b i n a t i o n o f e s r and v a r i a b l e t e m p e r a t u r e m a g n e t i c s u s c e p t i b i l i t y we h a v e d e t e r m i n e d tne J values f o r a s e r i e s of outer-sphere copper dimers: g

3

f

3

3

2

2

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2

EXTENDED

88

INTERACTIONS

[Cu (tren) (X) J(uPn ) 2

2

2

4

X

METAL

IONS

2

J (cm

SeCN sasT (I) OCiNi ( I I ) QCi\I CN CI Br

BETWEEN

0.05 0.09 0.05 1.9 3.2 3.5

)

0.07 0.17 0.06

F o r t n e a z i d e and s e l e n o c y a n a t as y e t , been a b l e t c a s e s no s i g n s o f e x c h a n g e i n t e r a c t i o n a r e s e e n i n t h e s u s c e p t i b i l i t y t o 4 . 2 ° K n o r a r e t h e r e any weak s i n g l e t t r i p l e t esr transitions. V a r i o u s p h y s i c a l d a t a on t h e SeCN~ compound i n d i c a t e t h a t i t i s i s o s t r u c t u r a l w i t h the c y a n i d e and c y a n a t e . Tne c r y s t a l l o g r a p h y o f t h e c o p p e r a z i d e s y s t e m i s b e i n g p u r s u e d by P r o f . C o r t Pierpont. X - r a y p r e c e s s i o n w o r k o n t h e SCft~, C l ~ , a n d Br"" compounds i n d i c a t e s t n a t t h e y a r e a l s o o u t e r sphere copper dimers. T h e t h i o c y a n a t e compound shows s i n g l e t - t r i p l e t t r a n s i t i o n s i n i t s X-band e s r s p e c t r a , f r o m w h i c h we c a l c u l a t e d |j|=0.05 c m ~ a t 345°K and IJI=0.06 cm a t 95°K. T h e r e a r e two c r y s t a l l i n e f o r m s o f t i i e c o p p e r c y a n a t e compound; we h a v e o n l y d e t e r m i n e d the X-ray s t r u c t u r e o f the form l a b e l l e d I. Both forms exhibit singlet-triplet transitions. I t i s perhaps relevant to p o i n t out that [Ni (tren) (WCO) ](BPh ) , e v e n though i t i s composed o f i n n e r - s p h e r e d i m e r s , a l s o h a s two d i f f e r e n t f o r m s . Apparently, there are two w a y s o f p a c k i n g s u c n d i m e r s w i t h t h e t e t r a p h e n y l borate ion. T h e h a l i d e - b r i d g e d o u t e r - s p h e r e d i m e r s show t h e strongest interactions. The a n t i f e r r o m a g n e t i c exchange i n t e r a c t i o n s a r e l a r g e enough t o e a s i l y d i s c e r n w i t h m a g n e t i c s u s c e p t i b i l i t y t o 4.2°K. However, i t i s i n t e r e s t i n g a n d a t t h e same t i m e p u z z l i n g t h a t t h e b r o m i d e and c h l o r i d e b r i d g e s s u p p o r t a p p r o x i m a t e l y t h e same m a g n i t u d e o f i n t e r a c t i o n i f t h e s e i n t e r a c t i o n s are propagated through hydrogen bonds. The X - r a y c r y s t a l s t r u c t u r e o f t h e c h l o r i d e compound i s p r e s e n t l y b e i n g worked on. Work ( b o t h e x p e r i m e n t a l a n d t h e o r e t i c a l ) c o n t i n u e s on t h e s e c o p p e r o u t e r - s p h e r e s y s t e m s t o understand the r e l a t i o n s h i p between the " o u t e r - s p h e r e " e x c h a n g e s t u d i e s a n d t h e many r e p o r t i n g s o f " a n i o n a s s i s t a n c e " of outer-sphere s o l u t i o n redox processes. x

2

2

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

4

2

8.

HENDRiCKSON

AND

N i , C u , Co

DUGGAN

89

Dimers

ο

<

τ

J

D

si

_

y\

Λ

/

H

ho>

Λ

2J

lo o>

: Λ Figure 9. Idealized energy level scheme and esr spectrum for two S = 1/2 systems interacting to show both zero-field splitting (D) and exchange coupling (J). Allowed transitions (AM = 1) are shown as large derivative feature in the spectrum whereas formally forbidden (ΔΜ = 2 and singlet to triplet state) transitions are shown at an in­ creased spectrometer gain (i.e., η times greater). 8

8

Figure

10.

Temperature dependence of all visible spectrum of [Cu^tren^lSlCO)*]

features in the X-band (BPh,,) , form I 2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

(9 GHz)

esr

90

EXTENDED

Cobalt(II)

and

Manganese(II)

INTERACTIONS

BETWEEN

METAL

IONS

Dimers

Schlenk tube techniques have been used t o p r e p a r e a s e r i e s o f h i g h - s p i n c o b a l t ( I I ) complexes w i t h the g e n e r a l c o m p o s i t i o n o f [ C o ( t r e n ) (X) «J ( B P h J . The e l e c t r o n i c a b s o r p t i o n and I n f r a r e a s p e c t r a o f t h e s e somewhat a i r - s e n s i t i v e c o b a l t ( I I ) c o m p l e x e s i n d i c a t e t h a t the complexes are f i v e - c o o r d i n a t e , probably tri­ g o n a l b i p y r a m i d a l (TBP). The m a g n e t i c s u s c e p t i b i l i t y c u r v e s i n d i c a t e an a t t e n u a t i o n o f m a g n e t i s m a t l o w temperatures. However, b e c a u s e z e r o - f i e l d i n t e r a c t i o n could be s u b s t a n t i a l i n s u c h a f i v e - c o o r d i n a t e C o ( I I ) s p e c i e s , i t i s n o t p o s s i b l e t o e a s i l y e x t r a c t the ex­ change parameter J from a f i t t i n g o f the d a t a . In agreement w i t h t h i statement i findin t h a t th magnetism f o r f i v e - c o o r d i n a t i n d i c a t e s a low-temperatur O u r a t t a c k on t h e c o b a l t ( I I ) s y s t e m s h a s b e e n one o f f i t t i n g the observed e l e c t r o n i c s p e c t r a t o l i g a n d f i e l d e q u a t i o n s f o r a t r i g o n a l b i p y r a m i d a l complex, f o l l o w e d by a t t e m p t s u s i n g the l i g a n d f i e l d p a r a m e t e r s to f i t the s u s c e p t i b i l i t y d a t a t o the equations f o r a m o n o m e r i c TBP c o b a l t ( I I ) c o m p l e x w i t h a reasonable spin-orbit interaction. P r e l i m i n a r y w o r k shows t h a t t h i s d o e s n o t work as w e l l as f i t t i n g t h e m a g n e t i c d a t a t o an e f f e c t i v e s p i n H a m i l t o n i a n f o r an S = 3 Co(II) dimer, i n c l u d i n g s i n g l e - i o n zero f i e l d s p l i t t i n g . The u l t i m a t e c h e c k w i l l be g a u g e d b y o u r s u c c e s s t o f i t t h e c o m p l i c a t e d l o w - t e m p e r a t u r e ( 4 . 2 - 9 0 ° K ) X- a n d Qb a n d e s r s p e c t r a we h a v e m e a s u r e d f o r t h e s e c o m p o u n d s . F i g u r e 11 shows t h r e e r e a s o n s ( X - b a n d a t ^ 15°K f o r A = Ν ~ , Β = 0CN~ a n d C = SCN~) why we a r e g o i n g t o have c o n s i d e r a b l e fun a n a l y z i n g these systems. Em­ p i r i c a l l y , we h a v e f o u n d an i n t e r e s t i n g t e m p e r a t u r e d e p e n d e n c e i n t h e s e e s r s p e c t r a a s c a n s e e i n F i g u r e 12 w h i c h s h o w s Q - b a n d s p e c t r a a t two t e m p e r a t u r e s f o r [ C o ( t r e n ) (NCO) 1 ( B P h ) . I t i s our a n t i c i p a t i o n a n d h o p e ( p e r h a p s u s i n g some o f t h e m a c h i n e r y s e t o u t i n an e a r l i e r symposium p a p e r by P r o f e s s o r R.L. B e l f o r d ) t h a t these e s r s p e c t r a h o l d the key t o a de­ termination of J . T h e c o b a l t c o m p l e x e s h a v e , a f t e r many a t t e m p t s , b e e n f o u n d t o be u n a v a i l a b l e i n t h e f o r m o f s i n g l e crystals. T h i s i s not the case w i t h the manganese(II) c o m p o u n d s w h e r e we h a v e o b t a i n e d c o l o r l e s s c r y s t a l s o f [ M n ( t r e n ) ( X ) ] (BPh.) , wheire X = NCO" and NCS~. X-ray c r y s t a l s t r u c t u r e work i s i n p r o g r e s s . These M n ( I I ) c o m p o u n d s a l s o show a n a t t e n u a t i o n i n m a g n e t i s m a t v e r y low t e m p e r a t u r e s . Large z e r o - f i e l d s p l i t t i n g s i n t h e e s r s p e c t r a and i n f r a r e d d a t a p o i n t t o the 2

2

2

1

2

2

2

2

2

4

2

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

HENDRiCKSON

A N D DUGGAN

τ

I 1

Figure 11. compounds,

1

Ni, Cu, Co

Dimers

1

1

Γ

I

I

ι

I

I

I

2

3

4 5 FIELD (kG)

I 6

I 7

8

L 9

X-band (—15°K) esr spectra for three high-spin Co(II) [Co (tren) Xi](B?h ) , where (A) X = N ~, (Β) X = OCN,and(C)X = SCN-. 2

t

h 2

3

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

92

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

Figure 12. Q-band (35 GHz) esr spectra for [Co,(tren),(NCO),](BPh ) at two different tem­ peratures k t

Ο

'

Figure

'

5,000 FIELD (G)

'

13. Room-temperature X-band [Mn,(tren)t(NCO),](BPh0

'

10,000

esr spectrum

of

2

FIELD (G)

Figure

14.

Room-temperature

Q-band

esr spectrum

of

[Mn (tren) (~NCS) ](BPh,,) 2

i

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

t

i

8.

HENDRICKSON

A N D DUGGAN

Ni, Cil, Ci) DîmCfS

93

presence o f outer-sphere dimers. C o m p l i c a t e d and i r r e g u l a r X-band s p e c t r a such as t h a t f o r [Mn (tren) (NCO) ](BPh ) d e p i c t e d i n F i g u r e 13 a r e c l e a r i n d i c a t o r s t h a t we a r e d e a l i n g w i t h d i m e r s . T h e r e a r e a c o n s i d e r a b l e number o f f e a t u r e s i n t h i s X-band spectrum, a p p a r e n t l y encompassing the whole 10,000 G a u s s r a n g e . The Q-band s p e c t r u m o f t h e t h i o c y a n a t e compound i n F i g u r e 14 i s s o m e w h a t s i m i l a r t o the Q-band s p e c t r u m o f t h e c y a n a t e and as s u c h i t shows t h a t t h e r e i s a n a p p r e c i a b l e c h a n g e i n a p p e a r ance upon c h a n g i n g microwave e n e r g i e s . 2

2

2

4

2

Literature Cited 1. 2. 3.

4. 5.

6. 7. 8. 9. 10. 11. 12. 13. 14.

Curtis, N.F., M c C o r m i c k J. Chem. Soc. D u g g a n , D.M. a n d Hendrickson, D.N., I n o r g . Chem., in press; J. Chem. Soc., Chem. Commun., 4 1 1 ( 1 9 7 3 ) . Shvelashvili, A.E., Porai-Koshits, M.A., a n d Antsyshkina, A.S., J. Struct. Chem. ( U S S R ) , (1969), 10, 5 5 2 . D u g g a n , D.M. a n d Hendrickson, D.N., Inorg. Chem., submitted for publication. Pierpont, C.G., Hendrickson, D.N., D u g g a n , D.M., Wagner, F., a n d Barefield, E.K., Inorg. Chem., submitted for publication. Ginsberg, A.P. Martin, R.L., Brookes, R.W., a n d S h e r w o o d , R.C., Inorg. Chem., (1972), 1 1 , 2884. D u g g a n , D.M. a n d Hendrickson, D.N., I n o r g . Chem., (1973), 12, 2422. C r a d w i c k , P.D. a n d Hall, D., Acta Cryst., (1970), B26, 1384 . Przystas, T.J. a n d Sutin, Ν., J. Amer. Chem. Soc., (1973), 95, 5545. D u g g a n , D.M. a n d Hendrickson, D.N., Inorg. Chem., in press. J a m e s , P.G. a n d Luckhurst, G.R., M o l . P h y s . , (1970), 18, 1 4 1 . M e r e d i t h , D.J. a n d Gill, J.C., Phys. Lett., (1967), 25A, 429. Birgeneau, R.J., Hutchings, M.T., a n d wolf, W.P., Phys. Rev. Letters, ( 1 9 6 6 ) , 1 7 , 308. Jeter, D.Y. a n d Hatfield, W.E., I n o r g . C h i m . Acta, (1972), 6, 440.

Ac^nowledgjnj^ri^. from

National

We

are grateful

Institutes

f o rp a r t i a l

funding

o f H e a l t h g r a n t HL13652.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

9 Structural and Magnetic Properties of Chromium(III) Dimers DEREK J. HODGSON U n i v e r s i t y of N o r t h C a r o l i n a , C h a p e l Hill, N.C. 27514

Introduction A number of s t r u c t u r a l i n v e s t i g a t i o n s in our l a b o r a t o r y and elsewhere have demonstrated that complexes of stoichiometry [Cu(L)OH]22+, where L is a bidentate l i g a n d , c o n t a i n a dimeric u n i t i n which two copper(II) centers are bridged by two hydroxo groups (1-5), and much of our recent research has been d i r e c t e d towards the c o r r e l a t i o n of the s t r u c t u r a l and magnetic p r o p e r t i e s of dimers of t h i s type (6,7). We have r e c e n t l y extended these s t u d i e s to chromium(III) complexes of the type [Cr(L) OH]2n+, where L i s again a bidentate l i g a n d , and i n t h i s paper I d e s c r i b e the r e s u l t s of our work i n t h i s area. The s t r u c t u r e s with which we are concerned are molecules or ions of the type shown i n f i g u r e 1, i n which we have two chromium ( I I I ) centers which are bridged by two hydroxo groups i n a planar array; the remaining c o o r d i n a t i o n s i t e s of the chromium octahedron are occupied by the bidentate l i g a n d s . For a symmetric bidentate l i g a n d , there are two p o s s i b l e geometries f o r t h i s dimer: if, i n f i g u r e 1, atoms AN(1) and AN(10), BN(1) and BN(10), e t c . form the c h e l a t e r i n g s , the dimer l a c k s an i n v e r s i o n center but has approximately D symmetry, but i f , f o r example, the c h e l a t i o n at C r ( l ) i s changed to AN(1)-BN(10) and BN(1)-AN(10) while that at Cr(2) i s unchanged the dimer has an i n v e r s i o n center and approximates C symmetry. Each of these geometries i s found. I t should be noted, however, that i n both of these geometries we maintain a planar b r i d g i n g u n i t and octahedral c o o r d i n a t i o n at the metal. Sinn (8) and G l i c k (9) have noted the i n f l u e n c e of the geometry at copper on the magnetic i n t e r a c t i o n s i n copper dimers, but here we are able to keep the geometry at the metal center approximately constant. 2

2

2h

The magnetic p r o p e r t i e s of these systems have been examined by my colleague, Professor W.E. H a t f i e l d . The Van Vleck equation f o r exchange coupled C r ( I I I ) ions (S = 3/2,3/2) can be w r i t t e n (10) as

94 In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

9.

Chromium(IIl)

HODGSON

2

2

Ng $ kT

Dimers

2 exp(2J/kT) + 10 exy(6J/kT) + 28 exy(12J/kT) 1+3 exp(2J/kT) + 5 exp(6J/kT) + 7 exp(12JA!T)

95

(1)

where J i s the exchange coupling constant and -2J represents the energy d i f f e r e n c e between the s i n g l e t ground s t a t e and the t r i p l e t f i r s t e x c i t e d s t a t e . As a r e s u l t of the presence of a manif o l d of r e l a t i v e l y low-lying paramagnetic e x c i t e d s t a t e s , however, i t has been suggested (11) that the Van Vleck expression should be modified by the i n c l u s i o n of b i q u a d r a t i c exchange.This gives r i s e to the Hamiltonian

H

Q

=

-2J(S S )-é(S V

2

)

(2)

and the expanded expressio m

X

m

NgH

2

χ

kT

(3)

2 exp[(2eT-6.5j)/feff}f 10 exp[ (6J-13.5.1)/kT] + 28 exp[ (12e7-9.7)/kT] 1.0+3 exp[(2J-6.5j)/kT] + 5 exp [ (6J-13.5j) /kT]+7 exp [ (12J-9j)/kT] In t h i s modified form of the Van Vleck equation, the energy se­ p a r a t i o n between the s i n g l e t ground s t a t e and t r i p l e t f i r s t e x c i t e d s t a t e , ΔΕ, i s -2J + 6.5j. Since i t i s our aim to c o r r e l a t e s t r u c t u r a l and magnetic p r o p e r t i e s , t h i s d i s c u s s i o n deals only with complexes whose s t r u c t u r e s have been p r e c i s e l y determined and does not include the l a r g e number of complexes (12-16) f o r which only magnetic data are a v a i l a b l e . G l y c i n a t o Complex The f i r s t s t r u c t u r e which was determined was that of the g l y c i n a t o complex [Cr(gly)2OH]2 > which c r y s t a l l i z e s i n the mono­ c l i n i c space group Ρ2χ/η with two dimers Jn a c e l l of dimensions a = 5.691(3), b= 16.920(9), a - 7.900(4) A, and 3 - 79.90(3)° (17,18). With only two dimers i n the c e l l , i t i s apparent that there must be an i n v e r s i o n center i n the middle of the dimer, and that the molecule must be of the approximately type; an examination of the s t r u c t u r e , which i s shown i n f i g u r e 2, v e r i ­ f i e s t h i s conclusion. The Cr-Cr and 0-0 separations i n the b r i d g i n g u n i t are 2.974(2) and 2.575(6) A, r e s p e c t i v e l y , and the s i m i l a r i t y of the two independent b r i d g i n g Cr-0 bond lengths of 1.966(4) and 1.968(4) A demonstrates that the b r i d g i n g i n t h i s u n i t i s symmetric. The s t r u c t u r a l parameter of greatest i n t e r e s t i s the value of the Cr-0-Cr b r i d g i n g angle, φ, and i n t h i s case i t i s 98.2(2)° (18).

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

96

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

Figure 1. Coordination about chromium(III) centers in a typical dihydroxo-bridged dimer. O(l) and G(2) are the oxygen atoms of the hydroxo bridges. Chelate rings are formed by joining AN(1) to AN(10), BN(1) to BN(10) etc. Data are for the [Cr(phen) OH], cation in [Cr(phen),OU],l, · 411,0. y

r

4

Inorganic Chemistry

Figure

2.

View

of the [Cr(gh/)^OH] atoms omitted

j molecule (18)

with

hydrogen

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

9.

Chromium(III)

HODGSON

Dimers

97

The low temperature magnetic s u s c e p t i b i l i t y data f o r [Cr( g l y ) O H ] are shown i n f i g u r e 3, i n which the dashed l i n e r e p r e ­ sents the best l e a s t - s q u a r e s f i t to the unmodified form of the Van V l e c k expression (equation (1)) while the s o l i d l i n e represents the best l e a s t - s q u a r e s f i t to equation (3). I t i s evident t h a t , i n t h i s case, the observed s u s c e p t i b i l i t y data are much more r e a d i l y approximated by the s o l i d l i n e , i.e. the i n c l u s i o n of b i q u a d r a t i c exchange i s s i g n i f i c a n t i n t h i s case. The magnetic s u s c e p t i b i l i t y of [ C r ( g l y ) O H ] maximizes near 20°K. The l e a s t - s q u a r e s f i t t i n g process leads to values of 2J = -7.4 cm~l and j = 0.04 cm~l, or ΔΕ = -10.0 cm~l. These values are i n good agreement with the value of 2J p r e d i c t e d by Earnshaw and Lewis (12) on the b a s i s of high temperature s u s c e p t i b i l i t y data. 2

2

2

2

Phenanthroline Complexe The second complex whose s t r u c t u r e was determined was the 1,10-phenanthroline complex [ C r ( p h e n ) 0 H ] C l i * 6 H 0 . T h i s complex c r y s t a l l i z e s i n the t r i c l i n i c space group PI with two dimers i n a c e l l of dimensions a « 14.056(7), b = 11.296(6), ο = 18.990(9) A, α = 87.15(3), 3 = 107.63(2), and γ = 74.68(3)° (19). With two d i ­ mers i n P I , no c r y s t a l l o g r a p h i c symmetry i s imposed on the system, and t h i s s t r u c t u r e i s an e i g h t y atom problem, not counting the hydrogen atoms! The s t r u c t u r e of the c a t i o n i s shown i n f i g u r e 4, and i t i s apparent that t h i s i o n c l o s e l y approximates Z? symmetry; there i s no i n v e r s i o n center, but there are three approximate two­ f o l d axes. I f the dimer attempted to adopt the approximately C h geometry found i n the g l y c i n a t o complex, there would be very se­ vere proton-proton i n t e r a c t i o n s across the dimer, e.g. between the phenanthroline group l a b e l e d G2 and that l a b e l e d G3. Hence, f o r the bulky phenanthroline l i g a n d , only the Z? geometry i s s t e r i cally feasible. The c o o r d i n a t i o n about the chromium(III) atoms i s shown i n f i g u r e 5. The Cr-Cr s e p a r a t i o n of 3.008(3) A i s a l i t t l e l a r g e r than that i n the g l y c i n a t o complex, and t h i s change i s due to an increase of approximately 4.5° i n the value of the b r i d g i n g angle, φ. Thus, i n t h i s phenanthroline complex the average value of φ i s 102.7°, while i n the g l y c i n a t o complex φ i s 98.2° (vide supra). The low temperature magnetic s u s c e p t i b i l i t y of [ C r ( p h e n ) OH] C l i · 6H 0 e x h i b i t s a maximum near 110°K, and the best l e a s t squares f i t to equation (3) gives a value of ΔΕ of approximately -55 cnT^ (20). S i n g l e c r y s t a l epr examinations of t h i s complex are c u r r e n t l y nearing completion. The magnetic data f o r t h i s c h l o r i d e s a l t of the phenanthro­ l i n e complex are of c o n s i d e r a b l e i n t e r e s t s i n c e , on the b a s i s of high temperature measurements, Earnshaw and Lewis (12) have c a l ­ c u l a t e d that the corresponding i o d i d e s a l t has a ΔΕ of a p p r o x i ­ mately -14 cm~l. Hence, i f our contention that the magnetic pro­ p e r t i e s of di-hydroxo-bridged dimers are p r i n c i p a l l y determined by the geometry of the bridge were c o r r e c t , i t appeared that the geo­ metry of the i o d i d e s a l t must be c o n s i d e r a b l y d i f f e r e n t from 2

2

+

2

c

2

2

2

2

2

+

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

98

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

04

Inorganic Chemistry

Figure 3. Temperature variation of the magnetic susceptibility of [Cr(gly) OH] . Dashed line represents the best jit to equation (1); solid line represents the best fit to equation (3) (see text) (18). É

Â

Acta Crystaltographica

Figure 4. View the cation in [Cr(phen) OH] Ch, • 6H 0 (19) 2

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2

of

9.

Chromium(III)

HODGSON

Dimers

99

that of the c h l o r i d e . Hence, we a l s o examined the s t r u c t u r e of the i o d i d e s a l t , [ C r ( p h e n ) 0 H ] I i f 4 H 0 . The i o d i d e s a l t a l s o c r y s t a l l i z e s i n the t r i c l i n i c space group PI with two dimeric u n i t s i n a c e l l of dimensions a = 11.464(12), b = 9.893(11), ο = 22.757(25) Â, α = 90.06(2), 3 = 93.04(2), and γ = 82.82(2)° (21). The s t r u c t u r e of the c a t i o n , which i s shown i n f i g u r e 6, i s very s i m i l a r to that found i n the c h l o r i d e s a l t , with the bulky phenanthroline l i g a n d s again f o r c ­ ing the dimer to adopt the roughly £> geometry. The inner coor­ d i n a t i o n sphere f o r t h i s dimer i s shown i n f i g u r e 1$ and a com­ p a r i s o n of t h i s f i g u r e with f i g u r e 5 demonstrates that the [Cr(phen) 0H] u n i t s i n these two s a l t s are s t r u c t u r a l l y sub­ s t a n t i a l l y s i m i l a r (21). Thus, f o r examgle, the Cr-Cr s e p a r a t i o n and Cr-O-Cr b r i d g i n g angle of 2.986(4) A and 102.2(2)°, r e s p e c t ­ i v e l y , i n the i o d i d e s a l from the values (19) o analog. T h i s s t r u c t u r a l r e s u l t i s , c l e a r l y j i n c o n s i s t e n t with the magnetic p r o p e r t i e s reported by Earnshaw and Lewis (12), and so we have reexamined the magnetic s u s c e p t i b i l i t y of the i o d i d e s a l t (22). The low temperature s u s c e p t i b i l i t y data are shown i n f i g u r e 7, i n which the s o l i d l i n e represents the best f i t to the Van Vleck equation modified by the i n c l u s i o n of b i q u a d r a t i c exchange. The magnetic s u s c e p t i b i l i t y of [Cr (phen) 0H] Ii «4H 0 i s seen to maxmize near 110°K, and the l e a s t - s q u a r e s f i t to equation (3) y i e l d s 2J = -43.8 cm" , j = +1.5 cm , and ΔΕ - -53.6 cm" (22). These values are very s i m i l a r to those obtained (20) f o r the c h l o r i d e but are s u b s t a n t i a l l y d i f f e r e n t from the value of ΔΕ = -14 cm"" reported by Earnshaw and Lewis (12). Moreover, the sim­ i l a r i t y between these r e s u l t s and those f o r the c h l o r i d e s a l t i s c o n s i s t e n t with the s i m i l a r i t y of the two s t r u c t u r e s noted above. I t i s , however, noteworthy that while the magnetic pro­ p e r t i e s of the corresponding n i t r a t e s a l t , [Cr (phen) 0H] (N03)i* · 7H 0, with values of 2J - -42.2 cm" , J = 0.0 cm" , and ΔΕ -42.2 cm" (23) are s u b s t a n t i a l l y s i m i l a r to those of the c h l o ide and i o d i d e s a l t , the bromide s a l t , [Cr(phen^ OH^ Br^ ·8Η2θ, apparently undergoes a weaker i n t e r a c t i o n with values of 2J = -29.2 cm" , J = 0.5 cm" , and ΔΕ - -32.5 cm" (23). I t would appear, t h e r e f o r e , that the c a t i o n i n the bromide s a l t may i n ­ deed be s t r u c t u r a l l y d i f f e r e n t from that i n the c h l o r i d e and i o d i d e cases, but no s t r u c t u r a l data are a v a i l a b l e to confirm or deny t h i s hypothesis. e

2

2

2

2

i++

2

2

2

1

2

+

2

-1

1

1

2

1

2

1

2

1

1

1

1

Oxalato Complex The f i n a l s t r u c t u r e of t h i s type, which has r e c e n t l y been completed i n our l a b o r a t o r i e s , i s that of the oxalato complex Na [ C r ( 0 X ) 0 H ] * 6 H 0 . T h i s m a t e r i a l c r y s t a l l i z e s i n the mono­ c l i n i c space group P 2 j / c with four dimers i n a c e l l of dimensions a = 19.530(12),b = 9.860(7), ο - 12.657(10) A, and 3 - 106.93(4)° 2

2

2

Q

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

100

EXTENDED

Figure

6.

View

of the cation

INTERACTIONS

in [Cr(phen),OII]J,,

BETWEEN

METAL

4HjO

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

9.

HODGSON

Chromium(III)

101

Dimers

While no c r y s t a l l o g r a p h i c symmetry i s r e q u i r e d f o r four dimers i n t h i s c e l l , i t t r a n s p i r e s that each dimer s i t s on a c r y s t a l l o g r a p ­ h i c i n v e r s i o n center so that there are, i n e f f e c t , two separate independent " h a l f - d i m e r s " i n the c e l l r a t h e r than one independent dimer. Hence, of course, the geometry of the anion must be of the ^2h y P r a t h e r than the D type. The s t r u c t u r e of the anion i s shown i n f i g u r e 8, the bond lengths and angles given being the average of the values obtained f o r the two independent halves; the agreement between these two independent measurements i s e x c e l l e n t (24). The values of the Cr-Cr s e p a r a t i o n of 3.000 A and the Cr-0Cr angle of 99.6(3)° are intermediate between those f o r the g l y ­ c i n a t o and phenanthroline complexes. Hence, while the only magnet­ i c data a v a i l a b l e at present are the room temperature values (y= 3.43 μΒ) obtained on the t e t r a h y d r a t e (13), i t i s evident that the value of ΔΕ f o r t h i s comple there i s a simple c o r r e l a t i o t

e

2

Interpretation The s t r u c t u r a l and magnetic data described above are c o r r e l ­ ated i n f i g u r e 9, i n which the s i n g l e t - t r i p l e t s p l i t t i n g ΔΕ i s p l o t t e d against the b r i d g i n g angle φ; a s i m i l a r p l o t f o r the analogous copper(II) dimers [ C u ( L ) 0 H ] i s a l s o included i n figure 9 examination of the chromium data suggests that the oxalato complex Na^[Cr(0X) 0H] *6H 0, which has a φ of 99.6° (vide supra), should have a ΔΕ of approximately -25 cm . F i g u r e 9 i s noteworthy f o r two separate reasons: f i r s t l y , because f o r a given metal there i s apparently an almost l i n e a r c o r r e l a t i o n between ΔΕ and φ, and secondly because the slope of the ΔΕ v s . φ p l o t f o r the chromium(III) complexes i s c o n s i d e r a b l y smaller than that f o r the copper(II) complexes. Each of these f e a t u r e s i s r e a d i l y explained i n terms of simple bonding theory. 2 +

2

#

2

2

2

-1

The c o r r e l a t i o n between ΔΕ and φ. The c o r r e l a t i o n noted can be explained i n terms of valence bond theory and the p r i n c i ­ p l e s of super exchange(25).If the o r b i t a l s used by the b r i d g i n g oxygen atoms are pure ρ o r b i t a l s , the bond angle i s expected to be 90° and the ground s t a t e i s p r e d i c t e d to be a t r i p l e t (i.e. ΔΕ > 0); i f the o r b i t a l s are purely s , the ground s t a t e i s pre­ d i c t e d to be a s i n g l e t (i.e. ΔΕ < 0). Hence, s i n c e an increased value of the b r i d g i n g angle i m p l i e s greater s character i n the b r i d g i n g o r b i t a l s , we would expect a decrease i n ΔΕ as the b r i d ­ ging angle i s increased from 90°. For the s i x copper and three chromium cases which have been studied i n d e t a i l , t h i s trend i s observed (_2). T h i s c o r r e l a t i o n can a l s o be expressed i n terms of molecular o r b i t a l theory. The M-O-M-O r i n g i s of approximate Z? symmetry i n these molecules, with the x-axis defined as the Cu-Cu d i r e c t ­ ion and y - a x i s p a r a l l e l to the 0-0 v e c t o r (26).Neglecting oxygen s o r b i t a l s , the eight σ-orbitals i n t h i s system transform i n D h 2h

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

102

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

X 006 |-

120 TEMPERATURE,

Figure

7.

ϊ·0 Κ

Temperature variation of the magnetic susceptibility 4H 0. Solid line represents the best fit to equation 2

of [Cr(phen) OH] (3) (see text). É

Figure 8. View of the anion in Na [Cr(ox) OH] · 6H 0. Data are the average values of the two crystallographically independent dimers. r

2

2

2

Oi

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

I

g

5

9.

HODGSON

Chromium(III)

Dimers

103

-500 h Figure 9. M-O-M bridging angle, φ (abscissa) v s . the singlet-triplet splitting energy, A E (ordinate) for dihydroxo-bridgecl complexes of Cu (steeper line) and Cr (flatter line) B2u

Big / A g ^ J

- dxy \

\\ V \

/

w w \\ w

/

\/

'dx^y metal ion

2

Px

\

\

/

\ \

/

\

11

\ \ \

/

// / /

liqand σ orbitals

/

/

/ /

B2u/ /

Agi - t molecular

orbitals

Figure 10. Molecular orbital diagram for the σ-orbitals in the CrO-Cr ring, assuming D symmetry and a Cr-O-Cr angle of 90° 2ll

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

104

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

Γ \

/

/

*

\B2u

/-

/ / \

Ν \

~^ 3u\\ B

—\ d x y

W W

y

^d 2- 2 \\ x

metal

ion

•i b a s i s

set

y

>w


w

\

//

//

"r//

\ "it \\

/

2=

' ι /

\

molecular

B3u

orbitals

Figure 11. Molecular orbital diagram for the σ-orbitah in the CrO-Cr ring, assuming D> symmetry and a Cr-O-Cr angle considerably greater than .90 ~ h

DO DO Cu(II) d

Cr(III) d

Figure 12. Comparison of the overlap between the metal orbitals containing the unpaired spin and the ρ orbitals on the bridging ligand for copper(II) and chromium(IU)

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

9.

HODGSON

Chromium(III)

Dimers

105

symmetry as 2Ag + 2 £ i + 2 B + 2 £ , and there are bonding and anti-bonding combinations with a l l r?our of these symmetries. On the assumption that only oxygen ρ o r b i t a l s p a r t i c i p a t e , the 4 and molecular o r b i t a l s would have i d e n t i c a l energies when the Cr-O-Cr angle was 90°, as would the # i and £ orbitals.This s i t ­ u a t i o n i s depicted i n f i g u r e 10, which demonstrates that t h i s ten e l e c t r o n system ( f o r chromium) must give r i s e to a t r i p l e t ground s t a t e . As the Cr-O-Cr angle i n c r e a s e s , however, the overlap of the i4g and £ ^ c o m b i n a t i o n s increases r e l a t i v e to that of the #3 and £ combinations. Hence, the o r b i t a l degenercies i n f i g u r e 10 are l i f t e d , and at s u f f i c i e n t l y l a r g e values of Φ the molecular o r b i t a l diagram shown i n f i g u r e 11 becomes o p e r a t i v e . At these l a r g e r b r i d g i n g angles, the s p l i t t i n g of the #3 * i ^g* o r b i t a l s i s s u f f i c i e n t to overcome the p a i r i n g energy, and the s i n g l e t s t a t e becomes the ground T h i molecula o r b i t a l view t h e r e f o r e , i s analogou the experimental data may interprete ; presum ably, f i g u r e 11 becomes appropriate at φ values greater than approximately 97.6°, while at angles between 90° and 97.6° the s p l i t t i n g between the #3 * and A * o r b i t a l s i s l e s s than the p a i r i n g energy, and so the t r i p l e t s t a t e remains lower i n energy than the s i n g l e t (i.e. ΔΕ remains p o s i t i v e ) (27). Unfortunately, f o r chromium(III) complexes of t h i s general type there are no examples of p o s i t i v e ΔΕ v a l u e s , but the presence of the t r i p l e t ground s t a t e complexes i n the copper(II) s e r i e s lends strong support to t h i s hypothesis. Moreover, of course, examination of the copper(II) l i n e i n f i g u r e 9 suggests that the ground s t a t e i s the t r i p l e t i f φ < 97.6°; hence, s i n c e f o r the chromium complexes the s m a l l e s t value of φ yet obtained i s 98.2°, none of the chrom­ ium complexes examined i n d e t a i l i s p r e d i c t e d to have a p o s i t i v e value of J (or Δ Ε ) . g

2 u

3

g

g

2 u

U

2 u

a n (

U

U

The Slopes of the Cr and Cu L i n e s . In the copper(II) complex­ es the unpaired s p i n r e s i d e s p r i n c i p a l l y i n atomic o r b i t a l s which point d i r e c t l y at the b r i d g i n g oxygen l i g a n d s (the d y o r b i t a l s i n f i g u r e 10, but more c o n v e n t i o n a l l y d 2 _ 2 s i n c e the usual a x i a l system i s d i f f e r e n t from that f o r c e d upon us i n P ^ symme­ t r y ) , while i n the chromium(III) complexes the unpaired s p i n i s i n t g o r b i t a l s which p o i n t between the b r i d g i n g atom; t h i s d i s ­ p a r i t y i s demonstrated p i c t o r i a l l y i n f i g u r e 12. Hence, s i n c e the overlap between the b r i d g i n g o r b i t a l s and the metal o r b i t a l s c o n t a i n i n g the unpaired s p i n i s much poorer f o r the chromium com­ plexes than f o r the copper complexes, we p r e d i c t that the magni­ tude of the s p i n - s p i n i n t e r a c t i o n should be greater f o r copper than f o r chromium. Hence, at a given value of φ, we p r e d i c t that the magnitude of ΔΕ f o r copper i s greater than that f o r chromium, i.e. that the slope of the l i n e f o r copper i s greater than that f o r chromium; t h i s r e s u l t , of course, i s e x a c t l y what i s seen i n f i g u r e 9. X

x

y

2

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

106

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

Acknowledgments I t i s a pleasure t o acknowledge the a s s i s t a n c e which I have r e c e i v e d from my present and former colleagues, e s p e c i a l l y Professor W.E. H a t f i e l d , Dr. J.T. V e a l , Dr. D.L. Lewis, Dr. D.Y. J e t e r , Dr. R.F. Drake, Ms. E.D. Estes, Ms. D.E. H a r t i s , Mr. R.P. Scaringe, Mr. R.P. Eckberg, and Mr. K.T. McGregor. Literature Cited 1. Lewis, D.L., H a t f i e l d . W.E., and Hodgson, D.J., Inorg. Chem. (1974), 13, 147. 2. Lewis, D.L., H a t f i e l d , W.E., and Hodgson, D.J., Inorg. Chem. (1972), 11, 2216. 3. Casey, A.T., Hoskins (1970), 904. 4. M i t c h e l l , T.P., Bernard, W.H., and Wasson, J.R., Acta Crystallogr. (1970), B26, 2096. 5. Majeste, R.J. and Meyers, E.A., J. Phys. Chem. (1970), 74, 3497. 6. McGregor, K.T., Watkins, N.T., Lewis, D.L., Drake, R.F., Hodgson, D.J., and H a t f i e l d , W.E., Inorg. Nucl. Chem. Letters (1973), 9, 423. 7. Estes, E.D., H a t f i e l d , W.E., and Hodgson, D.J., Inorg. Chem. (1974), in p r e s s . 8. Sinn, E. and Robinson, W.T., Chem. Commun. (1972), 359. 9. G l i c k , M.D. and L i n t v e d t , R.L., 167th. A.C.S. N a t i o n a l Meeting, Los Angeles, C a l i f o r n i a (1974). 10. Jezowska-Trzebiatowska, B. and Wojciedhowski, W., Transition Metal Chemistry (1970), 6, 1. 11. Ikeda, H., Kimura, I . , and Uryu, N., J. Chem. Phys. (1968), 48, 4800. 12. Earnshaw, A. and Lewis, J . , J. Chem. Soc. (1961), 396. 13. M o r i s h i t a , T., H o r i , Κ., Kyuno, Ε., and Tsuchiya, R., Bull. Chem. SOC. Japan (1965), 38, 1276. 14. Schugar, H.J., Rossman, G.R., and Gray, H.B., J. Amer. Chem. Soc. (1969), 91, 4564. 15. Kobayashi, H., Haseda, T., and M o r i , Μ., Bull. Chem. Soc. Japan (1965), 38, 1455. 16. J a s i e w i c z , Β., Rudolf, M.F., and Jezowska-Trzebiatowska, Β., Acta Physica Polonica (1973), A44, 623. 17. Hodgson, D.J., V e a l , J.T., H a t f i e l d , W.E., J e t e r , D.Y., and Hempel, J.C., J. Coord. Chem. (1972), 2, 1. 18. V e a l , J.T., H a t f i e l d , W.E., J e t e r , D.Y., Hempel, J.C., and Hodgson, D.J., Inorg. Chem. (1973), 12, 342. 19. V e a l , J.T., H a t f i e l d , W.E., and Hodgson, D.J., Acta Crystall­ ogr. (1973), B29, 12. 20. Eckberg, R.P., p r i v a t e communication. 21. Scaringe, R.P. and Hodgson, D.J., unpublished o b s e r v a t i o n s .

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

9.

HODCsox

Chromium(IH)

Dimers

107

22. Eckberg, R.P., Scaringe, R.P., Hodgson, D.J., and H a t f i e l d , W.E., unpublished o b s e r v a t i o n s . 23. Drake, R.F., Ph.D. D i s s e r t a t i o n , U n i v e r s i t y of North C a r o l i n a (1973). 24. Hodgson, D.J., and Scaringe, R.P., unpublished observations. 25. Goodenough, J.B., "Magnetism and The Chemical Bond", I n t e r s c i e n c e , New York, 1963. 26. Bertrand, J.Α., and Kirkwood, C.E., Inorg. Chim. Acta (1972), 6, 248. 27. Hodgson, D.J., Progr. Inorg. Chem. (1974), i n p r e s s .

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

10 Superexchange Interactions in Copper(II) Complexes WILLIAM E. HATFIELD U n i v e r s i t y of N o r t h C a r o l i n a , Chapel Hill, N . C . 27514

This paper represent a c t i o n s (1) i n copper(II) complexes with s p e c i a l emphasis on research at the U n i v e r s i t y of North C a r o l i n a , which i s princi­ pally concerned with s t r u c t u r a l , magnetic s u s c e p t i b i l i t y , and EPR measurements. The systems to be discussed i n c l u d e (I) a s e r i e s of hydroxo-bridged complexes of the general formula [CuL(OH)] X ·nH O, where L i s a bidentate amine such as 2,2'b i p y r i d i n e or an N-substituted 2-(2-aminoethyl)pyridine; (II) a s e r i e s of chloro-bridged dimers i n c l u d i n g [Co(en) ] [Cu Cl ]Cl · 2H O, [Cu (guaninium) Cl ], [Cu (α-picoline) Cl ], and [Cu (di­ methylglyoxime) Cl ]; and ( I I I ) the compound [Cu(pyrazine)(NO ) ] and r e l a t e d chains. Since most, if not all, of the copper(II) complexes to be considered have orbitally nondegenerate s i n g l e - i o n ground s t a t e s the Hamiltonian appropriate f o r the problem i s 2

2

2

3

2

2

2

2

6

2

4

2

4

2

8

2

2

4

3

H = -2JE S.-î. (1) K j For those cases i n which antisymmetric exchange and a n i s o t r o p i c exchange become important the f o l l o w i n g terms may be added to (1): 1

-> D

J

-> -> · S.xS. + S.' J 1

Γ..*

1

=

1 J

-> S. J

where D^. i s the antisymmetric vector coupling constant and r ^ . i s the a A i s o t r o p i c coupling tensor. For o r b i t a l s i n g l e t single** ions undergoing exchange, Moriya (2.) has estimated that D

^(Ag/g)J

and 2

—(Ag/g) J ij where Ag = |g-2|. For many copper compounds Ag i s approximately 108

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2

n

10.

HATFIELD

Superexchange

109

Interactions

0. 1, so | D | ~ 5 cm" f o r J = 100 cm~l. There i s no l i m i t on the magnitude of D and Γ f o r o r b i t a l l y degenerate s i n g l e i o n s t a t e s undergoing exchange, and a suggestion has been made that t h i s may be an a p p r o p r i a t e approach f o r the r a t i o n a l i z a t i o n of the magnetic p r o p e r t i e s of the tetramers [Cu^OXi+L^ ]. (3) These pro­ blems w i l l be discussed here. 1

1. Di-y-Hydroxo-bridged

Copper(II) Complexes

I t has been known f o r some time that copper (II) forms com­ plexes of the type [CuL0H]2X2 > where L i s a bidentate amine and X~ i s an a p p r o p r i a t e counterion. (4-8) These complex ions may be described as two planar or t e t r a g o n a l pyramidal u n i t s sharing an edge which i s defined by two b r i d g i n g hydroxo oxygen atoms. The magnetic p r o p e r t i e s o i n t e r a c t i o n s which d i f f e the s t r u c t u r a l and magnetic data f o r s i x of these compounds i t has been p o s s i b l e to i d e n t i f y some of the f a c t o r s which i n f l u e n c e the exchange i n t e r a c t i o n s , and the r e s u l t s of that study w i l l be reviewed here. The chemical and s t r u c t u r a l f e a t u r e s which w i l l be examined i n c l u d e 1) the nature of the c h e l a t i n g amine. 2) the copper-oxygen (hydroxo) bond d i s t a n c e . 3) the nature of any out-of-plane c o o r d i n a t i o n . 4) the geometry of the b a s a l plane. 5) the s i n g l e i o n ground s t a t e . 6) hydrogen bonding by the hydroxo-bridge hydrogen atom. 7) the Cu-Cu s e p a r a t i o n . 8) the Cu-O-Cu bridge angle. Magnetic parameters have been obtained from analyses of EPR s p e c t r a , and from the temperature v a r i a t i o n of the magnetic sus­ c e p t i b i l i t y . The l a t t e r i s c h a r a c t e r i s t i c of exchange coupled copper(II) p a i r s and the s i n g l e t - t r i p l e t s p l i t t i n g s have been determined by f i t t i n g the data to the Van Vleck equation X

m

-

{ l + ~ exp(-2J/kT)}'

1

+ Να

(2)

where the symbols have t h e i r usual meaning, Να i s the temperature independent paramagnetism, and the equation as w r i t t e n gives the s u s c e p t i b i l i t y per copper i o n . In some cases Τ has been replaced by (T-9) to account f o r interdimer i n t e r a c t i o n s . A. S t r u c t u r a l and Magnetic

Data.

f

1. Di-y-hydroxobis[N,N,N ^ ' - t e t r a m e t h y l e t h y l e n e d i a m i n e copper(II)] bromide. The f i r s t compound of t h i s type to be c h a r a c t e r i z e d by an X-ray c r y s t a l s t r u c t u r e determination was di-y-hydroxobis[N,N,N jN'-tetramethylethylenediaminecopper(II)] bromide, [Cu(tmen)0H]2Br .(9) The s t r u c t u r e of the formula u n i t viewed along the b-axis i s shown i n F i g u r e 1 along with some of f

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

110

EXTENDED

Figure

1.

Structure

INTERACTIONS

BETWEEN

of [Cu(tmen)OH] * viewed along the b axis from Ref. 9) 2 2

METAL

(adapted

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

10.

HATFIELD

Superexchange

111

Interactions

the important s t r u c t u r a l parameters. The s t r u c t u r a l data are a l s o c o l l e c t e d f o r a l l compounds of t h i s type i n Table I. The Cu-N bond d i s t a n c e of 2.030 A, the Cu-0 bond d i s t a n c e of 1.902 A, and the N-Cu-N angle of 86.7° are a l l normal f o r s u b s t i t u t e d e t h y l enediamine complexes. The c o o r d i n a t i o n about the copper i s square planar with the n i t r o g e n atoms being 0.14 A out of the plane of the CU2O2 unit.The bromide i o n i s involved i n hydrogen bonding with the hydroxo-bridge, s i n c e O-Br d i s t a n c e of 3.366 A i s com­ parable to the O-Br d i s t a n c e s of 3.39 and 3.37 A, r e s p e c t i v e l y , i n the hydrogen bonded systems M n B ^ ^ ^ O and CoBr *2H20. The i n f r a r e d spectrum a l s o i n d i c a t e s the presence of hydrogen bond­ ing i n that there i s a strong OH s t r e t c h i n g band at 3410 cm" , a value which i s approximately 200 cm" lower than that of a f r e e OH group.(10-12) The magnetic s u s c e p t i b i l i t 77-300°K both on a powdere the a,b, and c c r y s t a l l o g r a p h i c axes. (13) The data were f i t t e d to the Van Vleck equation (2) y i e l d i n g the magnetic parameters 2J = -509 cm" , g = 2.0, and Να = ~150xl0"6 cgs u n i t s . Within the p r e c i s i o n of the experimental measurement the c r y s t a l sus­ c e p t i b i l i t i e s were i s o t r o p i c . e

o

2

1

1

1

2. D i - y - h y d r o x o b i s t N ^ N * ,Ν'-tetraethylethylenediamine copper(II)] p e r c h l o r a t e . As a part of an i n v e s t i g a t i o n of the thermochromic p r o p e r t i e s of N - a l k y l s u b s t i t u t e d ethylenediamine complexes of c o p p e r ( I I ) , H a t f i e l d , P i p e r , and Klabunde(6) reported, i n 1963, the temperature v a r i a t i o n of the magnetic s u s c e p t i b i l i t i e s of the Ν,Ν,Ν ,Ν'-tetraethylethylenediamine (teen) and NjN-diethyl-N'-methylethylenediamine complexes of the general formula [Cu(diamine)OH] (CIO1J2. The data f o r the l a t t e r com­ pound were f i t t e d to Equation (2) f o r the determination of the magnetic parameters, while 2J f o r the teen compound was d e t e r ­ mined from the expression 2J = -1.11 T where T i s the temperature at which the magnetic s u s c e p t i b i l i t y a t t a i n s the max­ imum v a l u e , and the constant has u n i t s of cm" deg" . The s t r u c t u r e of [Cu(teen)0H] (C10i )2 has been completed only r e c e n t l y . ( 1 4 ) The s t r u c t u r e c o n s i s t s of [ C u ( t e e n ) 0 H ] 2 u n i t s and d i s c r e t e ClOi*" anions. (While t h i s geometry at the copper(II) i o n i s comparable to the s i t u a t i o n described above(9) f o r [Cu(tmen)0H]2Br2, i t i s i n marked contrast to the geometry of the s t r u c t u r e s of other compounds i n t h i s s e r i e s with oxyanions, v i d e post.) The best l e a s t squares plane of the N Cu0 CuN2 u n i t c a l l s a t t e n t i o n to the s l i g h t d i s t o r t i o n i n the molecule, the oxygen and n i t r o g e n atoms are approximately 0.15 Â out of the plane. The CU2O2 u n i t i s planar owing to the i n v e r s i o n center. The s t r u c t u r a l f e a t u r e s , which are given i n Table I, are very s i m i l a r to those determined f o r the tmen compound with the s i g n i f i c a n t exception being the decrease i n Cu-O-Cu angle from 104.1° i n [Cu(tmen)OH]2Br2 to 103.0° i n [ C u ( t e e n ) 0 H ] ( C l O ^ ) . Apparently there i s a hydrogen bonding i n t e r a c t i o n between 1

2

m a x

m a x

1

2

1

+

2+

2

2

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2

4

2

4

3

pyridine nitrogen

*

4

2

2

80.6(1)° 95.6(1)

2

2

97° 97°

96.3(5)° 98.8(3)° 93.0(5)° 99.5(2)°

[Cu(bipy)OH] (N0 )

2

81°

4

100.4(1)°

87.8(2)° 103.0(2)°

[Cu(bipy)0H] S0 -5H 0

2

[Cu(EAEP)0H] (C10 )

2

β-[Cu(DMAEP)0H] (C10 )

2

[Cu(teen)0H] (C10 )

2

[Cu(tmen)0H] Br

Cu-O-Cu

ο

in-plane

ο

1.99 2.00 2.00 2.02

4

2.379(2)

2

2.917(5) -130

2.847

+172

2.21(S0 ") 2.893(2) +48 2.24(H 0)

1.981(8)* 2.562(10) 1.998(10) 2.618(9) 2.001(8)* 2.053(10)

1.920(2) 1.998(2) 1.923(1) 2.000(2)

1.92 1.94 1.95 1.95

1.895(7) 1.913(7) 1.927(2) 1.930(8)

2.935(1) -201

-509

2.978(2) -410

x

-

2

-i cm

-

ο Cu-Cu,A 3.000

out-ofplane Cu-0,A

1.900(3) 2.003(3)* 2.721(4) 1.919(3) 2.066(3)

1.899(4) 2.013(5) 1.907(4) 2.024(15)

Cu-0,A Cu-N,A 86.7(8)° 104.08(.17)° 1.902(3) 2.030(10)

N-Cu-N

S t r u c t u r a l and Magnetic P r o p e r t i e s of [Cu(diamine)OH]

Table I

29,30

25,26,27, 28

17,20,21

15,17

6,14

8,9,13

References

10.

HATFIELD

Superexchange

Interactions

113

the hydroxo group and the p e r c h l o r a t e i o n , s i n c e the average Cl-0 bond digtance f o r the three oxygens which are not i n v o l v e d i s 1.389(5) A, while the Cl-0 bond d i s t a n c e f o r the oxygen which i s probably hydrogen bonded i s 1.434(5) Â . 3. 3-Di-y-hydroxobis[2-(2-dimethylaminoethyl)pyridinecopper( I I ) ] p e r c h l o r a t e . Two forms of the compound [Cu(DMAEP)0H]2" ( C l O i ^ can be i s o l a t e d . (15,16) The 3-form i s obtained, essent i a l l y uncontaminated with the α-form, by mixing equimolar q u a n t i t i e s of copper(II) p e r c h l o r a t e hexahydrate and DMAEP i n ethanol/ether while both forms may be found i f methanol/ether i s used. U h l i g and co-workers (17) had reported only one form of t h i s compound i n t h e i r study of the c o o r d i n a t i o n chemistry of Ns u b s t i t u t e d 2-(2-aminoethyl)pyridine, and from a comparison of the magnetic p r o p e r t i e they had the t r i c l i n i two expected hydroxo b r i d g e s , two p e r c h l o r a t e bridges and i t w i l l not be considered f u r t h e r here.(16) The 3-form has two hydroxo bridges (15) and, as shown i n F i g u r e 2, the copper ions are i n a t e t r a g o n a l pyramidal environment with oxygen atoms from p e r c h l o r ­ ate ions occupying the a x i a l p o s i t i o n s . The N2CUO2CUN2 u n i t i s e s s e n t i a l l y planar with the l a r g e s t d e v i a t i o n from the best l e a s t squares plane being 0.09 A. An unusual f e a t u r e s obtains here i n that the copper ions are not d i s p l a c e d out of the plane toward the a x i a l l i g a n d as has been observed i n many t e t r a g o n a l pyramidal copper(II) complexes. Presumably t h i s absence of d i s ­ placement i s a r e f l e c t i o n of the weak nature of the p e r c h l o r a t e c o o r d i n a t i o n . The bond d i s t a n c e s and angles are comparable to those observed i n other s i m i l a r complexes. The s t r u c t u r a l para­ meters p e r t i n e n t to t h i s d i s c u s s i o n are l i s t e d i n Table I where i t may be seen that the Cu-pyridine n i t r o g e n bond d i s t a n c e i s somewhat shorter than the Cu-amine n i t r o g e n bond. The Cu-O-Cu angle i s 100.4(1).° Hydrogen bonding i n v o l v i n g the hydroxo b r i d g i n g group i s i n d i c a t e d by the oxygen-oxygen s e p a r a t i o n of 2.993 A, which i s l e s s than twice the van der Waals r a d i u s of oxygen as given by Bondi (3.02 A) (18) but s l i g h t l y greater than the corresponding value given by P a u l i n g (2.80 Â ) . (19) That the bonding i s weak i s evident from the s t r o n g , sharp 0-H s t r e t c h i n g band at 3580 cm~l, a value which i s only 40 cm" l e s s than the value u s u a l l y asc r i b e d to f r e e hydroxyl groups. The temperature v a r i a t i o n of the magnetic s u s c e p t i b i l i t y of 3-[Cu(DMAEP)0H] (C10 ) from 50-300°K i s shown i n F i g u r e 3.(15) There i s a very broad maximum i n the magnetic s u s c e p t i b i l i t y at about 175°K, and the data may be f i t t e d to expression f o r ex-_^ change coupled p a i r s of copper(II) ions y i e l d i n g 2J = -195 cm and g = 2.00, where the c r i t e r i o n f o r the best f i t was the minimization of the f u n c t i o n 1

2

A

B F

l+

2

= Z[{ (exptl).X

X

(calcd) }T ] i

2

i

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

EXTENDED INTERACTIONS

114

BETWEEN M E T A L IONS

3h

01 50

I

1 100

1

1 150

1

J — 200

1

1

250

300

β

Τ ( Κ) Inorganic Chemistry

Figure 3. Susceptibility of fi-[Cu(DMAEP)OH] (ClO, )2 per Cu atom as a function of temperature. Solid line represents values calculated from equation (2) with g = 2.03 and 2] = 201 cm (15). 2

t

1

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

10.

HATFIELD

Superexchonge

115

Interactions

4

For t h i s set of parameters A = 2 . 1 x l 0 ~ . Since t h i s best f i t g-value i s somewhat lower than the average g-value (2.03) found from the EPR spectrum taken at 77°K, a second c a l c u l a t i o n was made i n which the g-value was held constant and only 2J was v a r i e d . The somewhat poorer f i t ( A B F " 5.7x10"^) gave 2J = -201 cm"l. In view of these r e s u l t s a value of = 2.03 i s probably accurate w i t h i n 2% and 2J - -200110cm" . B F

1

4. Du-y-hydroxobis[2-(2-ethylaminoethyl)pyridinecopper(II)]p e r c h l o r a t e . The s t r u c t u r e of the complex [Cu(EAEP)0H]2~ (010^)2 and c o o r d i n a t i o n geometry around copper i s very s i m i l a r to that described above f o r [Cu(DMAEP)OH] (CIO1J2 · The p e r t i n e n t s t r u c t u r a l d e t a i l s are tabulated i n Table I. In t h i s compound the copper ions are d i s p l a c e d approximately 0.12 A from the best least-squares b a s a l plan and the two b r i d g i n g oxyge the copper-oxygen(perchlorate) a x i a l i n t e r n u c l e a r separations are s i g n i f i c a n t l y shorter i n [Cu(EAEP)0H]2(CIO1J2 than i n the c o r r e s ponding complex [Cu(DMAEP)0H]2 (C10i*)2 where the copper ions are not d i s p l a c e d from the b a s a l plane. The two b a s a l planes are n e a r l y coplanar, with the angle between them being 1.4°. The average of the two Cu-O-Cu angles i s 99.2(3)° and the Cu-Cu s e p a r a t i o n i s 2.917(5) Â . The s t r u c t u r a l data i n d i c a t e that any hydrogen bonding i n v o l v i n g the b r i d g i n g hydroxo hydrogen atom i s very weak. The short oxygen(bridge)-oxygen(perchlorate) separations are 2.89 and 2.95 A, values which are l e s s than twice the van der Waals r a d i u s of oxygen as given by Bondi but greater than that given by P a u l i n g . The i n f r a r e d spectra shows a strong, sharp band at 3580 cm"" . The magnetic s u s c e p t i b i l i t y data (21) f o r [Cu(EAEP)0H] (010^)2 show a broad maximum at ~ 1 2 0 ° K and when the data are f i t t e d to Equation (2) the parameters 2J = -130cm~ and g = 2.04 result. 2

c

1

2

1

5. D i - y - h y d r o x o b i s [ 2 , 2 * - b i p y r i d i n e c o p p e r ( I I ) ] s u l f a t e pentahydrate. The complexes of the general formula [Cu(diamine)OH]2X "nH20 which are formed by 1,10-phenanthroline and 2 , 2 ' - b i p y r i d i n e are of c o n s i d e r a b l e i n t e r e s t s i n c e a v a r i e t y of counter ions may be used and the magnetic p r o p e r t i e s depend on the i d e n t i t y of the counter ion.(22-25) The f i r s t compound of t h i s type to be c h a r a c t e r i z e d f u l l y was [Cu(bipy)0H] S0i · 5H 0 (25-28). The s t r u c t u r a l d e t a i l s necessary f o r t h i s d i s c u s s i o n are l i s t e d i n Table I. In t h i s compound the copper ions are found i n d i s t o r t e d t e t r a g o n a l pyramidal environments with a water molecule c o o r d i n ated to one copper and the s u l f a t e i o n coordinated to the other coppeç. The copper ions are d i s p l a c e d approximately 0.18 and 0.23 A from the b a s a l planes formed by the n i t r o g e n atoms and the b r i d g i n g oxygens. I t i s i n t e r e s t i n g to note that the greater 2

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

+

2

116

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

displacement of 0.23 Â occurs i n the p o r t i o n of the molecule i n which the s u l f a t e i o n i s coordinated to copper and that that Cu-0 ( a x i a l ) bond d i s t a n c e i s the shorter of the two. The d i h e d r a l angle between the b a s a l planes i s 7.9°. There i s extensive hydrogen bonding with an 0(hydroxo)-0(sulfate) separation of 2.77 A. The magnetic s u s c e p t i b i l i t y of t h i s compound has been measured as a f u n c t i o n of temperature over the range 4.2-300°K, (26-28) and the χ vs.Τ p l o t , Figure 4, c l e a r l y i n d i c a t e s a d e v i a t i o n at low temperature from the Curie-Weiss behavior e x h i ­ b i t e d at higher temperatures. The measured s u s c e p t i b i l i t i e s deviate i n the manner expect f o r an exchange coupled p a i r of copper ions with a p o s i t i v e 2J value i n d i c a t i n g a ferromagnetic i n t e r a c t i o n and a t r i p l e t ground s t a t e with a low l y i n g s i n g l e t s t a t e . The best l e a s t square f i t of the data y i e l d 2J = +48(±10) cm"" and g = 2.2. The presence of th spectrum shown i n Figure 5. The spectrum may be described using the spin Hamiltonian - 1

1

H , = g J |BH S s

z

where the resonance \(z>

f

2

z

+ gj3(S^ H + s J H ) + D(S ' -2/3) y

f i e l d s are

= (g| | 3 )

- 1

^ ν -D| 1

H ( z ) = (g| j B ) " ^

+D)

2

1

Η ( χ , ) = (g^3)" [hv(hv - D ) ] χ

z

1 / 2

Υ

1

H (x,y) = (gjB)" [hv(hv + D ) ]

1 / 2

2

H ^ f o r b ) = ( 2 g | |Β)"\ν H ( f o r b ) = (2gj3) 2

-12 2 2 1/2 ( h V -D ) A

Z

± / Z

Here the s u b s c r i p t 1 designates the low f i e l d t r a n s i t i o n s and the s u b s c r i p t 2 designates the high f i e l d t r a n s i t i o n s , while the ΔΜ = 2 t r a n s i t i o n s are l a b e l e d forbidden. 3

1

6. D i - y - h y d r o x o b i s [ 2 , 2 - b i p y r i d i n e c o p p e r ( I I ) ] n i t r a t e . The s t r u c t u r e of [Cu(bipy)0H]2(N03) i s very s i m i l a r to that of the s u l f a t e s a l t with the d i f f e r e n c e being that n i t r a t e s are coor­ dinated i n the a x i a l p o s i t i o n s of the t e t r a g o n a l pyramids. (29) The copper(II) ions are d i s p l a c e d from the b a s a l planes toward the oxygen atom of the coordinated n i t r a t e where the Cu-0 ( a x i a l ) bond i s r e l a t i v e l y s h o r t , being 2.379(2) Â. S t r u c t u r a l d e t a i l s are l i s t e d i n Table I . The magnetic s u s c e p t i b i l i t y has been measured as a f u n c t i o n of temperature i n the range 1.6-300°K and can be f i t to Equation (2) y i e l d i n g 2J = 172 cm"! and g = 2.10. (30) The estimated 2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

10.

HATFIELD

Superexchonge

Interactions

117

20

40

60

80

300

Temperature (°K) Inorganic Chemistry

Figure 4. Inverse of magnetic susceptibility per cop­ per atom of [(hipy)Cu(OH) Cu(bipy)]SO · 511*0 as a function of temperature. Solid line represents valuescalculated from the Van Vleck equation; dashed line is extrapolation of Curie-Weiss law. Observed values are black dots. Data above 80K from Ref. 26 (28). 2

Figure

5.

X-band EPR spectrum of [Cu(bipy)OH] of/,Ο at 77°K, 0-10.0 kG

lf

,SO, ·

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

118

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

standard d e v i a t i o n on t h i s 2J value i s very l a r g e s i n c e the equa­ t i o n i s very i n s e n s i t i v e to v a r i a t i o n s i n l a r g e p o s i t i v e 2Jvalues. B. C o r r e l a t i o n of S t r u c t u r a l and Magnetic P r o p e r t i e s . 1. The nature of the c h e l a t i n g amine. Inspection of the data i n Table I r e v e a l s that the two complexes with the aromatic d i a ­ mine, 2 , 2 - b i p y r i d i n e , have t r i p l e t ground s t a t e s , the two com­ plexes with a l i p h a t i c diamines, tmen and teen, have s i n g l e t ground s t a t e s with l a r g e | 2 j | values, and that the two complexes with the mixed a l i p h a t i c / a r o m a t i c diamines, DMAEP and EAEP, have s i n g l e t ground s t a t e s with intermediate |2j| v a l u e s . There i s no c o r r e l a t i o n with the N-Cu-N angle, s i n c e these angles increase i n the order aromatic < a l i p h a t i c < mixed. Furthermore, Casey (25) has shown that [Cu(bipy)OH] and [ C u ( b i p y ) 0 H ] C l - 3 H values of -6, -34, and -39 cm. , r e s p e c t i v e l y . Since the N-Cu-N angle i s expected to be f a i r l y constant f o r a l l of the 2,2 b i p y r i d i n e compounds, and s i n c e there i s no apparent c o r r e l a t i o n of t h i s angle with 2J, then i t i s reasonable to conclude that changes i n t h i s angle are of secondary importance .However, the chemical nature of the bidentate amine i s probably important s i n c e as of yet there are no known overlaps of 2J values between the groups of compounds formed by the aromatic, mixed aromatic/ a l i p h a t i c , and a l i p h a t i c diamines. f

2

2

f

2. The copper-oxygen(hydroxo) bond d i s t a n c e . The copperb r i d g i n g oxygen bond d i s t a n c e s are a l l comparable and f a l l i n the range 1.9 to 1.95 A with the average bond d i s t a n c e being 1.915 A. For a l l p r a c t i c a l purposes the copper-oxygen bond d i s t a n c e i s constant i n t h i s s e r i e s of compounds. 3. The nature of any out-of-plane c o o r d i n a t i o n . The two com­ plexes with the a l i p h a t i c diamines, [Cu(tmen)OH] Br and [Cu(teen)OH] (ClO^) , are formed by two planar u n i t s sharing an edge. The c l o s e s t out-of-plane contacts to copper i n [Cu(tmen)O H ] B r are to bromide ions which are centered over the f i v e member c h e l a t e r i n g , these d i s t a n c e s are 4.778 and 4.933 Â , and are considered to be too long even foç semi-coordination. There are no out-of-plane atoms w i t h i n 4.0 A of copper(II) i n [Cu(teen)0H] (C10i ) . (31) The other four compounds are composed of t e t r a g o n a l pyramidal u n i t s sharing an edge with a p i c a l l i g a n d s on opposite s i d e s of the j o i n e d b a s a l planes. There i s an important trend here; the out-of-plane copper-oxygen bond d i s t a n c e s decrease with an i n c r e a s e i n the displacement of the copper i o n from the b a s a l plane toward the a p i c a l l i g a n d , v i z . , 2

2

2

2

2

2

2

+

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

10.

Superexchonge

HATFIELD

compound

Cu-0,

3-[Cu(DMAEP)OH] (C10 ) 2

4

[Cu(EAEP)OH] (C10 ) 2

4

[Cu(bipy)OH] (N0 ) 2

3

4

ο

ο

displacement, A

A

none

2.721(4)

2

2

2

2.618(9) 2.562(10) 2.379(2)

0.11 0.13 0.16

2

0.23 0.18

2.21(SO, ") 2.24(H 0)

[Cu(bipy)OH] S0 -5H 0 2

119

Interactions

2

2

There i s a general c o r r e l a t i o n of J with the copper-out-of-plane l i g a n d d i s t a n c e . Except f o r [Cu(bipy)0H] S0i 5H 0, the exchange coupling constant become d i s t a n c e decreases. e

2

+

2

4. The geometry of the b a s a l planes. To a good approximation the bases of the t e t r a g o n a l pyramids are a l l planar with d e v i a t i o n s f r o m the best l e a s t squares plane being on the order of 0.15 A or l e s s . The two b a s a l planes [or the c o o r d i n a t i o n planes i n the case of [Cu(tmen)0H] Br and [Cu(teen)0H] (C10i ) ]are f r e q u e n t l y coplanar, with the l a r g e s t d e v i a t i o n from c o p l a n a r i t y being a 7.9° d i h e d r a l angle between these planes i n [ C u ( b i p y ) 0 H ] SO^'5^0. Consequently, except f o r t h i s l a t t e r compound, the geo­ metry of the b a s a l plane remains constant throughout the s e r i e s . o

2

2

2

+

2

2

5. The s i n g l e i o n ground s t a t e s . I t i s very w e l l e s t a b l i s h e d that the unpaired e l e c t r o n i s i n the σ* o r b i t a l f o r square planar copper(II) complexes,(32) Although s p i n - o r b i t c o u p l i n g and the low symmetry c r y s t a l f i e l d components permit the mixing i n of other s t a t e s , to a good approximation the exchange mechanisms can be given i n terms of t h i s e l e c t r o n i c c o n f i g u r a t i o n . Although the displacement of the copper(II) i o n from the b a s a l plane toward the a x i a l l i g a n d i n the t e t r a g o n a l pyramidal complexes complicates even f u r t h e r the nature of the s i n g l e i o n ground s t a t e , i t w i l l be assumed that the unpaired e l e c t r o n i s i n the d >cr*, o r b i t a l i n these complexes. This assumption i s a d m i t t e d l y ïess tenable f o r [Cu(bipy)0H] S0i '5H 0 and [ C u ( b i p y ) 0 H ] ( N 0 ) where the d i s p l a c e ments are considerable and the a x i a l bond d i s t a n c e s are rather short. P r e c i s e d e s c r i p t i o n s of the ground s t a t e s must await the completion of d e t a i l e d EPR i n v e s t i g a t i o n s which are p r e s e n t l y underway. (33) 2

2

X

2

t

2

2

3

2

6. Hydrogen bonding by the h y d r o x o - b r i d g e h y d r o g e n atoms.The nature of the hydrogen bonding spans a wide range i n these compounds. A s u i t a b l e working hypothesis would suggest that the e l e c t r o n d e n s i t y on the b r i d g i n g oxygen atom should increase with the s t r e n g t h of the hydrogen bond and that t h i s v a r i a t i o n i n e l e c t r o n d e n s i t y should have a s i g n i f i c a n t e f f e c t on the exchange

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

120

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

coupling. I f e i t h e r the 0···Χ i n t e r n u c l e a r separation or Δν(ΟΗ) (the d e v i a t i o n from the " f r e e " hydroxyl s t r e t c h i n g energy) are taken as gauges of the hydrogen bond, then i t i s c l e a r from the data i n Table I I that there i s no simple c o r r e l a t i o n between e i t h e r of these parameters and the exchange coupling constant. Thus i t i s reasonable to conclude that hydrogen bonding i s of secondary importance i n determining the nature of the exchange coupling. 7. The copper-copper s e p a r a t i o n . There i s an i n t e r e s t i n g c o r r e l a t i o n between the Cu-Cu separation and the s i n g l e t - t r i p l e t s p l i t t i n g . As the Cu-Cu s e p a r a t i o n increases from 2.847 A i n [Cu(bipy)0H] (N0 ) to 3.000 Â i n [Cu(tmen)0H] Br ,_Jhe s i n g l e t t r i p l e t s p l i t t i n g changes from +172 cm" to -509 cm . I f the data f o r [Cu(bipy)0H] S0^·5H 0 omitted s i n c th b a s a l planes i n the compoun through the f i v e a v a i l a b l and^2J has a slope of -4545 c n r V A and an i n t e r c e p t of 13,130 cm 2

3

2

2

2

1

8. The copper-oxygen-copper bridge angle. Since the copDeroxygen (bridge) bond d i s t a n c e s are n e a r l y constant at 1.915 A, and s i n c e the C u 0 u n i t s are a l l n e a r l y planar there i s a s i m i l a r s t r i k i n g c o r r e l a t i o n between the 2J values and the Cu-0Cu bridge angle. T h i s l i n e a r c o r r e l a t i o n ( i n the range 95.6° < φ < 104.1°) i s i l l u s t r a t e d i n Figure 6, where the slope i s -79.5 cm" deg" and the i n t e r c e p t i s 7790 cm" . 2

1

2

1

1

C. R a t i o n a l i z a t i o n i n Terms of a Molecular O r b i t a l Model. The c o r r e l a t i o n s which have been observed can be understood i n terms of a simple molecular o r b i t a l model, which i s most appro­ p r i a t e f o r the two joined-planar compounds, but which i s s t i l l a good approximation f o r the j o i n e d t e t r a g o n a l pyramidal compounds. (34) I t i s necessary to consider the two oxygen o r b i t a l s , p , Py, and the two copper in-plane o r b i t a l s d _ , d . I f the f o l l o w ­ ing coordinate system i n D i s adopted x

2

v 2

x v

2 n

01

Cui X

i t can be seen that the o r b i t a l s transform as A

d

2

x -y

2

+

d

2

x -y

2

Yl " Ύ2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2

2

4

2

4

ο

" f r e e " hydroxyl

4

3

2

2

3620

2.877

2

2

-

2.89

2.993

3.00

3.366

X, A

[Cu(bipy)OH] (N0 )

3580

0

2.77

2

3580

1

-

4

2

3410

v(O-H),cnf

[Cu(bipy)OH] S0 '5H 0

2

[Cu(EAEP)OH] (C10 )

2

β-[Cu(DMAEP)] (C10 )

2

[Cu(teen)OH] (C10 )

[Cu(tmen)OH] Br

Compound

3

2

0N0 "

3

oso "

3

ocio ""

3

ocio "*

ocio "

Br"

X

Hydrogen Bonding Parameters f o r the B r i d g i n g Hydroxo Group.

Table I I

2

2

ο Bondi (3.02) Pauling (2.80)

MnBr ·2Η 0(3.39 A) CoBr -2H 0(3.37 A) ?

Comments

122

EXTENDED

B, lg

:

d x

B„ : 2u

l

d

"

xy

Yl

B

0

3u

+ d

xy

x

INTERACTIONS

BETWEEN

METAL

IONS

xy

2

- d

xy

+ Ύ2

: d 2 2 x -y z

^ 2

z

Χχ + x

2

2

where x^ and y^ designate the oxygen ρ c a l l y these symmetry r e l a t i o n s h i p

χ

and p

v

orbitals.

Symboli­

67

J

3u

Inspection of these w i l l show that the A and B^ molecular o r b i t a l s w i l l have i d e n t i c a l energies as w i l l ^ t h e B j ^ a n d u

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

10.

Superexchange

HATFIELD

200 -

\

*

Interactions

123

bipy NO,

100 ^bipy

S0

4

0

-100 τ

-

IAEP

ε ^

X.

-200

-300

-

-400

-

-500

-

-DM ΑΙ Ρ

\o

t

e

e

n

Ν.

1 95

Figure

$

1

6*. Correlation

1

! 97

of singlet-triplet

I 99

1

splittings

1 101

1—

i 103

—

with the Ctt-O-Cu

tmen

λ „

1 105

bridge

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

angle

124

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

o r b i t a l s . With t h i s information the molecular o r b i t a l diagram f o r t h i s fourteen e l e c t r o n system may be constructed; i t i s given i n Figure 7a. As the Cu-O-Cu angle increases the overlap of the A and B combinations increase r e l a t i v e to that of the B3 and 2u a t i o n s . At a s u f f i c i e n t l y l a r g e enough angle i t may be a n t i c i p a t e d that the separation between the B2 * and B i g * mole­ c u l a r o r b i t a l s would exceed the p a i r i n g energy and the s i n g l e t s t a t e with c o n f i g u r a t i o n ( b 2 * ) would r e s u l t as the ground s t a t e . The t r i p l e t s t a t e from the c o n f i g u r a t i o n ( b * ) ( b i * ) would be the low l y i n g paramagnetic s t a t e . g

l g

B

U

c o m D i n

U

2

u

2 u

II.Di-y-Chloro-bridged

g

Copper(II) Complexes

The p r o p e r t i e s of four d i - y - c h l o r o - b r i d g e d copper(II) com­ plexes w i l l be describe [Co(en)3] [Cu2Cl8]Cl2'2H copper(II) ions are i n d i s t o r t e d t r i g o n a l bipyramids which share an equator-to-apex edge, (35-40) and (Class II) [Cu(2-methyl2

p y r i d i n e ) 2 C l 2 l 2 and [Cu(dimethylglyoxime)Cl2]2»where the coordina­ t i o n about copper i s d i s t o r t e d t e t r a g o n a l pyramidal and the d i meric s t r u c t u r e i s formed by the sharing of the base-to-apex edge. (41-44) Unlike the hydroxo-bridged copper(II) complexes described i n Section I where the Cu-0 (bridge) bond lengths are constant, the chloro-bridged dimers have g r e a t l y d i f f e r e n t Cu-Cl (bridge) d i s t a n c e s . I t w i l l be shown here that the s i n g l e t t r i p l e t s p l i t ­ t i n g s are dependent on the b r i d g i n g bond lengths as w e l l as the Cu-Cl-Cu b r i d g i n g angle. A. S t r u c t u r a l and Magnetic Data. 1. T r i s ( e t h y l e n e d i a m i n e ) c o b a l t ( I I I ) D i - u - c h l o r o b i s [ t r i c h l o r o c u p r a t e ( I I ) ] D i c h l o r i d e Dihydrate. The unusual spec­ t r a l p r o p e r t i e s (45) of the compound Co(en)3CUCI5Ή2Ο, as f o r ­ mulated by Kurnakowin 1898, l e d to an X-ray c r y s t a l s t r u c t u r e examination (35,36) which revealed that the new and unusual [C^Cls] *"" ion was present. The compound c r y s t a l l i z e s i n the orthorhombic space group Pbca with four molecules i n a u n i t c e l l of dimensions a = 13.560(9), b = 14.569(9), and c = 17.885(12) A. The b r i d g i n g Cu-Cl distances are 2.325(5) and 2.703(5) A, the Cu-Cu separa­ t i o n i s 3.722(5) A, and the angle at the bridge i s 95.2(1)°. The exchange i n t e r a c t i o n (37) i n [ C u ( e n ) 3 ] [ C u C l 8 ] C l 2 ' 2 H 0 has been p r e c i s e l y c h a r a c t e r i z e d by s i n g l e c r y s t a l magnetic s u s c e p t i b i l i t y measurements. (38) Data were c o l l e c t e d i n the temperature range 4.2-80°K on a c r y s t a l ( 5 . 2 x 2 . 9 x 2 . 5 mm) with the magnetic f i e l d a p p l i e d along the c r y s t a l l o g r a p h i c axes. The data are given i n Figure 8, where i t may be seen that the s u s c e p t i b i l i t y maximizes at 12.9°K, and the usual g - f a c t o r a n i s o tropy i s observed ( x > x > χ ^ ) . The data may be f i t t e d to 1

2

c

2

a

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2

10.

HATFIELD

Superexchange

Interactions

125

», *

2u

a.

4 -

\

\

\

d 2 χ

-v

\

2

W

>

Λ-

\ X \ \ \

W e l e c t rons metal

molecular

ion orbitals

ligand

orbitals

orbitals

b.

/

/

/

/

Λ

*

Η—-χ. κ *

/

/

Figure 7. (a) Molecular orbital diagram for the in-plane orbitals in the CuO, ring, D symmetry, 90° Cu-O-Cu angle, (b) Antibonding molecular orbitals for Cu-O-Cu angle different from 90° (adapted from Ref. 34). 2ll

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

126

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

Equation (2) y i e l d i n g J / k = -10.7±.2° and g = 2.07±.02; J / k = -10.8±.2° and g = 2.03±.02; and J / k = -10.6±.2° and g = 2.18±.02. The data f i t the t h e o r e t i c a l curve very w e l l . For example, of the 44 data p o i n t s c o l l e c t e d with the f i e l d a p p l i e d along the c - a x i s , only f i v e deviated from the c a l c u l a t e d values by more than 1.5% while 34 of the p o i n t s d i f f e r e d by l e s s than 1%. No s i g n i f i c a n t improvement of the f i t s were observed when interdimer i n t e r a c t i o n s were included. Thus w i t h i n experimental e r r o r J = J = J = J , and = (1/3)(g +g +g ) = 2.09, a value which i s i n e x c e l l e n t agreement with the g value obtained from powder data c o l l e c t e d i n the temperature range 130-235°K. Here g was c a l c u l a t e d from the C u r i e constant using the formula a

a

b

a

b

b

c

c

c

a

g

2

b

c

2

= 3kC/N3 S(S+l)

The exchange i n t e r a c t i o i s almost c o l i n e a r with the copper(II)-copper(II) v e c t o r , and the exchange i n t e r a c t i o n observed along the a and b-axes, which are almost perpendicular to the copper(II)-copper(II) v e c t o r , are equal i n magnitude. A l s o , there i s no s i g n i f i c a n t long range interdimer exchange present i n the system. This r e s u l t i s not unexpected i n view of the l a r g e interdimer separation. The l a r g e c o p p e r ( I l ) - c o p p e r ( I I ) separation of 3.722 A precludes any through-space i n t e r a c t i o n s s i n c e no s i g n i f i c a n t o r b i t a l overlap can occur over t h i s d i s t a n c e , and d i p o l e - d i p o l e i n t e r a c t i o n s could not produce a s p l i t t i n g of the observed magnitude. Hence, i t seems reasonable to conclude that the i n t e r a c t i o n occurs v i a superexchange through the c h l o r i d e b r i d g e s . 0

2. D i - y - c h l o r o b i s [ d i c h l o r o ( g u a n i n i u m ) c o p p e r ( I I ) ] d i h y d r a t e . In 1970 Carrabine and Sundaralingam (39a) reported the s t r u c t u r e of d i - y - c h l o r o b i s [ d i c h l o r o ( g u a n i n i u m ) c o p p e r ( I I ) ] dihydrate, where the guaninium l i g a n d i s the c a t i o n formed by monoprotonating guanine, one of the bases bonded to the sugar residues i n the backbone of d i o x y r i b o n u c l e i c a c i d (DNA). The same authors presented a more complete s t r u c t u r a l a n a l y s i s the f o l l o w i n g year, (39b) and the s t r u c t u r e was then confirmed by Declercq, Debbaudt, and Van Meerssche (39c) i n an independent i n v e s t i g a t i o n . Both research groups reported the s t r u c t u r e to be that of a dimer c o n s i s t i n g of chloro-bridged, t r i g o n a l - b i p y r a m i d a l l y coordinated copper(II) ions, as shown i n Figure 9. The monoprotonation was shown to occur at the imidazole n i t r o g e n , N(7), of the purine r i n g system, and binding to the copper i o n , at N£9). The b r i n g ­ ing Cu-Cl distances were determined to be 2.447 A and 2.288 A, with a Cu-Cl-Cu b r i d g i n g angle of 98° and a Cu-Cu separation of 3.575 A. The temperature v a r i a t i o n of the inverse s u s c e p t i b i l i t y ( c a l c u l a t e d per copper(II) ion) of a powdered sample i n the temperature region 1.6 to 255°K i s represented by the black data p o i n t s i n Figure 10. (40) The maximum i n the curve occuring at 0

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

10.

Superexchange

HATFIELD

Figure

9.

Structure

of

Interactions

[Cu(guaninium)Cl. ]j {

(adapted

from

Ref.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

39a)

128

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

approximately 15°Κ i s probably due t o a small percentage of mono­ mer i c impurity which d i d not a f f e c t the percentage composition of the elemental a n a l y s i s . To c o r r e c t f o r t h i s impurity the data p o i n t s from 1 . 6 t o 11.2°K were f i t to the Curie-Weiss law χ = C/(T - Θ) Values of 7.86 χ 10 and -3.22° were obtained f o r the constants C and θ, r e s p e c t i v e l y . In t h i s temperature r e g i o n the s u s c e p t i ­ b i l i t y of the dimer i s n e g l i g i b l y small (vide p o s t ) . A l l the data p o i n t s were then c o r r e c t e d f o r the c o n t r i b u t i o n of the impurity to the observed s u s c e p t i b i l i t y , and the c o r r e c t e d p o i n t s a r e a l s o p l o t t e d i n F i g u r e 10 as the u n - f i l l e d c i r c l e s . The impurity was estimated to be present to the extent of 1% i n the f o l l o w i n g way: The s u s c e p t i b i l i t y f o d havin molecula weight equal t o one-hal the expression χ - N3 y /3kT 2

2

1/2 where μ = g$[S(S + 1) ] , a t a s e l e c t e d temperature, and the c a l c u l a t e d s u s c e p t i b i l i t y was compared with the experimental sus­ c e p t i b i l i t y a t that temperature. The s o l i d l i n e i n F i g u r e 10 i s the best f i t of the c o r r e c t e d data to Equation ( 2 ) , which y i e l d s the parameters 2 J « - 8 2 . 6 ± 1 . 0 cm" and g - 2.12±0.02. 3. D i - y - c h l o r o b i s [ c h l o r o ( d i m e t h y l g l y o x i m e ) c o p p e r ( I I ) ] . The s t r u c t u r e (43) of [Cu(DMG)Cl2l2 i s shown schematically i n Figure 11 with p e r t i n a n t molecular dimensions i n d i c a t e d thereon. The copper atom i s f i v e - c o o r d i n a t e d i n a square-based pyramidal a r ­ rangement c o n s i s t i n g of a n e a r l y square planar arrangement of two n i t r o g e n atoms and two t i g h t l y bound c h l o r i n e atoms with a c h l o r i n e atom from an adjacent u n i t i n the a p i c a l p o s i t i o n . The magnetic data (44) can be described by the s i n g l e t t r i p l e t equation ( 2 ) with 2 J = 6 . 3 cm" , g = 2 . 0 6 , and θ = - 1 . 7 ° . A d d i t i o n a l and convincing evidence f o r the t r i p l e t ground s t a t e i s provided by the magnetization s t u d i e s . The magnetization curves f o r S' = 1/2 and S' » 1 were evaluated from the B r i l l o u i n function (46) 1

1

1

τ>ίγ\ - ^ S ' * „ .. B(X) , coth n#

2 s

2S'+1

1 χ - 2s' v

c o t h

2S*

where X • (H/T)(S g3/k) and S i s the e f f e c t i v e s p i n . To account f o r interdimer i n t e r a c t i o n s the f i e l d H was s e t equal t o the e x t e r n a l f i e l d p l u s a molecular f i e l d , H , where i t was assumed that Hm was p r o p o r t i o n a l to the magnetization. Thus, 1 ^ = N N3

where <μ> = gS'B(X) and = (3ke )/[Ng 3 S (S +1)].(47) A best f i t ( l e a s t squares) of the experimental data was used to s e l e c t a value of -1.2 f o r Θ thereby i n d i c a t i n g an a n t i f e r r o f

f

m

f

2

2

f

f

W

1

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

10.

Superexchange

HATFIELD

129

Interactions

TEMPERATURE Inorganic Chemistry

Figure 10. Temperature vs. inverse susceptibility for the complex [(guaninium)CuClg]* · 2H 0 (38). Experimental points, · ; experimental points corrected for monomeric impurity, Q; Van Vleck equation best fit, . 2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

130

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IONS

1

magnetic interdimer i n t e r a c t i o n . The Θ term i n the magnetization s t u d i e s i s very s i m i l a r to the θ term i n Equation 2, but i n view of the approximate nature of the theory, i d e n t i c a l values f o r these interdimer i n t e r a c t i o n parameters are not expected. 4. D i - p - c h l o r o b i s [ c h l o r o b i s ( 2 - m e t h y l p y r i d i n e ) c o p p e r ( I I ) ] . In recent years many copper(II) complexes of the type CUL2X2» where X i s c h l o r i d e or bromide and L i s p y r i d i n e or s u b s t i t u t e d p y r i ­ dine, have been prepared and c h a r a c t e r i z e d . (48) These complexes are mainly polymeric, having s i x c o o r d i n a t i o n about the copper ion with h a l i d e l i g a n d s from adjacent molecules occupying the o u t of-plane c o o r d i n a t i o n p o s i t i o n s . However, the complexes of 2methylpyridine were found to have p r o p e r t i e s somewhat d i f f e r e n t to those found f o r the analogous p y r i d i n e complexes, (49)and i t was p o r t u l a t e d that th s t e r i c hindrance to th quently, Duckworth and Stephenson (41) determined that such was the case f o r d i c h l o r o b i s ( 2 - m e t h y l p y r i d i n e ) c o p p e r ( I I ) . The coor­ d i n a t i o n about copper i n t h i s complex i s t e t r a g o n a l pyramidal with the f i f t h p o s i t i o n (out-of-plane) occupied by a c h l o r i d e l i g a n d from an adjacent planar moiety, and the s i x t h p o s i t i o n i s e f f e c t i v e l y blocked by the methyl groups of the p y r i d i n e l i g a n d s . The s t r u c t u r a l d e t a i l s are summarized i n Table I I I . As shown i n F i g u r e 12, the magnetic s u s c e p t i b i l i t y (42) data f o r Cu(2-methylpyridine)2CI2 obey the Curie-Weiss law, χ = C / ( T - 9 ) , i n the range 295°K to approximately 30°K. For the c h l o r o complex. the C u r i e constant C = 0.394, θ = 1°K. and y f f 2.828C ' i s 1.78 B.M. However, a t the low temperature l i m i t i t i s apparent that the Curie-Weiss law f a i l s . There i s a d i s t i n c t minimum i n the χ"~^ versus Τ p l o t a t approximately 7°K. The data obey the Van V l e c k equation (2) f o r m a g n e t i c a l l y coupled p a i r s of copper ions y i e l d i n g g = 2.15 and -2J = 7.4 cm"^. =

e

1

2

B. C o r r e l a t i o n of S t r u c t u r a l and Magnetic P r o p e r t i e s . I t i s of i n t e r e s t to compare the magnetic parameters and s t r u c t u r a l data f o r the s t r u c t u r a l l y - and m a g n e t i c a l l y - c h a r a c t e r i z e d c h l o r o bridged b i m e t a l l i c copper(II) complexes discussed here. These data a r e compiled i n Table I I I . Both the guaninium complex and the [Cu2Cl3]^""anion are made up of t r i g o n a l bipyramids sharing equatorial-to-apex edges, w h i l e the other two complexes l i s t e d i n the t a b l e are square-based pyramids sharing base-to-apex edges. In the t r i g o n a l - b i p y r a m i d a l complexes i t i s l i k e l y that the un­ p a i r e d e l e c t r o n s a r e i n the d o r b i t a l s of the copper(II) ions i n the ground s t a t e , whereas i n the square-pyramidal complexes i t i s l i k e l y that they a r e i n the d _ 2 o r b i t a l s . A q u a n t i t a t i v e comparison of the exchange energies of the four complexes cannot be based on s t r u c t u r a l data because of the d i f f e r e n t o r b i t a l s involved i n superexchange, but a q u a l i t a t i v e comparison i s pos­ sible. The s t r u c t u r a l parameters of the guaninium complex and the z 2

x 2

y

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2

2

4-

2

2

1

DMG = dime thy lgloxime

2-mepy = 2-methylpyridine

2

[(DMG)CuCl ]

2

2

anion

[(2-mepy) CuCl ]

2

[Cu Clg] 2.26

2.24

101.4

88

squarepyramidal squarepyramidal

+6.3

2.70

2.45

43,44

41,42 3.37

2.70

35,36, 37,38

39,40

REF.

2.33

2.29

Cu-Cl BOND Cu-Cl BOND in-plane out-of-plane (A) (A)

-7.4

95.2

98

Cu-Cl-Cu ANGLE (degrees)

trigonalbipyramidal

trigonalbipyramidal

STRUCTURE

-14.6

-82.6

2

[(guaninium)CuCl ] ·2H 0

3

2J (cm )

COMPLEX X

Table I I I . Magnetic and s t r u c t u r a l data f o r c h l o r o - b r i d g e d copper(II) dimers.

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IONS

[ C i r c l e ] ^ " a n i o n are compared by superimposition i n Figure 13 with the s o l i d l i n e representing the guaninium complex. The smaller s i n g l e t - t r i p l e t s p l i t t i n g f o r the [ C ^ C l e l anion i n comparison to the guaninium complex^accompanies an increase i n the Cu-Cl-Cu bond angle from 95° to 98° and a decrease i n the b r i d g i n g bond lengths. Although i t has been demonstrated that the b r i d g i n g angle i s important i n determining the s i g n and magnitude of the s p l i t t i n g parameter 2J i n the s e r i e s of hydroxobridged copper(II) complexes i t i s u n l i k e l y t h i s e f f e c t can be presented as the s o l e explanation i n t h i s comparison because the b r i d g i n g bond lengths of these two chloro-bridged species are q u i t e d i f f e r e n t , ranging between 2.3 and 2.7 A, whereas they arg n e a r l y constant i n the hydroxo-bridged species at 1.90 to 1.95 A. (See Table I ) . The complexes [ ( 2 - m e t h y l p y r i d i n e g l y o x i m e ) C u C l 2 l 2 have d i f f e r e n t exchange coupling mechanism from that described above. In comparing these two square-pyramidal complexes i t should be noted that there i s a change i n ground s t a t e m u l t i p l i c i t y . The b r i d g i n g bond length i n [(2-mepy)2CUCI2]2 0.67 A longer than the comparable bond i n [(DMG)CuCl2]2 and angle at the b r i d ging c h l o r i d e i s 13.4° l a r g e r i n the 2-methylpyridine complex than i n the dimethylglyoxime complex. I t i s , t h e r e f o r e , not p o s s i b l e to a t t r i b u t e the change i n the exchange coupling constant only to bridge angle changes. C l e a r l y a number of a d d i t i o n a l chloro-bridged copper(II) dimers of both s t r u c t u r a l forms must be studied before the bridge-angle e f f e c t on 2J can be separated from the b r i d g i n g bond-length e f f e c t . i s

t n e

I I I . The Polymeric Compound [Cu(pyrazine)(N0q)?] Chains.

n

and

Related

The magnetic p r o p e r t i e s of the 1:1 copper(II) n i t r a t e pyrazine complex, [ C u i C i ^ H ^ ) ( ^ 3 ) 2 ] , r e f l e c t an exchange coupl i n g between the copper ions although as shown i n Figure 14 the copper(II) ions are separated by 6.712 A.(50) While exchange coupling across bidentate h e t e r o c y c l i c amine l i g a n d s had been suggested p r e v i o u s l y , (51) the p r e l i m i n a r y i n v e s t i g a t i o n (52) provided the f i r s t demonstration of an antiferromagnetic i n t e r a c t i o n i n a system which has been c h a r a c t e r i z e d by s t r u c t u r a l s t u d i e s , magnetic measurements c o l l e c t e d over a wide temperature range and EPR measurements. The c r y s t a l s t r u c t u r e of t h i s compound r e v e a l s a chemical chain p a r a l l e l to the a a x i s i n the orthorhombic c r y s t a l . Copper atoms are bridged along t h i s a x i s by the aromatic h e t e r o c y c l i c bidentate amine, pyrazine. Coordinated oxygen atoms from the n i t r a t e ions complete the h i g h l y d i s t o r t e d octahedron around each copper atom. The magnetic s u s c e p t i b i l i t y data c o l l e c t e d using a powdered n

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

10.

HATFIELD

Superexchange

133

Interactions

800h

0

10

30

50

100 200 3 0 0

T e m p e r a t u r e , °K Figure 12. Temperature variation imental inverse susceptibility of pyridine) Clt~\ t

of the exper­ [Cu(2-methyl-

g

Ν (CI)

Inorganic Chemistry

Figure 13. Comparison of the structural parameters for ninium)CuCl ] · 2H 0 and [Cu Cl ywith the solid line senting the guaninium complex (40) 3 2

£

â

8

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

[(guarepre-

134

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INTERACTIONS

BETWEEN

METAL

IONS

sample were d e s c r i b e d (52) by the I s i n g model f o r l i n e a r a n t i ferromagnetic i n t e r a c t i o n s i n chains using equations (3a,3b) which was developed by F i s h e r . (53) The equations are 2 2 XJL - ^ f j — [tanh ( φ Η φ sech (^)] (3a) 2

2

and 2 2 XII - ^ f i J —

exp

(2J/kT)

(3b)

where 1 s

^

2

3 XII +3

(4)

XI

The parameters whic data are = 2.22 an to be compared with the EPR r e s u l t s of Kokosζka and Reimann, (54) who reported g = 2.295, g = 2.054, and g = 2.070 ( = 2.133). In an attempt to determine more p r e c i s e l y the magnetic model which i s a p p r o p r i a t e f o r the d e s c r i p t i o n of the p r o p e r t i e s of t h i s compound and to c l a r i f y the exchange pathway, measurements have been performed on s i n g l e c r y s t a l s of t h i s m a t e r i a l between 1.7° and 60°K. (55) Reasonably l a r g e needle-shaped s i n g l e c r y s t a l s of Cu(pyrazine)(N03)2 were grown by slow-evaporation of an aqueous 1:1 s o l u t i o n of copper (II) n i t r a t e and pyrazine (Ci+Hi^). Because of the c r y s t a l morphology s e v e r a l of the l a r g e s t c r y s t a l s were s e l e c t e d and c a r e f u l l y o r i e n t e d under a microscope so that a l l a axes of the c r y s t a l s were c o l l i n e a r . The s u s c e p t i b i l i t i e s from 1.7° to 60°K both p a r a l l e l and perpendicular to the & a x i s are shown i n F i g u r e 15. A c l e a r maximum i s observed at approximately 6.8°K i n each d i r e c t i o n . These measurements r e v e a l t y p i c a l i s o ­ l a t e d antiferromagnetic l i n e a r chain behavior down to the lowest temperature achieved i n t h i s experiment. The I s i n g chain equations can not be used to f i t these data. Bonner and F i s h e r (56) have performed machine c a l c u l a t i o n s on f i n i t e Heisenberg r i n g s and have been able to estimate the l i m i t i n g behavior f o r an i n f i n i t e r i n g . Applying these r e s u l t s to the experimental data i n both d i r e c t i o n s J/k equals -5.30 (±0.05) °K where J i s as d e f i n e d by the term -2JS-j/Sj i n the Hamiltonian described by Bonner and F i s h e r . The g value g i v i n g the best f i t to theory f o r the perpendicular d i r e c t i o n was 2.10 (±0.01) and 2.03 (±0.01) f o r the p a r a l l e l d i r e c t i o n . As can be seen i n F i g u r e 15, the f i t f o r both d i r e c t i o n s down to 1.7°K i s i n q u i t e good agree­ ment with experiment. Commonly "4+2" t e t r a g o n a l l y d i s t o r t e d copper complexes e x h i b i t two small g values ( g ) and one l a r g e g value (S, J)· (57) I f i t i s assumed that the smallest g value l i e s along the a^ a x i s , corresponding to the s h o r t e s t bond d i s t a n c e , then one of the other small g values and the l a r g e s t g value l i e i n the plane perpendicular to t h i s a x i s . The s u s c e p t i b i l i t y perpendicular z

x

y

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

10.

HATFIELD

Superexchange

135

Interactions

Journal of the American Chemical Society

Figure

14.

Structure

(52)

of [CufC H N XN0 ) ']n 4

h

i

8

i

Tempera tu re *K

Journal of Chemical Physics

Figure 15. Best fit of the corrected magnetic susceptibility to the one-dimensional Heisenberg model with T/k = -5.30° for both directions and g == 2.03 and gj_ — 2.10 (55) n

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

136

EXTENDED

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BETWEEN

METAL

IONS

to the a a x i s should then r e f l e c t a g value intermediate between the two extremes detected by the EPR measurements i f one assumes a random o r i e n t a t i o n of the b and c axes. T h i s i s c o n s i s t e n t with the experimental o b s e r v a t i o n . The pathway of the exchange i n t e r a c t i o n must now be i d e n t i ­ f i e d . N i t r a t e ions bridge copper(II) ions i n Cu(N0 ) *2.5H 0 forming a crooked c h a i n . Even though t h i s chain i s present the magnetic s u s c e p t i b i l i t y , which d i s p l a y s a rounded maximum at 3.2 °K, has been adequately d e s c r i b e d as a r i s i n g from antiferromagn e t i c a l l y exchange coupled p a i r s (58,59) with the predominant mode of exchange thought to occur between the chemical chains.(60) The s h o r t e s t Cu-Cu s e p a r a t i o n (^4.7 A) i s between copper atoms i n the crooked c h a i n . Much weaker a n t i f err omagne t i c interdimer ex­ change e f f e c t s were i n c l u d e d to improve the f i t at low tempera­ t u r e s . As shown i n F i g u r 16 possibilit fo thi nitrat b r i d g e e x i s t s i n the bCu-Cu s e p a r a t i o n i s 5. i t i e s dimeric s u s c e p t i b i l i t y behavior was explored i n Cu(pyrazine)(Νθ3)2· When the dimer s u s c e p t i b i l i t y equation was used to f i t the data a value of J/k » -5.4°K r e s u l t s when the g v a l u e was r e s t r i c t e d to the range 2.00 to 2.20. However, the c a l c u l a t e d s u s c e p t i b i l i t i e s are c o n s i s t e n t l y lower than the experimental values with the d i f f e r e n c e i n c r e a s i n g as the temperature de­ creases. 3

2

2

Although an exchange pathway through the n i t r a t e i o n cannot be completely r u l e d out i n Cu(pyrazine)(N03)2> the c l e a r l i n e a r c h a i n behavior i n t h i s compound as compared to the p a i r i n t e r ­ a c t i o n i n Cu(N03)2·2.5H20 argues s t r o n g l y a g a i n s t t h i s pathway f o r exchange. The r e s u l t s with the s u b s t i t u t e d p y r i d i n e complexes to be d e s c r i b e d below o f f e r s convincing evidence against t h i s pathway, however, before that can be presented the p o s t u l a t e d mechanism of the exchange i n t e r a c t i o n through the pyrazine bridge w i l l be d e s c r i b e d . The p y r a z i n e r i n g i s p e r p e n d i c u l a r to the plane i n which the b i s ( n i t r a t o ) c o p p e r ( I I ) u n i t s l i e s , and the unpaired e l e c t r o n , i n the s i n g l e i o n approximation, i s i n the d 2 . 2 o r b i t a l . Now i t develops that the pyrazine r i n g i s canted wïth respect to the xy plane such that the highest energy occup i e d molecular o r b i t a l , the B^ π o r b i t a l , has the proper symme­ t r y to overlap with the d 2 „ 2 o r b i t a l . I t i s suggested here that the exchange i s propagated i n t h i s manner. To t e s t t h i s mechanism complexes have been prepared with s u b s t i t u t e d pyrazines with the expectation that the exchange c o u p l i n g would be a f f e c t e d by the e l e c t r o n i c nature of the s u b s t i t u e n t on the p y r a z i n e r i n g . Ex­ change by means of the a l t e r n a t e pathway through the n i t r a t e " b r i d g e " would be dependent on the s t e r i c p r o p e r t i e s of the sub­ s t i t u e n t and only s e c o n d a r i l y dependent on the e l e c t r o n i c nature. y

x

The experimental data which have been c o l l e c t e d (61,62) are tabulated i n Table IV. The s i m i l a r i t y of the s p e c t r a an3"tne g values can be taken as good evidence that the s t r u c t u r e s of the complexes are the same as that of the pyrazine complex. As can be

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

HATFIELD

Superexchange

Interactions

c

Journal of Chemical Physics

Figure 16. A view of Cu(pyrazineXNO$) in the be plane which illustrates the weak nitrate bridge between copper atoms. For clarity only copper atoms lying at (Oil) have been included (55). 2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

138

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IONS

seen i n Table IV there i s a s t r i k i n g c o r r e l a t i o n of the l i g a n d π«-ΗΓ* t r a n s i t i o n with 2J.There i s a l s o a c o r r e l a t i o n between 2J Table IV S p e c t r a l and Magnetic Data f o r [Cu(R-pyrazine) ( N C O J ligand

LIGAND

ΤΓ—TT* , cm

quinoxaline

42,300

pyrazine

38,460

methyl pyrazine

37,90

chloro pyrazine

33,700

^ 2J,cm" -9.0

1

2.15

complex d-d,cm" π -KJ*cm~ 18,500 29,000

-7.2

2.133

17,860

34,600

-2.8

2.153

17,800

36,400

and the charge t r a n s f e r band i n the complex which i s most l i k e l y 7r(ligand)-*J*(metal). Since there i s such a good c o r r e l a t i o n of 2J with the e l e c t r o n i c p r o p e r t i e s of the l i g a n d , and none at a l l with the s i z e of the s u b s t i t u e n t (which should separate the chains thereby a f f e c t i n g exchange through the n i t r a t e i o n ) , i t seems c o n c l u s i v e that the exchange pathway i s v i a the pyrazine bridge. IV. Acknowledgements T h i s research has been supported by the N a t i o n a l Science Foundation through grant number GP-22887 and by the M a t e r i a l s Research Center of the U n i v e r s i t y of North C a r o l i n a through grant number GH-33632 from the N a t i o n a l Science Foundation. I am g r a t e f u l f o r t h i s c o n t i n u i n g support. T h i s research e f f o r t has b e n e f i t e d g r e a t l y from c o l l a b o r a t i o n with P r o f e s s o r D.J. Hodgson and from the work of s e v e r a l i n d u s t r i o u s graduate students and research a s s o c i a t e s , many of whom are named i n the r e f e r e n c e s . T h e i r c o n t r i b u t i o n to t h i s program has been i n v a l u a b l e . V. L i t e r a t u r e C i t e d 1. For reviews see a. R.L. M a r t i n in New Pathways in Inorganic Chemistry,Edited by E.A.V. Ebsworth, A.G. Maddock, and A.G. Sharpe, Cambridge U n i v e r s i t y Press, 1968. b. E. Sinn, Coordn. Chem. Rev., 5., 313 (1970). c. G.F. Kokoszka and G. Gordon i n T r a n s i t i o n Metal Chemistry V o l . 5, E d i t e d by R.L. C a r l i n , Marcel Dekker, Inc., New York, 1969. d. J.B. Goodenough,Magnetism and the Chemical Bond, I n t e r s c i e n c e P u b l i s h e r s , Inc., New York, 1963.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

10.

HATFIELD

Superexchange

Interactions

139

2. T. Moriya, Phys. Rev., 120, 91 (1960); T. Moriya in Magnetism, T r e a t i s e Modern Theory of Matter, 1, 85 (1963). 3. M.E. L i n e s , A.P. Ginsberg, R.L. M a r t i n , and R.C. Sherwood, J. Chem. Phys., 57, 1 (1972). 4. P. Pfeiffer and H. G l a s e r , J. Prakt. Chem., 151, 134 (1938). 5. F.G. Mann and H.R. Watson, J. Chem. Soc., 2772 (1958). 6. W.E. H a t f i e l d , T.S. P i p e r , and U. Klabunde, Inorg. Chem., 2, 629 (1963). 7. D.W. Meek and S.A. Ehrhardt, Inorg. Chem., 4, 584 (1965). 8. J.R. Wasson, T.P. M i t c h e l l , and W.H. Bernard, J. Inorg. Nucl. Chem., 30, 2865 (1968). 9. T.P. M i t c h e l l , W.H. Bernard, and J.R. Wasson, Acta C r y s t . , B26, 2096 (1970). 10. J.R. F e r r a r o and W.R. Walker, Inorg. Chem., 4, 1382 (1965). 11. W.R. McWhinnie, J. 12. J.C.D. Brand and G Organic Chemistry, Oldbourne Press, London (1965). 13. B.J. Cole and W.H. Brumage, J. Chem. Phys., 53, 4718 (1970). 14. E.D. Estes, W.E. H a t f i e l d , and D.J. Hodgson, Inorg. Chem., i n press. 15. D.L. Lewis, K.T. McGregor, W.E. H a t f i e l d , and D.J. Hodgson, Inorg. Chem., 13, 1013 (1974). 16. D.L. Lewis, W.E. H a t f i e l d , and D.J. Hodgson, Inorg. Chem., 13, 147 (1974). 17. P. Krähmer, M. Maaser, K. S t a i g e r , and E. U h l i g , Z. Anorg. Allgem. Chem., 354, 242 (1967). 18. A. Bondi, J. Phys. Chem., 68, 441 (1964). 19. L. P a u l i n g , The Nature of the Chemical Bond, 3rd ed., C o r n e l l U n i v e r s i t y Press, Ithaca, New York (1960). 20. D.L. Lewis, W.E. H a t f i e l d , and D.J. Hodgson, Inorg. Chem., 11, 2216 (1972). 21. D.Y. J e t e r , D.L. Lewis, J.C. Hempel, D.J. Hodgson, and W.E. H a t f i e l d , Inorg. Chem., 11, 1958 (1972). 22. R.L. Gustafson and A.E. M a r t e l l , J. Amer. Chem. Soc., 81, 525, (1959). 23. D.D. P e r r i n and V.S. Sharma, J. Inorg. Nucl. Chem., 28, 1271, (1966). 24. C.M. H a r r i s , E. Sinn, W.R. Walker, and P.R. Woolliams, Aust. J . Chem., 21, 631 (1968). 25. A.T. Casey, Aust. J. Chem., 25, 2311 (1972). 26. A.T. Casey, B.F. Hoskins, and F.D. W h i l l a n s , Chem. Commun., 904 (1970). 27. J.A. Barnes, W.E. H a t f i e l d , and D.J. Hodgson, Chem. Commun., 1593 (1970). 28. J.A. Barnes, D.J. Hodgson, and W.E. H a t f i e l d , Inorg. Chem., 11, 144 (1972). 29. R.J. Majeste and E.A. Meyers, J . Phys. Chem., 74, 3497 (1970). 30. K.T. McGregor, N.T. Watkins, D.L. Lewis, R.F. Drake, D.J. Hodgson, and W.E. H a t f i e l d , Inorg. Nucl. Chem. L e t t e r s , 9, 423 (1973).

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

140

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

31. E.D. Estes, p r i v a t e communication. 32. See, f o r example, C. Chow, K. Chang, and R.D. W i l l e t t , J . Chem. Phys., 59, 2629 (1973). 33. K.T. McGregor and W.E. H a t f i e l d , to be p u b l i s h e d . 34. D.J. Hodgson, Progress in Inorganic Chemistry, E d i t e d by S.J. L i p p a r d , W i l e y - I n t e r s c i e n c e , New York, in p r e s s . 35. D.J. Hodgson, P.K. Hale, J.A. Barnes, and W.E. H a t f i e l d , Chem. Comm., 786 (1970). 36. D.J. Hodgson, P.K. Hale, and W.E. H a t f i e l d , Inorg. Chem., 10, 1061 (1971). 37. J.A. Barnes, D.J. Hodgson, and W.E. H a t f i e l d , Chem. Phys. Letters, 7, 374 (1970). 38. K.T. McGregor, D.B. Losee, D.J. Hodgson, and W.E. H a t f i e l d , Inorg. Chem., 13, 756 (1974). 39. a. J.A. Carrabine 92, 369 (1970);b. Molec. Biol., 61, 287 (1971); c. J.P. D e c l e r c q , M. Debbaudt, and M. Van Meerssche, B u l l . Soc. Chim. Belges, 80, 527 (1971). 40. R.F. Drake, V.H. Crawford, N.W. Laney, and W.E. H a t f i e l d , Inorg. Chem.,13, 1246 (1974). 41. V.F. Duckworth and N.C. Stephenson, A c t a C r y s t a l l o g r . , Sect. B, 25, 1795 (1969). 42. D.Y. J e t e r , D.J. Hodgson, and W.E. H a t f i e l d , Inorg. Chim. Acta, 5, 257 (1971). 43. D.H. Svedung, Acta Chem. Scand., 23, 2865 (1969). 44. N.T. Watkins, E.E. Dixon, V.H. Crawford, K.T. McGregor, and W.E. H a t f i e l d , Chem. Commun., 133 (1973). 45. W.E. H a t f i e l d and T.S. P i p e r , unpublished o b s e r v a t i o n s . 46. J.S. Smart, E f f e c t i v e F i e l d Theories of Magnetism, W.B. Saunders Co., P h i l a d e l p h i a , 1966. 47. For example, see J.A. Bertrand, A.P. Ginsberg, R.I. Kaplan, G.E. Kirkwood, R.L. M a r t i n , and R.C. Sherwood, Inorg. Chem., 10, 240 (1971). 48. W.E. H a t f i e l d and R. Whyman, T r a n s i t i o n Metal Chemistry, E d i t e d by R.L. C a r b i n , Marcel Dekker, Inc., New York, V o l . 5, p. 53ff (1969). 49. D.P. Graddon, R. Schulz, E.C. Watton, and D.G. Weeden, Nature, 198, 1299 (1963). 50. A. Santoro, A.D. M i g h e l l , and C.W. Reimann, A c t a C r y s t . , B26, 979 (1970). 51. See f o r example, D.E. Billing, A.E. U n d e r h i l l , D.M. Adams, and D.M. M o r r i s , J. Chem. Soc. (A), 902 (1966); M.J.M. Campbell, R. Grzeskowiak, and F.B. T a y l o r , ibid., 19 (1970). 52. J.F. Villa and W.E. H a t f i e l d , J . Amer. Chem. Soc., 93, 4081 (1971). 53. M.E. F i s h e r , J. Math Phys., 4, 124 (1963). 54. G.F. Kokoszka and C.W. Reimann,J. Inorg. Nucl. Chem., 32, 3229 (1970). 55. D.B. Losee, H.W. Richardson, and W.E. H a t f i e l d , J . Chem. Phys., 59, 3600 (1973).

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

10.

HATFIELD

Superexchange

Interactions

141

56. J. Bonner and M. F i s h e r , Phys. Rev. Sect. A, 135, 640 (1964). 57. E. König Magnetic P r o p e r t i e s of T r a n s i t i o n Metal Compounds., S p r i n g e r - V e r l a g , B e r l i n , 1966. 58. L. Berger, S. F r i e d b e r g , and J. Schriempf, Phys. Rev., 132, 1057 (1963). 59. B. Myers, L. Berger, and S. F r i e d b e r g , J. A p p l . Phys., 40, 1149 (1969). 60. J. Bonner, S. F r i e d b e r g , H. Kobayashi, and B. Myers, Proc. 12th I n t e r n a t i o n a l Conf. on Low Temp. P h y s i c s , Kyoto (1970). 61. H.W . Richardson and W.E. H a t f i e l d , t o be p u b l i s h e d . 62. H.W. Richardson, W.E. H a t f i e l d , H.J. S t o k l o s a , and J.R. Wasson, Inorg. Chem., 12, 2051 (1973).

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

11 Electronic and Magnetic Properties o f Linear Chain Complexes Derived from Biscyclopentadienyl Titanium(III) and o f the Infinite

RMX

Linear

3

Chain Complexes D.

SEKUTOWSKI,

R.

JUNGST,

and G .

D.

STUCKY

M a t e r i a l s Research L a b o r a t o r y , U n i v e r s i t y o f Illinois, U r b a n a , Ill. 61801

Introduction T h e f o c u s o f this p a p e r will b e o n the results of experimental studies of the structural and electronic properties o f two types o f linear chain systems containing m e t a l atoms:

and

The results we h a v e obtained for the biscyclopentadienyl c o m p l e x e s , w h i c h are recent a n d of a substantially m o r e preliminary nature, will be described first. A part o f o u r earlier work on the infinite linear tribridged chain systems will t h e n be described partly by way of introduction to the following two p a p e r s by Holt and McPherson on their studies of these materials. Biscyclopentadienyl

Titanium

Derivatives

T h e r e d u c t i o n c h e m i s t r y o f T i ( l V ) h a l i d e s was f i r s t e x t e n s i v e l y s t u d i e d as p a r t o f t h e development o f Z i e g l e r - N a t t a c a t a l y s t s i n t h e l a t e 1950 s. I t was recognized t h a t an important feature o f t i t a n i u m ( i l l ) chemistry i s t h e a b i l i t y of T i ( l l l ) t o a c t as a Lewis 1

142

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

11.

SEKUTOwsKi E T A L .

Linear

Chain

143

Complexes

a c i d , a n d one c a n o b t a i n a v a r i e t y o f c o m p l e x e s d e p e n d ­ i n g upon t h e s o l v e n t , r e d u c i n g agent and a n i o n . For example, t h e r e d u c t i o n o f C p T i C l w i t h Zn p r o c e e d s ac­ cording to: 2

2Cp TiCl

+

2

ZnCl

2 C

} or

2Cp TiCl 2

+

2

2

H

i

e

[Cp TiCl] ZnCl 2

2

2

Zn DME

[Cp TiCl] 2

+

2

ZnCl

[Cp Ti(DME)] [Zn Cl ] 2

2

2

2

6

[Cp TiCl] 2

+ 2

DM T h e 2:1 c o m p l e x was f i r s t r e p o r t e d b y S a l z m a n n i n 1968 (JL). T h e 1:1 complex has not been p r e v i o u s l y r e ­ p o r t e d a n d was o b t a i n e d b y a d d i t i o n o f d i m e t h o x y e t h a n e t o a s o l u t i o n o f t h e 2:1 complex. The s t r u c t u r a l c h e m i s t r y o f t h e s e mixed m e t a l T i ( l l l ) compounds i s i n t e r e s t i n g . The m o l e c u l a r s t r u c ­ t u r e o f t h e 1:1 z i n c c o m p l e x a s p r e p a r e d w i t h d i m e t h ­ o x y e t h a n e was r e c e n t l y d e t e r m i n e d b y u s a n d i s shown i n f i g u r e 1. I t contains [Cp Ti(DME)] c a t i o n s and the f i r s t example o f t h e Z n C l " a n i o n , w h i c h i s i s o e l e c tronic with Ga Cl . I t w i l l be i m p o r t a n t t o o u r l a t e r d i s c u s s i o n t o h a v e some i d e a o f w h a t one m i g h t e x p e c t f o r the i n t e r m o l e c u l a r magnetic i n t e r a c t i o n s of b i s cyclopentadienyl d systems. We h a v e e x a m i n e d t h e mag­ netic s u s c e p t i b i l i t y of [Cp Ti(DME)] [Zn Cl ]·C H to τ.5°Κ and found t h a t t h e i n t e r m o l e c u l a r exchange i s l e s s t h a n 1 cm"" . The m o l e c u l a r s t r u c t u r e o f t h e dibenzene s o l v a t e o f t h e 2:1 compound was d e t e r m i n e d b y u s a n d i n d e ­ p e n d e n t l y b y V o n k (_2). O u r c o o r d i n a t e s a r e shown i n f i g u r e 2. The p a r a m a g n e t i c m o l e c u l e s a r e s e p a r a t e d from each o t h e r i n t h e l a t t i c e by benzene m o l e c u l e s . T h e Z n - C l - T i a n g l e i s 90° w i t h i n e x p e r i m e n t a l e r r o r , w h i l e t h e C l - T i - C l a n g l e i s 8 l . 9 ( l ) . The T i - Z n - T i a n g l e i s 173.2(l)° and t h e T i - T i d i s t a n c e i s 6 820(5)S. An obvious q u e s t i o n o f i n t e r e s t i s whether or not t h e r e i s any e v i d e n c e o f exchange c o u p l i n g between t h e two d m e t a l atoms w h i c h a r e s e p a r a t e d by t h e diamagn e t i c z i n c atom. T h e r o t a t i o n o f t h e two C p T i g r o u p s t h r o u g h 90° w i t h r e s p e c t t o e a c h o t h e r s u g g e s t s t h a t any exchange pathway t h r o u g h t h e z i n c atom w h i c h i n ­ volves zinc o r b i t a l s which are orthogonal with respect t o a 90° r o t a t i o n a b o u t t h e l o n g o r t h o g o n a l a x i s o f t h e 2

2

2

2

6

6

1

2

2

2

6

e

2

1

#

1

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

6

144

EXTENDED

Figure

2.

INTERACTIONS

Molecular structure of [Cp TiCÏ] ZnCl Solvent molecules not shown. É

É

t

BETWEEN

METAL

· 2C H . R

6

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

11.

SEKUTOwsKi E T A L .

Linear

Chain

145

Complexes

m o l e c u l e must be f e r r o m a g n e t i c . Thus, exchange t h r o u g h ρ , ρ or s p h y b r i d o r b i t a l s on t h e z i n c atom would re­ sult i n ferromagnetic coupling. T o o u r k n o w l e d g e , t h e r e a r e no k n o w n e x a m p l e s o f l i n e a r o r n e a r l y l i n e a r t r i n u c l e a r m e t a l l i c systems i n w h i c h 1,3 m a g n e t i c e x c h a n g e h a s b e e n d e t e c t e d t h r o u g h a d i a m a g n e t i c m e t a l atom. S i n n (3) has r e p o r t e d on t h e m a g n e t i c i n t e r a c t i o n s o f a number o f t r i n u c l e a r s y s t e m s o f t h e t y p e shown b e l o w i n w h i c h t h e a p p r o x i m a t e a r ­ rangement o f t h e m e t a l atoms i s a n i s o s c e l e s t r i a n g l e 3

f

w h e r e M = Cu a n d M i s a v a r i e t y of metals including Mg. No 1,3 m a g n e t i c c o u p l i n g was d e t e c t e d , a l t h o u g h i t s h o u l d be p o i n t e d o u t t h a t t h e m a g n e t i c d a t a were measured o n l y t o l i q u i d n i t r o g e n t e m p e r a t u r e and t h e a b s e n c e o f m a g n e t i c e x c h a n g e was b a s e d o n t h e m a g n i ­ t u d e o f t h e W e i s s c o n s t a n t , Θ. S t u d i e s o f the tempera­ t u r e dependence o f t h e magnetic s u s c e p t i b i l i t y o f Ni (acac) h a v e shown t h a t a n a n t i f e r r o m a g n e t i c ex­ change between t h e t e r m i n a l n i c k e l atoms v i a t h e p a r a ­ m a g n e t i c n i c k e l ( I I ) c e n t r a l atom i s n e c e s s a r y t o f i t the experimental data Q). T h e 2:1 t i t a n i u m - z i n c c o m p l e x e s d o i n f a c t e x ­ h i b i t a n t i f e r r o m a g n e t i c b e h a v i o r a s shown b y t h e mag­ n e t i c s u s c e p t i b i l i t y d a t a ( f i g u r e 3) f o r t h e compound 3

6

[Cp TiBr] ZnBr *2C H 2

2

These d a t a were f i t by the s i n g l e t - t r i p l e t system:

X

m

= |^(τ?θ) t

1

+

x

/

3

2

Van

e x

P

6

6

Vleck

expression f o r a

(-2JAT)]'

1

The d a t a were measured a t a m a g n e t i c f i e l d g a u s s w i t h a Zeeman e n e r g y o f ^ 1 cm" . 1

+ Να o f 10 Κ In Table

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

1

146

EXTENDED

INTERACTIONS

BETWEEN METAL

IONS

2.00 r

Figure 3. Temperature depend­ ence of molar susceptibility and effective moment per titanium (BM) for lCp TiBr] ZnBr, · 2C H . Diamagnetic correction of —483 X lOr c.g.s. was applied. Theoretical curves cakuhted using: J = —15.66 cm , g = 1.94, θ 1.37°K, Να = 260 Χ lOr c.g.s. g

t

6

6

6

1

6

300

MO DIAGRAMS* FOR M(h -C H ) CI 5

5

0

ΠΊ4ρ

Tv4p

Figure 4. Molecular orbital di­ agrams for d° and d biscyclopentadienyl systems 1

Ti

d

v

V

cr

* ARROWS DESIGNATE NUMBER OF ELECTRONS IN HOMO

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

5

2

2

11.

SEKUTOwsKi E T A L .

Linear

Chain

Complexes

147

are the r e s u l t s of a s i m i l a r a n a l y s i s f o r the c h l o r i d e and f o r t h e d i m e r i c s p e c i e s ; [ C p T i X ] , X = CI, B r as r e p o r t e d b y C o u t t s , W a i l e s and M a r t i n (5). Martin (6) 2

Table

2

1 x

J(cm" ) [Cp TiCl] ZnCl -2C H

e

[Cp TiBr] ZnBr «2C H

6

2

2

2

2

[Cp TiBr]

2

2

2

2

e

6

-15.66

•125

a

From r e f e r e n c e

-8.93

J

l

J

B r

=

1.75

/ J ,C I =

I.60

*

/

J

C l

-78

a

[Cp TiCl]

a.

2

2

5

has p o i n t e d out t h a t r a n s f e r i n t e g r a l and hence t h e exchange c o u p l i n g s h o u l d become l a r g e r a s t h e e l e c t r o n e g a t i v i t y o f t h e a n i o n d e c r e a s e s a n d t h i s a p p e a r s t o be t h e c a s e f o r t h e t r i m e t a l l i c systems as w e l l . However, as i n d i c a t e d below, t h i s s i m i l a r i t y may v e r y w e l l be f o r t u i t o u s a n d , i n f a c t , t h e r e i s e v i d e n c e t h a t t h e exchange mechanisms i n t h e b i n u c l e a r and t r i n u c l e a r systems a r e q u i t e different. F o r t u n a t e l y , t h e r e i s some e x p e r i m e n t a l a n d t h e o r e c t i c a l information a v a i l a b l e concerning the e l e c t r o n i c d i s t r i b u t i o n about t h e m e t a l atom i n b i s c y c l o p e n t a dienyl d systems w h i c h l e a d s t o a p l a u s i b l e mechanism f o r the observed antiferromagnetic coupling. D a h l and P e t e r s e n ( 7 ) 5 ( 8 ) have r e c e n t l y d e m o n s t r a t e d by s i n g l e c r y s t a l e s r e x p e r i m e n t s and p h o t o e l e c t r o n spectroscopy that the unpaired e l e c t r o n i n the d system, C p V C l , i s p r i m a r i l y i n a molecular o r b i t a l of a symmetry w h i c h i s 3-5 ev below t h e l o w e s t u n o c c u p i e d molecular o r b i t a l ( f i g u r e 4). The u n p a i r e d e l e c t r o n d e n s i t y i s p r i m a r i l y i n a metal o r b i t a l which i s perpendicular t o t h e t w o f o l d symmetry a x i s o f C p V C l and i n t h e p l a n e o f t h e VC1 group. A s m a l l e r amount o f u n p a i r e d d e n s i t y i s i n a d o r b i t a l which l i e s along the b i s e c t o r of t h e C l - V - C l g r o u p ( T a b l e 2). T h e same o r d e r i n g o f l e v e l s was o b t a i n e d t h e o r e t i c a l l y f o r C p T i C l , a n d i t seems l i k e l y t h a t a s i m i l a r e l e c t r o n i c d i s t r i b u t i o n about t h e t i t a n i u m atom w i l l p r e v a i l i n [ C p T i X ] Z n X (X = C I , B r ) . I n f i g u r e 5* two a n t i f e r r o m a g n e t i c e x c h a n g e p a t h ways a r e shown w h i c h a r e c o n s i s t e n t w i t h t h e s y m m e t r y o f t h e m o l e c u l e and w i t h t h e t h e o r e t i c a l and e x p e r i m e n t a l r e s u l t s o f D a h l and P e t e r s e n . B o t h a r e o f symmetry b i n the p o i n t group D . A d o r b i t a l can a l s o 1

1

2

2

x

2

2

2

2

2

2

2

2

h

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974. 3.3

67.β

Cp

4.9

69.5

M

% Orbital

(a

y

22.3

z

0.3

0.4

ei (Humo) (Homo)

4.4

2

6.1

e

L. D a h l and J . P e t e r s e n P r i v a t e Communication

0.2

0.3

8-2

symmetry)

0.8

p

x

1-6.1

Cl

2

0.9

Character

Table

SEKUTOWSKi E T A L .

Linear

Chain

Complexes

x

Figure

5.

Exchange pathways for tallic Ti(III) complexes

trime-

0.40 r 0.35 h

ooo '

I

1

OO

'

600

'

'

I

1200

I

180.0

1

1

1

240.0

1

300.0

Degrees Kelvin Figure 6. Experimental data (A) and theoretical curve for tem­ perature dependence of molar susceptibility of [Cp TiCl] BeCk ' 2C H . Parameters used in curve generation: J = —6.89 cm' , g = 1.91, θ = 1.87°K, Να = 260 Χ ΙΟ c.g.s. t

6

1

2

G

6

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

150

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

be u s e d r a t h e r t h a n a ρ o r b i t a l g i v i n g a m o l e c u l a r orbital of a symmetry ( D . ) . We n o t e i n p a s s i n g t h a t the combination a t t h e bottom o f t h e f i g u r e which i n ­ v o l v e s t h e more h i g h l y p o p u l a t e d d o r b i t a l (d i n the C n o t a t i o n o f Dahl and Petersen) i s only p o s s i b l e i f a n s o r d o r b i t a l i s a v a i l a b l e on t h e c e n t r a l atom. I n order t o determine i ft h epresence o f a d o r b i t a l i s e s s e n t i a l f o r t h e presence o f magnetic exchange i n t h i s s y s t e m , t h e compound [ C p T i C l ] B e C l * 2 C H was p r e p a r e d and r e c r y s t a l l i z e d from a benzene s o l u t i o n . I t i s isos t r u c t u r a l w i t h t h e z i n c compounds a n d h a s a t e m p e r a ­ t u r e d e p e n d e n t m a g n e t i c s u s c e p t i b i l i t y , w h i c h i s shown in^figure_6 The value o f t h e coupling constant (-6.89 cm" ) i s comparable t o t h a t o f t h e z i n c c h l o ­ r i d e compound. I o r b i t a l s on the c e n t r a 1,3 e x c h a n g e . A s l i g h t l y d i f f e r e n t t y p e o f one d i m e n s i o n a l c h a i n system i s o b t a i n e d w i t h l a r g e r metal i o n s i n t h e central position. With Mn(ll) i ntetrahydrofuran ( T H F ) , t h e s t r u c t u r e , a s s h o w n i n f i g u r e 7, c o n t a i n s a s i x c o o r d i n a t e c e n t r a l m e t a l atom. The spin state cor­ r e l a t i o n diagram f o r a d - d - d s y s t e m ( f i g u r e 8) s u g ­ g e s t s two p o s s i b l e ground s t a t e s f o r < 1* i.e., forJ < J . Here, S represents the t o t a l s p i n s t a t e o f t h e s y s t e m a n d S* i s t h e s p i n s t a t e o b ­ t a i n e d b y c o u p l i n g t h e 1,3 a t o m s . J has been abbrevi­ a t e d as J . T h e (3/2, l ) ground s t a t e w i l l be o b t a i n e d i n t h e above range f o r a l l J < 0, w h i l e t h e (7/2,1) s t a t e w i l l be l o w e s t i n e n e r g y f o r J > 0. T h e e x ­ perimental μ curve f o r [ C p T i C l ] M n C l ( T H F ) i s shown i n f i g u r e 9 . The value o f M extrapolates t o ^ ~ 3 . 8 BM a t 0 ° K w h i c h i s w i t h i n e x p e r i m e n t a l e r r o r o f t h e s p i n o n l y v a l u e o f 3 . 8 7 BM f o r t h r e e u n p a i r e d e l e c ­ t r o n s and a q u a r t e t ground s t a t e , i . e . , J must be n e g a t i v e w i t h a n t i f e r r o m a g n e t i c c o u p l i n g between t h e t i t a n i u m a n d manganese atoms. I t was p o s s i b l e t o o b ­ t a i n an approximate value f o r J o f -o c m " b y f i t t i n g the experimental data, but J c o u l d n o t be determined w i t h any a c c u r a c y s i n c e o n l y s m a l l changes i n t h e c a l c u l a t e d magnetic s u s c e p t i b i l i t i e s over a narrow temperature range r e s u l t e d from l a r g e changes i n J . Zero f i e l d s p l i t t i n g a t t h e d i o n has been neglected i n t h e above c o n s i d e r a t i o n s . P o l y c r y s t a l l i n e e s r m e a s u r e m e n t s h a v e b e e n made a n d i n d i c a t e a z e r o f i e l d s p l i t t i n g o f 0.1 c m " f o r t h i s m a t e r i a l . Further studies o f c^-c^-d coupling arei n progress. I n a d d i t i o n t o t h eb r i d g i n g groups and t h e c e n t r a l m e t a l atom, a n o t h e r p a r a m e t e r w h i c h c a n i n f l u e n c e t h e exchange c o u p l i n g i n b i s c y c l o p e n t a d i e n y l d complexes x

2

2

2

2

2

2

e

6

#

1

1

5

1

IJ13/J12I

f

1

3

1

2

1

f

f

1

2

1

2

2

2

2

2

2

e f f

1

2

1

1

1

2

3

1

5

1

1

1

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

3

SEKUTOwsKi E T A L .

Figure

7.

Molecular

Linear

Chain

Complexes

structure of [Cp TiCl] MnCh(THF) . soids shown at 50% probability level. t

g

2

Thermal

ellip-

Figure 8. Spin state correlation diagram for (P-cP-d systems. S' is total spin state, S* is spin state obtained by coupling only the terminal paramagnetic centers, J is the exchange integral between terminal and central metals, and ] is exchange integral between terminal metals. 1

u

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

152

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

i s the c y c l o p e n t a d i e n y l r i n g . The m a g n e t i c s u s c e p t i b i l i t y data f o r [ C p T i C l ] shows a s m a l l a n o m a l y j u s t above the t r a n s i t i o n temperature which M a r t i n suggests may be due t o t h r e e d i m e n s i o n a l c o o p e r a t i v e i n t e r a c t i o n s (5)*(9). The most l i k e l y mechanism f o r t h e t h r e e dimensional c o o p e r a t i v e i n t e r a c t i o n would i n v o l v e the hindered r o t a t i o n of the c y c l o p e n t a d i e n y l groups. Do c o o p e r a t i v e p h e n o m e n a o f t h i s n a t u r e h a v e a n y e f f e c t on m a g n e t i c exchange i n t h i s m o l e c u l e o r i n t h e t r i nuclear molecules? When t h e m o n o - m e t h y l s u b s t i t u t e d derivative, [(MeCp) TiCl] i s e x a m i n e d , one f i n d s t h e r e s u l t s shown i n f i g u r e 10 f o r t h e m a g n e t i c s u s c e p t i bility. T h e r e i s more t h a n a t w o f o l d i n c r e a s e i n t h e v a l u e of J t o ^166 cm"" . No s t r u c t u r a l d a t a h a v e b e e n reported for [ C p T i C l ] however a model b a s e d on t h e structural results s t r o n g l y suggests tha be c o u p l e d v i a i n t r a m o l e c u l a r i n t e r a c t i o n s b e t w e e n c y c l o p e n t a d i e n y l r i n g s o n t h e same t i t a n i u m a t o m a n d b e t w e e n c y c l o p e n t a d i e n y l r i n g s on t h e d i f f e r e n t t i t a n i u m atoms w i t h i n a d i m e r . We w o u l d s u g g e s t t h a t t h e mean T i - T i d i s t a n c e w i t h i n a d i m e r f o r t h e v a r i o u s rotameric isomers which are obtained a t h i g h temperat u r e w i l l p r o b a b l y be g r e a t e r t h a n t h a t f o r t h e g r o u n d s t a t e low t e m p e r a t u r e r o t a m e r i c i s o m e r . I n t h e compound [ ( M e C p ) T i C l ] , t h e m e t h y l group " l o c k s i n " t h e ground s t a t e isomer, i . e . , r a i s e s the p o t e n t i a l b a r r i e r to r o t a t i o n . T h i s h a s some i n t e r e s t i n g i m p l i c a t i o n s . I t s u g g e s t s , f o r example, t h a t the o r d e r o f Br I ^ C1 y M a r t i n (5) f o r [ C p T i X ] c o m p l e x e s i s aue t o t h e a n o m a l o u s b e h a v i o r o f X = C I and not X = I , i . e . , t h e i n c r e a s i n g m a g n e t i c exchange does not f o l l o w d e c r e a s i n g e l e c t r o n e g a t i v i t y as noted above, but i n s t e a d f o l l o w s a decrease i n the T i - T i distance. The p r e d o m i n a n t mechanism f o r exchange w o u l d t h e n be a d i r e c t m e t a l - m e t a l i n t e r a c t i o n . T h i s i s cons i s t e n t with the unpaired s p i n d e n s i t y d i s t r i b u t i o n f o u n d by D a h l , w h i c h , however, does n o t e x c l u d e superexchange v i a the b r i d g i n g l i g a n d s . A d d i t i o n a l magnetic s u s c e p t i b i l i t y , h e a t c a p a c i t y , and s t r u c t u r a l s t u d i e s a r e now i n p r o g r e s s a t t h e U n i v e r s i t y o f I l l i n o i s i n o r d e r t o more c l e a r l y d e f i n e t h e a b o v e a n d r e l a t e d s y s tems. I f t h e a b o v e m o d e l i s v a l i d , one w o u l d n o t e x p e c t a s i g n i f i c a n t d i f f e r e n c e i n the magnetic exchange upon methyl s u b s t i t u t i o n i n the t r i n u c l e a r [Cp TiCl] ZnCl compounds. The m a g n e t i c s u s c e p t i b i l i t y data f o r [ ( M e C p ) T i C l ] Z n C l i s shown i n f i g u r e 12 a n d g i v e s a v a l u e o f J o f -7.3 cm"" w h i c h i s comparable to that of the unsubstituted m a t e r i a l . 2

2

2

2

1

2

2

J

>

J

J

o

2

b

s

e

r

v

e

d

b

2

2

2

2

2

2

2

1

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2

11.

SEKUTowsKi E T A L .

Linear

Chain

Complexes

Figure 9. Temperature dependence of effective moment (BM) per trimer of [Cp TiCl] MnCl (THF) . Diamagnetic correction of —416 X 10~ c.g.s. was applied. 2

2

2

2

6

002 h QOO « QO

ι

I

6O0

'

1

I

I

120.0

1800

I

I

24O0

1

1 3000

°K Figure 10. Temperature dependence of molar susceptibility of [(MeCp) TiCl] . Theoretical curve calculated using J = —166 cm , θ = 0.87°Κ,g = 2.06, Να = 260 Χ ΙΟ c.g.s. 2

2

1

Figure 11. Intramolecular of cyclopentadienyl rings in complexes

6

interactions [Cp TiX] 2

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

154

EXTENDED

INTERACTIONS

BETWEEN METAL

IONS

F i n a l l y , we would l i k e t o note t h a t numerous o t h e r c o m b i n a t i o n s o f m e t a l i o n s ( e . g . , z i r c o n i u m and vanadium) and b r i d g i n g group ( e . g . , - N R j -PR * -OR, -SR) a r e p o s s i b l e i n t h i s s e r i e s and a s y s t e m a t i c ex­ p e r i m e n t a l s u r v e y o f t h e exchange i n t h e s e compounds can be e x p e c t e d t o p r o v i d e , a t t h e v e r y l e a s t , a n em­ p i r i c a l u n d e r s t a n d i n g and c l a s s i f i c a t i o n o f t h e e l e c ­ t r o n i c c o u p l i n g o f t r i a t o m i c m e t a l systems. 2

RMX

3

2

Complexes

Compounds o f t h e t y p e RMX (R = u n i p o s i t i v e c a t i o n , M = divalent t r a n s i t i o n metal c a t i o n , X = halo­ gen) form a l a r g e c l a s s o f one d i m e n s i o n a l svstems w i t h the general s t r u c t u r dimensional propertie s t r a t e d by l o w t e m p e r a t u r e nmr and n e u t r o n d i f f r a c t i o n s t u d i e s ( l l ) . The f a c t t h a t one i s n o t d e a l i n g w i t h a c o m p l e t e l y i s o l a t e d one d i m e n s i o n a l c h a i n i s e v i d e n t i n t h a t t h e R group c a n be used t o "tune" t h e e l e c t r o n i c p r o p e r t i e s o f the t r a n s i t i o n metal as i l l u s t r a t e d i n T a b l e 3. Thus, one f i n d s t h a t t h e r e i s a change o f 0.141 i n t h e n i c k e l - n i c k e l d i s t a n c e g o i n g from R = ( C H ) N t o R = Κ . The c o r r e s p o n d i n g change i n 10 Dq i s 750 cm" and t h e C u r i e - W e i s s c o n s t a n t changes from 0 t o -112°K ( R b N i C l ) . There a r e few, i f any, systems i n w h i c h t h e f i r s t c o o r d i n a t i o n sphere o f t h e m e t a l atom c a n be s o s y s t e m a t i c a l l y and s u b t l y v a r i e d . An i m p o r t a n t q u e s t i o n w h i c h has become o b v i o u s t o many p e o p l e d u r i n g t h e p a s t y e a r i s , "Can one p r e d i c t i f a one d i m e n s i o n a l s t r u c t u r e s u c h a s t h a t shown i n f i g u r e 13 w i l l be o b t a i n e d ? " We have examined t h i s q u e s t i o n from a s t r i c t l y e m p i r i c a l p o i n t o f view w i t h t h e r e s u l t s shown i n f i g u r e 14 and T a b l e 4. There a r e a t l e a s t f i v e d i f f e r e n t configurât i o n s adopted b y RMX compounds ( f i g u r e 1 4 ) : p r i m i t i v e c u b i c , h e x a g o n a l , and v a r i o u s c o m b i n a t i o n s o f h e x a g o n a l (one d i m e n s i o n a l ) and p r i m i t i v e c u b i c packed s t r u c ­ t u r e s . The p u r e l y h e x a g o n a l c o n f i g u r a t i o n i s t h e 2L, one d i m e n s i o n a l s t r u c t u r e d i s c u s s e d p r e v i o u s l y . 2L r e f e r s t o the c r y s t a l l o g r a p h i c repeat d i s t a n c e i n t h e l i n e a r c h a i n d i r e c t i o n . The r a t i o o f h e x a g o n a l ( l i n e a r ) t o c u b i c p a c k i n g i n c r e a s e s f r o m 6 L t o 4L t o 9L. Remembering t h i s , t h e n , a n e x a m i n a t i o n o f known RMX s t r u c t u r e s r e v e a l s t h a t t h e h e x a g o n a l 1-d con­ f i g u r a t i o n i s f a v o r e d by: ( l ) I n c r e a s e d CFSE ( c r y s t a l f i e l d s t a b i l i z a t i o n energies) 2) L a r g e r X groups 3/ L a r g e r R groups ( t o a p o i n t ; ( C 5 H 5 ) N 3

3

4

1

3

3

3

+

4

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

SEKUTOwsKi E T A L .

Linear

Chain

Complexes

0.32 ρ 0.28 0.24 ^ χ

0.20 I

S 0.16 25 §" 0.121

C 3O

CO

0.08 004 0.00J

OO

600

1200

1800

Degrees Kelvin Figure 12. Temperature dependence of molar susceptibility of [(MeCp) TiCl] ZnCl . Theoretical curve calculated using J = -7.26cm , g = 1.92, θ = 0.69°Κ, Να = 260 Χ ΙΟ c.g.s. B

g

a

1

6

Figure 13. One-dimensional structure RMX compounds. R = (CH ),,N\ M = and X = Cl (10). 3

S

of Ni

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

4

3

3

CsNiBr

RbNiBr

3

(CH ) NNiBr

3

3

RbNiCl

KNiCl

3

4

CsNiCl

3

3

(CH ) NNiCl

3

3.10(1)

3.12(1)

11,300 11,300

6600

11,600

6500

6300

7100

2.91(1)

3.17(1)

(14),(15)

-112

12,400

7000

2.9^(1) 12,300

(14),(15)

-76

12,600

6800

2.96(1)

-156

(14),(15)

-101 ( 1 4 ) , ( 1 5 )

0 (13)

13,450

6350

C u r i e Weiss Θ(°Κ)

3.054(5)

- 1

15B'(cm )

-1

10Dq(cm )

Ni-Ni distance Ά

Table 3

11.

SEKUTOwsKi E T A L .

Linear

Chain

Table

Complexes

4

Summary o f S t r u c t u r e s o f C s M X

3

Compounds

F

Cl

Br

I

(1.36)

(1.81)

(1.95)

(2.16)

—

2L (16)

2L

(17)

2L (18)

V(0.87)

—

2L (19)

2L (20)

2L (18)

Cr(0.84)

—

2L (21)

2L (22)

2L (18)

Mg(0.65)

a

Mn(0.8o)

6L ( 2 3

Fe(0.76)

6L

(25) 2L (26)

Co(0.78)

9L

(21) 2L (28)

Ni(0.78)

2L (2g) 2L (30)

2L (31) 4L (22)

9L

(24)

Cu(0.69)

—

2L (32)

Cd(0.97)

—

6L (32)

2

(20)

2L (20)

—

—

—

2L (18)

Summary o f S t r u c t u r e s o f RMC1 Compounds 3

(Me) N

Cs

Rb

K

(2.60)

(1.69)

(1.48)

(1.33)

4

Vo(0.87) Cr(0.84) Mn(0.80)

a

—

2L (19)

—

—

2L (16)

—

2L (34)

2L (19) —

9L (18)

6L (35)

tetragonal

Fe(0.76)

—

2L (25)

2L (25)

—

Co(0.78)

—

2L (28)

2L (36)

—

2L (30)

2L (32)

Ni(0.78) Cu(0.69) Cd(0.97) a.

2L (11) —

2L ( 3 3 )

T h e numbers e a c h atom.

2L (38)

—

6L (33)

—

i n parentheses

3L (20) 4L (39)

are the ionic

radius

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

EXTENDED

(<)

INTERACTIONS

BETWEEN

(d)

METAL

(e) Acta Crystallographica

Figure 14. Perspective projections of octahedra network in a unit cell of RMX complexes (15). (a) P. (b) 2L. (c) 6L. (d) 4L·. (e) 9L. S

2

V+/CsMqBr

3

Journal of Chemical Physics

Figure

15.

Ligand

hyperfine CsMgBr,

in the esr spectra and CsMgh (41)

of V * 2

doped

into

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

11.

SEKUTOwsKi E T A L .

Linear

Chain

159

Complexes 2

favors tetrahedral N i C l ~ . We have examined t h e e l e c t r o n i c p r o p e r t i e s o f t h e s e complexes by a v a r i e t y o f methods, and some o f t h e more r e c e n t work i n t h i s a r e a w i l l be d e s c r i b e d i n t h e f o l l o w i n g two p a p e r s . I n c l o s i n g , two i n t e r e s t i n g o b s e r v a t i o n s w h i c h we have made w i l l be b r i e f l y described. CsMgX s a l t s p r o v i d e a c o n v e n i e n t i s o s t r u c t u r a l d i a m a g n e t i c m a t r i x i n t o w h i c h t o dope t h e p a r a m a g n e t i c t r a n s i t i o n metal ions ( l 6 ) , ( 4 l ) . Covalency e f f e c t s are s t r i k i n g l y a p p a r e n t i n THe l i g a n d h y p e r f i n e as i t i s shown superimposed on t h e |-1/2 > |+1/2 > t r a n s i t i o n of the esr s p e c t r a of the V bromine and i o d i n e com­ p l e x e s ( f i g u r e 1 5 ) . The s p i n H a m i l t o n i a n p a r a m e t e r s , e x c l u d i n g t h e l i g a n d h y p e r f i n e t e n s o r s a r e shown i n T a b l e 5. The i n c r e a s evident i n both th c o n s t a n t s and i n t h e changes i n t h e g t e n s o r . I n f a c t , we o b s e r v e f o r t h e i o d i n e complex a v e r y u n u s u a l ex­ ample o f an e a r l y t r a n s i t i o n element i n a n e a r l y o c t a ­ h e d r a l f i e l d w i t h both g and g g r e a t e r t h a n t h e f r e e e l e c t r o n g f a c t o r o f 2.0023. T h i s i s q u a l i t a t i v e l y e x p l i c a b l e i n terms o f M c G a r v e y s t h e o r y ( 4 2 ) , ( 4 3 ) o f c o v a l e n c y f o r d systems i n w h i c h t h e l i g a n d m o l e c u l a r o r b i t a l c o e f f i c i e n t s enter i n t o the expression f o r the g t e n s o r , w e i g h t e d by t h e l i g a n d s p i n o r b i t a l c o u p l i n g constant. The l a s t e x p e r i m e n t a l o b s e r v a t i o n t h a t w i l l be d e s c r i b e d i s i l l u s t r a t e d by t h e s i n g l e c r v s t a l e l e c ­ t r o n i c a b s o r p t i o n s p e c t r u m o f C s C r C l (21) ( f i g u r e 16). The c o n c e n t r a t i o n dependence o f t h i s s p e c t r u m i n C s M g C l i s a l s o shown. Two f e a t u r e s o f i n t e r e s t i n t h e s p e c t r u m a r e ( l ) a band a t 22,000 cm" w h i c h i s not ex­ p l a i n e d by l i g a n d f i e l d c a l c u l a t i o n s and (2) an en­ hanced i n t e n s i t y o f s p i n f o r b i d d e n t r a n s i t i o n s by ap­ p r o x i m a t e l y an o r d e r o f magnitude. The p o l a r i z a t i o n o f t h e 22,000 cm" band i s p a r t i c u l a r l y s t r o n g and i t s t e m p e r a t u r e dependence i s not t h a t e x p e c t e d f o r a v i b r o n i c mechanism. The e x p l a n a t i o n f o r t h i s e f f e c t was f i r s t p r o p o s e d by D e x t e r (44) and l a t e r e l a b o r a t e d upon by Day (45) and o t h e r s . In i t s s i m p l e s t form ( f i g u r e 17), a s i n g l e photon r e s u l t s i n t h e f o r m a t i o n o f e i t h e r two e x c i t o n s o r an e x c i t o n and a magnon. I n b o t h c a s e s , t h e t o t a l s p i n symmetry i s c o n s e r v e d . I n C s C r C l , the former r e s u l t s i n a band a t a p p r o x i m a t e l y t w i c e 10 Dq, t h e l a t t e r r e ­ s u l t s i n an enhanced a l l o w e d n e s s f o r t h e s p i n f o r b i d d e n q u i n t e t - t r i p l e t and q u i n t e t - s i n g l e t t r a n s i t i o n . I n summary, t h e i n f i n i t e one d i m e n s i o n a l complexes, RMX , p r o v i d e an u n u s u a l l y b r o a d and i n t e r e s t i n g c l a s s 4

3

->

2

fJ

x

1

3

3

3

1

1

3

3

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

h

(77°K)

(297°K)

4

l

+ 70 + 2.

4

+ 74. + 1.

B(xl0 cm"

x

+ 70. + 2.

4

A ( x l 0 c m " ) = + 76. + 1.

4

+ 2 + 65. + 2

+ 65.

+ 208O. + 30

2.035 + .002

1.999 + .002

= 1.9760 + .001

2.038 + .002

+ 2.

+ 67.

1.996 + .002

+ 2.

+ 67.

+ 2 1 6 0 + 50.

2.04 + .01

1.9740 + .0006

+ 128Ο + 15.

u

+ 70. + 2.

+ 1320. + 15.

1.999 + .002

3

2.04 + .01

CsMgI

+ 70+2.

+ 1.

+ 4.

3

1.996 + .002

CsMgBr

+ 74. + 1.

= + 76.

+ 961.

= 1.9750 + .0006

1.9730 + .0006

3

Parameters

D ( x l 0 c m " *) = + 8 5 8 . + 7.

g

B(xl0 cm" )

4

A(xl0 cm"

4

D(xi0 cm"

iι

g,,

CsMgCl

Spin Hamiltonian

Table 5

11.

SEKUTOWSKi E T A L .

Linear

Chain

Complexes

161

6.4% Cr^CsMgCI

3

5.0 ί

10.000

15.000

25.000

20.000

Figure 16. Electronic absorption spectrum at 77°K of CsCrCh and CsCrCls doped into CsMgCl (16). Spectra with Ε polarized parallel and perpendicular to the crystallographic axis

29% Cr^CsMgCl

s

5.000

5.000

10,000

10.000

6.000

20.000

25.000

15.000

20,000

25.000

mole

1

(cm ) 1

M°=l S =2 a

ΙΙΙΦ

m-

Cr'

Cr'

ΔΜ;=+Ι

ΔΜ =0

ΔΜ =0

8

8

AS=0

ν - · - exciton • exciton

Figure

17.

Simultaneous

T

ΔΜΪ=*1

ΔΜ!=

pair

AS=0 hi/ w r * - e x c i t o n • magnon

excitation

model

(44, 45)

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

cm . 1

162

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

of m a t e r i a l s f o r t h e i n v e s t i g a t i o n o f c o o p e r a t i v e electronic effects. Acknowledgment. The s u p p o r t o f t h e N a t i o n a l S c i e n c e F o u n d a t i o n under G r a n t s NSF-GH-33634 and GP-31016X a r e g r a t e f u l l y acknowledged. The e x p e r i m e n t a l m a g n e t i c s u s c e p t i b i l i t y measurements were made w i t h t h e g r e a t l y appreciated a s s i s t a n c e o f P r o f e s s o r David Hendrickson of t h e U n i v e r s i t y o f I l l i n o i s . Literature Cited 1. 2. 3. 4. 5. 6.

7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

Salzmann, J. J., H e l v . Chim. A c t a ( 1 9 6 8 ) , 5 1 , 5 2 6 . Vonk, C. G., J. Cryst. M o l . Struct. ( 1 9 7 3 ) , 3, 201. G r u b e r , S. J., J. Chem. Phys. (1968), 49, 2183. G i n s b e r g , A. P., Martin, R. L., and Sherwood, R. C., I n o r g . Chem. ( 1 9 6 8 ) , 7 , 932. C o u t t s , R. S. P., W a i l e s , P. C., and Martin, R. L., J . O r g a n o m e t a l l i c Chem. (1973), 47, 375.

M a r t i n , R. L., in "New Pathways in I n o r g a n i c C h e m i s t r y , " Pages 175-231, E. A. U. E b s w o r t h , A. G. Maddock and A. G. S h a r p e , Cambridge P r e s s , London ( 1 9 6 8 ) . D a h l , L. F. and P e t e r s e n , J. L., private communication. P e t e r s e n , J. L. and D a h l , L. F., J. Amer. Chem. Soc. ( 1 9 7 4 ) , 9 6 , 2248. M a r t i n , R. L. and W i n t e r , G., J. Chem. Soc. (1965),

1965,

4709.

S t u c k y , G. D., A c t a C r y s t . ( 1 9 6 8 ) , B24, 330. Y e l o n , W. B. and Cox, D. Ε., Phys. Rev. B. ( 1 9 7 3 ) , 7 , 2024, and included r e f e r e n c e s . Goodgame, D. M. L. and Weeks, M. J., J. Chem. Soc. ( 1 9 6 4 ) , 1 9 6 4 , 5 1 9 4 . Asmussen, R. W. and Soling, Η., Z. A n o r g . A l l g e m . Chem. (1965), 203, 3. Asmussen, R. W. and B o s t r u p , Ε., A c t a Chem. Scand. (1957), 11, 745.

Li, Ting-i, S t u c k y , G. D. and McPherson, G., A c t a C r y s t . (1973), B29, 1330. McPherson, G. L., K i s t e n m a c h e r , T. and S t u c k y , G. D., J. Chem. Phys. ( 1 9 7 0 ) , 5 2 , 815. McPherson, G. L. and S t u c k y , G. D., J. Chem. Phys. (1972),

57,

3780.

Li, Ting-i, S t u c k y , G. D. and McPherson, G. L., A c t a C r y s t . ( 1 9 7 3 ) , B 2 9 , 1330.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

11.

SEKUTOWSKI

19. 20. 21.

22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45.

E T AL.

Linear

Chain

Complexes

163

Seifert, H. J. a n d Ehrlich, P., Z. A n o r g . A l l g e m . Chem. (1959), 302, 286. Unpublished results. M c P h e r s o n , G. L . , K i s t e n m a c h e r , T. J., Folkers, J. B. a n d S t u c k y , G. D., J. Chem. P h y s . (1972), 57, 3771. Li, Ting-i a n d S t u c k y , G. D., Inorg. Chem. (1973), 12, 4 4 1 . Zalkin, Α . , L e e , K. a n d T e m p l e t o n , D. H., J. Chem. P h y s . ( 1 9 6 2 ) , 37, 697. G o o d y e a r , J. a n d K e n n e d y , D. J., Acta Cryst. (1972), B28, 1640. Kestigian, M., Leipzig, F . D., Croft, W. J. a n d Guidoboni, R. Che (1966) 5 1462 Seifert, H. J. Chem. ( 1 9 6 6 ) , , L o n g o , J. M. a n d Kafales, J . Α., J. Solid State Chem. ( 1 9 6 9 ) , 1, 1 0 3 . Seifert, H. J., Z. A n o r g . A l l g e m . Chem. (1960), 307, 137. Babel, D., Z. Naturforsch. (1965), 20A, 165. T i s h c h e n k o . G. N., Tr. Inst. Krist. A k a d . Nauk SSSR (1955), 11, 9 3 . S t u c k y , G. D., D'Agostino. S. a n d M c P h e r s o n , G. I., J. Amer. Chem. S o c . (1966), 8 8 , 4823. H e x a g o n a l , with a = 12.56, c = 11.56Å. See R e f . 27. Siegel, J . a n d Gebert, Ε . , Acta Cryst. (1964), 17, 790. M o r o s i n , B. a n d G r a e b n e r , E . J. Acta Cryst. (1967), 2 3 , 766. Seifert, H. J. a n d K o k n a t , F. W., Z. A n o r g . Allgem. Chem. (1965), 341, 269. E n g b e r g , A. a n d Soling, H., Acta Chem. S c a n d . (1967), 21, 168. A s m u s s e n , R. W. a n d Soling, H., Z. A n o r g . A l l g e m . Chem. (1956), 2 0 3 , 3 . S c h l u e t e r , A . W., J a c o b s o n , R. A. a n d R u n d l e , R. E . , Inorg. Chem. (1966), 5, 277. Willett, R. D., D w i g g i n s , C., K r u h a r d , R. a n d R u n d l e , R. E . , J. Chem. P h y s . ( 1 9 6 3 ) , 38, 2429. M o r o s i n , B., Acta Cryst. (1972), B28, 2303. M c P h e r s o n , G. L . , K o c h , R. C. a n d S t u c k y , G. D., J. Chem. P h y s . (1974), 6 0 , 1 4 2 4 . M c G a r v e y , B. R., J. Chem. P h y s . (1964), 41, 3743. M c G a r v e y , B. R., J . P h y s . Chem. (1967), 71, 51. D e x t e r , D. L . , P h y s . R e v . ( 1 9 6 2 ) , 126, 1962. D a y , P., Inorganica C h i m i c a Acta R e v i e w s ( 1 9 6 9 ) , 3, 8 1 .

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

12 Optical Properties of Linear Chains S. L. H O L T University of Wyoming, Laramie, Wyo.

82070

Compounds o f t h heavy alkali metal ion (Cs or Rb ) o r an organic cation s u c h as (CH3)4N+, M is a first row divalent transition m e t a l ion, and X is Cl¯, Br¯ o r I¯, display unusual magnetic behavior.(1,2) This b e h a v i o r i s predominately o n e - d i m e n s i o n a l and arises in large part because of the o n e - d i m e n s i o n a l m o l e c u l a r structure of these materials. F i g u r e 1 shows t h e chain-like con­ stitution exhibited by all compounds o f this type. These chains consist of face-sharing octahedra. The larger circles r e p r e s e n t the bridging halide ions while the smaller circles r e p r e s e n t the transition ions. As shown in Figure 2, t h e s e c h a i n s a r e physically separa­ ted by t h e cations, in this case, (CD ) N . These cations p r o v i d e the magnetic insulation between c h a i n s with the interchain distance and t h u s t h e strength of interchain interaction being d e p e n d e n t upon t h e size of cation, i.e., the larger the cation the smaller the interchain interaction. 3

4

+

Figure 3 shows t h e p a t h w a y o f t h e e x c h a n g e . As d r a w n , it s u g g e s t s t h a t t h e e x c h a n g e involves primarily the o v e r l a p of the dz orbital o f C a t i o n 1 w i t h t h e ρσ orbital of the bridging anion followed,by the inter­ action of the ρσ electrons w i t h t h e dz o f C a t i o n 2. Indeed, other orbitals may be involved as is schemat­ ically shown in Figure 4. In this c a s e , we h a v e s u p e r e x c h a n g e illustrated for b o t h 9 0 ° b o n d e d c h r o m i u m ( I I I ) i o n s and 90° b o n d e d iron(II) ions. In the upper p a r t o f the figure, we h a v e t h e c h r o m i u m ( I I I ) c a s e and h e n c e , a r e dealing with 3 d-electrons. The d-electrons on C a t i o n 1 a r e f o u n d in t orbitals, as a r e t h o s e on C a t i o n 2. An electron from the ρσ orbital of the ligand is shown as b e i n g virtually exchanged into the e orbitals o f C a t i o n 1. The coupling process of lowest energy requires t h a t the 2

2

2g

g

164

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

12.

HOLT

Linear

165

Chains

(hOl)

ORIENTATION

podia* j

.1200)

I «(IOO I (OOO #

I

NUCLEAR RECIPROCAL RECIPI LATTICE POINTS

Figure

1.

:(OOI)

(002)

I

MAGIN 'OETIC INTENSITY PLANES

ZONE BOUNDARY

A portion of the linear chain of an RMX. compound (adopted from Ref. l)

{

(003)4^

++

Figure

2.

Crystal structure

of (CDJMnCk

(adopted

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

from Réf. I)

166

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

Cation 2

Figure 3. The 90° exchange pathway between two interacting metal ions (adopted from Ref. 2)

Cotion I

Cation I Figure bonded

Anion

Anion

Cation 2

Cation 2

4. Superexchange pathway in (a) 90° C^-Cr *; (b) 90° bonded Fe +-Fe * (adopted from Ref. 2). 3

2

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

12.

HOLT

Linear

Chains

spin that

167

o f t h e v i r t u a l l y e x c h a n g e d e l e c t r o n be t h e same as i n the t2g s e t of C a t i o n 1. The P a u l i E x c l u s i o n P r i n c i p l e t h e n r e q u i r e s t h a t t h e o t h e r e l e c t r o n i n t h e ρσ o r b i t a l o f t h e l i g a n d h a v e a s p i n of o p p o s i t e s i g n ( i n t h i s c a s e down). Because t h e r e a r e no e l e c t r o n s i n t h e eg o r b i t a l s o f C a t i o n 2, o n l y one e x c h a n g e p r o c e s s i s p o s s i b l e , t h a t process t h a t g i v e s r i s e to a - J , i . e . , a n t i p a r a l l e l exchange c o u p l i n g b e t w e e n t h e ρσ e l e c t r o n o f t h e a n i o n and t2g e l e c t r o n s o f C a t i o n 2. The n e t e f f e c t i s t o a l l o w f e r r o m a g n e t i c coupling b e t w e e n C a t i o n 1 and C a t i o n 2 w i t h t h e o v e r a l l be­ h a v i o r shown by t h e compound b e i n g ferromagnetic. I n t h e l o w e r p a r t o f t h e f i g u r e , we h a v e t h e c a s e of 90° exchange betwee tw Fe2+ i o n s I thi t h i n g s are not so c l e a t h e ρσ e l e c t r o n o f eg o r b i t a l s o f C a t i o n 1 i s a n t i f e r r o m a g n e t i c i n n a t u r e . T h i s does not d i c t a t e the o v e r a l l s p i n a r r a n g e m e n t however. As we c a n s e e t h e r e e x i s t s t h e p o s s i b i l i t y for ferromagnetic as w e l l as a n t i f e r r o m a g n e t i c exchange b e t w e e n t h e e l e c t r o n i n t h e ρσ o r b i t a l o f t h e anion and t h e e l e c t r o n s i n t h e d - o r b i t a l s o f C a t i o n 2. The r e s u l t a n t m a g n e t i c b e h a v i o r d e p e n d s o n l y upon w h i c h i s s t r o n g e r , t h e c o u p l i n g b e t w e e n t h e e l e c t r o n i n t h e ρσ o r b i t a l and t h e eg e l e c t r o n s o r t h e c o u p l i n g b e t w e e n t h e ρσ e l e c t r o n and t h e t 2 g e l e c t r o n s . I f t h i s cou­ p l i n g i s s u f f i c i e n t l y s t r o n g to produce p r e f e r e n t i a l a l i g n m e n t and i n v o l v e s s e v e r a l c a t i o n c e n t e r s t h e n c o n d i t i o n s have been f u l f i l l e d f o r the c r e a t i o n of spin-waves. The d r a w i n g i n F i g u r e 5 i s a s c h e m a t i c d e p i c t i o n o f a s p i n - w a v e i n a one d i m e n s i o n a l ferromagnetically coupled system. As one c a n s e e , a s p i n d e v i a t i o n a t one end o f t h e c h a i n c a u s e s s p i n d e v i a t i o n s down t h e l i n e producing the "spin-wave". As a q u a n t u m o f e n e r g y c a l l e d a phonon e x c i t e s a v i b r a t i o n , a quantum of e n e r g y c a l l e d a magnon c a u s e s a s p i n - w a v e . I f one has i s o l a t e d c h a i n s w i t h i n a m a t e r i a l , magnons w i l l i n d u c e spin-waves i n these chains independent of each o t h e r . The p r e s e n c e o f m a g n e t i c c o u p l i n g and t h e induce­ ment o f s p i n - w a v e s c a n be shown e x p e r i m e n t a l l y through i n e l a s t i c - n e u t r o n - s c a t t e r i n g techniques. Figure 6 shows t h e v a r i a t i o n w i t h t e m p e r a t u r e o f t h e e x c i t a t i o n which i s a s s o c i a t e d w i t h the p r e s e n c e of spin-waves i n t h e compound ( C D 3 ) 4 M n C l 3 . As c a n be s e e n t h e s p i n - w a v e i s w e l l d e f i n e d at 1.9°K. T h i s d e f i n i t i o n d e c r e a s e s as one i n c r e a s e s t h e t e m p e r a t u r e t o 4 0 ° K . T h i s marked d e c r e a s e i n i n t e n s i t y i s t o be e x p e c t e d as t h e threedimensional o r d e r i n g t e m p e r a t u r e h a s b e e n shown t o be

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

168

EXTENDED

,., y

y

Figure

5.

y

y

γ

ψ ψ

INTERACTIONS

ψ

γ y

of a spin wave, (a) View in the ac plane, (adopted from Ref. 3).

Schematic

BETWEEN

METAL

y

IONS

y

y

(b) View in the ab plane

TMMC λ · 64 A EXCITATION

* ··· _J

I

I

•

—1

_l

I

I

I

I

L_

V

I

I

l_

Figure 6. Variation with tempera­ ture of the excitation at (0.3, 0,1.10), q,.* = 0.10 reciprocal lattice units compound is (CD ) ~NMnCl (adopted from Réf. 1).

L

Α

_l

_J 0.0

I 1.0

I

I 1 20 30 ENERGY (meV)

h

L.

1 4.0

L_ 5.0

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

A

12.

HOLT

Linear

Chains

169

0.84°K. T h u s , as one i n c r e a s e s t h e t e m p e r a t u r e above t h i s , c o n t i n u a l l y g r e a t e r d i s o r d e r and d e c o u p l i n g a l o n g the c h a i n s w i l l o c c u r . One w o u l d e x p e c t , as a g e n e r a l p h e n o m e n o n , t h e p r e s e n c e o f s p i n - w a v e s s h o u l d be l e s s e v i d e n t t h e h i g h e r Τ i s a b o v e TJJ . Our p r i m a r y i n t e r e s t h e r e w i l l be w i t h t h e o p t i c a l m a n i f e s t a t i o n of these spin-waves. F i g u r e 7 shows t h e v a r i o u s t y p e s o f e l e c t r o n i c p r o c e s s e s one i s l i k e l y t o encounter i n a m a g n e t i c a l l y coupled t r a n s i t i o n metal containing material. We s h a l l f o c u s o u r a t t e n t i o n p r i m a r i l y upon t h o s e t r a n s i t i o n s w h i c h a r e s p i n f o r b i d d e n i n n a t u r e , i . e . , those w h i c h undergo a change i n s p i n - m u l t i p l i c i t y , s i n c e i t i s here t h a t the e f f e c t s o f t h e s p i n - w a v e phenomenon on t h e e l e c t r o n i c t r a n s i ­ t i o n s w i l l be s e e n The f i r s t p a r o c c u r s when y o u h a v e a m a g n e t i c a l l y and o p t i c a l l y d i l u t e system i n which the s p i n - m u l t i p l i c i t y may change. T h i s i s a n o r m a l e x c i t a t i o n c a u s e d by a p h o t o n where the s u b l a t t i c e s a c t i n d e p e n d e n t l y o f each o t h e r . In t h e c a s e shown h e r e , we h a v e o n l y t h e e x c i t a t i o n o f an e l e c t r o n f r o m t h e g r o u n d s t a t e on s u b l a t t i c e A i n t o an e x c i t e d s t a t e w i t h t h e a c c o m p a n y i n g s p i n - f l i p . T h i s w i l l u s u a l l y be s e e n i n t h e s p e c t r u m o f a compound as a weak a b s o r p t i o n b a n d . The w e a k n e s s o f s u c h a t r a n s i t i o n a r i s e s b e c a u s e i t i s b o t h p a r i t y and s p i n f o r b i d d e n , AS^O. S u c h i s n o t t h e c a s e i n an o p t i c a l l y c o n c e n t r a t e d s y s t e m where c o o p e r a t i v e e x c i t a t i o n s such as we s e e i n t h e s e c o n d p a r t o f F i g u r e 7 may o c c u r . In t h i s c a s e , by e x c i t i n g an e l e c t r o n f r o m t h e g r o u n d s t a t e i n s u b l a t t i c e A and s i m u l t a n e o u s l y e x c i t i n g an e l e c t r o n w i t h t h e o p p o s i t e s p i n on s u b l a t t i c e B, we c a n c o n s e r v e t h e s p i n - a n g u l a r momentum, i . e . , AS=0. T h i s i s c a l l e d an e x c i t o n · e x c i t o n t r a n s i t i o n . The consequences of the c o n s e r v a t i o n of s p i n i s to p r o v i d e a band or t r a n s i t i o n which i s r e l a t i v e l y i n t e n s e i n comparison to a normal s p i n - f o r b i d d e n t r a n s i t i o n . (In t h e c a s e we h a v e c h o s e n b o t h t h e e x c i t a t i o n on s u b l a t t i c e A and t h e one on s u b l a t t i c e Β a r e by themselves spin-forbidden. T h i s i s n o t an e x c l u s i v e r e q u i r e m e n t f o r an e x c i t o n · e x c i t o n t r a n s i t i o n . ) Note a l s o t h a t i f t h i s e x c i t a t i o n o f A and Β a r e i n d i v i d u a l l y t h e same as t h e s i n g l e e x c i t o n e x c i t a t i o n shown i n t h e f i r s t p a r t of t h i s f i g u r e , then the c o o p e r a t i v e e x c i t o n * e x c i t o n t r a n s i t i o n w i l l o c c u r at a p p r o x i m a t e l y t w i c e the energy of the e x c i t o n - o n l y t r a n s i t i o n . In t h e t h i r d p a r t o f t h e f i g u r e , we h a v e an exciton+magnon t r a n s i t i o n . T h i s i s the f i r s t of these v a r i o u s phenomena w h i c h a r e s t r i c t l y c h a r a c t e r i s t i c o f a m a g n e t i c a l l y concentrated system. The f i r s t two

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

170

EXTENDED

—

* - t é A

INTERACTIONS

—

r

*

ι

- t é A

Β

1

- i é

B

EXCITON

EXCITON • EXCITON

-4-

—μ

1

METAL

r

•

-H-

BETWEEN

I

- t é Α

Β

EXCITON - MAGNON

EXCITON+MAGNON

Figure 7. Exciton and exciton · magnon processes A and B refer to opposite sublattices (adopted from Ref. 2)

FINE S T R U C T U R E O F \ TRANSITION

q

(*S) —

4

T ( G) | f

4

2.2*K

Figure 8. Exciton (El, E2) and exciton + magnon (VI, σΐ, σ2) transitions in MnF (adopted from Réf. 4) 2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

12.

HOLT

Linear

Chains

171

p r o c e s s e s , t h e e x c i t o n and e x c i t o n · e x c i t o n t r a n s i t i o n s a r e c h a r a c t e r i s t i c o f m a g n e t i c a l l y d i l u t e s y s t e m s as w e l l as m a g n e t i c a l l y c o n c e n t r a t e d s y s t e m s . The c a s e o f the exciton+magnon e x c i t a t i o n r e q u i r e s a m a g n e t i c a l l y c o u p l e d system, however. The exciton+magnon t r a n s i t i o n is a combination of a normal s p i n - f o r b i d d e n t r a n s i t i o n s u c h as shown on s u b l a t t i c e A a n d a s p i n - d e v i a t i o n o f low e n e r g y , s u c h as shown on s u b l a t t i c e B. Here a g a i n , j u s t as i n t h e e x c i t o n · e x c i t o n t r a n s i t i o n we s e e t h a t t h e s e l e c t i o n r u l e AS=0 i s o b e y e d . I n o t h e r w o r d s , we h a v e an u p - s p i n on A g o i n g t o a d o w n - s p i n s i m u l t a n e ­ o u s l y w i t h a d o w n - s p i n on Β g o i n g t o an u p - s p i n . The d i f f e r e n c e b e t w e e n t h i s and an e x c i t o n · e x c i t o n t r a n s i ­ t i o n i s t h a t t h i s p r o c e s s a r i s e s o n l y i n the case o f magnetically coupled system d th f th allowing excitatio n i t u d e than t h a t f o sition . The l a s t d i a g r a m i n F i g u r e 7 shows a s e c o n d pro­ c e s s by w h i c h t h e p r e s e n c e o f a c o o p e r a t i v e m a g n e t i c i n t e r a c t i o n i n t h e s y s t e m h e l p s t o a l l o w an e l e c t r o n i c excitation. This i s c a l l e d the exciton-magnon. In t h i s c a s e , we h a v e , as b e f o r e , a s p i n - f o r b i d d e n e x c i ­ t a t i o n on s u b l a t t i c e A b u t as o p p o s e d t o t h e e x c i t o n + magnon, we h a v e a l r e a d y c r e a t e d an e x c i t a t i o n on s u b l a t t i c e Β w h i c h t h e n d e c a y s a t t h e same t i m e as t h e e x c i t a t i o n o c c u r s on s u b l a t t i c e A. As we c a n s e e , t h e s p i n i s c o n s e r v e d i n t h i s s y s t e m as w e l l as i n t h e p r e v i o u s two s y s t e m s . One o b s e r v a b l e d i f f e r e n c e b e t w e e n t h e e x c i t o n + magnon a n d e x c i t o n - m a g n o n i s t h e i r p o s i t i o n r e l a t i v e t o the pure e x c i t o n l i n e . I t s h o u l d be c l e a r from F i g u r e 7 t h a t t h e exciton+magnon w i l l occur at h i g h e r e n e r g i e s than the pure e x c i t o n l i n e , w h i l e the exciton-magnon w i l l occur at lower e n e r g i e s than the pure e x c i t o n line. T h e c a s e f o r t h e e x c i t o n + m a g n o n i s shown q u i t e g r a p h i c a l l y i n F i g u r e 8. T h e s e l a t t e r t h r e e e x c i t a t i o n s a l l h a v e one t h i n g i n common and t h a t i s t h e i r u n u s u a l i n t e n s i t y . The v a r i a t i o n o f t h e s e i n t e n s i t i e s , as a f u n c t i o n o f Τ w i l l d i f f e r , however. F i g u r e 9 shows t h e c a l c u l a t e d temp­ e r a t u r e dependence f o r both the exciton+magnon ( c o l d band) and e x c i t o n - m a g n o n ( h o t band) c a s e s . This sug­ g e s t s t h a t a major change i n o s c i l l a t o r s t r e n g t h s h o u l d occur below TJT. The m a g n i t u d e o f t h i s change w i l l d e p e n d u p o n t h e r e l a t i v e c o n t r i b u t i o n o f magnon h o t and c o l d b a n d s and p h o n o n modes. F i g u r e 10 shows t h e c a l c u l a t e d t e m p e r a t u r e depend­ e n c e ( s o l i d l i n e ) f o r a two e x c i t o n t r a n s i t i o n i n RbMnF3. As c a n be s e e n f r o m t h e e x p e r i m e n t a l result

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

EXTENDED

i.

INTERACTIONS

BETWEEN

METAL

Spin Wovt Theofy

=> 2 >» w

α

!

5«

^

Hot Bond

100

200

Temperature

300

( K)

Figure 9. Calculated temperature dependence of magnon side bands in MnF> (adopted from Ref. 5)

Two Exciton Transition

Kid*

2[ A ( S)— T, ( G)] e

l e

e

4

4

e

7.0

60«έ

H50 ? H40

100

200

300 Κ

30

Temperature Figure 10. Temperature dependence of two-exciton absorption in RbMnF Ref. 6) {

of the intensity (adopted from

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

12.

Linear

HOLT

173

Chains

(dashed l i n e ) the agreement i s r e l a t i v e l y good. F i g u r e 11 p r o v i d e s us w i t h t h e e x p e r i m e n t a l l y d e t e r m i n e d i n t e n s i t y v a r i a t i o n of exciton-magnon t r a n s i t i o n s in RbMnF3 and MnF2. The c r i t e r i a t h e n f o r i d e n t i f y i n g magnon assisted transitions a r e : 1) t o l o o k t o e n e r g i e s s l i g h t l y h i g h e r and s l i g h t l y l o w e r t h a n t h e p a r e n t spin-forbidden t r a n s i t i o n f o r anomously i n t e n s e m a n i f o l d s and 2) ascertain i f an a n o m o l o u s i n t e n s i t y c h a n g e o c c u r s i n the r e g i o n of the Néel p o i n t . C r i t e r i o n 1) s h o u l d be q u a l i f i e d i n t h a t o n l y t h e magnon a s s i s t e d transitions may be v i s i b l e , t h e p a r e n t l i n e b e i n g t o o weak t o be observed. One o f t h e RMX3 s y s t e m s , i n w h i c h we h a v e h a d considerable interes metal ion n i c k e l . Tabl l o g r a p h i c p a r a m e t e r s f o r a number o f n i c k e l - c o n t a i n i n g compounds. A l s o i n c l u d e d a r e the room t e m p e r a t u r e m a g n e t i c moments, t h e i r W e i s s c o n s t a n t s a n d , w h e r e known, an i n d i c a t i o n o f t h e i r N é e l t e m p e r a t u r e s . As c a n be s e e n , t h e i n t e r c h a i n d i s t a n c e s v a r y f r o m 9.35 A i n t h e c a s e o f ( C H ^ ^ N i B ^ , down t o 6.85 A i n the case of ΤΙΝΙΟΙβ. T h i s t h e n s h o u l d g i v e us a l a r g e r a n g e i n which to look at the e f f e c t of i n t e r - versus intrachain coupling. To t h i s moment, t h e m e a s u r e m e n t s t h a t we h a v e made have been r e s t r i c t e d to ( C H 3 ) 4 N N i C l , C s N i C l 3 , CsNiBr3, RbNiCl3 * RbNiBr3. Indeed, even i n the case of the ( C H 3 ) 4 N i C l 3 we s e e f r o m T a b l e I t h a t i t h a s b e e n f o u n d t o be f e r r o m a g n e t i c , consequently s h o u l d n o t and, i n f a c t , d o e s n o t e x h i b i t any magnon a s s i s t e d transitions in i t s o p t i c a l spectrum. C o n s e q u e n t l y , our d i s c u s s i o n i s r e s t r i c t e d to the a l k a l i m e t a l s a l t s of the nickel halides. I n F i g u r e 12 we s e e t h e s p e c t r u m o f an ^ 2 % solid s o l u t i o n of C s N i C l 3 i n the c o l o r l e s s , non-magnetic diluent CsMgCl3. This i s b a s i c a l l y a normal spectrum f o r the Ni2+ i o n . The a b s o r p t i o n maximum o c c u r i n g a t approximately 7,000 cm" i s t h e ^A2g-*"^T2g(F) t r a n s i ­ tion. T h i s i s f o l l o w e d by t h e s p i n - a l l o w e d ^A2g"** 3 T i g ( F ) t r a n s i t i o n a t 12,000 wave n u m b e r s . Superim­ p o s e d u p o n i t i s t h e s p i n - f o r b i d d e n A2g- * Eg t r a n s i ­ tion. The 3 A 2 g + T 2 g t r a n s i t i o n t h e n l i e s a t ^ 1 8 0 0 0 cm" f o l l o w e d by t h e 3 A 2 - * l A i g t r a n s i t i o n a t 19000 cm" . The s p i n - a l l o w e d A2g^ Ti (P) transition is t h e n o b s e r v e d a t 22000 cm" . This i n turn i s followed by t h e s p i n - f o r b i d d e n A g - ^ T i g ( G) (^24,000 c m " ) , 3 A a - * E ( G ) (25 ,700 cm" ) and 3 A 2 g + T ( G ) (26 ,000 cm-ï) transitions. F i g u r e 13, t h e s p e c t r u m o f C s N i C l 3 a t 5 ° K , shows o

3

a n (

1

3

)

1

1

1

g

1

3

3

g

1

3

1

2

1

2

1

g

1

2 g

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

4

3

CsNiBr

R. W. A s m u s s e n

c.

Achiwa, J .

Ν.

b.

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Phys

43,

-156

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1932

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(1972).

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Z. a n o r g . u . a l l g e m . Chem., 2 8 3 , 1 ( 1 9 5 6 ) .

(1969).

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Willett, J.

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a n d H. S o l i n g ,

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P6 /mmc

3

Ρ6 /mmc

3

P6 /mmc

3

Ρ6 /mmc

3

P6 /mmc

3

P6 /mmc

Cmcm

3

v

Compounds

,Β .Μ·

o f Some R N i X

Interchain

Parameters

Intrachain

and M a g n e t i c

Group

P6 /m

Space

B. C. G e r s t e i n ,

3

RbNiBr

a.

3

RbNiCl

3

3

CsNiCl

CsNiI

3

3

TlNiCl

3

CH NH NiCl

3

3

3

(CH ) NNiBr

4

3

3

(CH ) NNiCl

Compound

Structural

TABLE I

12.

Linear

HOLT

1 0

,

175

Chains

ι 100

,

ι 200

ι

ι 300

Κ

I

Temperature

Figure 11. Experimental temperature dependence of oscillator strengths for exciton-magnon transition A ( S) -> E,( G), A ( G) in MnF and RbMnF, (adopted from Ref. 7) 6

lff

6

4

4

4

lfl

4

t

Figure

12.

The 5°K

spectrum of CsMgCly.Ni, from Réf. S)

±Z

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

(adopted

EXTENDED

_2i

22

IS ENERGY

Figure

13.

14

INTERACTIONS

IQ

BETWEEN METAL

g_

cnrf'XIO"*

The 5 ° Κ spectrum of CsNiCh, from Ref. 8)

±Z

(adopted

ENERGY cn-fXIO"'

Figure 14. The 5°K CsNiBr,, i Z (adopted

spectrum of from Ref. 8)

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

12.

HOLT

Linear

Figure

111

Chains

15.

Temperature dependence CsNiBr , ±Z

of the spectrum

of

s

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

3

3

CsNiCl

RbNiCl

Compound

u

1

w

1

Polarization

Oscillator

Α

3

A

A

3 2

3

8

g

^A

1

8

l

L

g

l g

(G)

(F)

( 0

( D )

(F)

2g

- T

3

T

3

^ Α

g

l g

2g

- T / E

2 g

(^)

3 A -*· Τ 2g 2g

A

3

3

2

2

2 g

T

3

g

l g

2.5

1.7

2.5

2.2 1.2 4.2

2.7

2.1

2.5

0.3

2.2

1.5

0.8 0.7 1.5

0.6

3.0

0.4

2.9

2.0

0.4

0.2

1.9

1.8

1.8

5.5

1.8

0.4

6.4

4.1

1.1

4.7

1.4

1.1

5

1.8

3

fxl0 (4°K)

i n RNiX

fxl05(77°K)

Transition

fxl0 (300°K)

5

f o r Selected

->\ (G)

3

3

- T

2g*

A

3

A

A

3

2 g

2 g

28-

3

A

A

3

3

A

2

A g^ T2g

3

3

Transition

Strengths

TABLE I I

2

Ο

25

I

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

II

CsNiBr

3

Compound

Table

\\

1

Polarization

Continued

/ E

3

g

- T

A

3

A

A

3

3

%

A

1

(G>

3

A

+ T

* S

2 g ^

2 g

e

A

G

)

(F)

l g

l g

g

(

(F)

l g

l g

v

( D )

l g

2g

- T

3

3 T

2 g ^

2 g

2g^

3

3 A

2

2 g

A „ -*-A, (G) 2g l g

A

3

3

A

3

Transition

·

6

9

7

11.7

·

8.1

'°

3

3

6.0

0.9

5

fxlQ (300°K)

3.6

5.2

2.4

5.4

4.8

0.7

4.8

2.8

8.5

1.4

5.4

1.8

1.9 4.4

1.0

0.9

0.5

2.6

3.3

5 5

fx xl lQ 0 ( (4 4° °K K) ) f

5 5

K) ) fxlQ ( (7 77 7° °K

I*

ο 5

180

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

a marked d i f f e r e n c e i n the r e l a t i v e i n t e n s i t i e s o f s e v e r a l o f t h e b a n d s when c o m p a r e d t o F i g u r e 12. Of p r i n c i p l e i n t e r e s t are t h o s e bands w h i c h are above 18,000 wave n u m b e r s . That band at a p p r o x i m a t e l y 19,000 w h i c h h a s b e e n i d e n t i f i e d as t h e s p i n - f o r b i d d e n ^A2g"** ^ A i g t r a n s i t i o n i s seen to have gained i n t e n s i t y rela­ t i v e t o t h e s p i n - a l l o w e d t r a n s i t i o n s b o t h a b o v e and b e l o w i t . The same may be s a i d f o r t h e h i g h e r e n e r g y s p i n - f o r b i d d e n t r a n s i t i o n s w h i c h l i e above the allowed 3A -* Tig(F) transition. T u r n i n g now to the spectrum of C s N i B r 3 , F i g u r e 14, one c a n a g a i n s e e t h a t t h e same i n t e n s i t y p a t t e r n i s repeated. F i g u r e 15 shows t h e t e m p e r a t u r e d e p e n d e n c e i n the lower energy r e g i o n f o r b o t h the spin-allowed and s p i n - f o r b i d d e n the p r i m a r y f e a t u r r a p i d i n c r e a s e i n the i n t e n s i t y of the f o r m a l l y s p i n f o r b i d d e n bands w i t h d e c r e a s i n g t e m p e r a t u r e . This i s i n c o n t r a s t to the d e c r e a s e i n i n t e n s i t y of the s p i n allowed t r a n s i t i o n s . This rapid increase i n i n t e n s i t y a t T>>T$j i s i n c o n t r a s t t o b o t h work r e p o r t e d by L o h r and M c C l u r e ( ^ ) on some m a n g a n e s e s a l t s and t o t h e w o r k o f F u j i w a r a et_ £ l (_7) c i t e d e a r l i e r , F i g u r e 11. That t h i s b e h a v i o r i s common t o a l l o f t h e antiferromagnetic RMX3 s t u d i e d i s g r a p h i c a l l y shown i n T a b l e I I w h e r e t h e o s c i l l a t o r s t r e n g t h s , both i n the JL C and 11C d i r e c ­ t i o n s , a r e shown as a f u n c t i o n o f t h r e e t e m p e r a t u r e s ; room, 80° and 5 ° K . As may be s e e n , i n a l l c a s e s a r a p i d i n c r e a s e of the i n t e n s i t y of the magnon-assis t e d e x c i t o n l i n e s i n the a l k a l i h a l i d e s i s n o t e d . This s u g g e s t s t h a t the i n t e n s i t y p r o d u c i n g mechanism i s s i m i l a r i n a l l c a s e s and p e r h a p s o f a d i f f e r e n t nature t h a n t h a t s e e n f o r RbMnF3 and MnF2. This l a t t e r point i s one w h i c h d e s e r v e s more s t u d y . Unfortunately, the l i m i t e d amount o f d a t a a v a i l a b l e do n o t a l l o w us t o s o r t out i n t e r - v e r s u s i n t r a c h a i n effects. Close i n s p e c t i o n a l s o shows t h a t t h e p a r e n t e x c i t o n l i n e d o e s n o t a p p e a r t o be d i s c e r n a b l e i n t h e c a s e s shown. The exciton+magnon are c l e a r l y seen, however. 3

2 g

I w o u l d l i k e t o e x p r e s s my t h a n k s t o t h e N a t i o n a l S c i e n c e F o u n d a t i o n f o r s u p p o r t o f t h i s r e s e a r c h and t o J . A c k e r m a n , G. M. C o l e , and Ε . M. H o l t who collabor­ a t e d i n t h i s work. Work p a r t i a l l y supported by the N a t i o n a l Science Foundation grants GP-15432A1 and GP-41506.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

12.

HOLT

Linear

Chains

181

Literature

Cited

1.

H u t c h i n g s , M. T., Shirane, G., Birgeneau, R. and S. L . Holt, P h y s . Rev., (1972), B5, 1999.

2.

A c k e r m a n , J. Inorg. Chim.

3.

Kittel, C., 4th edition,

4.

Sell, Phys.

5.

S h i n a g a w a , K. a n (1971), 30, 1280.

6.

Motizuki, K. a n d (1972), 11, 167.

7.

F u j i w a r a , T., G e b h a r d t , W., P e n t a n i d e s , K. a n d T a n a b e , Υ., J. P h y s . S c o . , J a p a n ( 1 9 7 2 ) , 33, 39.

8.

A c k e r m a n , J., Holt, Ε . M. a n d State Chem., (1974), 9, 279.

9.

L o h r , L . L . a n d M c C l u r e , D. S., J. Chem. ( 1 9 6 8 ) , 49, 3 5 1 6 .

F., Cole, G. M. a n d Acta, (1974),8, 3 2 3 . Introduction W i l e y , New

Holt,

Miyata,

S. L . ,

t o Solid S t a t e Y o r k , (1971) 539.

D. D., G r e e n e , R. L . a n d W h i t e , R e v . , ( 1 9 6 7 ) , 158, 489.

S.,

Sol.

Holt,

J.

Physics,

R. Μ . ,

State

Comm.,

S. L . , J. Sol.

Phys.,

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

13 Spectroscopic and Magnetic Properties o f CsMI

3

T y p e Transition Metal Iodides G. L. McPHERSON and L. J. SINDEL

Tulane University, New Orleans,La.70118

As t h e two previous been a great deal of interest in transition m e t a l salts of t h e general formula M(I)M' (II)X (where M(I) is a large univalent cation, Μ' (II) a divalent transi­ tion m e t a l ion, and X a halide ion). These materials often crystallize in hexagonal lattices in which t h e most prominent structural feature is a parallel array of infinite, linear c h a i n s of octahedra sharing faces. The c h a i n s r u n parallel t o t h e crystallographic c-axis with the transition metal i o n s at t h e centers and t h e halide i o n s at t h e c o r n e r s of t h e octahedra (see Figure 1). The magnetic properties o f these types of salts a p p r o a c h t h o s e o f a o n e - d i m e n s i o n a l system o f interacting spins. The hexagonal linear chain struc­ ture is observed with t h e widest variety o f transition metals and halogens when M(I) is a cesium ion. T h i s paper discusses t h e magnetic and spectroscopic proper­ ties o f several cesium metal triiodides which adopt this structure. A l t h o u g h t h e properties o f t h e s e salts are inherently interesting, it is especially informative to compare t h e iodides t o t h e analogous chlorides and b r o m i d e s . 3

The cesium metal triiodides, CsMgI , CsVI , CsCrI , CsMnI and CsNiI have been shown by X-ray studies to adopt the linear chain structure, Table I contains a summary of the crystallographic data for these salts. Although the space groups are not unambiguously determined it is very likely that a l l of the materials except the chromium salt are isostructural with CsNiCl (space group P6 /mmc). Crystallographic studies of CsCrCl3 and CsCrBr3 suggest that the structures of the chromium salts differ somewhat from that of CsNiCl ; however, the basic linear chain feature is still retained. Un­ doubtedly there are minor structural variations among 3

3

3

3

3

1,2

3

3

3

3

182 In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

4

13.

MCPHERSON

AND

siNDEL

Metal

183

Iodides

the CsMX s a l t s w h i c h r e s u l t f r o m t h e d i f f e r e n c e s i n the s i z e o f t h e h a l i d e i o n . C e r t a i n l y , a v e r y impor­ tant s t r u c t u r a l parameter i s the i n t r a c h a i n metalmetal separation. This separation i s equal t o half of the l a t t i c e dimension i n t h e c d i r e c t i o n and i s e x p e c t e d t o be l a r g e s t i n t h e i o d i d e s a l t s . 3

f f

Table

I.

Crystal

Structural

System:

n

Properties

Salts

2n

hhi,

Space Group:

P6 /mmc, P 6 / m c ,

Lattice

3

Hexagonal

Extinctions:

Mol./unit

of CsMI

3

3

cell:

o r P^2c

Ζ

Constants :

CsVI

a

c

8.21

6.81

a

8.12

6.85

b

8.18

6.95

a

8.00

6.76

a

8.20

7.01

a 3

CsCrI

3

CsMnI

3

CsNiI

3

CsMgI

3

^Reference "Reference

ίέϊ

In a d d i t i o n t o the f a i r l y subtle s t r u c t u r a l v a r i a t i o n s , s i g n i f i c a n t changes i n t h e n a t u r e o f t h e m e t a l - h a l o g e n bond would be e x p e c t e d i n g o i n g f r o m a c h l o r i d e t o a bromide and f i n a l l y t o a n i o d i d e lattice. These bonding d i f f e r e n c e s a r e d r a m a t i c a l l y demonstrat­ ed b y e l e c t r o n s p i n r e s o n a n c e m e a s u r e m e n t s . The e p r spectra of V , Μη , and N i doped i n t o t h e i s o s t r u c t u r a l magnesium s a l t s , C s M g C l , C s M g B r , and C s M g I , have b e e n s t u d i e d . The g- and m e t a l h y p e r ­ fine tensors indicate a considerable v a r i a t i o n i nthe metal-halogen bonding i n the three l a t t i c e s . Table I I g i v e s a summary o f h y p e r f i n e c o n s t a n t s and g - v a l u e s for the three l a t t i c e s . 2 +

2

+

3

9

3

6

3

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

EXTENDED INTERACTIONS

Jg4

Table

II.

H y p e r f i n e C o n s t a n t s * and

CsMgCl

V (77°K) g

obs Scale

Mn (77°K) 2 +

fobs

A

Ni (77°K)

4

g-Values "

CsMgBr

3

2 +

BETWEEN M E T A L IONS

CsMgI

3

1.975 1.957 75.

1.994 1.950 70.

2.056 1.942 65.

2.002

2.004 77.

2 .008 75.

2.25

2.25 2.40

2.16

80.

2 +

Sobs Scale

2.58

2.40

*The h y p e r f i n e c o n s t a n t p a r a l l e l and p e r p e n d i c u l a r components and of 10" cm" . (Data t a k e n from (6)) 4

3

are

in

units

1

+

T h e gobs v a l u e s r e p r e s e n t t h e a v e r a g e o f t h e p a r a l l e l and p e r p e n d i c u l a r components o f o b s e r v e d g - t e n s o r . The g a x values f o r the d and d systems are c a l c u ­ l a t e d from simple c r y s t a l f i e l d theory. (Data t a k e n from (6)) 3

C

8

c

One n o t i c e s t h a t t h e r e a r e c o n s i d e r a b l e d i s ­ c r e p a n c i e s between the observed g-values o f V and N i * and t h o s e c a l c u l a t e d f r o m t h e f o l l o w i n g s i m p l e crystal field expression. 2

2

(g = 2 . 0 0 2 3 - § λ ) F u r t h e r m o r e , t h e d i s a g r e e m e n t b e c o m e s more p r o n o u n c e d i n going from c h l o r i d e t o bromide t o i o d i d e . Presum­ a b l y t h e d i s a g r e e m e n t between t h e o b s e r v e d and c a l c u ­ l a t e d values a r i s e s from a l i g a n d c o n t r i b u t i o n t o the g-value. The l i g a n d c o n t r i b u t i o n i n c r e a s e s a s t h e s p i n o r b i t c o n s t a n t o f t h e l i g a n d i n c r e a s e s and a l s o as the d e r e a l i z a t i o n o f the u n p a i r e d e l e c t r o n s from the metal to the ligands i n c r e a s e s . " I n view o f the observed g-values, i t appears that the metalh a l o g e n b o n d i n g b e c o m e s more c o v a l e n t p r o c e e d i n g through the s e r i e s from c h l o r i d e t o i o d i d e . The Mn and V hyperfine constants support t h i s conclusion, s i n c e t h e c o n s t a n t s show a s t e a d y d e c r e a s e i n g o i n g from c h l o r i d e t o i o d i d e . A decrease i n the metal 7

1 0

5 5

5 1

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

13.

MCPHERSON

AND

siNDEL

Metal

185

Iodides

h y p e r f i n e constant suggests an i n c r e a s e i n the metal to ligand delocalization. (Reference ( 6 ) g i v e s a more thorough d i s c u s s i o n of the epr parameters.) Although the trend i n the nature of the metal-halogen bonding i n the CsMX s e r i e s i s p e r h a p s i n t u i t i v e l y obvious, the epr s t u d i e s p r o v i d e a very s a t i s f y i n g experimental verification. P l o t s of the r e c i p r o c a l of the molar s u s c e p t i b i l ­ i t y versus the a b s o l u t e temperature f o r C s N i I , CsMnI , and C s C r I a r e shown i n F i g u r e s 2, 3 , a n d 4, respec­ tively. Both CsNiI and C s M n I o b e y t h e C u r i e - W e i s s l a w a b o v e 190°K, b u t show s i g n i f i c a n t d e v i a t i o n a t 77°K. The l o w t e m p e r a t u r e d e v i a t i o n s a n d t h e l a r g e negative Weiss constants i n d i c a t e that these s a l t s are antiferromagnetic. t h e C u r i e - W e i s s la T h i s m a t e r i a l , h o w e v e r , has a l a r g e n e g a t i v e W e i s s constant which indicates that i t i s a l s o antiferromag­ netic. The m a g n e t i c p r o p e r t i e s o f C s V I d i f f e r f r o m those of the three p r e v i o u s l y mentioned s a l t s . The v a n a d i u m s a l t has a s m a l l p a r a m a g n e t i c s u s c e p t i b i l i t y ( 2 . 3 χ 10" esu/mole) which i s e s s e n t i a l l y independent of temperature. This o b s e r v a t i o n suggests that the antiferromagnetic i n t e r a c t i o n s i n this material are s i g n i f i c a n t l y s t r o n g e r than those of the other s a l t s . These i n t e r a c t i o n s a r e e f f e c t i v e e v e n a t room tempera­ ture. A l t h o u g h t h e s e s u s c e p t i b i l i t y s t u d i e s do n o t completely c h a r a c t e r i z e the magnetic behavior of the i o d i d e s , t h e r e i s l i t t l e doubt about the antiferromag­ n e t i c nature of these m a t e r i a l s . The m a g n e t i c s u s c e p t i b i l i t i e s o f a number o f t h e a n a l o g o u s b r o m i d e s and c h l o r i d e s have a l s o b e e n studied. D a t a has b e e n r e p o r t e d f o r C s V C l s , CsCrCl , CsMnBr , CsNiBr , and CsNiCl . ~ A f u n d a m e n t a l q u e s t i o n t o be c o n s i d e r e d when d i s c u s ­ s i n g the m a g n e t i c p r o p e r t i e s o f t h e CsMX s a l t s i s whether the magnetic exchange i n t e r a c t i o n s are d i r e c t (through space) or i n d i r e c t (through l i g a n d ) . In p r i n c i p l e b o t h mechanisms a r e p o s s i b l e , s i n c e t h e metal-metal separations w i t h i n a chain are f a i r l y s h o r t (~3Â) and e a c h m e t a l i o n s h a r e s t h r e e h a l i d e ligands w i t h the neighboring metal ions i n the chain. The c o m p a r i s o n o f t h e s u s c e p t i b i l i t y d a t a f o r t h e C s M X s a l t s s h o w n i n T a b l e I I I g i v e s some q u a l i t a t i v e insight into this question. 3

3

3

3

3

3

3

3

1 1

1 2

3

1 3

3

1 4

1 4

3

3

3

3

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

1 6

EXTENDED

INTERACTIONS

BETWEEN

METAL

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

MCPHERSON

AND

siNDEL

Metal

187

Iodides

100

50

-100°

0°

100° 200° 300° T(°K)

Figure 3. Reciprocal molar susceptibility of CsMnlg vs. absolute temperature. Curie-Weiss constants for the linear re­ gion: C = 4.97; θ = -165°.

Figure 4. Reciprocal molar suscepti­ bility of CsCrI vs. absolute temperature. Curie-Weiss constants: C = 3.08; θ = -163°. s

T(°K)

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

188

EXTENDED INTERACTIONS BETWEEN METAL IONS

Table I I I .

M a g n e t i c P r o p e r t i e s o f CsMX Cs7X χ,(297°Κ) 3

M-M D i s t a n c e

(Α)

Cl

3.01

1370

I

3.40

2220

Salts

3

Χ(77 Κ) ό

a

a

1440 2360

CsCrX (A.) X(297°K) 4500° 3

Cl Br I

M-M D i s t a n c e 3.11 3.25 3.42

(Α)

3.26 3.47

12800

X(297°K) 11000 10750

2.98 3.12 3.38

X(77°K) 18500 17200

e

CsNils (A) X(297°K)

M-M D i s t a n c e Cl Br I

D

67ΟΟ

M-M D i s t a n c e Br ι

X(77°K) 5300

3800 3650 2940

e

X(77°K) 9200^ 8l50 4350

α

d

d

X v a l u e s a r e i n u n i t s o f 10"" esu/mole ^Reference i l l ) R e f e r e n c e ΓΤ2) Reference Π 3 ) R e f e rence ( T i ) 6

D

c

d

D i r e c t exchange i s a f u n c t i o n o f t h e d i s t a n c e between i n t e r a c t i n g i o n s and would be expected t o d i m i n i s h as t h e m e t a l - m e t a l s e p a r a t i o n i n c r e a s e s . On the o t h e r hand, i n d i r e c t exchange depends more on t h e covalency of the metal-ligand-metal l i n k a g e . Since t h e m e t a l - m e t a l s e p a r a t i o n s i n t h e CsMX s e r i e s a r e d i r e c t l y dependent on t h e s i z e o f t h e h a l i d e i o n , t h e s t r e n g t h o f d i r e c t e f f e c t s would be e x p e c t e d t o f o l l o w the o r d e r : Cl>Br>I. I n c o n t r a s t , t h e i n d i r e c t e f f e c t s would be e x p e c t e d t o e x h i b i t t h e o p p o s i t e o r d e r . The data f o r the cesium n i c k e l t r i h a l i d e s i n d i c a t e t h a t the s t r e n g t h o f the a n t i f e r r o m a g n e t i c i n t e r a c t i o n s i s g r e a t e s t f o r C s N i I and s m a l l e s t f o r C s N i C l . This o b s e r v a t i o n s u g g e s t s t h a t i n d i r e c t exchange i s p r e ­ dominant i n t h e s e s a l t s . T h i s c o n c l u s i o n i s q u i t e reasonable i n l i g h t o f simple c r y s t a l f i e l d theory. 3

3

3

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

13.

MCPHERSON

AND

Metal

SINDEL

189

Iodides

8

The u n p a i r e d e l e c t r o n s o f a d system i n an o c t a h e d r a l complex o c c u p y t h e e~ s e t o f o r b i t a l s w h i c h a r e d i r e c t ed t o w a r d t h e l i g a n d s . For a d s y s t e m s u c h as Mn d i r e c t as w e l l as i n d i r e c t i n t e r a c t i o n s m i g h t be exp e c t e d s i n c e t h e u n p a i r e d e l e c t r o n s o c c u p y t h e t 2 g and e orbitals. The d a t a i n d i c a t e t h a t t h e c o u p l i n g i n CsMnBr i s perhaps a l i t t l e stronger than i n CsMnI , b u t t h e s u s c e p t i b i l i t i e s o f t h e two s a l t s a r e v e r y similar. I t appears that there are considerably stronger interactions i n CsCrCl than i n C s C r I . It i s p o s s i b l e t h a t d i r e c t exchange i s dominant i n a d system s i n c e the m a j o r i t y of the unpaired e l e c t r o n s o c c u p y o r b i t a l s ( t g ) w h i c h a r e d i r e c t e d away f r o m t h e ligands. We h e s i t a t e t o s p e c u l a t e o n t h e C s V X salts since the s u s c e p t i b i l i t i e s e n t i a l l y independen s u s c e p t i b i l i t i e s may r e s u l t f r o m a t e m p e r a t u r e i n d e p e n d e n t p a r a m a g n e t i s m w h i c h has n o t h i n g t o do w i t h t h e normal paramagnetism a s s o c i a t e d w i t h the unpaired e l e c t r o n s o f t h e V"*" i o n . Clearly, rather strong antiferromagnetic i n t e r a c t i o n s are p r e s e n t i n these vanadium s a l t s . One v e r y i m p o r t a n t p o i n t h a s b e e n n e g l e c t e d i n the q u a l i t a t i v e d i s c u s s i o n of the magnetic p r o p e r t i e s o f t h e CsMX s a l t s . I t has b e e n f i r m l y e s t a b l i s h e d t h a t t h e i n t e r a c t i o n b e t w e e n two p a r a m a g n e t i c i o n s i s c r i t i c a l l y d e p e n d e n t on t h e m e t a l - l i g a n d - m e t a l angle. W h i l e t h e s t r u c t u r e s o f t h e s a l t s t h a t have b e e n d i s cussed are a l l s i m i l a r , t h i s c r i t i c a l angle undoubtedl y v a r i e s t o some e x t e n t f r o m l a t t i c e t o l a t t i c e . Unfortunately, s u f f i c i e n t precise c r y s t a l l o g r a p h i c data are not p r e s e n t l y a v a i l a b l e t o d i s c u s s t h i s important point. The e l e c t r o n i c s p e c t r a o f C s V I , C s C r I , C s M n I , and C s N i I a r e shown i n F i g u r e s 5 and 6. In general, t h e s p e c t r a show t h e l i g a n d f i e l d t r a n s i t i o n s t h a t w o u l d be e x p e c t e d f r o m o c t a h e d r a l c o m p l e x e s o f t h e s e t r a n s i t i o n metal ions. The s p e c t r u m o f C s C r I shows an i n t e n s e a b s o r p t i o n edge a t a p p r o x i m a t e l y 10,000 cm" . The m a t e r i a l a b s o r b s s t r o n g l y t h r o u g h o u t t h e visible region. T h i s i n t e n s e a b s o r p t i o n may be due t o charge-transfer transitions. Charge-transfer absorpt i o n would be e x p e c t e d t o a p p e a r a t l o w e r e n e r g i e s i n these i o d i d e s than i n s i m i l a r c h l o r i d e or bromide complexes. The s h o u l d e r o n t h e a b s o r p t i o n e d g e o f t h e CsCrI s p e c t r u m has b e e n t e n t a t i v e l y a s s i g n e d t o t h e s p i n a l l o w e d , E-+ T , l i g a n d f i e l d transition. S i m i l a r l y , the C s N i I s p e c t r u m has a n i n t e n s e a b s o r p t i o n edge w h i c h a p p e a r s a t a p p r o x i m a t e l y 13000 cm" and p r e s u m a b l y r e s u l t s f r o m c h a r g e - t r a n s f e r t r a n s 5

2

g

3

3

3

3

4

2

3

2

3

3

3

3

3

3

1

3

5

5

2

3

1

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

190

EXTENDED

INTERACTIONS

BETWEEN

METAL

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

13.

MCPHERSON AND

Metal

siNDEL

191

Iodides

itions. The s p e c t r a o f C s V T a n d C s M n I do n o t s e e m t o be p a r t i c u l a r l y u n u s u a l . T a b l e I V g i v e s t h e band a s s i g n m e n t s f o r t h e i o d i d e s a l t s b a s e d on a n o c t a h e ­ dral ligand f i e l d . 3

Table

IV.

Spectroscopic CsVI

Assignment 4A

4 4

T

Assignments

3

CsCrI Energy 3

1

Energy

(cm"" )

7700 12000

2

2

-

3

Τχ(Ρ)

Assignment

1

(cm"" )

9000 (sh)

5E -* [ 5 T ] 2

13000(sh) 4

-

15300 1870

Ti(P)

CsMnI Assignment E n e r g y (cm" ) Αχ Ti(G) I78OO 3

1

6

4

-* « T (G) 2

4e,4

—

4

T

2

A 3 L

(g)

(d)

~» 4E (D)

Assignment 3

A

3 2

CsNils Energy

6500 8000

T [IE] 2

20900 22100 255ΟΟ

3

1

(cm" )

10900

T (F) X

26400

28600

4 T i (P)

s h = shoulder Brackets designate

assignments which are

uncertain.

3

4

Spectroscopic studies of C s C r C l , C s C r B r , CsMnBr CsNiCl and C s N i B r ' have b e e n r e p o r t e d . A com­ p a r i s o n o f t h e Dq v a l u e s o f t h e CsMX s a l t s i s p r e ­ s e n t e d i n T a b l e V. 3

1 7

3

3

1 3

3

1 8

3

3

Table

V.

Dq

CsVX

V a l u e s f o r the CsCrX

3

1000

CI Br

—

I

770

114 5

1150 900

b

3

CsMX CsMnX

3

3

Salts CsNiX

3

695*

a

680 605

c

655 650

d

Reference Reference ^Reference ^Reference

(13 (Tf

The t r e n d s i n t h e Dq v a l u e s a p p e a r t o f o l l o w t h a t w h i c h would be p r e d i c t e d b y t h e s p e c t r o c h e m i c a l series. I t s h o u l d be m e n t i o n e d t h a t t h e Dq v a l u e f o r C s M n I was d e r i v e d f o l l o w i n g t h e p r o c e d u r e p r e s e n t e d 3

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

,

192

EXTENDED

INTERACTIONS

BETWEEN METAL

IONS

in

reference (13). The e l e c t r i c a l r e s i s t i v i t i e s o f s i n g l e c r y s t a l s o f C s N i I have b e e n s t u d i e d a s a f u n c t i o n o f tempera­ ture. The m a t e r i a l a p p e a r s t o be a s e m i c o n d u c t o r w i t h a room t e m p e r a t u r e r e s i s t i v i t y o f 1 0 t o 10 ohm cm a n d a n e n e r g y o f a c t i v a t i o n o f a p p r o x i m a t e l y 0 . 7 ev. For a m a t e r i a l t h a t i s an i n t r i n s i c semiconductor the band gap ( t h e e n e r g y s e p a r a t i n g t h e v a l e n c e and c o n d u c t i o n bands) should be e q u a l t o twice t h e energy of a c t i v a t i o n o f c o n d u c t i o n . The i n t e n s e a b s o r p t i o n edge a p p e a r s i n t h e s p e c t r u m o f C s N i I a t a p p r o x i m a t e ­ l y 1.5 e v w h i c h i s a b o u t t w i c e t h e o b s e r v e d e n e r g y o f activation. T h i s s u g g e s t s t h a t a t room t e m p e r a t u r e the m a t e r i a l i s a n of the l i n e a r c h a i material i s essentially isotropic. I n c o n c l u s i o n , we h o p e t h a t we h a v e shown t h a t the c e s i u m m e t a l t r i i o d i d e s have r a t h e r i n t e r e s t i n g s o l i d s t a t e p r o p e r t i e s a n d t h a t t h e s e compounds w i l l be u s e f u l f o r f u r t h e r s t u d i e s i n t o t h e n a t u r e o f t h e linear chain M(I)M'(II)X compounds. 3

1 9

7

8

3

3

Literature Cited 1. Li, Ting-i, S t u c k y , G. D., a n d M c P h e r s o n , G. L . , Acta Crystallogr., (1973), B 2 9 , 1330. 2. M c P h e r s o n , G. L . a n d Data.

Quarls,

H.

F.,

Unpublished

3. B.,

M c P h e r s o n , G. L . , K i s t e n m a c h e r , T . K. Folkers, J. a n d S t u c k y , G. D., J. Chem. P h y s . , ( 1 9 7 2 ) , 57,

4.

Li,

3771.

Ting-i

and

Stucky,

G. D.,

Acta

Crystallogr.,

(1973), B29, 1529. 5. M c P h e r s o n , G. L . , K i s t e n m a c h e r , Τ . Κ . , a n d S t u c k y , G. D., J. Chem. P h y s . , (1970), 5 2 , 815. 6. M c P h e r s o n , G. L . , K o c h , R. C., a n d S t u c k y , G. J. Chem. P h y s . , (1974), 6 0 , 1424.

7.

M c G a r v e y , B . R., J. Chem. P h y s . ,

(1964),

41,

D.,

3743.

8. Garrett, B . B., D e A r m o n d , K., a n d G u t o w s k y , H. S., J. Chem. P h y s . , ( 1 9 6 6 ) , 4 4 , 3393.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

13.

MCPHERSON

AND

SINDEL

Metal

Iodides

193

9 . M e s e t i c h , A. A. and Buch, T., J. Chem. P h y s . , (1964), 4 1 , 2524. 10.

M e s e t i c h , A. A. and Watson, R. Ε., P h y s . Rev.,

(1966),

11.

143, 3 5 5 .

12.

H. J., Fink, Η., and Just. Ε., Naturwiss.,

Seifert,

(1968), 55,

297.

L a r k w o r t h y , L. F. and Trigg. J. Κ., Chem. Commun.,

(1970),

1221.

13. McPherson, G. L., Aldrich, H. S., and Chang, J. R., J. Chem. P h y s . ( 1 9 7 4 ) 6 0 534 14. Asmussen, R. W. Chem., (1956), 2 8 3 , 1. 15.

A c h i w a , N., J. Phys. S o c . ( J a p a n ) , (1969), 2 7 ,

561.

16. S m i t h J., Gerstein, B. C., Liu, S. Η., and S t u c k y , G., J. Chem. Phys., (1970), 53, 4 1 8 . 17.

McPherson, G. L. and S t u c k y , G. D., J. Chem.

Phys.,

(1972),

57,

3780.

18. Ackerman, J., Holt, Ε . M., and Holt, S. L., J. Solid S t a t e Chem., in press. 19. McPherson, G. L., Wall, J. E., Jr., and Hermann, A. M., I n o r g . Chem., in press.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

14 Magnetic and Thermal Properties of the Linear Chain Series

J. N.

3 3

McELEARNEY,

R. L .

MX ·2H O

[(CH ) NH]

G.

E.

3

SHANKLE,

2

D. B.

LOSEE,

S.

MERCHANT,

and

CARLIN

U n i v e r s i t y of Illinois, C h i c a g o , Ill. 60680

Introduction Several previous p a p e r s in this s y m p o s i u m h a v e discussed properties of members o f t h e linear chain series RMX (where M=transition metal, X = h a l i d e a n d R= (CH ) N, Cs or Rb). T h i s p a p e r will be c o n c e r n e d w i t h t h e new linear chain series [(CH ) NH]MX ·2H O, where M=(Mn, Co, Ni, Fe o r Cu) a n d X = ( B r o r Cl). Large single crystals suitable for optical and oriented mag­ netic field studies may be easily obtained for most members o f t h e series. Single crystal near-zero-field magnetic susceptibility d a t a ( m e a s u r e d using a m u t u a l inductance t e c h n i q u e ) and heat capacity data (measured using standard heat pulse techniques) are presented h e r e w h i c h show several of the interesting features of this series: anisotropic magnetic b e h a v i o r greater than n o r m a l with t h e p r e s e n c e o f low-dimensional character­ istics a s well a s spin-canting in the ordered state. 3

3

4

3

3

3

2

Crystal Structures To a large e x t e n t the magnetic b e h a v i o r o f these compounds is quite clearly related to their structure. N o t all t h e members o f t h e series are isomorphic, al­ though they probably are a l l isostructural, as inferred from their magnetic properties. Structures have been obtained o n l y for t h e n o n - i s o m o r p h i c (M=Co, Cu; X=C1) compounds (1,2.), although X-ray studies indicate that t h e (M=Co, Mn; X=C1) compounds b o t h crystalize in the s p a c e g r o u p Pnma a n d a r e p r o b a b l y i s o m o r p h i c . The m o s t i m p o r t a n t structural characteristics of t h e s e compounds may be s e e n by considering the projec­ tions o f t h e (M=Co; X=C1) p r o t o t y p e s h o w n in P i g s . 1 a n d 2. The structure consists of chains of edge-shar­ ing trans-[CoCl (0Η ) ] octahedra. The cobalt atoms 4

2

2

194

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

14.

MCELEARNEY

Ο

Cl

New

E T AL.

a

H 0

·

2

Linear

Chain

195

C

Figure 1. Projection of the unit cell of [(CH ) NH]CoCl * 2H O onto the ac plane. Height above this plane of several of the atoms is indicated. (All cobalt atoms are at the same height.) s 3

3

s

Ο

,

ο

-Ο

Figure 2. Projection of a portion of the crystal structure of [(CH ) NH] CoCl · 2H 0 onto the be plane. Portion used was a 3.33-A thick layer taken parallel to the be plane and centered about the cobalt atoms. Dashed lines give unit cell bounda­ ries. S 3

s

2

Ο ci

· Co

β

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

HO 2

r

196

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

a r e 3-637 A a p a r t ( C o - C l d i s t a n c e s a r e 2.456 and 2.503 A) and the i n t e r n a l c h l o r i n e b r i d g i n g a n g l e s a r e 93.14° and 95-52°. A n i o n i c c h l o r i n e s t i e t o g e t h e r c h a i n s which l i e i n t h e be p l a n e v i a hydrogen b o n d i n g . I t i s i m p o r t a n t t o note the r e l a t i v e t i l t i n g o f t h e [ C o C l ι» ( 0 H ) 2 ] m o l e c u l a r u n i t s w i t h r e s p e c t t o each o t h e r as seen i n the p r o j e c t i o n s . I t i s t h i s t i l t i n g , t a k e n w i t h t h e tendency o f the s p i n s t o a l i g n i n some manner c o n s i s t e n t w i t h the O-Co-0 v e c t o r s , w h i c h l e a d s to the u n u s u a l magnetic p r o p e r t i e s o f t h i s s e r i e s . 2

C(CH )3NH]CuCl3'2ΚιΟ

Properties of

3

The most u n u s u a l p r o p e r t y o f t h i s compound ( i n view o f the p r o p e r t i e f t h o t h e compounds) i t h a t i t behaves as a norma o n l y s l i g h t s i g n s o magneti exchang prio o r d e r i n g a t 0.15°K (3) and t h a t thus i n t r a c h a i n magnet­ i c exchange i n t h i s m a t e r i a l must be q u i t e s m a l l . Thus i t s measured heat c a p a c i t y (which v e r y n e a r l y f o l l o w s a T law) has been used i n c o n j u n c t i o n w i t h a c o r r e s p o n d ­ i n g s t a t e s p r o c e d u r e t o determine l a t t i c e c o n t r i b u t i o n s to the heat c a p a c i t i e s o f the o t h e r members o f t h e series. 2

P r o p e r t i e s o f C ( C H ) *NH]CoX 3'2H 0 3

3

2

(X=C1, B r )

Both o f t h e s e compounds behave v e r y s i m i l a r l y . The (M=Co; X=C1) compound m a g n e t i c a l l y o r d e r s a t 4.14°K, w h i l e the B r a n a l o g does so a t 3.86°K. The d a t a and t h e o r e t i c a l f i t s f o r b o t h a r e n e a r l y i d e n t i c a l so o n l y the r e s u l t s f o r the (X=C1) compound w i l l be g i v e n h e r e . The measured heat c a p a c i t y i s shown i n P i g . 3 and the e x t e n s i v e amount o f s h o r t - r a n g e o r d e r above t h e o r d e r ­ i n g t e m p e r a t u r e i s c l e a r l y e v i d e n t . More t h a n 9056 o f the e x p e c t e d e n t r o p y change o c c u r s above t h e t r a n s i ­ tion. This short-range order i s i n d i c a t i v e of the l o w e r e d d i m e n s i o n a l i t y o f the s p i n system and i s c o n ­ s i s t e n t w i t h the s h e e t - l i k e nature of the s t r u c t u r e . Thus s i n c e C o ( I I ) u s u a l l y has I s i n g - l i k e c h a r a c t e r i s ­ t i c s , Onsager s s o l u t i o n t o t h e a n i s o t r o p i c two-dimen­ s i o n a l I s i n g model (k) has been used t o f i t t h e d a t a . B o t h the magnetic heat c a p a c i t y d e r i v e d from t h e d a t a and the f i t t e d curve a r e shown i n P i g . 4. The v a l u e s f o r the exchange parameters d e t e r m i n e d from t h e f i t are J/k=7.7°K and J /k=0.09°K. (The f i t o f t h e B r a n a l o g r e s u l t s i n J/k=7.0°K and J /k=0.09°K.) C l e a r l y the s p i n system i s not f a r from b e i n g o n e - d i m e n s i o n a l . Thus t h e measured magnetic s u s c e p t i b i l i t i e s , shown i n F i g s . 5 and 6, s h o u l d be n e a r l y one-dimension1

f

f

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

14.

New Linear

MCELEARNEY E T AL.

197

Chain

(C H

3) 3 Ν Hi'C 0 C 1 3 · 2 H 0 2

9 T E M P E R A T U R E

Figure

3.

12

13

3

14

15

( K)

3

3

2.5

Π

Zero-field heat capacity data for [(CH,) NH"\CoCh · 2H 0 and 15°K. Solid line represents the lattice contribution.

[ (C Η >

«

10 e

2

Ν Η] C 0 C Ι · 2

between

3°

H 0

3

2

Ι­

ο

ε ^

2.0

J 0

,

2

3

4

5

6

L. 7

8

T E M P E R A T U R E

Figure

ι

I

9

10

_J

L_ Π

12

13

I

1

14

15

(·Κ)

4. Magnetic heat capacity (solid line) of l(CH ) NH]CoCl · 2H 0. Dashed line is the fit to Onsagers two-dimensional anisotropic Ising model solution. 3 3

s

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

198

EXTENDED

INTERACTIONS

BETWEEN

f[CH ) NH]CoCI - 2 3

5

3

x

METAL

IONS

18

20

H 0 2

· · ·

h

0. I 6 h

4

6

θ

10

12

TEMPERATURE

Figure

5.

14

16

(°K )

Zero-field magnetic susceptibility of [(CH )sNH] 2H 0 measured parallel to the b axis S

2

[(CH,) NH]CoCI,' 2 s

H 0 2

Xc

è * f

ΔΔΔΔ X

e

•••• Χ

β

*

; .'i

*l

m

8

I I 10

ί

t 12

T E M P E R A T U R E (°K )

Figure 6. Zero-field magnetic susceptibilities of [(CH ). NH"\CoCl 2H 0 measured along a and c axes. Results of measurements the a axis on two different crystals are shown. s

2

l

3

· along

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

CoCl

s

·

14.

MCELEARNEY

New

ET AL.

Linear

199

Chain

al i n character. The n e a r l y d i s c o n t i n u o u s b e h a v i o r n e a r t h e t r a n s i t i o n t e m p e r a t u r e s h o u l d be n o t e d . It i s e v i d e n t t h a t t h e r e i s a n e x t r e m e amount o f a n i s o t r o p y , X b e i n g g e n e r a l l y v e r y much s m a l l e r t h a n X and X , a n d t h a t t h e r e a r e i n d i c a t i o n s o f a f e r r o m a g n e t i c moment p a r a l l e l t o t h e a a x i s i m m e d i a t e l y b e l o w t h e transition. A d d i t i o n a l l y , at the lowest temperatures X and are constant, while X decreases with dec r e a s i n g t e m p e r a t u r e , as m i g h t be e x p e c t e d f o r a n a n t i ferromagnetic m a t e r i a l f o rwhich c i s t h e easy a x i s . These f a c t s l e a d t o a s p i n arrangement model i n w h i c h s p i n s l i e n e a r l y a l o n g t h e c a x i s w i t h some c a n t ing towards the a a x i s . A l lspins are perpendicular to the b a x i s . A l l t h e s p i n s i n e a c h b e s h e e t o f Co i o n s are a l i g n e d p a r a l l e and between c h a i n s b o r i n g p a i r o f be s h e e t s a r e a l i g n e d s o a s t o g i v e a n e t moment a l o n g t h e a a x i s w i t h n o n e t moment a l o n g the c a x i s . Thus t h e s p i n arrangement i s t h a t o f a c a n t e d a n t i f e r r o m a g n e t o r a weak f e r r o m a g n e t . The i n t r a e h a i n exchange v a l u e determined from t h e heat c a p a c i t y d a t a has been used w i t h t h e e q u a t i o n s d e r i v e d (5.) f o r a o n e - d i m e n s i o n a l I s i n g m o d e l t o f i t t h e Xb a n d Xc d a t a . To t a k e a c c o u n t o f t h e a a x i s c a n t i n g , t h e r e s u l t s d e r i v e d b y M o r i y a (6_) f o r t h e s u s c e p t i b i l i t y o f a c a n t e d a n t i f e r r o m a g n e t have been a p p l i e d t o the X data. T h e r e s u l t s o f t h e f i t s a r e shown i n Fig. 7 a n d a r e s e e n t o be e x t r e m e l y g o o d . b

a

a

c

c

a

Properties

o f [ (CH 3) 3NH]MnX 3 *2H 2O

(X=C1, B r )

B o t h o f t h e s e compounds b e h a v e s i m i l a r l y , w i t h t h e (X=C1) compound e x h i b i t i n g a t r a n s i t i o n a t 0 . 9 8 ° K , w h i l e t h e ( X = B r ) compound o r d e r s a t 1.58°K. Because o f t h e s i m i l a r i t y o f t h e d a t a f o r t h e s e compounds o n l y t h e l a t t e r w i l l be d i s c u s s e d h e r e . The m e a s u r e d h e a t c a p a c i t y i s shown i n F i g . 8. The s h a r p s p i k e c o r r e s p o n d s t o t h e m a g n e t i c o r d e r i n g w h i l e t h e b r o a d hump i s i n d i c a t i v e o f the short-range order expected f o ra low-dimensional system. More t h a n 80% o f t h e m a g n e t i c e n t r o p y c h a n g e e x p e c t e d f o r a n S=5/2 s y s t e m i s g a i n e d above 1.58°K. Likewise, the single c r y s t a l susceptib i l i t i e s m e a s u r e d f o r t h i s compound, shown i n F i g . 9* show t h e e f f e c t s o f e x t e n s i v e s h o r t - r a n g e o r d e r . The b r o a d maximum i n X s i g n i f i c a n t l y a b o v e t h e o r d e r i n g temperature i s e s p e c i a l l y i n d i c a t i v e o f t h e behavior w h i c h s h o u l d be e x p e c t e d f o r a l i n e a r c h a i n . It is a l s o i m p o r t a n t t o n o t e t h e amount o f a n i s o t r o p y b e l o w 10°K and t h a t X and X behave as p a r a l l e l a n d p e r p e n dicular antiferromagnetic s u s c e p t i b i l i t i e s , respecc

c

a

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

200

Figure

EXTENDED

7.

INTERACTIONS

Results of the fits (solid lines) to the [(CH, ),NH]CoCl. susceptibility data t

t

BETWEEN

· 2H 0 2

METAL

principal

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

axis

MCELEARNEY

New Linear

E T AL.

Chain

201

4.8 ~

[(CH ) NH]MnBr -2H 0 3

3

3

2

4.0 -

32 24

0-8 I

_i

0.0

I

ι

ι

I

ι

I

»

ι

•

I

ι

I 8

ι

I

ι

I 9

ι

! 10

e

TEMPERATURE ( K)

Figure

8.

Zero-field

heat capacity

data for [(CH ) NH]

MnBr

3 3

3

·

2H 0 2

1.12 |CH ) NH]MnBr - 2 H 0 3

3

3

2

0.96

0.80

°

0.64

Φ

0.48

x 0.32 Xo-».«.—~

~rr ·.·.*· .* ; · « * «

0.16

0.00

I

I

i

1

I

I

6

I

I

8

I

I

i

10

TEMPERATURE

Figure

9.

Zero-field

I

12

14

16

_l

I I 18

I 20

( K) e

magnetic susceptibilities of [(CH ) NH"]MnBr sured along the a, b, and c axes 3 3

3

· 2H 0

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2

mea­

202

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

tively. As M n ( I I ) h a s a n S g r o u n d s t a t e , i t m i g h t b e e x p e c t e d t h a t the H e i s e n b e r g l i n e a r c h a i n model p r e v i o u s l y a p p l i e d t o o t h e r M n ( I I ) c h a i n s (7.,<8) c o u l d d e s c r i b e the d a t a . However, a r a t h e r s i g n i f i c a n t m o l e c u l a r f i e l d c o r r e c t i o n (9.) t o t h a t m o d e l i s r e q u i r e d when i t i s used to f i t the data. The f i t o f X achieved with an i n t r a c h a i n i n t e r a c t i o n o f J / k = - 0 . 4 l ° K and a m o l e c u l a r f i e l d p a r a m e t e r o f z J V k = - 0 . 7 4 ° K i s shown i n F i g . 10. (The c o r r e s p o n d i n g C I a n a l o g p a r a m e t e r s a r e J/k= -0.36°K and z J / k = - 0 . 5 5 ° K . ) The m o l e c u l a r f i e l d p a r a m e t e r must be i n t e r p r e t e d as i n c l u d i n g a l l t h e i n t e r a c t i o n s i g n o r e d by t h e m o d e l H a m i l t o n i a n , s u c h a s i n t e r c h a i n i n t e r a c t i o n s and n o n - i s o t r o p i c exchange i n t e r actions. I t i s obviou chain interactions c a t e t h a t t h e s p i n H a m i l t o n i a n m u s t n o t be p e r f e c t l y i s o t r o p i c b u t i n s t e a d must i n c l u d e a l a r g e I s i n g - l i k e anisotropy. Actually, this i s t o t a l l y consistent with the I s i n g - l i k e b e h a v i o r of X i n w h i c h a b r o a d maximum a p p e a r s more t h a n 100? a b o v e t h e o r d e r i n g t e m p e r a t u r e . T h i s i s a l s o c o n s i s t e n t w i t h r e c e n t r e s u l t s (10) w h i c h s u g g e s t a n i s o t r o p i c d i p o l a r i n t e r a c t i o n s must be p r e s e n t i n a l i n e a r c h a i n compound w h i c h p o s s e s s e s isot r o p i c n e a r e s t - n e i g h b o r exchange. Such i n t e r a c t i o n s w o u l d add an I s i n g - l i k e t e r m t o t h e H a m i l t o n i a n . Thus, a s m i g h t be e x p e c t e d , t h e X d a t a c a n be w e l l d e s c r i b e d by a s s u m i n g a n I s i n g H a m i l t o n i a n a n d c o r r e c t i n g t h e calculated s u s c e p t i b i l i t y with a molecular f i e l d term which then accounts f o r the i s o t r o p i c i n t e r a c t i o n s i g n o r e d by t h e I s i n g m o d e l . Even c o n s i d e r i n g a n i s o t r o p i c i n t e r a c t i o n s , s p e c i a l c o n s i d e r a t i o n must be g i v e n t o t h e b e h a v i o r o f X^. The sharp r i s e with decreasing temperature o f X^ b e l o w 3°K i s r e m i n i s c e n t o f the b e h a v i o r o f X i n t h e (M=Co; X= C I ) compound. T h e r e a r e two i m p o r t a n t d i f f e r e n c e s : t h e s e d a t a do n o t r i s e t o v e r y h i g h v a l u e s , a n d i m m e d i a t e l y below the t r a n s i t i o n the data p o i n t s drop w i t h decreasing temperature. T h i s b e h a v i o r may be e x p l a i n e d i f i t i s assumed t h a t t h e r e i s a h i d d e n c a n t i n g o f the s p i n s a l o n g t h e b a x i s i n t h i s compound. That i s , the s p i n s i n a g i v e n c h a i n a r e a l i g n e d more o r l e s s p a r a l l e l t o t h e c a x i s a n d a r e c a n t e d i n t h e ±b d i r e c t i o n w i t h ant i f e rromagne t i c c o u p l i n g between them. Since t h e r e i s no i n d i c a t i o n o f c a n t i n g - l i k e b e h a v i o r i n X a , t h e s p i n s must l i e i n t h e be p l a n e . It i s interesting to note t h a t t h i s s i t u a t i o n i s o p p o s i t e t o t h a t seen i n t h e (M=Co) c o m p o u n d s . The s u p p o s i t i o n o f c a n t i n g i s s u p p o r t e d by t h e g o o d f i t t o t h e XTD d a t a , shown i n P i g . 11, o b t a i n e d when t h e m o l e c u l a r f i e l d m o d i f i e d l i n e a r a

f

c

c

a

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

14.

MCELEARNEY

0.36

New Linear

E T AL.

Chain

203

Γ

[(CH ) NH]Mn Br - 2 H 0 3

3

3

2

0.30

v ο E ^

0.24

0.18

3

E «u

0.12

X

0.06 0

0

0

0

2

4

6

8

10

12

TEMPERATURE Figure

10. Fitted

results

14

16

(solid line) for the a axis magnetic [(CH ),NHlMnBr · 2H O 3

18

20

(°K)

3

susceptibility

of

z

.1 2 r [ÎCH ) Ν H] Μη Br - 2 H 0

0.96

3

3

3

2

0.80

ο Ε

0.64

0.48

0.32

0.16

0.00'— — — — — — 1

1

1

1

1

1

6

8

10

TEMPERATURE

Figure

11. Fitted

result

(solid line) [(CH jjNH] S

14

12

_ι

ι

ι

1

18

1

»

20

( K) e

for the b axis magnetic MnBr · 2H 0 3

susceptibility

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

of

204

EXTENDED INTERACTIONS BETWEEN METAL IONS

chain m o d e l u s e d for t h e X fit is include the susceptibility behavior for c a n t e d m a g n e t s . a

Properties

this 1°K,

of

further predicted

modified to by M o r i y a

[(CH ) NH]NiBr .2H 0 3

3

3

2

P r e l i m i n a r y s u s c e p t i b i l i t y m e a s u r e m e n t s made on m a t e r i a l i n d i c a t e p a r a m a g n e t i c b e h a v i o r as l o w as with indications of antiferromagnetic exchange.

Conclusions Clearly t h e l o w - d i m e n s i o n a l nature o f t h e s t r u c ­ t u r e s o f t h e s e compounds i s r e f l e c t e d i n t h e i r mag­ netic behavior. Th served i n the paramagneti one- o r t w o - d i m e n s i o n a l models. One i n t e r e s t i n g d i f f e r e n c e b e t w e e n t h e (M=Co) a n d (M=Mn) compounds i s t h e r e l a t i v e change i n the s t r e n g t h o f t h e i n t r a c h a i n ex­ c h a n g e when C I i s r e p l a c e d by B r - i n c r e a s i n g f o r (M= Mn) a n d d e c r e a s i n g f o r (M=Co). U n f o r t u n a t e l y , no i n ­ terpretation i s possible until detailed crystal struc­ tures are a v a i l a b l e . Indeed, the s i m i l a r i t i e s and d i f ­ f e r e n c e s among t h e members o f t h e s e r i e s , a l o n g w i t h t h e a n i s o t r o p i c b e h a v i o r make t h e m i d e a l c a n d i d a t e s f o r further experimental studies. Literature Cited 1.

L o s e e , D. Β . , M c E l e a r n e y , J. N., S h a n k l e , G. Ε . , Carlin, R. L . , Cresswell, P. J., R o b i n s o n , W a r d T., P h y s . R e v . Β ( 1 9 7 3 ) , 8, 2185. 2. L o s e e , D. Β . , M c E l e a r n e y , J. Ν . , Siegel, Α. Ε . , Carlin, R. L . , K h a n , Α . , R o u x , J. P., J a m e s , W. J., P h y s . Rev. B ( 1 9 7 2 ) , 6, 4 3 4 2 . 3. Stirrat, C. R., Dudzinski, S., Owens, A. H., Cowen, J. Α . , P h y s . R e v . Β ( 1 9 7 4 ) , 9, 2183. 4. O n s a g e r , L . , P h y s . R e v . ( 1 9 4 4 ) , 6 5 , 1 1 7 . 5. Fisher, M. E . , J. M a t h . P h y s . (1963), 4, 124. 6. M o r i y a , T., P h y s . R e v . ( 1 9 6 0 ) , 120, 9 1 . 7. S m i t h , T., Friedberg, S. Α . , P h y s . R e v . ( 1 9 6 8 ) , 176, 660. 8. Dingle, R., Lines, M. E . , Holt, S. L . , P h y s . R e v . ( 1 9 6 9 ) , 187, 643. 9· M c E l e a r n e y , J. N., L o s e e , D. B., M e r c h a n t , S., Carlin, R. L . , P h y s . R e v . B ( 1 9 7 3 ) , 7, 3314. 10. W a l k e r , L . R., Dietz, R. E . , A n d r e s , K., D a r a c k , S. Solid S t a t e Commun. ( 1 9 7 2 ) , 1 1 , 593.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

15 T h e Ferromagnetic Ordering of Ferrous Chloride Polymers of Diimine Ligands W.

M.

REIFF*,

B.

DOCKUM,

and C.

TORARDI

Northeastern U n i v e r s i t y , Boston, Mass. 02115 S.

FONER,

R. B .

FRANKEL,

a n d M. A .

WEBER

F r a n c i s B i t t e r N a t i o n a l M a g n e t L a b o r a t o r y , Massachusetts Institute of Technology, C a m b r i d g e , Mass. 02139

Magnetization and Mössbauer studies show that F e ( b i p y r i d i n e ) C l and Fe(phenanthroline)Cl order ferromagnetically at Tc ~ 5 K . Methyl substituted bipyridine derivatives appear to be slowly relaxing paramagnets in the range of 12 to 2 K. These results are correlated with near— and f a r - i n f r a r e d and x-ray spectroscopic data which suggest six-coordinate chloro-bridged polymeric structures for the unsubstituted d i imine systems, and five-coordinate dimeric structures for the methyl substituted compounds. 2

2

Introduction In recent years there have been extensive studies of magnetic behavior of simple dimeric o r s m a l l multimetal cluster compounds containing a variety of organic ligands. The driving force for such studies i s the hope of gaining a better understanding of exchange interactions i n magnetically condensed inorganic salts such as anhydrous and hydrated metal halides which exhibit extended cooperative magnetic behavior. F o r instance, anhydrous ferrous chloride has the cadmium chloride structure with i n t r a - and inter-chain chloro-bridging. The intra-chain interaction for this compound i s ferromagnetic (J>O) where as the inter-chain interaction i s weaker and antiferromagnetic. A s a consequence, anhydrous ferrous chloride exhibits(1,2) "meta-magnetic" phase transition f r o m an antiferromagnetic to paramagnetic state in an external field of about 11 kG. Extended (lattice) ferromagnetic interaction in transition metalorganic ligand systems i s much less common than for simple inorganic a

* Please address correspondence to this author. 205

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

206

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

salts. In this article we present a M6ssbauer and magnetic susceptibility study of such behavior in polymeric octahedral complexes of the type F e ( d i - i m i n e ) C l where the di-imine i s either 2,2'-bipyridine, hereafter bipyridine o r 1,10-phenanthroline and substituted derivatives. We also present x-ray and spectroscopic data bearing on the molecular structure of these compounds. It i s of interest to see if there are any significant magnetic dilution effects on placing organic ligands " i n " ferrous chloride and at the same time maintaining a polymeric structure. The weakening of inter-chain interaction by the ligand dilution allows the possibility of lower dimensionality magnetic interaction and weak metamagnetic behavior. In this connection we make comparisons of the mag­ netic behavior of Fe(bipyridine)Cl2 and Fe(phenanthroline)Cl2 to our preliminary results for F e ( p y r i d i n e ) C l The latter system i s also a chloro-bridged p o l y m e r ^ rather than the analogou chromophor Fe(bipyridine)Cl2 effects of preparative technique are also considered in the discussion of Fe(5,5'-(U-CH3-bipyridine )C1 prepared by high vacuum thermolysis. 2

2

Chemical Preparation Analytical data f o r the compounds studied are given i n Table I. A l l of the complexes were studied as powders as the preparations do not yield appropriate single crystals. The synthetic methods used con­ sisted of rapid precipitation f r o m aqueous-hydrochloric acid solution o r vacuum thermolysis of [Fe(di-imine)^ C l yielding fine dust-like powders. Attempts at conductivity or molecular weight measurements by the usual solution methods results in disproportionation to the thermodynamically more stable Fe(di-imine>3 . 2

Table I: Analytical Data Compound

Calculated C

Η

Ν

Fe 18.20

Fe (phenanthroline )C1

2

46.95 2.63

9.12

Zn (phenanthroline )C1

2

45. 50 2.55

Fe(bipyridine)Cl ,

Fe(4,4 -di-CH bipyridine)Cl 3

2

Observed C

Η

Ν

46.77 2.66

9.34

8.85

45. 55 2.41

8.79

42.42 2.86

9.91

42.26 2.86

10.10

46.31

3.90

9.01

46.36 3.86

8.69

46.31

3.90

9.01

45.81

3.87

8.35

2

,

Fe(5,5 -di-CH bipyridine )C1 3

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

Fe 18.60

15.

REiFF E T A L .

Ferwus

Chloride

Polymers

207

Susceptibility and Magnetization Studies A p r e l i m i n a r y study of the magnetic susceptibility^) showed that a solution preparation of Fe(phenanthroline)Cl2 orders ferromagnetically with T = 8 ± 2 K. F i t s with a Curie-Weiss law correspond to a p a r a ­ magnetic C u r i e temperature θ = 12 ± 4 Κ and the Curie-Weiss constant C = 3.81 emu/mole. In this section we present magnetic data for Fe(bipyridine)Cl2 also prepared in solution and the substituted derivative Fe(5, S'-di-CHg-bipyridineX^ obtained by vacuum thermolysis. Fe (bipyr idine )Cl2 clearly i s s i m i l a r to Fe (phenanthroline )Cl2 but has a stronger ferromagnetic interaction. Evidence f o r a ferromagnetic inter­ action i s seen (Fig. 1) i n the X g vs Τ plot f o r which the intercept i s large 20 K) and positive. The plot of X g vs Τ i s linear from ~60 Κ to 200 Κ and a fit with the Curie-Weis C = 5.26 emu/mole. Th tion σ at low field (Fig. 2) shows the expected rapid r i s e in the vicinity of T which i s estimated to be ^ 8 K. A m o r e precise estimation of T will be discussed in connection with the Mtfssbauer data. Between 0 and ~ 1 0 k G at 4.2 Κ there i s a rapid r i s e i n the magnetic moment per gram, σ. However, above ~ 1 0 k G there i s a gradual increase in σ and the compound i s clearly not saturated for applied field B as large as 200 kG, F i g . 3. It i s interesting to point out that the sharp r i s e in σ vâ Τ seen i n F i g . 2 i s quite s i m i l a r to that found(^) for the linear chain polymer Co (pyridine )2 CI2 · In the latter compound the sharp r i s e in σ i s attributed to strong one-dimensional ferromagnetic intra-chain interaction (corre­ lation) above the Néel temperature at which the complex undergoes three-dimensional antiferromagnetic ordering. A s i m i l a r positive intrachain correlation may well be o c c u r r i n g in Fe(bipyridine)Cl2 and Môssbauer data bearing on this possibility will be discussed subsequentc

1

1

c

c

0

iy. The field and temperature dependence of the magnetic properties for Fe(5,5 -di-CH3-bipyridine) CI2 are somewhat different from those of Fe(bipyridine)Cl2. A plot of Xg* vs Τ (Fig. 4) appears to have a near zero o r s m a l l negative temperature intercept. A fit of the data yields 6 OK o r slightly negative with C = 4.3 emu/mole. Thus this material appears to exhibit very weak, possibly antiferromagnetic interactions. A weak exchange interaction i s also indicated by the gradual r i s e in σ vs B for this compound. The difference in magnetic behavior f o r the two bipyridine systems i s related to a difference i n molecular structure. f

Q

Magnetic Moments. Effective magnetic moments in Bohr mag­ netons (fig) are calculated using the relation μ

= Λ/357ΝΟ Ά

Μ

(Τ-β)

=

2.828

Je

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

208

EXTENDED

INTERACTIONS

BETWEEN

METAL

200

Figure

1. χ,," vs. Τ for Fe(bipyridine)Cl>, applied field B 1.14 kG for Ί<80Κ, B„ = 16.8 kG for Τ >80K 1

Fe(Bipyridine)CI

u

=

2

Η = 1.14 kilo-gauss

-ημη-Ο-ηίη-

20

30

40

50

60

70

T(°K)

Figure

2.

Magnetic

moment per gram, σ, vs. Τ at 1.14 kG for Fe( bipyridine )C l>

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

REiFF E T A L .

Ferrous

Chloride 1

~~Ί

100

1

Polymers 1

1

1

1

1

r

1

ο ο ο ο ο ο ο ο

Fe (Bipyridine ) C l

50

_J

ο

Figure

1

I

L

J

40

σ vs. applied

3.

I

field, B dine)Cl

2

L Ι­

at 4.2 Κ for

0>

Fe(bipyri-

2

Fe(5,5'-Di-CH -Bipy)CI 3

2

20

15

X 10

OK

Figure 6

L_

4.

y„~ vs. Τ for l

100

Τ (Κ)

FefrS'-di-CHs-bipyridinefih, 16.8 kG

200

B

0

=

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

210

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

where k i s the Boltzmann constant, N Avogadro's number and X ^ j i s the corrected m o l a r susceptibility. The values of M f f obtained f o r the preceding values of C are: Fe(phenanthroline)Cl2 (5. 5 μ ) , Fe(bipyridine)Cl (6.4 μ ) and Fe(5,5*-οϋ-αΗ3-Μρ^(ΐ1ηβ)αΐ2 (5.8 μ ) . Since the spin-only value of the magnetic moment f o r high-spin ferrous i s 4.9 μ g, the foregoing values of μ ^ represent i n part unusually large orbital contributions to the moment,in particular f o r the last two com­ pounds. There i s no evidence of any high-spin f e r r i c o r other impurity that could account f o r the magnetic moment values we observe. F o r instance, an high-spin iron III impurity of ~ 3 0 % would be required to increase the room temperature moment from 5.1 to 5.4 μ . Such an impurity content would be readily observed in both the analytical data and Mflssbauer spectra (Fig 5) N evidenc f o iro III i detected The Mo'ssbauer spectru single iron II environment. Furthermore, for i r o n III complexes of d i imine ligands, there i s generally instability and a strong tendency f o r reduction of f e r r i c to diamagnetic ferrous even on exposure to ordinary light. It i s interesting to note that in a study of Fe(bipyridine)Cl and Fe(phenanthroline)Cl at 300 K, Dwyer et al(6) also observed large moments, 5.79 μ and 5.72 μ respectively. Thus we believe that the observed large effective moments are " r e a l " and probably reflect;in part, an orbital contribution. Difficulty i n saturating the compounds at low temperatures (Fig. 3) suggests a large magnetic anisotropy consistent with large, anisotropic orbital contributions to the moment. F o r instance, the moment μ c a l ­ culated using the relation: Q

e

Β

2

Β

Β

Θ

Β

2

2

Β

Β

where M i s the molecular weight and σ the magnetization i n emu/gram are given in Table II. It i s evident that with increasing B the moments of a l l three systems are increasing. However, the high field behavior shown in F i g . 3 and a s i m i l a r data for Fe(phenanthroline)Cl2 indicate that these systems are not saturated even at 200 kG. The spin-only value of μ i s given by 0

When g = 2 and S = 2 for high-spin ferrous, μ = 4 μ , the expected saturation moment. T h e r e i s , thus, substantial enhancement of the moment for Fe(bipyridine)Cl2 and this i s reflected in its high μ ^ and μ . The somewhat l a r g e r value f o r the moment (Table II) of Fe(bipyridine)Cl than f o r F e (phenanthroline)Cl2 i s consistent with the Β

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

REiFF E T A L .

15.

Ferrous

Chloride

211

Polymers

Table II: Magnetic Moments Compound Fe(bipyridine)Cl

2

Fe(phenanthroline)Cl

2

,

Fe(5,5 -di-CH -bipyridine)Cl 3

2

T(K)

B (kG)

^

4.2

16.8

4.7

3.0 4.2

48.4 215

3.1 3.9

4.2 4.2 3.0

14.7 16.8 48.4

1.9 2.1 3.4

Q

f

l a r g e r ferromagnetic interaction of the former; a ratio of 6 s f o r these complexes is ~2:1. To conclude this section it i s important to mention that the second o r d e r Zeeman effect and spin-orbit coupling can also contribute to the enhancement of magnetic moments to values greater than the spin-only. However, this problem has been studied^) i n some detail for simple tetrahedral iron complexes such as [ ( C H 5 > N ] F e C l ^ whose effective moments (~5.4 to 5.6 μ%) are high. It i s found that the combination of the foregoing effects increases the moment to only ~5.2 Mg. F o r the rather distorted systems of this investigation these effects are expected to be less important. Thus the effective moments for the present com­ pounds are unusually high and we have no simple explanation f o r their origin. It is difficult to envision a large direct orbital contribution. A small amount of configuration interaction of the ground 3d^ with a nearby 4s*3d having a much higher spin-only moment could greatly enhance the moments but this would be difficult to demonstrate. 2

4

2

Mflssbauer Studies The Mëssbauer spectra of Fe (bipyridine )C1 and Fe(phenanthroline)Cl made in solution are v e r y s i m i l a r . The onset of magnetic order i s easily seen in the Môssbauer spectra, as illustrated in F i g . 6 f o r Fe(phenanthroline)Cl . The C u r i e points of Fe(bipyridine)Cl and Fe(phenanthroline)Cl are T = 3.8 Κ and T = 5.0 Κ respectively. Magnetization data previously reported f o r the Fe (phenanthroline )C1 suggested a transition temperature of about ~8 K. The higher apparent T observed by the magnetization measurements 2

2

2

2

2

c

c

2

c

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

212

EXTENDED

INTERACTIONS

BETWEEN METAL

Figure 5. Mossbauer spectrum of Fe(5,5'-di-CH bipyridine)Cl at 78 s

2

-2.00

0

2.00

K,B =0 o

4.00

VELOCITY (mm/sec) RELATIVE TO IRON

(a)

'

Figure 6. Mossbauer spectra of Fe(phenanthroline)Cl at (a) 9.0 K, (b) 5.3 K, (c) 5.0 K, and (d) 3.9 Κ g

-2

0

2

VELOCITY (mm/sec)

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

15.

REiFF E T A L .

Ferrous

Chloride

Polymers

213

may be ascribed to the difficulty in defining the magnetic transition f o r the i r r e g u l a r l y shaped powder sample. The large susceptibility above T i s illustrated i n F i g . 7 where spectra of Fe(bipyridine)Cl2 ( T = 3.8 K) at 4.2 Κ are shown at zero magnetic field and in longitudinal magnetic fields of 4 and 35 kG. It i s seen that the small external field of 4 kG induces a hyperfine field without polarizing the moment along the ex­ ternal field direction. T h i s i s indicated by the presence of Am = 0 lines in the spectrum and by the fact that the angle 3 between the principal component of the electric field gradient and the magnetic hyperfine field as deduced from the spectrum i s unique,and close to that observed i n the ordered state below T in zero external field. At B = 35 kG, the appearance of the spectrum i s considerably altered due to the polariza­ tion of the ferromagnetic moment by the external magnetic field, with a consequent randomizatio Below their respectiv Fe(phenanthroline)Cl have essentially s i m i l a r spectra. F o r the f o r m e r at 1.5 K, = -60 kG, Δ Ε = +1.70mm/sec and & « 60° while f o r the lat­ ter 1% » - 75 kG, Δ Ε = +2.03 mm/sec and 8 « 60°. A s discussed previously the temperature and field dependence of magnetization of Fe(5,5 -di - C H3 -bipyridine )C1 (prepared by thermoly­ sis) suggests weak,possibly negative,magnetic exchange interactions. The temperature dependence φη zero field) of the Mo'ssbauer spectra f o r this material also suggests weak magnetic interactions. Instead of a sharp ferromagnetic transition over a s m a l l (^ 0.5 K) interval as in F i g . 6, Fe(5,5 -di-CH3-bipyridine)Cl exhibits gradual changes of the magnetic hyperfine splitting over a much l a r g e r temperature range. The transitions of the quadrupole doublet start broadening at ~12 Κ and a fully resolved Zeeman spectrum i s not observed until ~2 Κ indicating slow paramagnetic relaxation rather than a cooperative ordering p r o c e s s It will be shown that this compound contains high-spin iron II in a highly distorted 5-coordination environment. F o r such a low symmetry, longer spin-lattice relaxation times and slow paramagnetic relaxation are not unexpected. The observation of this phenomenon is f a r less common f o r high-spin ferrous than f o r f e r r i c complexes. c

c

c

Q

2

1

2

,,

,,

,

2

Molecular Structure Studies F a r Infrared Spectra and X - r a y Data. The difference in the mag­ netic behavior of Fe(bipyridine)Cl and Fe(5, S ' - d i - C ^ - b i p y r i d i n e J C ^ i s probably due to the preparative method and a difference of basic molecular structure resulting therefrom, rather than from just simply a substituent effect. Figure 8 shows the far-infrared spectra of Fe (phenanthroline )Cl2 (prepared in solution) and Zn(phenanthroline )C1 . The latter zinc complex i s known by a single crystal x-ray study^) to be a pseudo-tetrahedral monomer. We have compared the x-ray powder 2

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

214

EXTENDED

Figure

8.

INTERACTIONS

BETWEEN

METAL

WAVENUMBER Far-infrared spectra of Fe and Zn (phenanthroline)C^-min­ eral oil mull on polyethylene at 300 Κ

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

15.

REiFF E T A L .

Ferrous

Chloride

215

Polymers

patterns of Fe(phenanthroline)Cl2 and the zinc analogue and they are not s i m i l a r , indicating these systems are not isomorphous. T h i s i s reflected in the f a r - i n f r a r e d spectrum of Zn(phenanthroline)Cl2. The two strong terminal Z n - C l stretching vibrations expected for a monomer having approximate 0 γ symmetry are seen as a broad band centered ~325 cm"*. In the i r o n analogue these vibrations are shifted to con­ siderably lower energy (~255 cm" ); this is also true f o r the zinc and i r o n bipyridine complexes. These results are consistent with chlorobridging(9> 10) as shown i n F i g . 9. A s i m i l a r polymer structure has been proposed as part of a recent study(H) of Sn(bipyridine)Cl2, i . e . , an infinite linear polymer chain with chloro-bridging. 2

1

The f a r infrared spectrum of Fe(5,5'-CU-CH3-bipyridine)C1 is shown in F i g . 10. Stron band t 322 d 240 l indicat th ence of both terminal an involving five-coordinate i r o n II. Fe(4,4*-di-CH3-bipyridine)Cl2 ex hibits a s i m i l a r spectrum and the 0 U - C H 3 substituted systems probably have a dimeric structure as shown in the structure of F i g . 11. That i s , a combination of steric effects f r o m the methyl substituents and high vacuum thermolysis preparation results i n dimeric rather than the extended polymeric structure proposed for the solution preparation of Fe (bipyridine )C1 . A dimeric structure s i m i l a r to that presented in F i g . 11 has been found in a single crystal x-ray s t u d y ( ^ ) of [Ni(2,9-di-CH -phenanthroline)0 ]9· R e c e n t ^ ) magnetic studies of other s i m i l a r chloro-bridged nickel II dimers suggest weak (intradimer) antiferromagnetic exchange and i s consistent with our results for the methyl substituted derivative. 2

2

13

2

3

Near Infrared Spectra. The near-infrared spectrum of Fe(phenanthroline)Cl2 given in F i g . 12 i s t y p i c a l ^ * ) of a pseudooctahedral FeN2Cl4 chromophore and strongly supports the proposed structure. Fe(pyridine) Cl2 i s an octahedral p o l y m e r ^ ) containing the same chromophore but with trans-nitrogens. As expected, it ex­ hibits a near-infrared spectrum quite s i m i l a r to that of Fe(phenanthroline)Cl2 and F e ( b i p y r i d i n e ) C l . The sign of the quadrupole interaction for these systems i s positive and consistent with a d^y ground orbital. We thus tentatively assign the transitions observed at 6000 and 10,300 c m " as ^ ( c L ^ ) - Β ι ( ά ) 2< xy) - ^ l ^ z ) respectively. ^ F o r a five-coordinate F e ^ C L j chromophore as suggested for the methyl substituted complexes, one expects the d-d transition at some­ what lower energies. We observe a ligand-field band for Fe(4,4*-di-CH3-bipyridine)C1 at 9100 and 5000 cm" . 4

1 5

2

2

1

5

a

χ 2 -

n

d

5B

d

2

2

x

1

2

Mflssbauer Isomer Shifts.

In Table ΠΙ we present some isomer

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

216

EXTENDED

INTERACTIONS

BETWEEN

METAL

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

REiFF E T A L .

Ferrous

Chloride

Polymers

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

218

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

shift (δ) and quadrupole splitting (ΔΕ) data f o r compounds (1,2,3,4) investigated in this

work and compare these data to those for other

compounds (5 through 8) of known structure. Table ΠΙ: Mtfssbauer Data Compound T(K)

Structure - Chromophore 1. Fe (phenanthroline )C1 polymeric - o c t a h e d r a l - c i s - F e N C l ^ 2

2

2. Fe(phenanthroline)Br polymeric -octahedral -cis - F e N

_,_ 6+

Reference

ΔΕ

300 78

1.04

1.45 2.17

*

1.16

78

1.13

1.98

*

300 78

0.93 1.06

3.51 3.72

*

300 78

1.07

0.56 1.28

15

1.20

300 78

1.05

3.15

15

1.14

3.35

300 78

0.86 0.98

2.76 3.07

16

300

0.80

78

2.66 2.77

*

0.91

*

2

2

Cl

4

3. Fe(4,4'-di-CH -bipyridine)C1 3

dimer-five-coordinate F e N C l 2

3

,

4. Fe(5,5 -di-CH -bipyridine)Cl9 dimer-five-coordinate F e N C l 3

2

3

5· F e ( p y r i d i n e ) C l polymeric -octahedral -trans - F e N C l ^ 2

2

2

6. F e ( p y r i d i n e ) C l monomeric-octahedral-trans-Fe 4

2

7. F e (quinoline ) C l monomeric -tetrahedral - F e N C l 2

2

2

2

8.

C1 N

2

Fe(2,9-di-CH -phenanthroline)C1 3

monomeric -tetrahedral - F e N C l 2

2

2

4

*

*

* This work *

mm/sec, relative to iron f o i l , source at 300 K.

It i s seen that f o r the six-coordinate FeClo-imine nitrogen systems a more positive isomer shift i s observed.(I*) With decreasing coordination number the isomer shift generally d e c r e a s e s ^ a s evidenced by table entries 7 and 8 f o r known tetrahedral complexes. (-^) The intermediate i s o m e r shift^O) and large quadrupole interaction of compounds 3 and 4 are reasonable f o r the proposed five-coordinate structure. Magnetic Behavior and Structure In view of the proposed structure (Fig. 9) and considerable support­ ing data, the observation of ferromagnetism f o r Fe (bipyridine)C1 and Fe(phenanthroline)Cl i s not unreasonable. Various m o d e l a i » ) p r e ­ dict ferromagnetic interaction f o r such a structure when (a) the metal and bridging atoms l i e in the same plane; (b) bridge angles are ~ 9 0 ° ; (c) the exchange involves bridge atom p- and metal d-orbitals; 2

21

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

22

15.

REIFF ET AI. Ferrous

Chloride

219

Polymers

and (d) the exchange is between metal atoms i n oxidation states c o r r e ­ sponding to a more than half-filled t g manifold. A study of the ferromag­ netic behavior f o r other bridging anions ( F , B r , I ) m a y enable a s s e s s ­ ment of the contribution of direct metal-metal exchange as opposed to "superexchange" via the bridging ligands since the metal-metal distance can be v a r i e d considerably. T h i s work i s now in progress. A l s o inter­ esting is the absence of meta-magnetic behavior for the systems of this investigation in studies to as low as 1.4 Κ and for fields up to 200 kG. We have observed metamagnetic behavior i n Fe(pyridine)2Cl2 and Fe (pyridine ) ( N C S ) . This work will be discussed in a forthcoming pub­ lication. ( ) 2

-

2

-

-

2

2 3

1. Wilkinson, M.K., Cable, J.W., Wollan, E.O., and Koehler, W.C., Phys. Rev., (1959) 113,497. 2. Simkin, D.J., Phys. Rev., (1968) 177, 1008. 3. Dunitz, J.D., Acta Cryst. (1957) 10, 307. 4. Reiff, W.M.,and Foner, S., J. Amer. Chem. Soc., (1973) 95, 260. 5. Takeda, K., Matsukawa, S., and Haseda, T., J. Phys. Soc.Japan, (1971) 30, 1330. 6. Broomhead, J.A., Dwyer, F.P., Austr.J. Chem., (1961) 14,250. 7. Clark, R.J.H., Nyholm, R.S. and T a y l o r , F.B., J. Chem. Soc., (A) (1967) 1802. 8. Reimann, C. W., Block, S., Perloff, A., Inorg. Chem., (1966) 5, 1185. 9. Postmus, C., F e r r a r o , J.R., and Wozniak, W. (1967) 6, 2030.

Inorg. Chem.,

10. Wilde, R. F., Srinivasan, T.K.K., Ghosh, S.N., J. Inorg. Nucl. Chem., (1973) 35, 1017. 11. Fowles, W.A., and Khan, I.Α., J. L e s s Common Metals, (1968) 15, 209. 12. Preston, H.S., and Kennard, C.H.L., J. Chem. Soc. (A) (1969), 2682. 13. Hendrickson, D. private communication. 14. Goodgame, D.M.L., Goodgame, M., Hitchman, M.A., and Weeks, M.J., Inorg. Chem., (1966) 5,635.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

EXTENDED INTERACTIONS BETWEEN METAL IONS

220 15.

Long, G.J., Whitney, D.L.,

and Kennedy, J.E., Inorg. Chem.,

(1971) 10, 1406. 16.

Long, G.J., and Whitney, D.L., 33, 1196.

J. Inorg. Nucl. Chem., (1971)

17.

Burbridge, C . D . , Goodgame, D.M.L., Goodgame, Μ . , J. Chem. Soc. (A) (1967), 349.

18.

E r i c k s o n , Ν . E. i n , "The Mössbauer Effect and Its Application i n Chemistry", Advances in Chemistry Series, No. 88, American Chemical Society, Washington, D.C. (1967).

19.

Edwards, P. R., Johnson, C.E., Phys., (1967) 47, 2074

20.

Reiff, W.M., Erickson (1969) 8, 2019.

21.

Anderson, P.W., "Magnetism", (1963) Volume 1, Ch. 2, Rado, G. Τ., and Suhl, Η., Ed., Academic Press, New York, Ν. Y.

22.

Goodenough, J.B., "Magnetism and the Chemical Bond", Interscience, New Y o r k (1963), pp. 165-185.

23.

Reiff, W.M., Long, G.J., Little, B.F., in preparation.

and Williams, R.J., J. Chem.

Foner, S., Frankel,

R.B.,

Acknowledgements W. M. Reiff i s pleased to acknowledge the partial support of the Research Corporation and the Petroleum Research Fund administered by the American Chemical Society. He thanks the National Science Foundation (NSF Grant No. G H 39010) for the major support of the investigation. Finally, he thanks Dr. Graham Hunt of A F C R L for use of the Perkin-Elmer 180 IR-spectrometer. M. A. Weber was supported by an Organization of American States Fellowship. The F r a n c i s Bitter National Magnet Laboratory i s supported by the National Science Foundation.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

16 T h e Unusual Magnetic Properties o f an Imidazolate B r i d g e d P o l y m e r o f an Iron(III) H e m i n I R W I N A. C O H E N Brooklyn College, The City University of New York, Brooklyn, N.Y.

11210

DAVID OSTFELD Seton Hall University, South Orange N.J For the l a s t s e v e r a t r o n t r a n s f e r processes i n v o l v i n g metalloporphyrins because o f the important b i o l o g i c a l r o l e of the cytochromes (1,2). This has l e d us t o consider the stability o f l i g a n d b r i d g i n g between metalloporphyrins and the utilization o f such bridges i n e l e c t r o n t r a n s f e r and s p i n coupling processes. The antiferromagnetic coupling of the two i r o n s i n [ T P P F e ( I I I ) ] O due to the oxo bridge, shown s c h e m a t i c a l l y i n F i g u r e 1, i s now w e l l known ( 3 ) . But other bridges, e s p e c i a l l y those i n v o l v i n g l i g a n d s g e n e r a l l y a s s o c i a t e d w i t h heme p r o t e i n s have not been as w e l l i n v e s t i g a t e d . H i s t i d i n e has been found bound to the heme i r o n in hemoglobin (4) and cytochrome c (5) and t h i s has generated a great d e a l of i n t e r e s t in imidazole adducts of i r o n porphyrins. Thus the [porphyrin F e ( I I I ) ( I m H ) ] i o n has been w e l l studied by s e v e r a l research groups (6-13). The s t r u c t u r e o f that c a t i o n is shown s c h e m a t i c a l l y i n F i g u r e 2. Imidazolate complexes of porphyrins have not been w e l l examined i n s p i t e of the i n t e r e s t i n g possibility of bridge formation and mediation o f e l e c t r o n t r a n s f e r . Reported imidazolate complexes are g e n e r a l l y assumed to be polymeric (14) as confirmed by s i n g l e c r y s t a l s t r u c t u r e determinations on the Z n ( I I ) (15) and Cu(II) (16) complexes. The ability o f t h i s l i g a n d t o conduct e l e c t r o n s i s not shown by the electrical c o n d u c t i v i t y of the imldazolates s i n c e c r y s t a l l i n e samples o f C o ( I I ) , Cu(II) and Zn(II) imidazolates have been found t o possess g r e a t e r resistivities than imidazole itself (17)· However, the antiferromagnetic behavior found f o r copper imidazol a t e (18) has i n d i c a t e d the p o s s i b l e r o l e of the anion i n s p i n p a i r i n g between metal i o n s . Thus, when we observed that the a d d i t i o n o f base to s o l u t i o n s of hemin f l u o r o b o r a t e and imidaz o l e caused the formation o f compounds w i t h porphine t o i r o n to imidazolate r a t i o s o f 1:1:1, we embarked on a study o f t h e i r magnetic p r o p e r t i e s . A d d i t i o n o f a s o l u t i o n of base to one c o n t a i n i n g TPPFeBF and imidazole c o n s i s t e n t l y produced an i n s o l u b l e product. 2

+

2

4

221 In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

222

EXTENDED, INTERACTION'S

BETWEEN

ΜΈΤΛΙ.

ION'S

E x a m i n a t i o n o f t h e X - r a y powder d i f f r a c t i o n p a t t e r n s o f the p r o ­ ducts i n d i c a t e d t h a t exact d u p l i c a t i o n o f the r e a c t i o n c o n d i t i o n s reproduced i d e n t i c a l products but that v a r i a t i o n s i n the solvents lead t o products w i t h d i f f e r e n t s o l i d phases. In a d d i t i o n , the powder p a t t e r n s i n d i c a t e d t h a t i r r e v e r s i b l e c h a n g e s o c c u r r e d u p o n g r i n d i n g the s o l i d phases, a f t e r which a l l the products f a i l e d t o produce any o b s e r v a b l e r e f l e c t i o n s and thus appeared t o b e ­ come a m o r p h o r o u s . No common s o l v e n t ( e . g . HgO, ClfeOH, ( C % ) G Ô DMSO, C % C i , C % C l CeHe) was f o u n d c a p a b l e o f d i s s o l v i n g a n y o f the p r o d u c t s , although long exposure t o methanol g e n e r a l l y caused d e c o m p o s i t i o n o f t h e s o l i d s and produced t h e oxo d i m e r , (TPPFe) 0„ I n no case d i d t h e i n f r a r e d ( K B r ) spectrum o f t h e p r o d u c t r e v e a l a n y a b s o r p t i o n a t 870" . T h i s shows t h e a b s e n c e o f (TPPPe)pO i n e a c h o f t h e p r o d u c t s . The i r a b s o r p t i o n bands c h a r a c t e r i s t i c o f t h e TPPF f r e q u e n c i e s a n d t h e Raman s p e c t r u m o f t h e o n e s o l i d p r o d u c t e x a m i n e d was e x t r e m e l y s i m i l a r t o t h a t o b s e r v e d f o r s o l i d [TPPFe(lmH)p]Cl. 2

f

2 f

2

1

P r e p a r a t i v e r e a c t i o n s w e r e c a r r i e d o u t u n d e r two s e t s o f c o n d i t i o n s which d i f f e r e d mainly i n t h e time allowed f o r addit i o n o f t h e b a s i c s o l u t i o n t o t h a t o f t h e hemin and i m i d a z o l e . T h e r e a c t i o n s c o n s i d e r e d t o be r a p i d i n v o l v e d a d d i t i o n o f t h e basic βοlut ions dropwlse w i t h s t i r r i n g over a p e r i o d o f l e s s t h a n 30 m i n . T h e r e a c t i o n s considered t o be s l o w i n v o l v e d two p h a s e s y s t e m s w h i c h w e r e c a r e f u l l y a d d e d t o t h e same f l a s k s o a s to preserve an i n t e r f a c e . T h e p r o d u c t formed a t t h e i n t e r f a c e o v e r a p e r i o d o f more t h a n two d a y s . V a r i a t i o n o f the composi­ t i o n o f t h e s o l v e n t o r changes i n t h e t i m e a l l o w e d f o r t h e r e ­ a c t i o n d i d n o t a p p e a r t o change t h e o v e r a l l e t o i c h i o m e t r y o f t h e product. A n a l y t i c a l data indicated that the r e a c t i o n product was TPPFeIm · S w h e r e Im i s t h e i m l d a z o l a t e a n i o n a n d S i s a s m a l l m o l e c u l e , u s u a l l y s o l v e n t , s u c h a s igCJ o r C*%OH. F o r one s e t o f r e a c t i o n c o n d i t i o n s , S was o b s e r v e d t o be a m o l e o f ImH. I n n o c a s e was a n y a n i o n e x c e p t i m l d a z o l a t e f o u n d t o be p r e s e n t i n any s o l i d porphine r e a c t i o n product. I t was a l s o f o u n d p o s ­ s i b l e t o c o n v e r t a s a m p l e o f TPPFe Im · ImH t o TPPFe Im · % 0 s i m ­ p l y by w a s h i n g t h e i n s o l u b l e s o l i d w i t h w e t m e t h a n o l a n d d i c h l o r o m e t h a n e t o remove t r a p p e d i m i d a z o l e * The s t r u c t u r e o f t h e p r o d u c t i s p r o p o s e d a s i n d i c a t e d i n F i g u r e 3 a n d i s j u s t i f i e d by t h e f o l l o w i n g : A ) t h e low s o l u b i l ­ i t y o f t h e p r o d u c t i n d i c a t e s a h i g h m o l e c u l a r w e i g h t ; B) t h e a b ­ sence o f a d d i t i o n a l anions r e q u i r e s t h e presence o f i m l d a z o l a t e a n i o n ( r a t h e r than imidazole) f o r charge compensation o f t h e f e r r i c h e m i n ; C ) t h e s i m i l a r i t y o f t h e Raman s p e c t r u m o f t h e product t o that o f the TPPFe(ImH) c a t i o n reveals the hexacoord i n a t i o n o f t h e h e m i n i r o n ; D) t h e o v e r a l l e t o i c h i o m e t r y o f t h e s o l i d s i s a l w a y s T P P / F e / l m = 1/1/1; E ) t h e a d d i t i o n a l s m a l l m o l e ­ c u l e ( e q . % 0 , C % O H ) c o n t a i n e d i n t h e s o l i d s a r e e a s i l y removed without changing the r a t i o o f TPP/Fe/lm; F) other metal i m i d a z o l a t e s h a v e b e e n f o u n d t o be p o l y m e r i c a n d none h a v e b e e n p r o v e n 2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

16.

COHEN

AND

Iwn(III)

OSTFELD

Fe—Ο—Fé

Hemin

223

/

Figure 1. Schematic of the structure (3b) of the μ oxo bridge in [TPPFe~\ 0. Vertical lines represent the plane of the porphine ring. 2

+ Η

Η

Χ ι

Η

\

Ν Ν — F e — Ν

Ν Η Η

\

C

^

C

H

Η

Figure 2. Schematic of the structure (12) of the cation, TPPFe(lmH);. Vertical lines repre­ sent the plane of the porphine ring..

-Fe

-

Fe

— N ^ Q

Η —

—In/2 Figure 3. Proposed structure of [TPPFelm] „. Vertical lines represent the plane of the porphine ring.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

224

EXTENDED

INTERACTIONS

B E T W E E N

M E T A L

IONS

to be momeric. The magnetic p r o p e r t i e s o f s o l i d TPPFe Im · HgO were exam­ ined and a f i e l d dependence o f the s u s c e p t i b i l i t y was observed, f i g u r e k shows a p l o t o f vs 1/1 a t 300°K and includes only those data p o i n t s with H > 6kG. T h i s type o f p l o t i s o f t e n used to determine the paramagnetlc s u s c e p t i b i l i t y o f a sample i n the presence o f ferromagnetic i m p u r i t i e s which can be saturated by the a v a i l a b l e magnetic f i e l d s (19). The i n t e r c e p t i n Figure h as determined by a l e a s t squares f i t o f the data i s 1 0 χ . * 1980 ± 60 cgs/mole at 300°K. À study o f the temperature dependence o f the f i e l d dependent s u s c e p t i b i l i t y would a s c e r t a i n the p o s s i b l e maintenance o f the Curie Law f o r X^. This r e q u i r e s l a r g e f i e l d s t o magnetic a l l y saturate the sample, but the use o f a Cryostat r e s t r i c t s our a v a i l a b l e f i e l d s t use o f a few data p o i n t TT°K. A p p l i c a t i o n o f the above technique t o two d i f f e r e n t s o l i d samples a t TT°K produced values o f l O * ^ o f 69IO and 5990 cgs/ mole. The values o f χ^ r e v e a l an i n t e r e s t i n g value of the mag­ n e t i c moment. At 300°K u = 2.15 BM/Fe and a t 7T°K = 1.9 t o 2.1 HM/Fe. Therefore, w i t h i n experimental e r r o r , the polymers e x h i b i t a paramagnetic moment of 2.1 BM/Fe which i s temperature indepen­ dent between 77 and 300°K. T h i s agrees with the measured moment (between h.2 and 50°K) o f TPPFe ( ImH) C 1 where u » 2.36 BM/Fe (13). These values a r e a l l c o n s i s t e n t with low s p i n F e ( l l l ) ex­ h i b i t i n g some s p i n o r b i t coupling. The l i n e a r i t y shown i n Figure h implies that the f i e l d de­ pendent p o r t i o n o f the s u s c e p t i b i l i t y o f the sample i s saturated when H > 6kG. This i s shown i n Figure 5 as molar magnetization vs f i e l d . In t h i s case the magnetization ( l =* X^H) i s d e t e r ­ mined from the observed χ^ l e s s the f i e l d independent χ^; taken as I920 χ 1 0 ~ cgs/mole i n accord w i t h the r e s u l t o f Figure k. The room temperature s a t u r a t i o n magnetization, f o r TPPFe Im · HgO i s found as 11.5 cgs units/mole. This corresponds to an extremely small moment a t s a t u r a t i o n (300°K) o f 2.06 χ 1CT BM/Fe, i f a l l the i r o n present i s assumed to be i n v o l v e d . As small as t h i s value i s , TPPFeIm · HgO shows magnetic o r d e r i n g a t room temperature. The study o f the temperature dependence o f the s u s c e p t i b i l ­ i t y o f TPPFeIm · HgO was c a r r i e d a t a f i x e d f i e l d o f 6*kk KGauss and the r e s u l t s are presented i n Table I. The d e t * i n d i c a t e s a n o n - l i n e a r r e l a t i o n s h i p between l/χί and Τ w i t h l/χ^ decreasing f a s t e r than the Curie-Weiss Law p r e d i c t s as temperature de­ creases. T h i s behavior (downward curvature o f a l/χ vs Τ p l o t ) i s reminiscent o f a f e r r i m a g n e t i c m a t e r i a l above i t s Neel tem­ perature ( 2 0 ) . However, that i n t e r p r e t a t i o n can be r e j e c t e d s i n c e the Neel temperature according t o t h i s data would have t o β

8

f

f

u

2

e f f

e

3

c

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

COHEN

AND

OSTFELD

Iron(IH)

Hemin

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

EXTENDED

INTERACTIONS

BETWEEN

METAL

Figure 5. Molar magnetization at 300°Κ vs. Η for TPPFelm * H 0. The independent portion of the susceptibility, χι = 1920 X 10' cgs/mole, subtracted from the observed χ °, M = H(x — χι)2

6

c

Μ

M

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

field was

IONS

16.

C O H E N

A N D

Iron(III)

OSTFELD

Hemin

227

be b e l o w 100° K. I b i s i t i n c o n s i s t e n t w i t h the f i e l d dependent behavior of the saae s a m p l e , b e c a u s e s a t u r a t i o n can be o b s e r v e d at room t e m p e r a t u r e . TABLE I . M a g n e t i c P r o p e r t i e s o f TPPFelm · % 0 T°K

10%

~ff 89 10k 127 15* 185 217 251

1%

8200

122

7719 Tlk6 6372 5766

129 ΐ4θ

1*300 1*200 i*08i* 3978

319 3^ β. =

0

%

157 m 189 204

5295 1*908

286 300

1

i o %

c l c / 7792 67^1 5769 472% 3986

4o8 8 9 T

1377 1648 1870 2052

3243 2765

238

2143

2200 2203

2000 1881 L744

245

b

t l i e

% cale Paramagnetic s u s c e p t i b i l i t y c a l c u l a t e d f o r a s y s t e m w i t h u = 2·2 BM. b c % " % cale ' d e p e n d e n t p o r t i o n o f the s u s s

t

l

i

e

f

ceptibility. Inasmuch a s the p a r a m a g n e t i c moment o f T P P F e l m · HgO was shown t o remain between 2 and 2.2 BM from 77 t o 344°K; t h e c o r ­ c r e s p o n d i n g f i e l d independent s u s c e p t i b i l i t y c a n be c a l c u l a t e d (χ^ ) e a c h t e m p e r a t u r e . S u b t r a c t i o n o f | from leaves~>N the f i e l d dependent p o r t i o n o f at each temperature a

t

c

a

c

(Table I j . T h e v a r i a t i o n o f χ^ a t c o n s t a n t f i e l d w i t h Τ i s shown i n F i g u r e 6. It i s apparent that the extent o f magnetiza­ t i o n decreases c o n s i d e r a b l y w i t h d e c r e a s i n g t e m p e r a t u r e . An ex­ t r a p o l a t i o n o f t h e c u r v e i m p l i e s t h a t χ^ s h o u l d become n e g l i g i b l e b e l o w 70°Κ a n d t h e n t h e t o t a l o b s e r v e d s u s c e p t i b i l i t y s h o u l d be due t o χ^ a l o n e . The o b s e r v a t i o n o f t h e l o s s o f m a g n e t i z a t i o n a t low temperature i s i n d i c a t i v e o f a f e r r o m a g n e t i c system w h i c h u n d e r g o e s a m a g n e t i c p h a s e change a t low t e m p e r a t u r e o r a f e r r i magnetic system which possesses a h i g h Neel temperature and a low t e m p e r a t u r e c o m p e n s a ! i o n p o i n t (20). The d e p e n d e n c e is fit

of ^

shown i n F i g u r e 7 ·

It

on Τ a t low temperature is evidently

t h e C u r i e - W e i s s Law i n a n y r e g i o n o f T .

ledge o f the value

of χ β

that the t o t a l

is o n l y about 2 BM.

observed

T h e l a c k o f know­

the constant

It

i s also rather

susceptibility

10$ o f t h e χ^ c a l c u l a t e d

Clearly,

field

but t h e anomalous b e h a v i o r o f

s a m p l e a t a b o u t 6θ Κ i s e v i d e n t .

to note

and h i g h

complex and does n o t

b e l o w 77°Κ p r o h i b i t s s e p a r a t i o n o f

i n t o χ^ a n d χ^ a t e a c h t e m p e r a t u r e the

quite

important

a t low t e m p e r a t u r e

on the basis

of

=

moment o b s e r v e d a b o v e 77°Κ i s n o t

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

228

EXTENDED

INTERACTIONS

BETWEEN

METAL

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

COHEN

Iwn(IH)

A N D OSTFELD

ΗβΤΠΪΠ

-

7.0

" G G

G

— G

ο

o

o

<*>

Q

G G ) -

G

G

G

Θ G

3.( 0

Figure 7.

1 40

Observed

1 80

1 1 120 160 TEMPERATURE °K

χ ° for TPPFelm Μ

1 200

· H,Ο vs. temperature

1 240

at 15 KGauss

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

230

E X T E N D E D

constant

INTERACTIONS

B E T W E E N

M E T A L

IONS

b e l o w 77° K.

The a p p r o a c h used here to determine χ. i n the presence o f a f i e l d dependent moment i s u s u a l l y r e q u i r e d f o r t h e study o f sam­ ples c o n t a i n i n g f e r r o m a g n e t i c i m p u r i t i e s . In a d d i t i o n , the very s m a l l moment o b s e r v e d by d e t e r m i n i n g t h e s a t u r a t i o n m a g n e t i z a ­ t i o n o f TPPFeIm · HgO s u g g e s t s t h a t t h e o r d e r e d s y s t e m i s i n f a c t o n l y an i m p u r i t y i n the sample. I t c o u l d be a r g u e d t h a t the [TPPFeIm] s y s t e m i s b e h a v i n g l i k e [ T P P F e ( l m H ) ] * and p o s ­ ses a temperature independent o f a b o u t 2.3 BM a n d t h e i m ­ p u r i t y a l o n e i s r e s p o n s i b l e f o r the f i e l d dependent e f f e c t s . The a n a l y s e s c e r t a i n l y do n o t r u l e out s u c h a p o s s i b i l i t y . On t h e o t h e r h a n d , t h e s t a r t i n g m a t e r i a l T P P F e B F d o e s n o t show any 2

4

and s i m p l y g r i n d i n g t h e s o l i d p r o d u c t w h i c h i s n o t e x p e c t e d t o e f f e c t t h e amount o f a n i m p u r i t y d o e s c h a n g e t h e v a l u e o f y^ It i s also rather s i g n i f i c a n w i t h imidazole produce f i e l d d e p e n d e n t moment and t h e t r e a t m e n t o f TPPFe B F w i t h OH" produces ( T P P F e ) 0 w h i c h a l s o d o e s n o t e x h i b i t any f i e l d d e p e n ­ dent magnetic p r o p e r t i e s . We do n o t t h e r e f o r e e x p e c t t h a t t r e a t ­ ment o f T P P F e B F by ImH a n d O H * t o g e t h e r w i l l p r o d u c e a f e r r o ­ magnetic i m p u r i t y i n the p r o d u c t . A n o t h e r s t r o n g argument a g a l n s t an i m p u r i t y b e i n g r e s p o n s i b l e f o r the f i e l d dependence f o l l o w s a c o n s i d e r a t i o n o f t h e l o w t e m p e r a t u r e d e p e n d e n c e o f y^. Below 60°R t h e v a l u e o f y^ d r o p s w e l l b e l o w t h a t c o n s i s t e n t w i t h a u = 2 system. T h e r e i s n o way t h a t a m a g n e t i c i m p u r i t y c a n s u b t r a c t from t h e t o t a l s u s c e p t i b i l i t y o f a s a m p l e ; t h a t I s , t h e minimum χ f o r a n i m p u r e s a m p l e must be due t o t h e p a r a m a g n e t i t ­ self. E i t h e r t h e m a g n e t i c b e h a v i o r a n d t h e a n o m a l y a t 60°K i s c h a r a c t e r i s t i c o f t h e b u l k s a m p l e o r we must r e q u i r e t h a t t h e i m ­ p u r i t y a n d t h e p o l y m e r b o t h c o i n c i d e n t a l l y u n d e r g o some m a g n e t i c p h a s e c h a n g e a t a b o u t 60°K. I t i s a l s o s i g n i f i c a n t t o n o t e t h a t n o EPR s p e c t r u m c o u l d be o b s e r v e d a t room t e m p e r a t u r e o r 7 7 ° Κ f o r T P P F e Im · ΗβΟ. How­ ever even 1 p a r t per thousand o f e i t h e r i r o n metal o r magnetite c o u l d be o b s e r v e d i n t h e EPR s p e c t r u m when t h o s e m a t e r i a l s w e r e s p e c i f i c a l l y added t o a s o l i d p o r p h i n e . F i n a l l y , e x a m p l e was p r e p a r e d w h i c h showed a l a r g e f i e l d d e p e n d e n c e o f y^ ( i . e . 133$ change i n when Η v a r i e d f r o m 2 t o 6 K G a u s s ) . The s a m p l e was suspended i n C % C 1 a n d g a s e o u s HC1 was b u b b l e d t h r o u g h t h e s u s ­ pension for 1 mln. The s o l i d i m m e d i a t e l y d i s s o l v e d . The s o l u ­ t i o n was n o t d i s t u r b e d f o r 5 m i n . and t h e n t h e s o l v e n t was r e ­ moved i n a stream o f N . T h e s o l i d was d r i e d i n v a c u o a t room t e m p e r a t u r e and was n o t p u r i f i e d o r a d d i t i o n a l l y t r e a t e d i n any manner. T h e f i n a l s o l i d a s was o b t a i n e d was e x a m i n e d m a g n e t i ­ c a l l y and t h e was o b s e r v e d t o change by o n l y 6.3$ between h i g h and l o w f i e l d . F u r t h e r m o r e , t h e y?. o b s e r v e d c o r r e s p o n d e d t o a u £ £ b e t w e e n 5.6 and 5.8 B M / F e . The v i s i b l e s p e c t r u m i n ­ d i c a t e d t h a t t h e t r e a t m e n t w i t h HC1 h a d c o n v e r t e d t h e p o l y m e r t o T P P F e C l [known u = 5.9 BM ( 2 1 ) ] . 4

2

4

c

2

2

e

e

f

f

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

16.

C O H E N

A N D

osTFELD

Iron(III)

231

Hemin

We c o n c l u d e that the f i e l d dependent m a g n e t i c behavior o f the sample i s c h a r a c t e r i s t i c o f t h e p o l y m e r b e c a u s e m i l d t r e a t ­ ment w h i c h b r e a k s u p the polymer a l s o e l i m i n a t e s t h e f i e l d d e p e n ­ dence o f χ^. The very high s u s c e p t i b i l i t i e s and f i e l d dependence observed i n d i c a t e s that [ T P P F e l m ] i s capable o f magnetic order­ ing. The f a c t t h a t the f i e l d dependent magnetization can be c

n

s a t u r a t e d ( a t room t e m p e r a t u r e ) e l i m i n a t e s a n t i f e r r o m a g n e t i c c o u p l i n g and i m p l i e s t h a t f e r r o - o r f e r r i - m a g n e t i c c o u p l i n g i e

o c c u r r i n g , The temperature dependence o f χ^ does n o t a l l o w t h e s t r a i g h t f o r w a r d a s s i g n m e n t o f e i t h e r f e r r o - o r a n t i f e r r o m a g n e t ism t o t h i s eyetern* A f e r r i m a g n e t i c model r e q u i r e s the presence o f two d i f f e r e n t t y p e s o f m a g n e t i c s i t e s f o r t h e i r o n w i t h a n a n t i ­ f e r r o m a g n e t i c i n t e r a c t i o n b e t w e e n them (20). I t i s e x t r e m e l y d i f f i c u l t t o e n v i s i o n how t h i s c o u l d o c c u r f o r t h i s s y s t e m . In particular, interchai l a r g e s e p a r a t i o n betwee phine r i n g . However, t h e u n u s u a l p r o p e r t i e s o b s e r v e d may be due t o the l i n e a r s t r u c t u r e o f the polymer. The p r o b l e m o f u n i d i m e n s i o n a l m a g n e t i c a l l y o r d e r e d systems has been o f c o n s i d e r a b l e t h e o r e t i c a l and e x p e r i m e n t a l i n t e r e s t for several years. T h i s i s i n p a r t d u e t o t h e p r o o f by M e r m i n and Wagner (22) t h a t a n i n f i n i t e o n e d i m e n s i o n a l s y s t e m a t f i ­ n i t e temperature cannot e x h i b i t l o n g range o r d e r and thus c a n ­ not a c t a s a p e r f e c t f e r r o - o r ant i ferromagne t . I t i s however s t i l l possible f o r s h o r t range i n t e r a c t i o n s t o o c c u r which can lead t o both types o f magnetic o r d e r i n g . S e v e r a l groups have s u b s e q u e n t l y d i s c u s s e d t h e t h e o r y o f t h e s e s y s t e m s (23%24). Most o f t h e e x p e r i m e n t a l work has concerned a n t i f e r r o m a g n e t i c systems (25) b u t C s N i F h a s b e e n f o u n d t o a c t a s one d i m e n s i o n a l f e r r o magnet b y s u s c e p t i b i l i t y a n d n e u t r o n d i f f r a c t i o n s t u d i e s (26). The r e s u l t s o b s e r v e d f o r [ T P P F e I m ] a p p e a r t o o c o m p l i c a t e d t o attempt any q u a n t i t a t i v e f i t t o t h e t h e o r i e s a t t h i s t i m e . How­ e v e r t h e v e r y l o w s a t u r a t i o n moment d e r i v e d f r o m t h e f i e l d d e p e n ­ dent magnetization o f [TPPFeIm] i s c o n s i s t e n t w i t h the o c c u r ­ r e n c e o f o n l y weak, s h o r t range o r d e r i n g o f the f e r r o m a g n e t i c type. I f o n l y a s m a l l p o r t i o n o f the b u l k sample i s o r d e r e d t h e n the remainder o f the i r o n s i t e s could continue t o e x h i b i t normal p a r a m a g n e t i s m a n d c o n t r i b u t e t o a f i e l d i n d e p e n d e n t moment o f reasonable magnitude as observed. T h i s model c o u l d a l s o e x p l a i n t h e e x t r e m e v a r i a b i l i t y o f χ^ b e t w e e n s a m p l e s b e c a u s e t h e c o n d i ­ t i o n s o f p r e c i p i t a t i o n and presence o f s o l v e n t c o u l d g r e a t l y change t h e e x t e n t o f t h e s m a l l o r d e r i n g t h a t o c c u r s . In o r d e r t o b e t t e r a s c e r t a i n the a p p l i c a b i l i t y o f a l i n e a r f e r r o m a g n e t i c m o d e l f o r [ T P P F e l m ^ we w i l l r e q u i r e c o n s i d e r a b l y m o r e i n f o r m a t i o n d e r i v e d f r o m low t e m p e r a t u r e s u s c e p t i b i l i t y a n d Mossbauer s t u d i e s . Knowledge r e g a r d i n g t h e u b e l o w 60°K a n d t h e n a t u r e o f t h e a n o m a l y a b o u t 60°K i s c u r r e n t l y b e i n g s o u g h t . T h e f a c t s t h a t t h e p o l y m e r c a n be p r e p a r e d a n d t h e i m i d a z o ­ l a t e bridges allow metal-metal Interactions to occur, support our i n t e r e s t i n t h i s system. Thus o u r c u r r e n t s t u d y o f t h e r o l e o f 3

n

n

e

f

f

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

232

E X T E N D E D

I N T E R A C T IONS

B F T W E E N

M E T A L

IONS

imidazole i n oxidation reduction reactions i n v o l v i n g i r o n porphy­ r i n s may p r o v i d e a model f o r p a r t o f t h e b i o l o g i c a l e l e c t r o n transport c h a i n * We thank A r t h u r T a u b e r a t t h e U . S . Army E l e c t r o n i c s Command f o r t h e measurement o f s u s c e p t i b i l i t y b e l o w 77° Κ a n d t h e P u b l i c H e a l t h S e r v i c e f o r s u p p o r t o f t h i s w o r k t h r o u g h G r a n t AM-11355·

Literature Cited 1. Cohen, I. A., Jung, C. and Governo, T., J. Amer. Chem. Soc. (1972)94,3003. 2. Abbreviations: TPP=the tetraphenylporphine dianion, C H N4=; Im=the imidazolate anion, C3H3N2¯;X =the field independent magnetic susceptibility; X = the field dependent magnetic susceptibility; X calc. = the field in­ dependent magnetic susceptibility calculated from an as­ sumed moment;X susceptibility. 3.(a)Nicholas, Μ., Mustacich, R. and Jayne, D., J. Amer. Chem. Soc. (1972) 94, 4518. (b) Hoffman, A. B., Collins, D. Μ., Day, V. W., Fleischer, E. B., Srivastava, T. S. and Hoard, J. L., ibid (1972) 94, 3620. (c) Cohen, I. A., ibid (1969) 91, 1980. Also see references found within (a) and (b). 4. Bolton, W. and Perutz, M. F., Nature (1970) 228, 551. 5. Dickerson, R. E., Tarkano, T., Eisenberg, D., Kallai, O. Β., Samson, L., Cooper, A. and Margoliash, E., J. Biol. Chem. (1971) 246,1511. 6. Ogoshi, H., Watanabe, E., Yoshida, Z., Kincaid, J. and Nakamoto, K., J. Amer. Chem. Soc. (1973)95,2845. 7. LaMar, G. N. and Walker, F. Α., ibid (1973)95,1782. 8. Duclos, J. Μ., Bioinorganic Chem. (1973) 2, 263. 9. Coyle, C. L., Rofson, P. A. and Abbott, Ε., Inorg. Chem. (1973) 12, 2007. 10. LaMar, G. N. and Walker, F. A., J. Amer. Chem. Soc. (1972) 94, 8607. 11. Kolshi, G. B. and Plane, R. A. ibid (1972)94,3740. 12. Collins, D. Μ., Countryman, R. and Hoard, J. L., ibid (1972) 94, 2066 13· Epstein, L. M., Straub, D. K. and Maricondi, C., Inorg. Chem. (1967) 6, 1720. 14.(a)Baraniak, E., Freeman, H. C. and Nockolds, C. E., J. Chem. Soc. (1970) A, 2558. (b) Eilbeck, W. J . , Holmes, F. and Underhill, Α. Ε., ibid (1967) 757. (c) Yamane, T. and Davidson, Ν., J. Amer. Chem. Soc. (1960) 82, 2118. 15. Freeman, H. C., Advan. Protein Chem. (1967) 22, 257. 16. Jarivis, J. A. J. and Wells, A. F., Acta Crystallogr. (1960) 13, 1028. 17. Brown, G. P. and Aftergut, S., J. Polym. Sci. (1964) 2, Part A-2, 1839. 18. Inuve, M., Kishita, M. and Kubo, Μ., Inorg. Chem. (1965) 4, 626. 44

28

i

d

i

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

16.

19.

20.

21.

COHEN

AND

OSTFELD

Iron(III)

288

Hemin

W i l l i a m s , D . E . G., "The M a g n e t i c P r o p e r t i e s o f M a t t e r " , p. 9 9 , E l s e v i e r P u b l i s h i n g Co., I n c . , New Y o r k , Ν. Y. 1966. M o r r i s h , A. Η . , "The P h y s i c a l Principles o f M a g n e t i s m " , pp. 486-498, J o h n W i l e y a n d S o n s , Inc., New York, Ν. Y. 1965. Maricondi,

C.,

Swift,

W.

and

Straub,

D . Κ . , J. A m e r .

Chem.

Soc. (1969) 91, 5205. M e r m i n , N . D . a n d W a g n e r , Η . , P h y s . R e v . Lett. (1966) 17, 1133. 23. K o n d o , J. a n d Y a m a j i , Κ . , P r o g . T h e o r . P h y s . (1972) 47, 807. 24. B o n n e r , J. C . a n d Fisher, M . F., P h y s . R e v . (1964) 135, A640. 25. F o r a few e x a m p l e s s e e : ( a ) M c E l e a r n e y , J. Ν., M e r c h a n t , S . and Carlin, R. L., D. Y . a n d Halfield, 3055. ( c ) H u t c h i n g s , Μ. Τ., Shirane, G., B i r g e n e a u , R. J. and Holt, S . L., P h y s . R e v . B . (1972) 5, 1999. ( d ) S w e e n e y , W. V . a n d C o f f m a n , R . E., B i o c h i m . B i o p h y s . A c t a (1972) 286, 26. ( e ) C a s e y , Α. Τ., Morris, B . S., Sinn, E. a n d T h a c k e r a y , J. R., Aust. J. Chem. (1972) 25, 1 1 9 5 · (f) Griffiths, R. Β., P h y s . R e v . (1964) 135, Α659. 2 6 . ( a ) S t e i n e r , M . a n d D o r n e r , Β . , Solid State Comm. (1973) 12, 537. (b) Steiner, Μ., ibid (1972) 11, 73. ( c ) Steiner, M. and D a c k s , H., ibid (1971) 9, 1603. (d) Steiner, Μ., Kruger, W. a n d Bakel, D., ibid ( 1 9 7 1 ) 9, 227. (e) Steiner, Μ., Z. Angew. Physik (1971) 32, 116.

22.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

17 Local and Collective States i n Single and M i x e d Valency Chain Compounds P. DAY

University of Oxford, South Parks Rd., Oxford ΟΧ1 3QR, England

Abstract Compounds c o n t a i n i n g chains o f metal atoms are found with four d i s t i n c t c l a s s e s of s t r u c t u r e , each of which, as a r e s u l t of the widely v a r y i n g strength of the metal-metal i n t e r a c t i o n , has a s s o c i a t e d w i t h it its own c h a r a c t e r i s t i c p a t t e r n of p h y s i c a l p r o p e r t i e s . Thus one may have e i t h e r s i n g l e or mixed v a l e n c y phases w i t h near neighbor contact between metal ions formed e i t h e r d i r e c t l y or through anion b r i d g e s . The o p t i c a l , X-ray photo e l e c t r o n , magnetic and t r a n s p o r t p r o p e r t i e s of examples of each c l a s s are surveyed, with p a r t i c u l a r emphasis on work c a r r i e d out at Oxford, to h i g h l i g h t the r e l a t i v e importance of s i n g l e center and c o l l e c t i v e e x c i t e d s t a t e s in each category, in r e l a t i o n to t h e i r s t r u c t u r e s and the magnitude of the metal-metal i n t e r a c t i o n . I t is pointed out that a c o l l e c t i v e d e s c r i p t i o n may be a p p r o p r i a t e f o r magnetic and e l e c t r o n i c e x c i t a t i o n s even without e l e c t r o n delocalization.

1.

Introduction

Only i n the l a s t few years have i n o r g a n i c chemists i n t e r e s t e d i n the e l e c t r o n i c s t r u c t u r e s of metal complexes p a i d much a t t e n t i o n to the consequences of the i n t e r a c t i o n s which can take place between molecules and ions placed next to each other i n the s o l i d s t a t e . At t h e i r weakest such I n t e r a c t i o n s may manifest themselves only i n r e l a t i v e l y s u b t l e s h i f t s and s p l i t t i n g s of e l e c t r o n i c absorption bands, and i n magnetic o r d e r i n g at low temperatures. On the other hand, strong i n t e r m o l e c u l a r i n t e r a c t i o n s lead to spectacular c o l o r changes or the appearance of new a b s o r p t i o n bands not found i n the spectra of the i s o l a t e d 234

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

17.

DAY

Chain

Compounds

235

molecules or i o n s . F i n a l l y , i n extreme cases the e n t i r e range of p h y s i c a l p r o p e r t i e s of the c r y s t a l may be profoundly d i f f e r e n t from those expected of ordinary i o n i c or molecular s o l i d s . Of the l a t t e r s i t u a t i o n , the most famous instance i s c e r t a i n l y the one-dimensional m e t a l l i c conducting behavior of the p a r t i a l l y o x i d i z e d platinum compounds such as K2Pt(CN)4Bro,3 3H20. The e x c e p t i o n a l p r o p e r t i e s of t h i s m a t e r i a l have served to focus a t t e n t i o n on metal atom chain compounds i n general. For a number of years we ourselves have been i n t e r e s t e d i n both magnetic and charge t r a n s f e r i n t e r a c t i o n e f f e c t s i n i n o r g a n i c c r y s t a l s . The purpose of the present paper i s to b r i n g together some of these observations, p r i m a r i l y concentrating on one-dimensional examples, to exemplify how the o p t i c a l , magnetic and e l e c t r o n transport p r o p e r t i e s are influenced by the strengt atom chain compounds ar and are formed by a l a r g e number of d i f f e r e n t elements. I t may be u s e f u l t h e r e f o r e to attempt some general c l a s s i f i c a t i o n of the p a t t e r n of p h y s i c a l behavior c h a r a c t e r i s t i c of each s t r u c t u r e type, and of the strength of the metal-metal i n t e r a c t i o n . One reason why t h i s c l a s s of m a t e r i a l s i s so i n t e r e s t i n g i s that they span the e n t i r e range from l o c a l i z e d to c o l l e c t i v e behavior and thus should be a good t e s t i n g ground f o r t h e o r e t i c a l models of the s o l i d s t a t e . e

2.

L o c a l i z e d and

C o l l e c t i v e States

Before d i s c u s s i n g the v a r i o u s c l a s s e s of metal chain compound, a general point which needs to be touched on concerns the meaning of the words ' l o c a l i z e d and ' c o l l e c t i v e as used i n t h i s context. U n l i k e s o l i d s t a t e p h y s i c i s t s , i n o r g a n i c chemists are not u s u a l l y accustomed to c o n s t r u c t i n g wavefunctions which a r e i n v a r i a n t to the operations of t r a n s l a t i o n w i t h i n a p e r i o d i c l a t t i c e . Nevertheless, i f a c r y s t a l such as ^ P t (CN)^ absorbs a photon of such an energy that one of the c o n s t i t u e n t complex anions undergoes a l i g a n d f i e l d t r a n s i t i o n , i t i s an inescapable f a c t that we do not know which of the 1023 anions i n the c r y s t a l has been e x c i t e d , and that as a r e s u l t , the e x c i t e d s t a t e wavefunction must allow an equal p r o b a b i l i t y of each i o n being e x c i t e d : i n other words the wavefunction i s a Bloch f u n c t i o n . In one sense the s t a t e which i t describes i s ' c o l l e c t i v e , since the e x c i t a t i o n belongs to the whole l a t t i c e . How f a r and how f a s t i t can move i n p r a c t i c e , however, depends on the magnitude of i n t e r - i o n i c coupling or t r a n s f e r i n t e g r a l s compared with e i t h e r i n t r a - i o n i c e l e c t r o n c o r r e l a t i o n or r e p u l s i o n e f f e c t s on the one hand, or e l e c t r o n i c - v i b r a t i o n a l i n t e r a c t i o n s on the other. Crudely, i f the e x c i t a t i o n i s capable of being t r a n s f e r r e d from molecule to molecule f a s t e r than each molecule can accommodate i t s geometry to the e x c i t a t i o n , then the e x c i t a t i o n , or 'exciton,' 'belongs' to the whole l a t t i c e . 1

1

1

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

236

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

Otherwise, i t s range of t r a v e l i s l i m i t e d to a few adjacent molecules (or f i n a l l y , only to one), and i t i s l o c a l i z e d . D e r e a l i z a t i o n of e x c i t a t i o n , however, i s f a r from implying d e r e a l i z a t i o n of e l e c t r o n s , or the occurrence of e l e c t r o n i c c o n d u c t i v i t y . E x c i t o n m i g r a t i o n over hundreds of angstroms i s f a m i l i a r i n aromatic molecular c r y s t a l s , f o r example, yet i n t h e i r ground s t a t e s they are e x c e l l e n t i n s u l a t o r s . For c o n d u c t i v i t y i t i s obvious that we need to form c r y s t a l s t a t e s i n which the e l e c t r o n occupancy at d i f f e r e n t s i t e s has been a l t e r e d , e.g., c r e a t i n g Pt(CN)|~ and Pt(GN)^ i n K ^ P t i C N ) ^ In o r d i n a r y i o n i c and molecular c r y s t a l s such processes r e q u i r e a l a r g e energy i n p u t , and i t i s t h e r e f o r e i n a p p r o p r i a t e to t a l k of c o l l e c t i v e e l e c t r o n s t a t e s i n these m a t e r i a l s . I n t e r i o n i c e l e c t r o n t r a n s f e r s t a t e s are only formed r e a d i l y i n s o l i d s c o n t a i n i n g an i n t e g r a e l e c t r o n concentratio e l e c t r o n from the p o s i t i v e hole l e f t on the i o n from which i t o r i g i n a t e d . In c o n t r a s t , i f d i f f e r e n t s i t e s i n the c r y s t a l are already occupied by d i f f e r e n t numbers of e l e c t r o n s when the l a t t i c e i s formed, much l e s s energy i s needed to form s t a t e s i n which these s i t e s are merely interchanged. I t may then be p o s s i b l e to form conducting s t a t e s at lower e l e c t r o n c o n c e n t r a t i o n s , or with smaller overlap between i o n s , than one would need to generate energy bands of f i n i t e width i n s t o i c h i o m e t r i c s i n g l e valence s o l i d s . T h i s i s the s i g n i f i c a n c e of mixed valency, as we s h a l l see i n the survey of metal i o n chain compounds which follows." 3.

Types of T r a n s i t i o n Metal Chain Compound

T r a n s i t i o n metal compounds i n which each c a t i o n has only two nearest neighbors are formed both i n s i n g l e and mixed valency s i t u a t i o n s . In t u r n , each of these may be formed e i t h e r with or without anion b r i d g e s between the neighboring c a t i o n s . Some examples of each type of compound are given i n Table 1. The p r i n c i p a l c l a s s of anion bridges s i n g l e valence chains i s that of the hexagonal p e r o v s k i t e s ABX3. When X i s a h a l i d e i o n and A a u n i v a l e n t i o n the l a t t i c e can be thought of as made up of a c l o s e packed a r r a y of A and X, i f they are of comparable s i z e , e.g., Cs* and C l ~ . Now i n a c l o s e packed assembly of ions the number of o c t a h e d r a l holes equals the number of c l o s e packed atoms, but i n the system AXo o n l y οne_quarter of the o c t a h e d r a l holes are surrounded e x c l u s i v e l y by X ions and are hence a v a i l a b l e f o r occupation by the B^+ c a t i o n s . The ABX3 etoichiometry would then be generated by f i l l i n g a l l the holes of t h i s type. I f the l a t t i c e of (A+3X) i s cubic c l o s e packed we have the cubic p e r o v s k i t e s t r u c t u r e formed by many oxides and f l u o r i d e s . However another p o s s i b i l i t y i s hexagonal c l o s e packing of (A+3X). In that case the o c t a h e d r a l BX5 groups are stacked i n columns s h a r i n g p a i r s of opposite faces so that the

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

Direct

Anion Bridged

n

2

2

4

2

Ir(C0) acac

APt(CN).

3

Pt(NH ) PtCl

2

4

2

(M - N i , Pd, P t )

K PtCl^, PtenCl

M(DMG)

8 8 8 Square planar 3d , 4d , 5d

3

CsMCl .2H 0(M » Mm, Fe, Co)

M « V, Cr, Mh, Fe, Co, N i )

3

AMC1 (A « a l k a l i metal or NR*,

Hexagonal p e r o v s k i t e s , ABX^:3d

Valence

3

A

2

n

P t ( C

4

4

n

A _ Pt(CN) 2-n 2°4>2

2

A Pt(CN) X

n

l i n e a r / s q u a r e planar h a l i d e s

CsAuCl

:d

8

Wolframs r e d s a l t

3

Square planar 5d**

d

10

Valence

square p l a n a r / o c t a h e d r a l h a l i d e s

Mixed

Compounds

PtenCl

6

d :d

8

Examples o f T r a n s i t i o n Metal Chain

Single

Table 1.

238

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

Β to Β separation i s much smaller p a r a l l e l to the c - a x i s o f the hexagonal u n i t c e l l than perpendicular to i t . I f the BX* octahedra were u n d i s t o r t e d , and i f the i o n i c r a d i u s of A+ were equal to that of X~ the r a t i o o f the nearest neighbor B-B d i s t a n c e along the stacks to the d i s t a n c e between stacks would be 2.449. In many examples, however, the r a t i o i s l a r g e r s i n c e one can use c a t i o n s such as Ν ζ Ο Η β ) ^ which a r e bigger than X". On the other hand, with u n d i s t o r t e d B X 5 the nearest neighbor B-B d i s t a n c e i s 2//3 times the B-X bond l e n g t h . I t i s c l e a r t h e r e f o r e that the anions w i l l p l a y an important r o l e i n any exchange processes between the Β i o n s . Whatever the pathway o f the B-B i n t e r a c t i o n though. A convincing demonstration o f i t s one-dimensionality i s provided by the r a t i o of i n t r a - c h a i n and i n t e r - c h a i n exchange i n t e g r a l s i n a compound such as N ( Œ ) 4 M n C l 3 , which ha The other c l a s s o anion b r i d g i n g p l a y s a r o l e a r e the hydrated ternary t r a n s i t i o n metal h a l i d e s . Here the c o o r d i n a t i o n of each b i v a l e n t metal i o n i s again o c t a h e d r a l , c o n s i s t i n g of four h a l i d e ions and two cis-water molecules (3). The octahedra a r e j oined i n t o chains through approximately 180° bridges i n v o l v i n g the trans h a l i d e ions. H a l i d e bridges between c a t i o n s o f d i f f e r i n g o x i d a t i o n s t a t e are mainly confined t o s i t u a t i o n s i n which one of the c a t i o n s has a low s p i n d^ c o n f i g u r a t i o n , and hence b a s i c a l l y square planar c o o r d i n a t i o n . The vacant s i t e s perpendicular to the plane c o n t a i n i n g the l i g a n d s may then be occupied by h a l i d e ions which a r e themselves coordinated t o another c a t i o n . I n the most common examples of t h i s type, the second c a t i o n has a low s p i n c o n f i g u r a t i o n and thus octahedral c o o r d i n a t i o n . The h a l i d e bridges may a l s o be the t e r m i n a l groups of l i n e a r l y coordinated d*0 c a t i o n s such as A u . I n f a c t , a famous example of the l a t t e r i s the b l a c k compound known as Wells' s a l t , which has the e m p i r i c a l formula CSAUCI3. The s t r u c t u r e , determined many years ago by E l l i o t and P a u l i n g (4) contains chains o f a l t e r n a t i n g l i n e a r A u C l and square planar AuClT._ However, each A u C l a l s o has four c h l o r i d e ions from four AuCl^ coordinated a t r i g h t angles t o i t s p r i n c i p a l a x i s , so i n t o t o , the l a t t i c e could be viewed as a d i s t o r t e d v e r s i o n of the cubic c l o s e packed perovskite. To achieve a c l o s e enough approach between metal ions f o r d i r e c t i n t e r a c t i o n between them to outweigh i n t e r a c t i o n s through b r i d g i n g l i g a n d groups, the most f a v o r a b l e s i t u a t i o n i s c l e a r l y to have complexes which are c o o r d i n a t i v e l y unsaturated, so that another metal i o n , or complex, can a c t , as i t were, as a l i g a n d . The most f a m i l i a r examples of c o o r d i n a t i v e unsaturation a r e square planar complexes, so i t i s among low s p i n d$ compounds that some o f the most spectacular i n t e r m e t a l l i c i n t e r a c t i o n e f f e c t s are found, both i n s i n g l e and mixed valency compounds. A l s o , because i t i s the trans s i t e s i n such a geometry, which 3

1

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2

17.

DAY

Chain

Compounds

239

are a v a i l a b l e f o r c o o r d i n a t i o n , polymerization i s bound to g i v e l i n e a r systems. S i n g l e valence examples are found with 3d**, 4 d and 5d8 c o n f i g u r a t i o n s , both where t h e _ u n i t s are n e u t r a l molecules l i k e P t e n C l , anions l i k e Pd(CN)?~ or a l t e r n a t i n g anions and c a t i o n s , as i n Magnus' Green S a l t . In a l l of these, important i n t e r m e t a l l i c i n t e r a c t i o n e f f e c t s are observed, as we s h a l l i n d i c a t e l a t e r . At the present time examples of d i r e c t metal-metal i n t e r a c t i o n s i n mixed valence chains are only known f o r systems based on p a r t i a l o x i d a t i o n of square planar complexes i n the t h i r d t r a n s i t i o n s e r i e s . We and others have made repeated e f f o r t s to prepare N i and Pd analogues of the p a r t i a l l y o x i d i z e d Pt c h a i n compounds, but w i t h no success. Even attempts to dope K P t ( C N ) B r . 3 0 · 3 H 0 with Pd(CN) %~ or Ni(CN)f" by c o - c r y s t a l l i z i n g i t i n the presence of l a r g e excesses of these two ions do not lead to an d e t e c t a b l e i n c o r p o r a t i o n of 3 d or 4 d complex. Whether extension of the 5d o r b i t a overlap between the c a t i o n s t o s t a b i l i z e the band s t r u c t u r e , or whether i t i s connected with the smaller nd-(n+l)p s e p a r a t i o n i n the t h i r d t r a n s i t i o n s e r i e s we cannot say. Following t h i s b r i e f and g e n e r a l i z e d survey of the main s t r u c t u r e types of the metal chain compounds, some account can now be given of the p h y s i c a l p r o p e r t i e s a s s o c i a t e d with each type. In order to avoid t h i s having too much of the character of a review, reference w i l l mainly be made to work on chain compounds which has been c a r r i e d out i n Oxford i n the l a s t few years. 8

2

2

4

0

2

8

8

4.

S i n g l e Valence Metal Chain Compounds

(a) Anion Bridged Compounds. An o u t l i n e of the p r o p e r t i e s of s i n g l e valence metal chain compounds i s given i n Table 2, which h i g h l i g h t s the d i f f e r e n c e s found between compounds c o n t a i n i n g anion bridges r a t h e r than d i r e c t l y i n t e r a c t i n g metal i o n s . The key observation i s that none of the anion bridged compounds have any low energy e x c i t e d s t a t e s of metal-to-metal charge t r a n s f e r type. Since, as we have noted, i t i s mixing of t h i s k i n d of s t a t e i n t o the ground s t a t e which leads to c o l l e c t i v e e l e c t r o n i c behavior, i t i s no s u r p r i s e that a l l known examples are i n s u l a t i n g , with l o c a l i z e d ground s t a t e s . Ligand-to-metal charge t r a n s f e r s t a t e s e x i s t i n the u l t r a v i o l e t , however, and i n s o f a r as these i n v o l v e the b r i d g i n g l i g a n d s , t h e i r mixing i n t o the ground s t a t e provides a mechanism f o r superexchange, l e a d i n g to magnetic o r d e r i n g at low temperatures. At l o t has been w r i t t e n about one-dimensional magnetic o r d e r i n g , but a s i n g l e r e p r e s e n t a t i v e example w i l l i l l u s t r a t e some of the c h a r a c t e r i s t i c s of t h i s k i n d of system. U n l i k e CsCuCl^, which d i s t o r t s from the hexagonal p e r o v s k i t e s t r u c t u r e so as to provide each Cu atom with four short and two long bonds, the other f i r s t t r a n s i t i o n s e r i e s i o n which

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974. charge t r a n s f e r states

(unusual temperature depen-

Transport

Magnetic

Insulator

Weak superexchange

magnetic a t low temperatures

(anisotropic?)

(probably e x t r i n s i c )

Semiconductor

Diamagnetic

t r a n s i t i o n s only

transfer) Ferromagnetic or a n t i f e r r o -

when En chain

Usual h a l i d e ·*· metal

(b) C o l l e c t i v e

(d-*p or charge

dence of i n t e n s i t y )

s t r o n g l y perturbed by UV

exciton-magnon i n t e r a c t i o n

Large Davydov s h i f t s

field)

Vibronic transitions

Direct Interaction

Weakly perturbed by

Anion Bridged

P r o p e r t i e s of S i n g l e Valence T r a n s i t i o n Metal Chains

None a t low energy

(ligand

(a) L o c a l l y e x c i t e d

Optical

Table 2.

17.

DAY

Chain

241

Compounds

1 1

c u s t o m a r i l y shows strong J a h n - T e l l e r d i s t o r t i o n , C r , forms a compound CsCrCl3 which, a t l e a s t above 172 K, has an u n d i s t o r t e d hexagonal p e r o v s k i t e s t r u c t u r e (5). That one-dimensional s p i n c o r r e l a t i o n s are important i n t h i s compound f o l l o w s a t once from the temperature v a r i a t i o n of the magnetic s u s c e p t i b i l i t y which has a broad maximum near 170 K, then decreases with the onset of antiferromagnetism. In f a c t though, three dimensional magnetic o r d e r i n g does not take place u n t i l the Neel temperature of 16 Κ i s passed, so between 170 and 16 Κ only antiferromagnetic i n t e r a c t i o n s w i t h i n the chains are of any importance. An even more d i r e c t measure of the one-dimensionality of the s p i n c o r r e l a t i o n s i s the d i s p e r s i o n of the spin-wave e x c i t a t i o n s p a r a l l e l and perpendicular to the chain ( 6 ) . J u s t as we have to w r i t e the e l e c t r o n i c e x c i t e d s t a t e wavefunctions of a c r y s t a l as Bloch f u n c t i o n s , s t allo th e x c i t a t i o l p r o b a b i l i t y of r e s i d i n the same way, must a magneti (tha , of a s p i n from the d i r e c t i o n i t would have i n the t o t a l l y ordered l a t t i c e ) be d e l o c a l i z e d . Spin waves of d i f f e r i n g wavelengths (or wave v e c t o r s ) have d i f f e r e n t energies, which can be determined by i n e l a s t i c neutron s c a t t e r i n g , thus b u i l d i n g up a p i c t u r e of the d i s p e r s i o n curve experimentally. The experimental spin-wave d i s p e r s i o n of C s C r C ^ near 4 Κ p a r a l l e l and perpendicular to the chains i s shown i n F i g u r e 1. Although the r e s u l t s do not cover the e n t i r e B r i l l o u i n zone, because of the small s i z e of the c r y s t a l a v a i l a b l e to us, they do show very c l e a r l y that the d i s p e r s i o n perpendicular to the £-axis i s e s s e n t i a l l y zero. The exchange i n t e g r a l between neighboring Cr ions i n the b a s a l plane i s t h e r e f o r e n e g l i g i b l y small w h i l s t w i t h i n the chains curve f i t t i n g to the experimental p o i n t s on the d i s p e r s i o n curve gives J ^ ^ 26.7 cm""*. Since customarily there are no charge t r a n s f e r s t a t e s i n the v i s i b l e or near u l t r a v i o l e t i n t h i s c l a s s of compound, the lowest energy e x c i t e d s t a t e s are l i g a n d f i e l d i n type, and can t h e r e f o r e be described as F r e n k e l , or t i g h t - b i n d i n g , e x c i t o n s . Many l i g a n d - f i e l d e x c i t e d s t a t e s have s p i n p r o j e c t i o n s d i f f e r e n t from the ground s t a t e and t r a n s i t i o n s to them should consequently be e l e c t r i c d i p o l e forbidden. I t has been known f o r a number of years, however, that such t r a n s i t i o n s can be rendered allowed i n antiferromagnetic compounds by u s i n g the exchange I n t e r a c t i o n to couple an e x c i t o n formed w i t h decrease of s p i n (e.g., sextet to quartet) with a s p i n d e v i a t i o n among the r e s t of the ions i n the l a t t i c e , which remain i n t h e i r ground s t a t e s . The most famous examples of such 'exciton-magnon' combination bands are found i n manganese s a l t s ( 7 ) , so i t i s i n t e r e s t i n g to see how the e f f e c t manifests i t s e l f i n M n s a l t s of the hexagonal p e r o v s k i t e type, which c o n t a i n chains of a n t i f e r r o m a g n e t i c a l l y coupled ions l i k e that found above i n C s C r C ^ . In o v e r a l l appearance the l i g a n d f i e l d spectra of Mh chain compounds are much l i k e those of other s i x - c o o r d i n a t e Mh 11

11

11

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

242

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

complexes, confirming a g a i n that the metal-metal i n t e r a c t i o n s i n anion bridged s i n g l e valence chains are weak. Two f e a t u r e s stand out, however. F i r s t , the o s c i l l a t o r strengths of the_ t r a n s i t i o n s are greater than i n non-bridged ('outer-sphere') s a l t s where magnetic i n t e r a c t i o n s are n e g l i g i b l e . Second, the temperature dependences of the o s c i l l a t o r strengths f o l l o w a r a t h e r c u r i o u s p a t t e r n , q u i t e u n l i k e that of the u s u a l 'coth' p l o t f o r a simple v i b r o n i c a l l y induced t r a n s i t i o n . Some experimental examples are shown i n F i g u r e 2(a) (8). In a l l cases the o s c i l l a t o r strength increases r a p i d l y a t f i r s t when the temperature i s r a i s e d above 4.2 K, then passes through a broad maximum a t a temperature which v a r i e s from one compound to another, but appears t o be the same f o r a l l the bands i n each compound. Then a f t e r dropping s l i g h t l y i t f i n a l l y i n c r e a s e s slowly and monotonically towards room temperature This v a r i a t i o n has some g e n e r a s u s c e p t i b i l i t y v s . temperatur f a c t , the r e s u l t s i n F i g u r e 2(a) stimulated Tanabe and Ebara t o c a l c u l a t e the i n t e n s i t y , frequency and l i n e w i d t h v a r i a t i o n of the magnon sidebands i n l i n e a r antiferromagnets, w i t h the r e s u l t shown i n F i g u r e 2(b) (10). I t can be seen that the o v e r a l l form of the v a r i a t i o n i s n i c e l y reproduced. Tanabe's theory r e q u i r e s that the temperature a t which the broad maximum occurs i n the i n t e n s i t y i s approximately |J/k|S(S+1). For the three compounds we i n v e s t i g a t e d , the v a l u e s of J/k derived from the spectra are l i s t e d i n Table 3, which a l s o shows that f o r the two compounds f o r which we have s u s c e p t i b i l i t y data (9,11), the l e v e l of agreement between the l a t t e r and the o p t i c a l l y derived v a l u e of J/k i s v e r y s a t i s f a c t o r y . (b) D i r e c t l y I n t e r a c t i n g Metal Ions. When metal ions are brought c l o s e enough together i n a c h a i n to i n t e r a c t d i r e c t l y , without the intermediary of a b r i d g i n g l i g a n d , the e f f e c t of the neighboring ions on the e l e c t r o n i c s t a t e s of each metal i o n i s n a t u r a l l y i n c r e a s e d . U n f o r t u n a t e l y no examples of such chains are known i n which the c o n s t i t u e n t ions have unpaired spins because, as we noted a l r e a d y , they are a l l based on square planar complexes. The s i n g l e exception known to us i s P t Q i R ^ ^ C u C l ^ which, however, c o n t a i n s chains of a l t e r n a t i n g Pt(NH3)^ and CuCl^" and so has a diamagnetic i o n between each p a i r of i o n s . Consequently i t has a Neel temperature of o n l y 0.5°K, from which near neighbor exchange i n t e g r a l of about 0.25 cm~l can be estimated (12). Turning to o p t i c a l p r o p e r t i e s , one f i n d s two d i f f e r e n t types of s i t u a t i o n s i n the s i n g l e valence chains w i t h d i r e c t l y i n t e r a c t i n g metal i o n s , depending on whether the lowest energy e x c i t e d s t a t e s of the c o n s t i t u e n t complexes are forbidden or allowed. The former a r e most l i k e l y t o be l i g a n d f i e l d s t a t e s , which are p a r i t y forbidden from the ground s t a t e i n centrosymmetric complexes l i k e PtCl£~ and may i n a d d i t i o n be +

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

17.

DAY

Chain

Compounds

243

Figure 1. Dispersion of spin waves propagating (a) par­ allel and (b) perpendicular to the Cr chains in the linear antiferromagnet CsCrCl>

ο

ο

ο

ο

ο

ο

ο

Ο '

Λ

ο

ο

ο

ο ο ο

Ο

?0

60

ΙΟΟ

τικ

ο ΙΘΟ

2όΟ Figure 2(a). Temperature depend­ ence of oscillator strengths of ligand field transitions in linear antiferromagnets NtCHsh MnCl and CsMnX, · 2H 0 (X = Cl, Br) (8) 3

2

Figure 2(b). Calculated temperature variation of oscillator strengths of exciton-magnon transitions in linear antiferromagnets and ferromagnets (10)

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

244

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

spin-forbidden. The l a t t e r may be e i t h e r 'atomic' (e.g., 5d ·> 6p i n P t complexes) or charge t r a n s f e r . Cases where the lowest e x c i t e d s t a t e s are of l i g a n d f i e l d type are the s a l t s I ^ P t O ^ , PtenX (X-Cl,Br) and Pt ( N H ^ P t C l ^ , Magnus' Green S a l t (MGS), although some of the e a r l i e r workers thought that the unusual c o l o r of the l a s t example pointed to 'metal-metal bonding,' and the existence of new e x c i t e d s t a t e s which could not be traced back i n parentage to any intramolecular t r a n s i t i o n s of the c o n s t i t u e n t anion or c a t i o n (13). At f i r s t s i g h t i t does seem p e c u l i a r that MGS should be green, s i n c e P t ( N H ) | i s c o l o r l e s s , both i n s o l u t i o n and i n P t i N ^ ^ C l ? , while P t C l | ~ i s pink i n s o l u t i o n and i n I ^ P t C l ^ . Nevertheless, by l o o k i n g a t the way the p o l a r i z e d c r y s t a l spectra vary along a s e r i e s of MGS analogues Pt (RNH^^PtClA as the R group i s made more bulky, we showed i n MGS are a l l t r a c e a b l i n the spectrum of P t C l | i n I ^ P t C l ^ , a l b e i t w i t h red s h i f t s up to 4000 cm~l. Examples of these are shown i n Figure 3. They are a l s o considerably i n t e n s i f i e d compared to l ^ P t C l ^ , most d r a m a t i c a l l y when the i n c i d e n t e l e c t r i c vector i s p a r a l l e l to the metal stack (15). The reason f o r the i n t e n s i f i c a t i o n i s assumed to l i e i n the f a c t that the t r a n s i t i o n s borrow t h e i r i n t e n s i t y v i b r o n i c a l l y from allowed t r a n s i t i o n s out i n the u l t r a v i o l e t which are themselves s t r o n g l y r e d - s h i f t e d by the intermolecular i n t e r a c t i o n . Intense absorption when the i n c i d e n t e l e c t r i c v e c t o r i s p a r a l l e l to the metal chains has o f t e n been taken as evidence for metal-metal bonding, but as we f i r s t pointed out (16), t h i s conclusion may be based on a f a l s e i n t e r p r e t a t i o n of the a v a i l a b l e absorption mechanisms. I t i s undoubtedly t r u e , of n i c k e l dimethylglyoximate f o r example, that the c r y s t a l , which contains metal chains, has an Intense absorption band i n the v i s i b l e , p o l a r i z e d p a r a l l e l to the chains, which does not appear i n the s o l u t i o n spectrum. I t i s equally t r u e , however, that the f i r s t intense band of n i c k e l N-methyl-salicycaldimine, which does not form chains i n the c r y s t a l , i s a l s o p o l a r i z e d perpendicular to the molecular planes. In both cases the t r a n s i t i o n i s most probably a d ττ* charge t r a n s f e r , but the f e a t u r e d i s t i n g u i s h i n g them i s f h a t when the molecules are stacked plane to plane the t r a n s i t i o n d i p o l e s are a l l p a r a l l e l , so i n t e r a c t i o n s between them are maximized and very l a r g e Davydov s h i f t s occur. No intermolecular e l e c t r o n exchange need be invoked i n e i t h e r case. In f a c t , we b e l i e v e that i n general, when the lowest energy e x c i t e d s t a t e s of the molecular u n i t s are p o l a r i z e d perpendicular to t h e i r planes, the lowest e x c i t e d s t a t e s of the stacks of molecules i n the c r y s t a l are always n e u t r a l Frenkel excitons r a t h e r than i o n i c e x c i t o n s . This p r e j u d i c e i s based on the magnitude of the Davydov s p l i t t i n g to which the n e u t r a l excitons w i l l be subject (a 'back of an envelope' c a l c u l a t i o n suggests around 14,000 cnT^ f o r t r a n s i t i o n 1 1

2

+

3

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

17. D A Y

Chain

Table 3.

Temperature Dependence of I n t e n s i t y of Magnon Sidebands i n L i n e a r Antiferromagnets

Compounds

Τ

max

245

J/k

(exp.) r

20-40 Κ

CsMnBr .2H 0 2

30-60

4.5

(NMe )MnCl

3

50-80

6.7

2

3

4

(susceptibility) 3.0 Κ

3.3 Κ

CsMnCl .2H 0 3

J/k

6.3

Figure 3. Polarized absorption spectra of (a) Pt(CH iNH ),,PtCl Pt(C H NH ) PtCl and (c) K PtCl Dotted lines are Ε c, full Ε c (15). 2

2

5

2 >l

f>

2

r

h

(b) lines

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

246

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

d i p o l e length of U i n a stack of molecules separated by 3.25Â). On the other hand, i f the parent t r a n s i t i o n i s p o l a r i z e d w i t h i n the molecular planes, Davydov s p l i t t i n g of i t s n e u t r a l excitons i n the c r y s t a l i s smaller ( a c t u a l l y one h a l f of the other i n the p o i n t d i p o l e approximation) and i o n i c e x c i t o n s may become observable. An example here i s the work of M a r t i n and h i s colleagues on P t e n X (17,18). One way of v e r i f y i n g that 'new electronic transitions appearing i n chain compounds with p o l a r i z a t i o n s p a r a l l e l to the stacks are a c t u a l l y Davydov components of t r a n s i t i o n s which take p l a c e i n the u l t r a v i o l e t i n the i s o l a t e d u n i t s i s to determine how t h e i r energy v a r i e s as one changes the i n t e r m o l e c u l a r spacing i n the stacks. In the p o i n t d i p o l e approximation the separation between the Davydov components i s p r o p o r t i o n a l to |M|2/R3 where M i s the t r a n s i t i o n d i p o l the centers of the molecule set of molecules to exemplify t h i s expression i s that of the t e t r a c y a n o - p l a t i n i t e s . Many s a l t s of t h i s anion contain stacks of Pt(CN)|~ w i t h t h e i r planes p a r a l l e l , though w i t h changing c a t i o n the Pt-Pt spacing can be v a r i e d from 3.13Â (Mg) to 3.69Â (Rb) (20). Although the lowest e x c i t e d s t a t e of Pt(CN)|~ i n s o l u t i o n l i e s a t 35,600 cm"*, i n a l l such s a l t s there i s an extremely intense absorption band i n the v i s i b l e or near u l t r a v i o l e t , p o l a r i z e d e n t i r e l y along the d i r e c t i o n of the stacks. With i n c r e a s i n g Pt-Pt s e p a r a t i o n the band moves to higher energy, and i n F i g u r e 4 we p l o t i t s energy against 1/R**. Both f o r the t e t r a c y a n o - p l a t i n i t e s and the isomorphous t e t r a c y a n o p a l l a d i t e e , which are included on the same p l o t , there i s a very s a t i s f a c t o r y c o r r e l a t i o n between these two q u a n t i t i e s . Furthermore, e x t r a p o l a t i n g the two s t r a i g h t l i n e s of Figure 4 to R » », i . e . , to the i s o l a t e d molecule, we f i n d energies of 44,800 cm" (Pt(CN)|~) and 52,900 cm"" (Pt(CN)|~) which, bearing i n mind that the aqueous s o l u t i o n s p e c t r a of the ions do not s t r i c t l y represent those of the i s o l a t e d molecules, i s w e l l w i t h i n the energy range of the z - p o l a r i z e d allowed t r a n s i t i o n s assigned by Mason and Gray (21). 2

1

f

1

5.

1

Mixed Valence Metal Chain Compounds

About f o r t y elements form compounds i n which, to s a t i s f y the observed s t o i c h i o m e t r y , one has to a s s i g n d i f f e r e n t o x i d a t i o n numbers to d i f f e r e n t metal ions of the same element i n the same lattice. These s o - c a l l e d 'mixed valence' compounds o f t e n have p r o p e r t i e s f a r from a simple s u p e r p o s i t i o n of those we would p r e d i c t f o r each of the o x i d a t i o n s t a t e s taken s e p a r a t e l y . In p a r t i c u l a r , i f the s i t e s occupied by the metal ions of d i f f e r i n g valency have s i m i l a r c o o r d i n a t i o n numbers and l i g a n d environments, the energy needed to t r a n s f e r an e l e c t r o n from one to the other, i . e . , to interchange the v a l e n c i e s , may be v e r y low. Indeed, i f the metal ion s i t e s i n the l a t t i c e of our

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

(ESCA, Mossbauer

Class

I n s u l a t o r a t 4K

(probably

Mixed Valence

M e t a l l i c a t 300K

Semiconductor

Transport

(Metal atoms equivalent)

(Valences

trapped)

IIIB ( I n c i p i e n t l y II??)

II

intrinsic)

Dlamagnetic

Plasma edge i n v i s i b l e

f o r En chains

Opaque a t a l l frequencies

E J L chains

Weak allowed t r a n s i t i o n s when

Dlamagnetic

etc)

metal charge

t r a n s f e r f o r Ε it chains

Metal

when Ex chain

No l i g a n d f i e l d s t a t e s seen

Direct Interaction

Chains

Magnetic

or m e t a l l i c )

(charge t r a n s f e r

(b) C o l l e c t i v e

(ligand f i e l d )

Bridged

Only weakly perturbed

Anion

P r o p e r t i e s of Mixed Valence T r a n s i t i o n Metal

(a) L o c a l l y e x c i t e d

Optical

Table 4.

2* ο ο

248

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

mixed valence compound a l l t u r n out to be c r y s t a l l o g r a p h i c a l l y i d e n t i c a l , then one may have a n o n - i n t e g r a l number of e l e c t r o n s at each s i t e , even i n the ground s t a t e . Searching out the known mixed valence compounds from a l l corners of the P e r i o d i c T a b l e , and c o r r e l a t i n g t h e i r p r o p e r t i e s with the observed s t r u c t u r e s , we f i n d that they can be d i v i d e d i n t o three broad c l a s s e s (22). I f the l i g a n d f i e l d s around the ions of d i f f e r i n g valence are v e r y d i f f e r e n t , i o n i z e d e x c i t o n s , or i n chemists language metal-to-metal charge t r a n s f e r s t a t e s , l i e a long way above the ground s t a t e whose p r o p e r t i e s , along with those of many lower e x c i t e d s t a t e s , are very c l o s e to being j u s t a s u p e r p o s i t i o n of the p r o p e r t i e s of the c o n s t i t u e n t i o n s . T h i s category, c a l l e d c l a s s I, i s of l i t t l e i n t e r e s t here. At the opposite extreme, a t r u l y n o n - i n t e g r a l number of e l e c t r o n s at each metal i o n s i t e , which would be r e q u i r e i d e n t i c a l , can only b continuous. T h i s category we c a l l c l a s s I I I . Between c l a s s e s I and I I I l i e s a continuous spectrum of cases ( c l a s s II) i n which two types of metal i o n s i t e s can be d i s t i n g u i s h e d by c r y s t a l l o g r a p h y , but i n which i o n i z e d e x c i t o n s l i e c l o s e enough to the ground s t a t e that they c o n t r i b u t e to the o p t i c a l spectrum i n the v i s i b l e or even the near i n f r a r e d . Some of the i o n i z e d excitons may even have the c o r r e c t symmetry to mix with the ground s t a t e , p a r t l y smudging out the formal o x i d a t i o n numbers defined by counting e l e c t r o n s a t the two metal i o n s i t e s . We w i l l now examine the p r o p e r t i e s of the known mixed valence metal c h a i n compounds i n the l i g h t of t h i s simple c l a s s i f i c a t i o n . J u s t as i n the s i n g l e valence chains, we d i s t i n g u i s h the two cases of anion b r i d g i n g and d i r e c t l y i n t e r a c t i n g metal i o n s . 1

(a) Anion Bridged Compounds. A l l the anion bridged Pt and Au compounds have two s e t s of c r y s t a l l o g r a p h i c a l l y d i s t i n c t metal i o n s i t e s , and may t h e r e f o r e be c h a r a c t e r i z e d as c o n t a i n i n g trapped valences i n t h e i r ground s t a t e s , (II,IV) i n the former and ( I , I I I ) i n the l a t t e r . X-ray photoelectron (XPS) spectroscopy should be a powerful t o o l f o r d e c i d i n g whether d i f f e r e n t p a r t i a l changes must be assigned to the ions or each type of s i t e , because we might hope to see core i o n i z a t i o n s from each type. I f we do not (assuming that the reason i s not simply the poor r e s o l u t i o n of the technique), then the p a r t i a l charges are equal o r , s a i d another way, the e l e c t r o n s are exchanging between the s i t e s more r a p i d l y than the time taken f o r the core e l e c t r o n to be e j e c t e d . F i g u r e 5 shows some XPS spectra of Pt chain compounds i n the 4f r e g i o n (23), i n c l u d i n g the mixed valence PtenCl3 (en: ethylenediamine). Crystallographically, the l a t t e r can be thought of as made up of a l t e r n a t i n g square planar P t e n C l and o c t a h e d r a l P t e n C l 4 molecules. Comparison between the spectra of PtenCl3 and i t s separated c o n s t i t u e n t molecules i s made more d i f f i c u l t by the ease with which PtenCl4 and PtenCl3 reduce i n the X-ray beam. However, by c a r e f u l l y I I

I v

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

17.

DAY

Chain

Compounds

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

249

250

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

f o l l o w i n g the spectra of these two compounds as a f u n c t i o n of i r r a d i a t i o n time one f i n d s that the PtenCl3 spectrum i s indeed c l o s e to a s u p e r p o s i t i o n of those of PtenC±2 and PtenCl/ (Figure 6). However, the same cannot be s a i d of the o p t i c a l p r o p e r t i e s , f o r i n a d d i t i o n to weak absorption bands, p o l a r i z e d perpendicular to the chains, which do i d e n t i f y c l o s e l y with l o c a l l y e x c i t e d l i g a n d from t r a n s i t i o n s of the c o n s t i t u e n t s , PtenCl3 and other s i m i l a r s a l t s a l l have intense absorption across most of the v i s i b l e , p o l a r i z e d e n t i r e l y p a r a l l e l to the chains (24)» This we a s s i g n as due to metal to metal charge t r a n s f e r , i . e . , to i o n i z e d e x c i t o n s t a t e s , and the compounds belong to c l a s s I I defined above. Because the valences i n the ground s t a t e are trapped, though, the c r y s t a l s are semiconducting r a t h e r than metallic. (b) D i r e c t l y I n t e r a c t i n g Metal Ions. In the s i n g l e valence chains when the metal ions were brought c l o s e enough together to i n t e r a c t d i r e c t l y , i n s t e a d of through a b r i d g i n g group, l a r g e Davydov s h i f t s of the n e u t r a l e x c i t o n s t a t e s showed that the metal-metal i n t e r a c t i o n was very much increased. A comparable e f f e c t on the i o n i z e d e x c i t o n s t a t e s of a mixed valence chain might b r i n g them c l o s e enough to the ground s t a t e f o r the system to f i n d i t s e l f on the other s i d e of a Mott t r a n s i t i o n , and to have a conducting ground s t a t e i n which the e l e c t r o n s t a t e s were t r u l y c o l l e c t i v e . T h i s i s the background to the recent i n t e r e s t i n compounds l i k e K2Pt(CN) Br ,3o3H 0(KCP) (25). At f i r s t s i g h t i t has a l l the c h a r a c t e r i s t i c s expected of a c l a s s I I I mixed valency compound: d e s p i t e the n o n - i n t e g r a l o x i d a t i o n number, a l l the Pt atoms appear to occupy c r y s t a l l o g r a p h i c a l l y equivalent s i t e s ; w i t h the i n c i d e n t e l e c t r i c v e c t o r p a r a l l e l to the chains i t i s opaque throughout the v i s i b l e and i n f r a r e d down to very low f r e q u e n c i e s , and has a plasma edge i n the r e f l e c t i v i t y i n the v i s i b l e ; a t room temperature i t i s a m e t a l l i c conductor. Unfortunately, i f more i n t e r e s t i n g l y , however, there are some f e a t u r e s which complicate such a simple view. At low temperature, the compound i s not m e t a l l i c , but semiconducting. There i s a l s o evidence of a tendency f o r the Pt atoms to become i n e q u i v a l e n t as a r e s u l t of long wavelength a c o u s t i c phonon instabilities. I f such an i n s t a b i l i t y , which showed up by i n e l a s t i c neutron s c a t t e r i n g as a Kohn anomaly i n the a c o u s t i c phonon d i s p e r s i o n became locked i n as a s t a t i c d i s t o r t i o n , i t would provide enough reason f o r the energy gap a t low temperature. A c r u c i a l experiment would t h e r e f o r e be to examine the c r y s t a l s t r u c t u r e a t l i q u i d helium temperature. We r e c e n t l y c o l l e c t e d X-ray d i f f r a c t i o n data from a s i n g l e c r y s t a l of KCP a t room temperature, 77° and 4.2°K (26). The arrangement of the d i f f r a c t o m e t e r and c r y o s t a t r e q u i r e d one to concentrate on one c r y s t a l a x i s at a time, and we chose the (0,0,A). Within that l i m i t a t i o n we found no s i g n of Bragg peaks 4

0

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

17.

DAY

Chain

Compounds

Figure

251

6. XPS spectrum of PtenCl in the 4f region after surface cleaning 3

Pt

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

252

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

corresponding to ' l o c k i n g i n ' a P e i e r l s d i s t o r t i o n at the repeat d i s t a n c e (6·67 times the Pt-Pt spacing) suggested by the neutron s c a t t e r i n g r e s u l t s . What we d i d f i n d , however, i s a set of seven peaks indexing as (0,0,ε), where ε •= η/ζ. The peaks w i t h η odd are weaker than those with η even, and a l e a s t squares f i t to the Bragg angles of those with η - 1,2,3,4,6,8,10 y i e l d s a v a l u e f o r ζ of 8.97. A key o b s e r v a t i o n i s that the peaks are present a t room temperature as w e l l as a t 4.2°K, and that they do not vary i n any discontinuous way as the temperature i s lowered. They must t h e r e f o r e r e s u l t from some s u p e r l a t t i c e o r d e r i n g which has not p r e v i o u s l y been detected. Our present view i s that the s u p e r l a t t i c e i s probably connected with o r d e r i n g among the bromide i o n s , supposed i n the o r i g i n a l c r y s t a l s t r u c t u r e determination to be randomly d i s t r i b u t e d on three f i f t h s of the a v a i l a b l of Pt(CN>4 groups. Fro such as the hexagonal tungsten bronzes, we b e l i e v e a random d i s t r i b u t i o n of anions i s a. p r i o r i u n l i k e l y , a view r e i n f o r c e d by the v e r y narrow composition range which KCP e x h i b i t s . Since i t has become such an important model compound f o r one-dimensional m e t a l l i c behavior the s u b t l e r d e t a i l s of i t s s t r u c t u r e o b v i o u s l y deserve the c l o s e s t a t t e n t i o n . L i t e r a t u r e Cited 1.

W e l l s , A. F., "Structural Inorganic Chemistry," 3rd ed., p. 375, Oxford U n i v e r s i t y Press, Oxford, 1962.

2.

Hutchings, M. T., Shirane, G., Birgeneau, R. J . and H o l t , S. L., Phys. Rev. (1972), B5, 1999.

3.

Jensen, S. J . , Andersen, P. and Rasmussen, S. Ε., Acta Chem. Scand. (1962), 16, 1890.

4.

Elliot, 1846.

5.

McPherson, A. L., Kistenmacher, T. J . , F o l k e r s , J . B. and Stucky, G. D., J . Chem. Phys. (1972), 57, 3771.

6.

Hutchings, M. T., Day, P., Gregson, Α. Κ., Leech, D. H. R a i n f o r d , B. D., P h y s i c a l Society Meeting, Manchester, England, January 1974.

7.

S e l l , D. D., Greene, R. L. and White, R. Μ., Phys. Rev. (1967), 158, 489.

8.

Day, P. and D u b i c k i , L., J.C.S. Faraday I I (1973), 69,

N. and P a u l i n g , L., J . Amer. Chem. Soc. (1938), 60,

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

and

363.

17. D A Y

9.

Chain

Compounds

253

D i n g l e , R., L i n e s , M. E. and H o l t , S. L., Phys. Rev. (1969), 187, 643.

10. Ebara, K. and Tanabe, Υ., J. Phys. Soc. Japan (1974), 36, 93. 11. Smith, T. and F r i e d b e r g , S. Α., Phys. Rev. (1968), 176, 660. 12. Soos, Z. G., Huang, T. Z., V a l e n t i n e , J. S. and Hughes, R. C., Phys. Rev. (1973), B8, 993. 13. Yamada, S. J. Amer. Chem. Soc. (1951), 73, 1579. 14. Day, P., Orchard, A. F., Thomson, A. J. and W i l l i a m s , R. J. P., J. Chem. Phys. 15. idem.,

ibid.

(1965), 43, 3763.

16. Day, P., Inorg. Chim. Acta Rev. (1969), 3, 81. 17. M a r t i n , D. S., Hunter, L. D., Kroening, R. F. and Coley, R. F., J . Amer. Chem. Soc. (1971), 93, 5433. 18. Kroening, R. F., Hunter, L. D., Rush, R. Μ., Clardy, J. C. and M a r t i n , D. S., J. Phys. Chem. (1973), 77, 3077. 19. C r a i g , D. P. and Walmsley, S. Η., "Exitons in Molecular C r y s t a l s , " W. A. Benjamin Inc., New York, 1968. 20. Moreau-Colin, M. L., S t r u c t u r e and Bonding

(1972), 10, 167.

21. Mason, W. R. and Gray, H. B., J. Amer. Chem. Soc. (1968), 90, 5721. 22. Robin, M. B. and Day, P., Adv. Inorg. Chem. and Radiochem. (1967), 10, 247. 23. McGilp, J. Chemistry, Part I I T h e s i s , Oxford, 1973. 24. Yamada, S. and Tsuchida, R., 29, 894.

Bull.

Chem. Soc. Japan

(1956),

25. See papers by K. Krogman and H. R. Z e l l e r i n t h i s symposium. 26.

Griffiths,

D., Day, P. and Wedgwood, F. Α., unpublished.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

18 Evidence for Extended Interactions between Metal Atoms from Electronic Spectra o f Crystals with Square Complexes DON S. MARTIN, JR. A m e s L a b o r a t o r y of the U . S . A . E . C .

Abstract

Frequently, the square-planar molecular or ionic complexes of platinum(II) or palladium(II) in crystals are aligned directly over one another in one dimensional stacks. The electronic ab­ sorption spectra of the crystals for polarized light in favorable circumstances provide information about the crystal interactions and possible intermolecular electron transfers in the solid state. For crystals with large metal-metal separations, >4.1Å, the d-d spectra correlate closely in energies and intensities with solu­ tion spectra. The spectra of crystals with Pt Br 2- ions indi­ cate metal-metal electron transfer between metals at 3.55Å, al­ though the transfer may be influenced by the bromide bridges. For Magnus green salt the spectral changes of PtCl 2- are con­ sistent with large energy shifts of Frenkel excitons by crystal effects. However, for some molecular complexes with separations of 3.4-3.5Å, electron transfers to ionic exciton states occur. 2

6

1

4

Introduction The electronic absorption spectra have been measured recently for single crystals of a number of ionic or molecular square planar complexes of platinum(I I). Frequently, such complexes stack face to face in linear arrays in crystals to provide strong anisotropics in the absorption of polarized light and in the electrical conductivities. Our concern here will be for the ev­ idence of transfer of electrons between the platinum atoms. If such electron transfer transitions can be identified, their wave lengths provide directly the energy required for the transfer. Our work to the present has been restricted to the complexes with halide and amine ligands. The absorption bands for the platinum complexes at room tem­ perature are rather broad; half-widths of approximately 2,000 cm"* are common at 300°K. A transition bandais normally characterized by the wave length, λ, or wave number, v, of the maximum, by the 254 In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

18.

Crystals

MARTIN, JR.

with

Square

Complexes

255

intensity, € , m o l a r a b s o r p t i v i t y a t t h e maximum o r as t h e o s c i l l a t o r s t r e n g t h : f = 4.32 χ 10" J*e dv and by t h e t e m p e r a t u r e dependence o f t h e i n t e n s i t y . For s i n g l e c r y s t a l s t h e r e i s a l i m i t upon t h e i n t e n s i t y imposed by t h e p r a c t i c a l lower l i m i t s o f c r y s ­ t a l t h i c k n e s s , w h i c h has been e x t e n d e d down t o 1-20μ. T h u s , t h e s t u d i e s a r e l i m i t e d t o bands w i t h e l e s s t h a n 1,000-2,000 cnP-M" . These a r e the n o r m a l l y f o r b i d d e n t r a n s i t i o n s . Hence, t h e a l l o w e d t r a n s i t i o n s a r e not c h a r a c t e r i z e d by c r y s t a l a b s o r p t i o n s p e c t r o s ­ copy except i n t h e i r absence. The t e m p e r a t u r e dependence o f i n t e n s i t y i s an e s p e c i a l l y powerful d i a g n o s t i c property. For t h e d«-d t r a n s i t i o n s , f o r exam­ p l e , w h i c h a r e symmetry f o r b i d d e n , an a s y m m e t r i c v i b r a t i o n may s e r v e as a p e r t u r b a t i o n t o g i v e a v i b r o n i c wave f u n c t i o n i n t o w h i c h i s mixed an a s y m m e t r i c e l e c t r o n i c wave f u n c t i o n . Thus t h e t r a n s i t i o n s may o b t a i able intensity. Sinc w h i c h s e r v e s as t h e p e r t u r b a t i o n , temperatures, t h e r e i s n o r m a l l y a d e c r e a s e i n i n t e n s i t y . £or a b a n d , e x c i t e d vibronic ly by a v i b r a t i o n o f wave number, V j , t h e i n t e n s i t y dependence (\) i s g i v e n by t h e e q u a t i o n : m a x

9

1

f ( T ) = f(0°K) c o t h ( h V j / 2 k T ) .

(I)

However, f o r a band w i t h a n o n - z e r o t r a n s i t i o n d i p o l e , s i n c e t h e i n t e g r a t e d i n t e n s i t y i s not t e m p e r a t u r e d e p e n d e n t , t h e peak height, e , w i l l i n c r e a s e as t h e band n a r r o w s a t l o w e r t e m p e r a ­ ture. F o r t h e h a l i d e and a m i n e - h a l i d e c o m p l e x e s o f p l a t ? n u m ( 1 1 ) t h e c r y s t a l s p e c t r a have i n d i c a t e d t h e o n e - e l e c t r o n o r b i t a l o r d e r i n g i n F i g u r e 1, w h i c h a p p l i e s t o t h e t e t r a g o n a l symmetry 4hThe l o w e s t u n o c c u p i e d o r b i t a l i s t h e σ" o r b i t a l b a s e d on the d _ y 2 w i t h b symmetry. W i t h t h e o t h e r d - o r i b t a l s f i l l e d , t h e g r o u n d s t a t e w i l l be a A i g ; and d-d t r a n s i t i o n s t o t h e s i n g l e t and t r i p l e t s t a t e s o f A g , Eg and B i g r e s p e c t i v e l y s h o u l d o c c u r , the t r i p l e t s t a t e s l y i n g below the corresponding s i n g l e t s . W i t h t h e h i g h s p i n - o r b i t c o u p l i n g o f a heavy P t - a t o m t h e s p i n d e s i g n a t i o n s a r e not e x a c t , and t h e s t a t e s must be d e s c r i b e d by t h e d o u b l e symmetry g r o u p , i n t h i s c a s e D4. Interactions along a s t a c k of square p l a n a r ions w i l l normally i n v o l v e the d s o r b i t a l , which possesses sigma c h a r a c t e r about the s t a c k i n g a x i s . F o r c o m p l e x e s w i t h low e n e r g y π a c c e p t o r o r b i t a l s s u c h as c y a n i d e and o x a l a t e , t h e d o r b i t a l may be much h i g h e r , and i n d e e d has been c o n s i d e r e d by S c h a t z and c o w o r k e r s (2) t o be t h e h i g h e s t f i l l e d orbital for Pt(CN) ". Even i n t h e h a l i d e c o m p l e x e s t h e assignment of the t r a n s i t i o n t o s t a t e (σ * - d 2 ) i s somewhat mx

n

x 2

i g

1

2

z

z 2

3

4

z

open t o q u e s t i o n s i n c e t h e a s s i g n e d band i s s u r p r i s i n g l y weak. B e l o w t h e d o r b i t a l s i n F i g u r e 1 a r e shown t h e o r b i t a l s d e r i v e d f r o m π and σ o r b i t a l s on t h e l i g a n d s . O n l y t h e odd o r b i t a l s a r e d e s i g n a t e d f o r t h e s e can p r o v i d e i n t e n s e P t ( a " ) g * ~ L t r a n s i t i o n s . A p o s s i b l e a l t e r n a t i v e a s s i g n m e n t o f i n t e n s e t r a n s i t i o n s may u

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

256

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

be 6p «-5d, f o r t h e 6 p o r b i t a l i s t h e s e c o n d l o w e s t u n f i l l e d orb i t a l . F u l l y a l l o w e d m o l e c u l a r t r a n s i t i o n s i n D^h must be A s ( z p o l a r i z a t i o n ) or Ε (χ,ν^ p o l a r i z a t i o n ) . A l l o t h e r t r a n s i t i o n s a r e s e e n a s a c o n s e q u e n c e o f v i b r o n i c and s p i n - o r b i t p e r t u r b a t i o n s , w h i c h m i x t h e s e s t a t e s i n t o t h e wave f u n c t i o n s . In a c o n s i d e r a ­ t i o n of a l l t h e v i b r a t i o n s o f an i o n such as P t B r " i t i s found t h a t t h e ΙΚ2^ s t a t e i s v i b r o n i c l y forbidden i n z-polarization but a l l o w e d i n x , ^ . The E and * B i g t r a n s i t i o n s a r e a l l o w e d i n t h e t h r e e p o l a r i z a t i o n s x , ^ and z. Since the s i n g l e t spin func­ t i o n s have t h e symmetry, Α χ , and t r i p l e t s a r e A^ and E i în t h e d o u b l e g r o u p , Oi, t h e t r i p l e t s t a t e s e l e c t i o n r u l e s a r e d i f f e r e n t from those o f the s i n g l e t s t a t e s . For c r y s t a l l i n e s t a t e s i n which extended i n t e r a c t i o n s a r e small, there i s n e g l i g i b l c o m p l e x e s , and e l e c t r o n l o c a l l i z e d , however, and o t h e members o t h e c r y s t a influence t r a n s i t i o n energies. In s u c h c a s e s t h e t h e o r y o f F r e n k e l t y p e e x c i t o n s f o r m o l e c u l a r c r y s t a l s c a n a p p l y . The c r y s t a l ground s t a t e wave f u n c t i o n w i l l have t h e f o r m z

z

1

u

τ

υ

2

4

τ

1

Φ°

0

* -
(2)

Ν

where
The

<ρί,ρ

^h,N/h'

^

where t h e r e a r e h m o l e c u l e s p e r u n i t c e l l . c r y s t a l s t a t e s w i l l c o n s t i t u t e a band o f f u n c t i o n s : *ί

κ

- ( N / h ) " * Σ exp(«k-r ) 0/ ρ ρ ιρ

(k)

where k i s t h e wave v e c t o r . However, f o r t h e a b s o r p t i o n o f l i g h t t h e s e l e c t i o n r u l e a p p l i e s : §°~*§o 9 _*.·£· l y the t r a n s i t i o n t o k = 0 i s allowed. For the c a s e o f o n l y one m o l e c u l e p e r u n i t c e l l , t h e t r a n s i t i o n e n e r g y i s g i v e n by ( 3 ) : o r |

ν = ψ

+

(5)

D' + I"

where v° i s t h e e n e r g y f o r t h e f r e e i o n . D i s t h e d i f f e r e n c e i n van d e r Waal s e n e r g y between t h e e x c i t e d m o l e c u l e and t h e g r o u n d s t a t e m o l e c u l e . T h i s t e r m i s f r e q u e n t l y not w e l l c h a r a c t e r i z e d b u t i s i n t e n s i t y i n d e p e n d e n t . The I t e r m has r e c e i v e d e x t e n s i v e i n v e s t i g a t i o n s i n c e s u c h terms p r o v i d e t h e Davydov s p l i t t i n g o f s t a t e s . I c o n t a i n s a sum o f i n t e r a c t i o n 1

1

1

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

18.

Crystals

MARTIN, JR.

with

Square

Complexes

257

terms, £ l F r e q u e n t l y , t h e s e terms a r e t a k e n as t h e i n t e r ­ a c t i o n Bf t r a n s i t i o n d i p o l e s on t h e two a t o m s , m and n, w i t h f o r m . m n

ι

mn

R-3

=

e

mn

2

(x'x m n

x

1

+ y'y n m 7

1

- 2z'z' ) n m '

7

v

(6) '

where R i s the s e p a r a t i o n of the molecules m and n, e x i s t h e χ component o f t h e t r a n s ­ i t i o n d i p o l e on t h e m-th m o l e c u l e , e t c . x , x ^ and y^, y a r e d i r e c t e d a l o n g s e t s o f p a r a l l e l a x e s on t h e two m o l e c u l e s an8 t h e ζ d i r e c t i o n i s a l o n g t h e a x i s o f c e n t e r s o f the t r a n s i t i o n d i p o l e s . For a l i n e a r s t a c k of m o l e c u l e s separated by 3 . % and w i t h a t r a n s i t i o n moment o f 1Â i n t h e ζ o r s t a c k i n g d i r e c t i o n the l t e r m s o f t h e two adj'acent m o l e c u l e s i n t h e s t a c k p r o v i d e a r e d s h i f t o f 12,000 cm" . A s i m i l a r v a l u e f o r an χ o r 2 t r a n s i t i o n moment w i l l g i v e a s h i f t t o s h o r t e wave l e n g t h o f 6,000 cm" . Hence t r a n s i t i o n e n e r g i e s ca t e n s i t y , e v e n w i t h no e l e c t r o n d e l o c a l i z a t i o n . m n

m

1

m

m n

1

1

K PtBr 2

4

and

KsPtCl : 4

Minimum E x t e n d e d I n t e r a c t i o n s

K P t B r p o s s e s s e s t h e K P t C l s t r u c t u r e shown i n F i g u r e 2 w i t h a = 7 . 3 5 A and c = 4 . 3 3 Â . The P t - B r bond l e n g t h i s 2.45JL The P t B r " i o n s s t a c k d i r e c t l y o v e r one a n o t h e r . They a r e s e p a r a t e d by t h e K i o n s and a r e a t t h e r e l a t i v e l y l a r g e d i s t a n c e o f 4.33A f r o m one a n o t h e r s o i n t e r a c t i o n s a r e s m a l l . The a b s o r p ­ t i o n w i t h l i g h t p o l a r i z e d i n the c - d i r e c t i o n provides the i o n i c z - p o l a r i z a t i o n w h e r e a s t h e p o l a r i z a t i o n normal t o c, j[.£. (a_), p r o v i d e s t h e i o n i c χ,^-polarization. T h i s i s t h e r e f o r e an i d e a l c r y s t a l l i n e system t o study, f o r the ion occupies a s i t e of f u l l D h symmetry s o t h a t t h e s i t u a t i o n i s not compromised by a l o w e r s i t e symmetry. The c r y s t a l s p e c t r a f o r K P t B r a r e shown i n F i g u r e 3 . The bands a t e n e r g i e s b e l o w 2 9 , 0 0 0 cm" a r e t h e n t h e ch-d t r a n s i t i o n s . The l a r g e d e c r e a s e i n i n t e n s i t y a t 15°K i s c h a r a c t e r i s t i c o f v i b r o n i c e x c i t a t i o n s . T h e _ J o c a t i o n s o f t h e maxima i n t h i s r e g i o n correspond c l o s e l y t o the v's r e p o r t e d f o r s o l u t i o n s p e c t r a of P t B r " (4,5) s o a p p a r e n t l y t h e D and I t e r m s o f e q u a t i o n 5 match c l o s e l y t h e s o l u t i o n e f f e c t s . For a_-polar i z a t i o n t h e r e i s a s h o u l d e r a t 300°K w h i c h i s r e s o l v e d as a p e a k w i t h v i b r a t i o n a l s t r u c t u r e a t 2 4 , 0 0 0 cm" i n t h e 15°K s p e c t r u m . T h i s band i s c o m p l e t e l y a b s e n t i n t h e c - p o l a r i z a t i o n , and i t can t h e r e f o r e be a s s i g n e d unambigous1 y as A g . The band i n b o t h p o l a r i z a t i o n s a t 2 7 , 0 0 0 cm" i s a s s i g n e d as E g f r o m a n a l o g y t o t h e P t C l " s p e c ­ t r u m where an Α-term i n t h e MCD s p e c t r u m i s c l e a r l y d i s c e r n a b l e (6,2)· The s h o u l d e r and p e a k i n e a c h p o l a r i z a t i o n a t c a . 1 7 , 0 0 0 and 1 9 , 0 0 0 cm" a r e p r e s u m a b l y components o f A and ^"Êg. The s h o u l d e r a t 2 2 , 7 0 0 cm" w i t h v i b r a t i o n a l s t r u c t u r e , which is a p p a r e n t i n a - p o l a r i z a t i o n and a l s o i s v e r y f a i n t i n c-polar i z a t ion, i s a s s i g n e d as t h e Big(a*«-d 2). W i t h t h i s a s s i g n m e n t o f t h e 2

4

2

4

2

4

+

4

2

4

1

2

1

1

4

1

T

2

1

1

2

4

1

3

2

g

1

3

z

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

EXTENDED

INTERACTIONS

BETWEEN

METAL

->4h

,— e

(xy)

u

/

/

'

ep­

*

ι

ι— ig ι-—a (z) Q

JU

os—

br (x -y ) 2

2

g

ZrH-bzgCxy) 2)

5d— ---+>-a g /

b

2

u

-*-"—TT

^ a

2g

—a

2U

/

Figure 1. Molecular orbital scheme for platinum(II) complex under D symmetry as in PtClf- or FtBrf: Only the ρ orbitals of the halides are included, and gerade orbitals aris­ ing from ligand π and σ orbitals are omitted. On the right are excited symmetry states aris­ ing from d « - d transitions from the A ground state. Both singlet and triplet states will occur for each indicated symmetry state. } h

1

lg

Figure 2. Unit cell for the tetragonal K PtCl, structure. Κ ions are at ζ = Vi; others at ζ = 0. x

f

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

18.

Crystals

MARTIN, JR.

with

Square

Complexes

259

1

t r i p l e t s t a t e the B g s t a t e i s e x p e c t e d t o l i e a t some 9,0ftù 12,000 cm" higher energy. Consequently, the t r a n s i t i o n seen at 33,800 cm i n £-polarization i s a s s i g n e d t o t h i s B i g t r a n s i t i o n . However, t h e i n t e n s i t y o f t h i s t r a n s i t i o n a p p e a r s uncomfortably small f o r t h i s assignment. I t i s h a r d l y more i n t e n s e t h e n t h e s p i n - f o r b i d d e n t r a n s i t i o n s d e s p i t e i t s g r e a t e r p r o x i m i t y t o the s t r o n g bands f r o m w h i c h i n t e n s i t y i s b o r r o w e d , and t h e a s s i g n m e n t c l e a r l y i s subj'ect t o q u e s t i o n . T h e r e i s a c l o s e c o r r e s p o n d e n c e between t h e d*-d s p e c t r a o f KsPtBr and K P t C l w h i c h i s c l e a r f r o m F i g u r e 4 where t h e e n e r g i e s o f t h e t r a n s i t i o n s a r e c o m p a r e d . The t r a n s i t i o n i n t e n s i ­ t i e s a r e i n d i c a t e d by t h e l e n g t h o f l i n e s d e s i g n a t i n g t h e s t a t e s , which f o r the c r y s t a l s are p r o p o r t i o n a l t o log e a t 15°K. It c a n be s e e n t h a t e a c h d«-d t r a n s i t i o n i n K P t B r f a l l s 1,500-2,200 cm" below a corresponding t r a n s i t i o n i n KjaPtCU In a d d i t i o n the v i b r a t i o n a l s t r u c t u r transitions. However quite different. F i ^ r s t o f a l l , t h e s o l u t i o n s p e c t r u m P t B r " has a band a t 31 ,500 cm" , e - 600 c m ^ M " . S i n c e t h e c r y s t a l s p e c t r a f o r b o t h p o l a r i z a t i o n s have a deep v a l l e y a t 29,000 cm" , this band i s p r e s e n t l y a s s i g n e d t o a s o l u t i o n s p e c i e s o t h e r t h a n P t B r " , probably P t B r " . The s o l u t i o n does e x h i b i t s t r o n g bands a t 34,200 and 37,200 cm" with e o f 3,000 and 8 , 5 0 0 cm" M r e s p e c t i v e l y . The c - p o l a r i z e d c r y s t a l s p e c t r u m , w h i c h can be r e c o r d e d t o 37,000 cm" , r e q u i r e s t h a t b o t h t h e s e t r a n s i t i o n s be p o l a r i z e d x,^. The p r e s e n t a s s i g n m e n t a t t r i b u t e s them t o s i n g l e t and t r i p l e t s t a t e s o f t h e same i r r e d u c i b l e r e p r e s e n t a t i o n i n D , v i z . Eîg, w i t h n e a r l y t h e same e n e r g y w h i c h m i x , a r e s p l i t and share i n t e n s i t y . The s e p a r a t i o n o f 3,000 cm" f o r these are c o n s i s t e n t w i t h the expected s p i n - o r b i t c o u p l i n g f o r the Pt i o n . The s i n g l e t s t a t e i s p r o b a b l y t h e E a r i s i n g f r o m M(a' )*~L. A c c o r d i n g t o J o r g e n s e n ( 8 ) , t h e ^-Ey t r a n s i t i o n g a i n s i n t e n s i t y by m i x i n g o f π and σ c h a r a c t e r i n t h e e l i g a n d o r b i t a l s . . There s h o u l d be a A s t a t e c o r r e s p o n d i n g t o Μ(σ*)Η- a t r o u g h l y t h i s e n e r g y . However, i t a r i s e s f r o m Q * ( b i g ) * - b u . Since b i s pure π i n c h a r a c t e r , t h e t r a n s i t i o n s h o u l d be, a c c o r d i n g t o J o r g e n s e n , much w e a k e r . I t may l i e j'ust above 37,000 cm" . However, t h e r e can not be a s t r o n g A u t r a n s i t i o n b e l o w 48,000 cm" . With P t C l " t h e MCD s t u d i e s o f S c h a t z and c o w o r k e r s (2) show t h a t a s h o u l d e r a t 43,000 cm is E w h e r e a s an i n t e n s e p e a k a t 46,000 cm" must be ^-Asu- The d i f f e r e n c e between P t C l " and P t B r " suggests that the intense t r a n s i t i o n s i n P t C l " are p r i m a r i l y 6pH>d. The A u s t a t e w o u l d c o r r e s p o n d t o 6p «-5d . The v e r y weak peak a t c a . 3 0 , 5 0 0 cm" seen i n both p o l a r i ­ z a t i o n s d e s e r v e s some comment. In c - p o l a r i z a t i o n , a t l e a s t , i t seems t h a t a v i b r o n i c l y e x c i t e d t r a n s i t i o n w o u l d be a p p a r e n t i n t h e 300°K s p e c t r u m . Even t h o u g h t h e a b s o r p t i o n i s r i s i n g r a p i d l y i n t h i s r e g i o n , i t can not be d i s c e r n e d a t a l l . I t a p p e a r s t h e r e ­ f o r e t o have a s m a l l but n o n - z e r o t r a n s i t i o n d i p o l e , and i t i s a s s i g n e d as E . The t r a n s i t i o n t o t h i s s t a t e i s d i p o l e - a l lowed 1

1

1

1

4

2

4

m x

2

4

1

2

4

1

1

1

2

2

4

2

6

1

1

m x

1

1

4

1

1

v

U

u

1

2 U

2

2 u

1

1

1

2

2

4

1

1

u

1

2

2

4

4

2

4

1

2

z

z2

1

3

U

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

260

EXTENDED

ι ι ι ι I

I

I

I

I

j

1

I

I

I

I

I

!

I

INTERACTIONS

BETWEEN METAL

IONS

!

Kj>PtBr

4

100

15

Figure

3.

20

Polarized

25

crystal absorption

30

35

spectra for

K PtBr 2

k

50i J

45

A

2

u

40 :,

E - B, (E,' ); U

3

U

U

235 χ

3

B,u

ε 30

-/

.—'Eg

υ - — 'A2a . - B , Q (Ei )

la

3

fl

25 20 h

K PtBr 2

4

K PtCI 2

4

Figure 4. Excited states for K PtBr,, and KjPtCl,. Length of the line for each state is 15 proportional to log €„,„, (15°K) for the crystal transitions and log «,„„, (soin) for the intense transitions. 2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

18.

Cry stab with

MARTIN, JR.

by

Square

Complexes

virtue of the spin-orbit coupling

Magnus

1

Green S a l t .

261

i n both p o l a r i z a t i o n s .

PtÇNHaUPtCU.

I t h a s l o n g been r e c o g n i z e d t h a t t h e r e must be s t r o n g c r y s ­ t a l e f f e c t s on t h e s p e c t r a i n Magnus' g r e e n s a l t (MGS), Pt(NH3) PtCl . The component c a t i o n , P t ( N H 3 ) , i s c o l o r l e s s i n aqueous s o l u t i o n o r h a l i d e s a l t s w h e r e a s t h e a n i o n , P t C l " , i s r e d . A s t h e name i m p l i e s t h e MGS i s a d a r k g r e e n , v e r y i n s o l u b l e salt. I t p o s s e s s e s a t e t r a g o n a l c r y s t a l s t r u c t u r e (9) w i t h two of each ion p e r u n i t c e l l . There a r e s t a c k s o f a l t e r n a t i n g ions w h i c h a r e i l l u s t r a t e d i n F i g u r e 5 w i t h a s p a c i n g o f 3.24A between t h e a n i o n s and c a t i o n s i n t h e c h a i n s . The p o l a r i z e d c r y s t a l s p e c ­ t r a a r e p r e s e n t e d i n F i g u r e 6. The c r y s t a l s a r e h i g h l y d i c h r o i c . A c r y s t a l w h i c h a p p e a r s d a r k g r e e n i n c_-polar i z a t i o n i s p a l e y e l low i n ^ - p o l a r i z a t i o n much h i g h e r i n t h e c - p o l a r i z a t i o r e s u l t s f r o m a window a t a b o u t 20,000 c m " . The waves i n t h e r e c o r d i n g a t l o w a b s o r b a n c e a r e due t o i n t e r f e r e n c e by m u l t i p l y r e f l e c t e d l i g h t (10). A l t h o u g h t h e i n s t r u m e n t a t i o n d i d n o t p e r ­ m i t measurements b e l o w 17,000 c m " , i t i s c l e a r t h a t t h e l o w e n e r g y band i n £-polarization i s v i b r o n i c i n c h a r a c t e r . A t h i g h e r e n e r g i e s t h e a b s o r p t i o n i n c r e a s e s beyond a n ε o f 700 cm" Μ " , t h e l i m i t o f t h e measurements. In a_-polar i z a t i o n t h e s p e c ­ t r u m i s d o m i n a t e d by a s i n g l e peak a t a b o u t 25,000 c m " , a l t h o u g h t h e r e i s a s h o u l d e r a t 23,000 cm" and a weak s p i n - f o r b i d d e n band b e l o w 17,000 c m " . The wave numbers o f t h e t h r e e p e a k s i n K s P t C l s o l u t i o n a r e shown by t h e 3 a r r o w s i n F i g u r e 6. I t i s concluded t h a t t h e s p i n - f o r b i d d e n p e a k o f P t C l " has been r e d - s h i f t e d b y a b o u t 4,000 cm" i n MGS. The AQQ t r a n s i t i o n and t h e trans­ i t i o n s have c o a l e s c e d t o t h e s i n g l e peak a t a b o u t 25,000 cm" . T h i s c o r r e s p o n d s t o a r e d s h i f t o f n o t more t h a n 1,000 cm" f o r t h e ^ g s t a t e a n d a b o u t 4000-5000 cm" f o r t h e ^-Eg s t a t e . The r e l a t i v e s h i f t o f t h e s e t r a n s i t i o n s i s a t t r i b u t a b l e t o t h e D' t e r m o f e q u a t i o n 5. The a n g u l a r d e p e n d e n c e o f t h e d o r b i t a l s i n ­ v o l v e d i s shown i n F i g u r e 7 . T h u s , f o r t h e t r a n s i t i o n t o t h e A s g s t a t e , a"(d 2_y2)*~d y, both o r b i t a l s a r e concentrated i n the plane o f t h e i o n and t h e i r e l e c t r o n s s u f f e r s i m i l a r i n t e r a c t i o n s w i t h t h e e l e c t r o n c l o u d s o f t h e adj'acent i o n s . For the d o r b i t a l , however, t h e r e i s a c o n s i d e r a b l e e l e c t r o n d e n s i t y o u t o f t h e p l a n e o f t h e i o n . The e n e r g y o f t h e e l e c t r o n i n t h i s o r b i t a l i s t h e r e f o r e r a i s e d by t h e e l e c t r o n - e l e c t r o n r e p u l s i o n s , and a red s h i f t f o r t h e t r a n s i t i o n o c c u r s . The h i g h a b s o r p t i o n i n t h e c - p o l a r i z a t i o n i s c l a r i f i e d f r o m d i f f u s e r e f l e c t a n c e s p e c t r a f o r K P t C l and MGS w h i c h a r e shown i n F i g u r e 8. I t c a n be s e e n t h a t t h e maximum, w h i c h c o r r e s p o n d t o a v e r y i n t e n s e t r a n s i t i o n , has been s h i f t e d f r o m c a . 44,000 cm" i n K P t C l t o 35,000 cm" i n MGS. T h i s band i s a p p a r e n t l y i n t h e c - p o l a r i z a t i o n . Such a l a r g e r e d s h i f t must be l a r g e l y c a u s e d b y t h e I t e r m o f e q u a t i o n 5. I t w o u l d r e q u i r e s t r o n g 2 +

4

4

4

2

4

1

1

1

1

1

1

1

4

2

4

1

1

1

1

1

X

X

y

2

1

4

1

2

4

1

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

z

262

EXTENDED

Figure

Figure 6. Polarized crystal absorption trum for Magnus' green salt

INTERACTIONS

5.

Alternate Magnus'

BETWEEN

METAL

stacking of green salt

spec-

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

ions

in

18.

MARTIN, JR.

Figure

7.

Angular

Crystals

with

Square

Complexes

portions of ά-orbitals involved tral transitions of MGS

Figure

8.

263

in spec­

Diffuse reflectance spectra and K.FtCh, at 300°Κ

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

for

MGS

264

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

2

2 +

t r a n s i t i o n s o f c o m p a r a b l e e n e r g y i n b o t h P t C l ~ and P t ( N H 3 ) . T h i s i s the a l l o w e d t r a n s i t i o n from which the v i b r o n i c e x c i t a t i o n s i n c - p o l a r i z a t i o n b o r r o w i n t e n s i t y , and t h e p r o x i m i t y o f t h i s band e n h a n c e s t h e i r i n t e n s i t y by a b o u t an o r d e r o f m a g n i t u d e . The s p e c t r a o f MGS t h e r e f o r e i n d i c a t e t h a t t h e s t r i k i n g c o l o r changes r e s u l t f r o m c r y s t a l i n t e r a c t i o n s , p r i m a r i l y w i t h a d j a c e n t n e i g h b o r s , w h i c h m o d i f y t h e m o l e c u l a r t r a n s i t i o n s but w i t h o u t d e l o c a l i z a t i o n of e l e c t r o n s . 4

Pt Br 2

2 6

"

4

Spectra

A d i m e r complex p r o v i d e s an o p p o r t u n i t y f o r t h e d e m o n s t r a ­ t i o n o f e l e c t r o n d e l o c a l i z a t i o n on a l i m i t e d s c a l e by a b s o r p t i o n spectroscopy. The d i m e r i c a n i o n c r y s t a l l i z e s i n t h e t e t r a e t h y l ammonium s a l t w h i c h i w h i c h t h e s t r u c t u r e wa t i o n o f d i m e r i c i o n on t h e c l e a v a g e f a c e s show Figure 9. T h e r e i s one a n i o n p e r u n i t c e l l . The p l a t i n u m atoms i n a d i m e r a r e s e p a r a t e d 3.55Â, and t h e a n i o n s a r e s e p a r a t e d 7 . 6 i f r o m t h e i r n e a r e s t n e i g h b o r s by t h e b u l k y c a t i o n s . The l i g h t beam t h e r e f o r e e n t e r s d i m e r a n i o n n e a r l y end on. The m o l e c u l a r a x e s o f heavy a n i o n a p p a r e n t l y e s t a b l i s h the axes of the i n d i c a t r i x , so t h e r e i s v e r y l i t t l e d i r e c t i o n a l change o f t h e s e a x e s w i t h wave l e n g t h . One e x t i n c t i o n f o r c r y s t a l s was a l i g n e d , as a c c u r a t e l y as c o u l d be d e t e r m i n e d , w i t h t h e p r o j e c t i o n o f t h e y m o l e c u l a r a x i s upon t h e c r y s t a l f a c e o v e r t h e e n t i r e wave lengtTh r e g i o n w h i c h was s t u d i e d . The a b s o r p t i o n i n t h i s e x t i n c t i o n d i r e c t i o n gave a v e r y c l e a n intermediate a x i s or ^ - p o l a r i z a t i o n . The o t h e r e x t i n c t i o n p r o v i d e d p r i m a r i l y the s h o r t a x i s or x - p o l a r i z a t i o n . Preliminary s p e c t r a a r e shown i n F i g u r e 10. The a b s o r p t i o n i n ^ - p o l a r i z a t i o n i s much h i g h e r t h a n i n t h e x - p o l a r i z a t i o n . T h u s , a t room tempera­ t u r e t h e r e i s a peak i n γ p o l a r i z a t i o n a t 2 3 , 5 0 0 cm" very c l o s e t o t h e 24,000 cm" f i r s t spin-allowed transition in K P t B r . However, t h e e f o r t h e band a t t h i s V i n K P t B r was o n l v c a . 100 c n P - M . In t h e P t B r " s a l t i t was 800 cm' /H(?t Br ~J7 But when t h e c r y s t a l was c o o l e d , t h e peak h e i g h t i n c r e a s e d t o 1600 c n T V M , w h i c h i n d i c a t e d t h a t t h e t r a n s i t i o n was d i p o l e a l l o w e d r a t h e r than v i b r o n i c . In t h e x - p o l a r i z a t i o n t h e r e was not much i n d i c a t e d s t r u c t u r e i n t h e s p e c t r u m b u t t h e a b s o r p t i o n i n c r e a s e d as ν i n c r e a s e d . A peak a t 3 6 , 5 0 0 cm" occurs j u s t where t h e MH. c h a r g e t r a n s f e r band o c c u r s i n P t B r " s o l u t i o n . T h i s may be t h e n an x - p o l a r i z a t i o n o r o u t - o f - p l a n e M*-L w h i c h a p ­ p a r e n t l y i s v e r y weak i n P t B r " as w e l l . A t 15°K, band maxima a t 29,000 cm" and 32,000 cm" a r e s e e n as w e l l , and t h e r e i s a s h o u l d e r a t 26,000 cm" . In c o n s i d e r a t i o n o f t h i s s p e c t r u m e a c h p l a t i n u m atom can be c o n s i d e r e d t o b r i n g i n t o t h e d i m e r i t s complement o f f i v e 5 d orbitals. M o l e c u l a r o r b i t a l s f o r t h e d i m e r a r e c o n s t r u c t e d by t a k i n g t h e sum o r d i f f e r e n c e o f c o r r e s p o n d i n g d - o r b i t a l s on d i f ­ f e r e n t p l a t i n u m atoms. W i t h t h e c h o i c e o f a x e s t h e σ'* o r b i t a l s o

1

1

2

m a x

2

- 1

2

2

1

2

6

1

2

4

2

4

1

4

4

1

1

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

e

u

Cry stab with

MARTIN, JR.

Square

Complexes

[N(C H ) ]JPt Br ] 2

5

4

2

6

b

Figure

9.

Projection

of the dimeric ions, Pt Br ~ of[N(C H )J [Pt Br l 2

2

I

2000 "

1

1

1

I

J

1

1

5

2

1

2

6

upon the 001 cleavage

2

(

1

ι

ι

l—ι

j

ι

ι

ι

2

5

4

2

2

ι

Ϊ

1

6

300°Κ/^

- x polarization ^-<^X

0 ··-[

ι

» ι

ι j-i

ι

y - polarization

ι ^

ι

t

1

^7 /

300V7 κy J

ι

1η ι

{

\ ^ y

20

Polarized

1

ι

I

ι

ι

I

ι

1 1

1

1 I

—

J

25 V

ι

1 I ι

15

1

15° Κ

/

—

10.

ι

[N(C H ) ] [Pt Br ]

1000

Figure

face

6

(cm x10 _ 1

crystal absorption

3

I 30

ι

ι

ι

ι

I 35

1

)

spectrum

for \N(C H ) '\ t

s k

t

[Pt Br ]

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

t

6

266

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

a r e formed f r o m t h e p l a t i n u m d ' s , and t h e o r b i t a l s w i t h ττc h a r a c t e r l o c a t e d i n the molecular plane are the d y _ s. The two l i n e a r c o m b i n a t i o n s f o r t h e s e atoms w i t h t h e i r symmetry d e s i g n a t i o n under D a r e shown i n F i g u r e 11. This f i g u r e a l s o shows t h e LC o f l i g a n d o r b i t a l s w i t h t h e same symmetry a s t h e LC o f p l a t i n u m o r b i t a l s and w h i c h c o n t r i b u t e t o t h e σ and π b o n d s . Thus t h e r e a r e two f i l l e d o r b i t a l s and two empty o r b i t a l s and a p o s s i b i l i t y of k t r a n s i t i o n s . I f t h e p l a t i n u m atoms were e s s e n ­ t i a l l y i s o l a t e d and t h e o r b i t a l s were o f p u r e d - c h a r a c t e r , t h e n o n l y a s i n g l e s p e c t r a l band w i t h v i b r o n i c c h a r a c t e r i s e x p e c t e d . However, e a c h p a i r o f t h e f i l l e d and empty M.0. s c o n t a i n s a £ and a u s t a t e . T h e r e f o r e two o f t h e f o u r e x c i t e d s t a t e s w i l l have _u c h a r a c t e r and two w i l l have £ c h a r a c t e r . As i n d i c a t e d i n F i g u r e 11, b o t h u n g e r a d e e x c i t e d s t a t e s have B symmetry and s h o u l d have d i p o l e - a l l o w e which the i n t e n s i t i e s o v e r l a p of the d o r b i t a l s . However, t h e i n t e n s i t i e s may be m o d i f i e d by t h e b r i d g i n g l i g a n d o r b i t a l as w e l l . The h i g h i n t e n s i t y and d i p o l e c h a r a c t e r o f t h e band i n ^ - p o l a r i z a t i o n i n d i c a t e s t h a t t h e r e i s e l e c t r o n d e l oca 1 i z a t i o n i n t h e t r a n s i t i o n . The i n v e s t i ­ g a t i o n of t h i s system i s c o n t i n u i n g . ,

2

z 2

2n

!

2 u

Pt(en)Cl

2

and

Pt(en)Brs.

Transitions to Ionic States

F o r t h e m o l e c u l a r c r y s t a l s o f t h e two compounds, P t ( e n ) C l and P t ( e n ) B r , where en i s e t h y l e n e d i a m i n e , t h e r e i s now e v i d e n c e for e l e c t r o n i c t r a n s i t i o n s in which e l e c t r o n s are t r a n s f e r r e d f r o m one m o l e c u l e t o an a d j a c e n t m o l e c u l e (12,13)» The e x c i t e d s t a t e f o r s u c h a t r a n s i t i o n i s t h e r e f o r e a bound i o n i c e x c i t o n state. In b o t h c r y s t a l s , w h i c h a r e o r t h o r h o m b i c , t h e m o l e c u l e s s t a c k f a c e t o f o r m o n e - d i m e n s i o n a l c h a i n s as shown i n F i g u r e 12. The c h e l a t i n g g r o u p i n a d j a c e n t m o l e c u l e s l i e on a l t e r n a t e s i d e s s o t h e r e a r e two m o l e c u l e s i n a prîmativé c e l l . However, t h e c o r r e s p o n d i n g molecular axes of the non-equivalent molecules are parallel. Under s u c h c o n d i t i o n s t h e t r a n s i t i o n p r o b a b i 1 i t y t o one o f t h e two Davydov s t a t e s f o r e a c h t r a n s i t i o n w i l l be e x a c t l y z e r o , and o n l y one c r y s t a l t r a n s i t i o n i s o b s e r v a b l e f o r e a c h molecular t r a n s i t i o n . The m o l e c u l a r symmetry i s C . However t h e l o c a l symmetry a t t h e P t i s and t h e m o l e c u l a r symmetry d e v i a t e s f r o m t h i s o n l y by t h e p u c k e r i n g o f t h e c h e l a t e r i n g . It i s l i k e l y t h a t t h e s e l e c t i o n r u l e s a r e e s t a b l i s h e d by t h e C v symmetry. The c h o i c e o f a x e s i s shown i n F i g u r e 12. The x,^ and ζ m o l e c u l a r a x e s a r e a l i g n e d w i t h t h e o r t h o r h o m b i c £ , b, and c a x e s , r e s p e c t i v e l y . S p e c t r o s c o p i c q u a l i t y c r y s t a l s grew w i t h w e l l d e v e l o p e d 100 f a c e s s o a b s o r p t i o n s f o r c and _b p o l a r i z a t i o n s were m e a s u r a b l e t o p r o v i d e t h e m o l e c u l a r ζ and ^ p o l a r i z a t i o n s respectively. The symmetry o f t h e d - o r b i t a l s a r e shown i n F i g u r e 13. Under t h e c h o i c e o f a x e s t h e d o r b i t a l i s a part of the σ*-Μ0. The C2ν symmetry s p l i t s t h e t g d e g e n e r a c y o f D h and f o u r d i s t i n c t d«-d t r a n s i t i o n s a r e p o s s i b l e . S i n c e Qgy does not c o n t a i n 2

2

2

2

x y

4

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

.18.

Crystals

MARTIN, JR.

with

Square

Complexes

«b.

2u

•) b 3g

B

(-) b

Wa t)

t) Br

ν

ν

2U

l u

g

Λ) ^3Γ'

Figure 11. Symmetry adapted LCAO's for the d a n d i h e d * . / orbitals upon the two platinum atoms of Pt Br ~. LCAO's of ligand orbitals also shown. Alge­ braic sign of the LC is in parenthesis; also given is the orbital symmetry under D . Excited states for the two indicated d <- d transitions are both B and both transitions are dipole allowed in y-polarization.

y z

y

2

7

2 6

2h

iu>

•

x,- a Figure

12.

Stacking Pt(en)Cl

2

of the molecules crystals

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

in

268

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

a c e n t e r o f symmetry, t h e l i g a n d f i e l d p e r t u r b a t i o n s can c o n t r i b ­ u t e d i p o l e - a l lowed c h a r a c t e r t o t h e d«-d t r a n s i t i o n s . The s t a t e s a t t a i n e d f r o m s p i n - a l l o w e d d*-d t r a n s i t i o n s t o g e t h e r w i t h t h e a l l o w e d p o l a r i z a t i o n by t h e l i g a n d f i e l d p e r t u r b a t i o n s i n t h e o r d e r o f i n c r e a s i n g e n e r g y as i n d i c a t e d by t h e o r b i t a l scheme o f F i g u r e 13 a r e : B , d* <-d _ , x p o l a r i z a t i o n ; , d**^ » f o r b i d d e n ; B i , d * *-d , ζ-polar i z a t i o n ; and B , d" «-d ; x * polarization, f o r e , t h e r e s h o u l d be no l i g a n d -a 11 owed d*-d t r a n s i t i o n w i t h ^ o r l b - p o l a r i z a t i o n . The s p e c t r u m o f a s o l u t i o n o f P t ( e n ) C l 2 i s i n F i g u r e 14. It i s q u i t e c h a r a c t e r i s t i c o f o t h e r s o l u t i o n s p e c t r a o f h a l i d e - a m i ne complexes of p l a t i n u m ( 1 1 ) . Two s p i n - f o r b i d d e n bands can be r e ­ s o l v e d between 23,000 and 29,000 cm" . T h e r e a r e t h e n two s p i n a l l o w e d bands d i s c e r n a b l e . The t r a n s i t i o n a t 33,000 cm" can be a s s i g n e d t o t h e B s an t r a n s i t i o n s ^ 3 and B l o w i n g a r a t h e r deep v a l l e y a t 40,000 cm" t h e r e i s an i n t e n s e band a t 4-9,000 cm" . For comparison the b - p o l a r i z e d c r y s t a l s p e c t r u m a t 300°K_is i n c l u d e d i n F i g u r e lïï. There i s a sharp p e a k a t 33,000 cm" w h i c h i s n e a r l y 3 t i m e s as i n t e n s e a s _ t h e one i n s o l u t i o n a t t h a t e n e r g y . A v a l l e y o c c u r s a t 37,000 cm" and a r a t h e r d e f i n i t e s h o u l d e r i s s e e n a t 39,000 cm" , very close t o t h e v a l l e y o f t h e s o l u t i o n s p e c t r u m . The b - p o l a r i z e d i n t e n s i t i e s h o w e v e r , a r e an o r d e r o f m a g n i t u d e b e l o w t h e c - p o l a r i z e d s p e c t r u m . B o t h c r y s t a l s p e c t r a f o r 300°K and 15°K a r e i n c l u d e d i n F i g u r e 15 t o g e t h e r w i t h a room t e m p e r a t u r e d i f f u s e r e f l e c t a n c e s p e c t r u m . The i n t e n s i t y f e a t u r e s o f t h e c - p o l a r i z a t i o n and t h e r e f l e c t a n c e s p e c t r a a r e v e r y r e m i n i s c e n t o f t h o s e f o r MGS. There i s a very s t r o n g enhancement i n t h e i n t e n s i t y o f v i b r o n i c t r a n s i t i o n s w h i c h a r e d e r i v e d f r o m t h e r e d - s h i f t o f an i n t e n s e band p o l a r i z e d i n t h e s t a c k i n g d i r e c t i o n f r o m an e n e r g y a t l e a s t as h i g h as 49,000 cm" i n t h e s o l u t i o n down t o 37,500 cm" i n the c r y s t a l . T h i s s h i f t must be due t o a c o n s i d e r a b l e e x t e n t t o t h e I t y p e o f t e r m i n e q u a t i o n 5. There i s a r e d - s h i f t of the s p i n - f o r b i d d e n bands as w e l l , p e r h a p s n o t q u i t e as g r e a t as i n MGS b u t h e r e t h e stacking distance is 0J5A greater. The u n p r e d i c t e d f e a t u r e o f t h e s p e c t r u m was thé d i p o l e a l lowed c h a r a c t e r o f t h e two t r a n s i t i o n s i n b - p o l a r i z a t i o n a t 33,000 and 39,000 cm" i n d i c a t e d by t h e t e m p e r a t u r e dependence o f their intensity. In t h i s d i r e c t i o n t h e l i g a n d f i e l d p e r t u r b a t i o n d o e s not c o n t r i b u t e any t r a n s i t i o n d i p o l e t o d«-d t r a n s i t i o n s . T h e r e f o r e , i t has been c o n c l u d e d t h a t t h e s e two t r a n s i t i o n s c o r ­ r e s p o n d t o t h e t r a n s f e r o f e l e c t r o n s between m o l e c u l e s , to i o n i c s t a t e s . The t h e o r y f o r e l e c t r o n d e l o c a l i z a t i o n between e l e m e n t s o f a one d i m e n s i o n a l c h a i n was t r e a t e d by M e r r i f i e l d (14). O v e r l a p between o r b i t a l s o f a d j a c e n t atoms i s r e q u i r e d , and i n g e n e r a l t h e e l e c t r o n f r o m one m o l e c u l e can be t r a n s f e r r e d t o an u n o c c u p i e d o r b i t a l , β m o l e c u l e s removed f r o m t h e h o l e w h i c h r e ­ sults. The l o w e r e n e r g y s t a t e s a r e bound i o n i c s t a t e s w i t h a f i x e d s e p a r a t i o n between t h e e l e c t r o n and h o l e , w h i c h a r e however 1

2

xy

x2

y 2

v z

1

1

2

2

1

1

1

1

1

1

1

1

1

1

1

1

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

MARTIN,

18.

Crystals

JR.

with

Square

Complexes

°4h °2u

b

P

- b,

*

2g * * x

b

2 Figure 13. for Pt(en)Cl x

blg»^

II

•g «ig

*

2

2

1

7r , x

y

2

H " °i

—4j: U H " «i

0

»

Orbital symmetries and Pt(en)Br mole­ cules

2

"H" 1+- b, jjtt ° 52

ΙΟ"

3

ν cm-'

Figure 14. Absorption spectrum for an aqueous solution Pt(en)Cl and the b-polarized crystal absorption spectrum Pt(en)Ch g

of for

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

EXTENDED

INTERACTIONS

WAVENUMBERS (cm-')x 10 " Figure

y

y

2

11 *

15. Diffuse

reflectance spectrum sorption spectrum for

—

—

—

-tf-

- i f - -Vh

- t f -

- ( N - l)/2

-H-

j-l

- Θ

j

IONS

crystal ab­

t

—

—

u'

- t f - -tf-

-H-

p t f

-ft- °

j f t f ::::::·.£&

- f t - - t f -

H

j+2

u

+ (N-l)/2

ire 16. Schematic of an ionic excitation in a one-dimensional stack of molecules: θ (β = -j-l) based on (d ) i <- (ά *) }

METAL

3

and polarized ?t(en)Cl

-4-

::::::: ^ ± t

BETWEEN

xy }

+

7

}

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

Pt(en)Cl

g

]8.

Crystals

MARTIN, JR.

with

Square

Complexes

271

mobile i n the chain. B e c a u s e o f t h e coulomb a t t r a c t i o n o f t h e h o l e and e l e c t r o n t h e l o w e s t i o n i c s t a t e s r e s u l t when t h e h o l e and e l e c t r o n a r e on adj'acent m o l e c u l e s . A s y s t e m w i t h s u c h a o n e - m o l e c u l e t r a n s f e r f r o m a d _ o r b i t a l t o an adj'acent d * o r b i t a l , β = +1, i s i l l u s t r a t e d i n F i g u r e 16. A t h i g h e n e r g i e s t h e t r e a t m e n t d e s c r i b e s a c o n d u c t i o n band f o r t h e f r e e h o l e and e l e c t r o n . A c r y s t a l wave f u n c t i o n f o r a β s e p a r a t i o n o f t h e h o l e and e l e c t r o n and f o r w h i c h t h e o n e - d i m e n s i o n a l wave v e c t o r , k, i s z e r o (a r e q u i r e m e n t f o r a d i p o l e a l l o w e d c r y s t a l t r a n s i t i o n j i s : 2

Φ4(β) = M - * E j 0 j ( p ) .

(7)

Now t h e wave f u n c t i o n f o r t h e t r a n s f e r o f t h e e l e c t r o n i n t h e o p p o s i t e d i r e c t i o n (-β) w i l l be d e g e n e r a t e w i t h §ο(β)· The two c r y s t a l s t a t e s formed f r o l i n e a combinatio f thes degenerat f u n c t i o n s w i l l be f o

HO)

= 2 - W + l ) + W(-D],

(8)

= 2"*[M(+1) - Φ ' ( - 1 ) ] .

(9)

The f u n c t i o n Y + ( l ) has t h e same symmetry as t h e F r e n k e l s t a t e , § 0 ( e q u a t i o n 4) and t h e s e two s t a t e s may i n t e r a c t a s i s shown i n F i g u r e 17. I f t h e F r e n k e l s t a t e i s a l l o w e d , t h e n b o t h s t a t e s w i l l mix and s h a r e i n t e n s i t y . U n l e s s t h e i r e n e r g y i s v e r y c l o s e , Ψ+(1) w i l l be much weaker t h a n $ . However, i f t h e F r e n k e l s t a t e and t h e r e f o r e Ψ4- "e f o r b i d d e n , t h e n i t may be p o s s i b l e f o r Ψ ! t o be a l l o w e d , s i n c e i t i s o f d i f f e r e n t symmetry. The r e q u i r e m e n t for allowed character i s that: 9

0

ar

r

j j - 1 - - j . j +l ' r

0

'

(Ό)

where r J . , i s t h e t r a n s i t i o n moment f o r t h e t T a T i s f e r o f an e l e c t r o n f r o m t h e j t o t h e j + 1 member o f t h e s t a c k , j_.e. +

r-'

ι*! " f u ? r u '

,dT,

(11) 1

where u and u a r e t h e f i l l e d and empty orbitals respectively. Such a n o n - z e r o t r a n s i t i o n moment does r e q u i r e s i g n i f i c a n t o v e r ­ l a p o f t h e o r b i t a l s . An e x a m i n a t i o n o f t h e symmetry o f t h e d o r b i t a l s r e v e a l s t h a t t h e r e i s o n l y one d*-d e l e c t r o n t r a n s f e r t r a n s i t i o n v i z . (d£ ) . ,*-(d ). , f o r w h i c h t h e c o n d i t i o n o f e q u a ­ t i o n 10 a p p l i e s f o r a v_ and hefSce a b_ p o l a r i z a t i o n . T h e r e f o r e t h i s a s s i g n m e n t has been g i v e n t o t h e c r y s t a l t r a n s i t i o n a t 33,000 cm" i n t h efc>p o l a r i z a t i o n . I t i s i n t e r e s t i n g t o n o t e t h a t i t i s t h e a b s o r p t i o n w i t h p o l a r i z a t i o n normal t o t h e s t a c k i n g d i r e c t i o n which provides evidence f o r d e l o c a l i z a t i o n of the e l e c t r o n s along t h e s t a c k i n t h e t r a n s i t i o n . However, t h e r e i s t h e s e c o n d t r a n s J

1

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

272

EXTENDED

INTERACTIONS

BETWEEN

METAL

1

Figure 17. Energy state diagram illustrating mixing of the Frenkel Φ,/ and the Ψ wave functions with an indicated transition Ψ/( 1) <- Φ„ +

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

18.

Crystals

MARTIN, JR.

with

Square

Complexes

273

îtîon a t 39,000 cm" w h i c h must be a s s i g n e d . Of a l l p o s s i b l e i o n i c s t a t e t r a n s i t i o n s i n t o t h e o' (d } or i n t o the 6p orbital, f o r t h a t m a t t e r , t h e r e i s o n l y one, a σ' ητ-01 ( d * *-a ) w h i c h w i l l p r o v i d e a n o n - z e r o t r a n s i t i o n moment i n a c c o r d a n c e w i t h e q u a t i o n 10. The 39,000 cm" t r a n s i t i o n has been g i v e n t h i s a s s i g n m e n t . In t h e r e c e n t w o r k w i t h t h e P t ( e n ) B r s y s t e m ( 1 3 ) , the s t a c k i n g d i s t a n c e o f t h e m o l e c u l e s i s 3 . 5 0 A compared t o t h e 3.39A f o r t h e P t ( e n ) C l . The s o l u t i o n s p e c t r u m f o r P t ( e n ) B r , F i g u r e 1 8 , e x h i b i t s s i m i l a r f e a t u r e s o f t h a t f o r P t ( e n ) C l . However, t h e s p i n f o r b i d d e n and s p i n a l l o w e d components have been r e d s h i f t e d a b o u t 2 , 0 0 0 cm" and a maximum i n t h e i n t e n s e s p e c t r u m was n o t r e a c h e d b e l o w 5 0 , 0 0 0 cm" , where a m o l a r a b s o r p t i v i t y o f 1 2 , 0 0 0 c m ^ M " was a t t a i n e d . The c r y s t a l s p e c t r u m f o r P t ( e n ) B r i s i n F i g u r e 19. In t h e d i f f u s e r e f l e c t i o n t h e h i g h e s t maximum i s a t 36,500 cm" , ver corresponding spin-forbidde o f a t l e a s t 10 more i n t e n s e t h a n i n l b - p o l a r i z a t i o n . In t h e J> p o l a r i z a t i o n a component band a t 3 1 , 0 0 0 cm" has v i b r o n i c c h a r a c ­ t e r and p r o b a b l y c o r r e s p o n d s t o B ( d ' «-d^ 2 ) . The s m a l l bump a t 33,500 cm" however has d r a m a t i c a l f ^ i n c r e a s e d i n h e i g h t as t h e band n a r r o w s . I t has t h e r e f o r e been a t t r i b u t e d t o t h e c o r ­ r e s p o n d i n g i o n i c s t a t e ( d ) | + ι * ~ ( < * ) \ · S i n c e n o r m a l l y t h e bands f o r bromide complexes are r e t l - s h i f t e d i n comparison t o c h l o r i d e , t h e occurrence o f t h i s band a t n e a r l y t h e same wave number as i n Pt(en)Cl i s c o n s i s t e n t w i t h the g r e a t e r s e p a r a t i o n of the mole­ c u l e s and t h e h i g h e r coulomb e n e r g y f o r s e p a r a t i o n o f t h e e l e c t r o n and h o l e . The s p e c t r u m c o u l d not be e x t e n d e d t o a h i g h enough e n e r g y t o o b s e r v e a s e c o n d p e a k as was t h e c a s e f o r P t ( e n ) C l . In t h e i o n i c c r y s t a l s t a t e t h e i n t e r a c t i o n o f t h e e x c i t e d i o n s w i t h t h e r e m a i n d e r o f t h e c r y s t a l i s an i m p o r t a n t e n e r g y t e r m . Thus t h e e l e c t r o n t r a n s f e r b e t w e e n t h e n e u t r a l m o l e c u l e s was o b s e r v e d w h e r e a s t h e r e was no e v i d e n c e f o r a s i m i l a r t r a n s ­ i t i o n i n MGS. In t h a t c a s e an e l e c t r o n t r a n s f e r f r o m P t C U " t o Pt(NHa)4 w o u l d r e p l a c e a p a i r o f d o u b l y c h a r g e d i o n s i n an i o n i c l a t t i c e by a p a i r o f s i n g l y charged ions w i t h a c o n s i d e r ­ a b l e r e d u c t i o n i n t h e l a t t i c e e n e r g y . T h i s e f f e c t t h e r e f o r e removes t h e e l e c t r o n t r a n s f e r s t o h i g h e r e n e r g i e s t h a n c a n be o b s e r v e d . 1

v

z

ν

2

1

2

2

2

2

1

1

1

2

1

1

x

v

2

1

χζ

x

2

2

2

2

Summary The a b s o r p t i o n s p e c t r a f o r t h e s y s t e m s d e s c r i b e d i n t h i s r e ­ v i e w i n d i c a t e t h a t i f t h e m e t a l i o n s i n t h e p l a t i n u m ( l l ) complexes a r e s e p a r a t e d by g r e a t e r t h a n 4 . 0 - 4 . I A , t h e e x t e n d e d i n t e r a c t i o n s have n e g l i g i b l e i n f l u e n c e on t h e m o l e c u l a r e l e c t r o n i c t r a n s i t i o n s . When t h e s e p a r a t i o n s a r e r e d u c e d t o 3.25 t o 3.5Â t h e r e a r e o b s e r v ­ a b l e e f f e c t s . A t t h i s p o i n t the c r y s t a l e f f e c t s , even i n the a b s e n c e o f e l e c t r o n d e l o c a 1 i z a t i o n do p r o v i d e s t r i k i n g changes i n some o f t h e t r a n s i t i o n e n e r g i e s . F i n a l l y , the i d e n t i f i c a t i o n of e l e c t r o n t r a n s f e r t r a n s i t i o n s i n P t B r e " and t h e m o l e c u l a r c r y s ­ t a l s P t ( e n ) C l and P t ( e n ) B r i n d i c a t e the energy r e q u i r e d , 3-4 2

2

2

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

274

EXTENDED

1

ι ι ι I ι ι ι ι I ι ι ι ι I I

1

ι ι I I I I I j I I

INTERACTIONS

1 1

BETWEEN

METAL

IONS

I I I I I

1200

Pt(en)Br

2

1000 8 00

15

1I I I 1 I I 1I 1 M 1 I 20 25 30 (cm"

V Figure

1

I

i I M 1ι ι 1I 1I I I ill ι ι ι 35 40 45 50

x IO

3

)

18. Absorption spectrum of an aqueous solution of Pt(en)Br the b-polarized crystal absorption spectrum of Pt(en)Br

2

and

2

ID

ι ι ιι

^

1I I

I 1

Pt(en)Br

CÛ <

11

ι ι ι

1

ι ι ι ι ι M ι ι ι 11 ι ι ι 1

I I1

2

n

k

b - polarization ι ι ι ι I ι ι ι ι I ι ι ι ι I ι ι ι ι

15

Figure

19.

20

Diffuse

25

30_ V

1

ι ι ι ι

1

35 40 (cm-'xIO )

ι ι ι ι I ι ι ι ι

45

50

3

reflectance and polarized spectra for Pt(en)Br

crystal

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

absorption

18.

MARTIN, JR.

Crystals

with

Square

Complexes

275

e.v, t o p r o v i d e t h e e l e c t r o n t r a n s f e r s between t h e m e t a l i o n s . Such h i g h e n e r g i e s f o r electron transfer require, of course, the c h a r a c t e r i z a t i o n o f the m a t e r i a l s as i n s u l a t o r s . The a b s o r p t i o n s p e c t r a w h i c h a p p l y t o weak t r a n s i t i o n s a r e c o m p l i m e n t e d by t h e p o l a r i z e d s p e c u l a r r e f l e c t i o n s p e c t r a f o r t h e i n t e n s e t r a n s i t i o n s , w h i c h w i l l be d i s c u s s e d b y P r o f e s s o r A n e x i n the f o l l o w i n g paper. Literature Cited 1. 2.

B a l l h a u s e n , C. J., "Introduction t o L i g a n d Field Theory", p . 1 8 6 , McGraw Hill, New Y o r k , 1 9 6 2 . P e i p h o , S. P., Schatz, P. N., McCafferty, A. J., J. Amer. (1969),

Chem. S o c . ,

3. 4.

91,

5994.

C r a i g , D. P., W a l m s l e y tals", p. 5 4 , W. A Mason, W. R. I I I , G r a y , H. B., J. Amer. Chem.

Soc.,

(1968),

90, 5722.

5. 6. 7. 8. 9.

J o r g e n s e n , C. K., U. S. Army R e p o r t , DA-91-506-EUC 241, (1951). M a r t i n , D. S. Jr., Foss, J. G., McCarville, M. E., T u c k e r , Μ. Α., Kassman, A. J., I n o r g . Chem. (1966) 5 , 491. M c C a f f e r t y , A . J., Schatz, P. Ν., S t e p h e n s , P. J., J. Amer. Chem. Soc., (1968), 90, 5 7 3 0 . J o r g e n s e n , C. K., Private C o m m u n i c a t i o n . A t o j i , J., R i c h a r d s o n , J. W., R u n d l e , R. E., J. Amer. Chem. Soc,

10. 11. 12.

(1957),

J. Amer. Chem.

13.

3017.

Soc.,

(1971),

93,

5433.

K r o e n i n g , R. F., H u n t e r , L. D., Rush, R. Μ., Martin,

14.

79,

M a r t i n , D. S. Jr., R u s h , R. Μ., Kroening, R. F., F a n w i c k , P. F., I n o r g . Chem. (1973), 12, 301. S t e p h e n s o n , N. C., Acta Cryst., (1964), 17, 5 8 7 . M a r t i n , D. S. Jr., H u n t e r , L. D., Kroening, R., Coley, R. F.,

D.

Merrifield,

S. Jr., J. P h y s .

Chem.,

R. E., J. Chem. P h y s . ,

(1973),

(1961),

Clardy,

77, 3077.

34,

1835.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

J.

C.,

19 T h e Nature of the Lowest-Energy

A l l o w e d Electronic

Transition in Crystals of Certain d Transition Metal 8

Complexes that Possess Extended Metal Chains BASIL G. ANEX University

of

New

Orleans,

New

Orleans,

La.

70122

Abstract The M a g n u s' salts and g l y o x i m a t e s and alkaline-earth cyanides of Pt(II), Pd(II). and Ni(II) are representatives o f a group o f d transition-metal c o m p l e x e s that show striking solid-state optical properties. The m o s t o b v i o u s manifestations of the solid-state effects involved here are either directly or indirectly related t o the lowest-energy allowed electronic transition displayed by t h e s e crystals. A series of single-crystal optical studies involving primarily polarized specular reflection t e c h n i q u e s has thus been carried o u t with t h e a i m o f characterizing the crucial low-energy transition in each group o f compounds listed above. The results o b t a i n e d for crystals containing only one m e t a l species combined with s e p a r a t e w o r k on t h e individual ions that constitute t h e M a g n u s' salts have a l l o w e d the identification o f the single-molecule origin o f the crystal transitions. Evidence is thus provided regarding the atomic orbitals involved in the crystalline excited state. The extension of these investigations to include studies o f systems containing m o r e t h a n one kind o f m e t a l a t o m h a s a l l o w e d one t o ascertain the extent to which the crystal excited state involves delocalization o v e r m o r e t h a n one center. A rather comprehensive picture of the experimental situation for these systems with regard to their optical properties has t h u s emerged and provides a critical standard by which proposed models for their electronic structure may be evaluated. 8

The w o r k reported h e r e h a s b e e n in part the National Science Foundation and t h e Institutes of Health.

supported National

276

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

by

19.

ANEx

Complexes

with

Extended

Chains

277

Introduction F o r a number o f y e a r s o u r l a b o r a t o r y has had an i n t e r e s t i n a r a t h e r b r o a d group o f p l a n a r d" systems w h o s e c r y s t a l s show a b s o r p t i v e b e h a v i o r t h a t i s m a r k e d l y and o f t e n d r a m a t i c a l l y d i f f e r e n t t h a n t h a t d i s p l a y e d by t h e i r component i o n s o r m o l e c u l e s as i s o l a t ed e n t i t i e s . The c o m p o u n d s s t u d i e d i n t h i s w o r k h a v e i n c l u d e d t h e g l y o x i m a t e s and a l k a l i n e - e a r t h c y a n i d e s o f P t ( I I ) , P d ( I I ) , and N i ( I I ) a n d a v a r i e t y o f M a g n u s salts. These systems a l l p o s s e s s c r y s t a l s t r u c t u r e s c h a r a c t e r i z e d by the component p l a n a r s p e c i e s s t a c k i n g one on t o p o f t h e o t h e r a n d t h e m e t a l a t o m s f o r m i n g p a r a l l e l c h a i n s extending the l e n g t h o f the c r y s t a l . The i n t e r p l a n a r s p a c i n t o be l i m i t e d b y r e p u l s i v component m o l e c u l e s , s i n c e i n e a c h c l a s s o f compounds one f i n d s r o u g h l y t h e same l o w e r l i m i t t o t h e m e t a l m e t a l s p a c i n g , 3.2 - 3 . 3 i?, and a n o p e n i n g up o f t h e s t a c k when one i n t r o d u c e s r e l a t i v e l y b u l k y substituents i n t o the l i g a n d s i n v o l v e d ( o r , i n the case o f the c y a n i d e s j i n t r o d u c e s more b u l k y c a t i o n s ) . The o p t i c a l s t u d i e s we h a v e c a r r i e d o u t on t h e s e s u b s t a n c e s h a v e h a d two b r o a d o b j e c t i v e s : to e s t a b l i s h the nature o f the c h a r a c t e r i s t i c s o l i d - s t a t e absorpt i o n t h a t one f i n d s i n t h e s e s y s t e m s a n d t o i d e n t i f y t h e r e l a t i o n s h i p , i f any. between t h e s e absorptions and t h e single-molecule (or "isolated-molecule ) s p e c t r a o f t h e component complexes. Throughout, p o l a r i z e d s i n g l e - c r y s t a l s p e c t r o s c o p y has p l a y e d a key role. The p o l a r i z a t i o n s o f t h e e l e c t r o n i c a b s o r p t i o n b a n d s t h u s d e t e r m i n e d n o t o n l y a l l o w e d one t o r e l a t e t h e o b s e r v e d t r a n s i t i o n s t o t h e o r y and t o r e s o l v e o v e r l a p p i n g bands, but a l s o - most i m p o r t a n t l y - p r o v i d e d " l a b e l s " t h a t one m i g h t u s e t o f o l l o w a g i v e n t r a n s i t i o n a s he moved f r o m one compound t o a n o t h e r w i t h i n a given series. One may a l s o n o t e t h a t i n a d d i t i o n t o t h e d e v e l opment o f e v i d e n c e b e a r i n g on t h e q u e s t i o n s stated above, t h i s work has a l s o l e d t o the a c c u m u l a t i o n o f much v a l u a b l e i n f o r m a t i o n c o n c e r n i n g t h e spectroscopy o f t h e a l l o w e d bands o f the complexes i n v o l v e d as such. Although l i g a n d - f i e I d spectra of t r a n s i t i o n m e t a l complexes have been r a t h e r e x t e n s i v e l y s t u d i e d , and p o l a r i z a t i o n d e t e r m i n a t i o n s a r e n o t uncommon f o r them, d e t a i l e d w o r k on t h e s t r o n g l y - a l l o w e d b a n d s ( o f t e n l o o s e l y c a l l e d " c h a r g e - t r a n s f e r " bands) i s l e s s f r e q u e n t l y c a r r i e d o u t , and p o l a r i z a t i o n a s s i g n m e n t s f o r them h a v e b e e n l a r g e l y n o n - e x i s t e n t . These bands, h o w e v e r , c o n t a i n a w e a l t h o f v a l u a b l e a n d , a s w i l l be 1

I f

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

1 1

278

seen

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INTERACTIONS

BETWEEN

METAL

IONS

sometimes s u r p r i s i n g i n f o r m a t i o n . The r e a s o n s f o r t h e p a u c i t y o f i n f o r m a t i o n c o n c e r n i n g the i n t e n s e bands i n these systems i n c l u d e the f a c t t h a t t h e y a r e o f t e n s t r o n g l y o v e r l a p p i n g and t h u s d i f f i c u l t t o r e s o l v e , and t h e y a r e s u f f i c i e n t l y i n t e n s e t h a t p o l a r i z a t i o n d e t e r m i n a t i o n s by d i r e c t a b s o r p t i o n become f o r m i d a b l e u n d e r t a k i n g s w h i c h r e q u i r e v e r y t h i n s a m p l e s , v e r y good s p e c t r o m e t e r s , o r , more l i k e l y , a c o m b i n a t i o n o f t h e s e two f a c t o r s (1). Except f o r the glyoximate s t u d i e s , the techniques o f s i n g l e - c r y s t a l r e f l e c t i o n s p e c t r o s c o p y p r o v i d e d the key i n the p r e s e n t work f o r a v o i d i n g t h e s e d i f f i c u l t i e s . F o r t h i s r e a s o n , i n w h a t f o l l o w s , we w i l l f i r s t b r i e f l y r e c a p i t u l a t e t h e manner i n w h i c h t h e r e f l e c t i o n a p p r o a c h i s a p p l i e d i n our l a b o r a t o r y . m a r i z e o u r w o r k on p r e v i o u s l y f o l l o w i n g an o r d e r t h a t r e f l e c t s t h e s e q u e n c e i n w h i c h t h e r e s e a r c h was a c t u a l l y d o n e . 3

Specular

Reflection

Spectroscopy

E v e n i f one h a s a v a i l a b l e a s p e c t r o m e t e r w i t h r e a s o n a b l y l o w l e v e l s o f s t r a y l i g h t , a n d t h u s may study samples o f r e l a t i v e l y h i g h o p t i c a l d e n s i t y , o b t a i n i n g c r y s t a l s p e c t r a f o r t r a n s i t i o n s that i n sol u t i o n s t u d i e s show m o l e c u l a r e x t i n c t i o n c o e f f i c i e n t s o f e v e n a few t h o u s a n d may r e q u i r e r a t h e r e x o t i c methods o f sample p r e p a r a t i o n . F i g u r e 1 o f R e f e r e n c e 1 p r e s e n t s some e x t r e m e e x a m p l e s o f t h e d i s t o r t i o n s t h a t one c a n o b t a i n i f t h e s a m p l e s e m p l o y e d i n a n a b s o r p t i o n s t u d y a r e s u f f i c i e n t l y t h i c k t h a t one e s s e n t i a l l y measures the s t r a y l i g h t i n h i s spectrometer o v e r much o f t h e a b s o r b i n g r e g i o n o f t h e s p e c t r u m . One can have s i t u a t i o n s where d i s t o r t i o n s o f the s p e c t r a due t o o p a q u e n e s s o f t h e s a m p l e a r e much m o r e s u b t l e t h a n t h o s e shown i n R e f e r e n c e 1. C a r e f u l d i r e c t a b s o r p t i o n s t u d i e s on h i g h a b s o r b e r s t h u s usually involve a f a i r l y elaborate set of self-consistent s t u d i e s , s u c h as a s c e r t a i n i n g the o p t i c a l d e n s i t y as a function of c r y s t a l thickness at c r i t i c a l wavelengths. A l t h o u g h t h e sample p r e p a r a t i o n n e c e s s a r y f o r the s t u d y o f r e l a t i v e l y h i g h l y - a b s o r b i n g c r y s t a l s c a n somet i m e s be a c c o m p l i s h e d ( 2 , 3 ) , t h e t e c h n i q u e s t h a t h a v e b e e n d e v e l o p e d a r e n o t g e n e r a l l y a p p l i c a b l e and t y p i cally involve considerable effort. For t h i s reason i t i s fortunate that specular r e f l e c t i o n spectroscopy prov i d e s a r e a s o n a b l e approach t o the study o f such s y s tems (1). S p e c u l a r , o r m i r r o r - l i k e , r e f l e c t i o n f r o m t h e s u r f a c e o f a s o l i d ( t o be d i s t i n g u i s h e d f r o m d i f f u s e r e f l e c t a n c e , where t h e i n c i d e n t r a d i a t i o n a c t u a l l y

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

19.

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279

t r a v e r s e s the i n t e r i o r o f a u s u a l l y - p o w d e r e d sample) c l o s e l y p a r a l l e l s the b e h a v i o r of the i n d e x of refrac­ t i o n (n) i n t h e v i c i n i t y o f an a b s o r p t i o n . Since η t y p i c a l l y r i s e s t o r e l a t i v e l y h i g h v a l u e s a s one ap­ p r o a c h e s an a b s o r p t i o n f r o m t h e l o w - e n e r g y s i d e and t h e n f a l l s t o r e l a t i v e l y low v a l u e s on t h e h i g h - e n e r g y s i d e , s i m i l a r b e h a v i o r b y t h e r e f l e c t i v i t y may be u s e d t o i n f e r the e x i s t e n c e o f a b s o r p t i o n bands. Moreover, s i n c e the s t r e n g t h o f a r e f l e c t i o n band i s , g e n e r a l l y s p e a k i n g , d e p e n d e n t on t h e s t r e n g t h o f t h e absorption, t h e o b s e r v a t i o n o f r e f l e c t i o n b a n d s becomes e a s i e r as d i r e c t a b s o r p t i o n methods become d i f f i c u l t . The rela­ t i o n s h i p b e t w e e n r e f l e c t i o n and a b s o r p t i o n a l s o i m p l i e s t h a t i f one u s e s a s i n g l e c r y s t a l and p o l a r i z e d inci­ dent r a d i a t i o n , th have a p o l a r i z a t i o related absorption. Polarized reflection studies may t h u s be u s e d i n a manner c o m p l e t e l y a n a l o g o u s t o p o l a r ­ i z e d a b s o r p t i o n s t u d i e s to a s c e r t a i n the polarization o f the t r a n s i t i o n s involved. F i g u r e 1 shows t h e r e l a t i o n s h i p that exists be­ tween the a b s o r p t i v e p r o p e r t i e s o f a c r y s t a l , the index o f r e f r a c t i o n , and the r e f l e c t i o n c o e f f i c i e n t i n a representative situation. The extinction coefficient, k, p l o t t e d i n F i g u r e 1 i s r e l a t e d t o t h e u s u a l m o l a r extinction coefficient through €

=

(47rk)/(2.303AC)

(1)

w h e r e λ i s t h e w a v e l e n g t h t o w h i c h e and k r e f e r and C i s the m o l a r c o n c e n t r a t i o n o f the a b s o r b e r i n the crystal. The o p t i c a l c o n s t a n t s ( n and k ) shown i n F i g u r e 1 are i n f a c t not measured q u a n t i t i e s , but have been d e r i v e d by a p p l i c a t i o n o f the K r a m e r s - K r o n i g a n a l y s i s (1) t o the r e f l e c t i o n spectrum. In t h i s procedure, one f i r s t computes t h r o u g h the a p p l i c a t i o n of the integral expression ^

the phase change t h a t i n c i d e n t r a d i a t i o n o f the fre­ q u e n c y o f i n t e r e s t u n d e r g o e s when i t i s r e f l e c t e d from the c r y s t a l s u r f a c e . I n E q u a t i o n 2, θ(αχ.) i s t h e p h a s e c h a n g e f o r l i g h t o f c i r c u l a r f r e q u e n c y CDJ_ (ω = 2irv, where ν i s the f r e q u e n c y of the l i g h t i n q u e s t i o n ) and R(a>) i s the r e f l e c t i o n c o e f f i c i e n t f o r l i g h t o f c i r c u ­ l a r f r e q u e n c y ω. O n c e one knows t h e p h a s e c h a n g e and

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r e f l e c t i o n c o e f f i c i e n t f o r a g i v e n f r e q u e n c y , the c a l ­ c u l a t i o n o f η and k i s a s t r a i g h t - f o r w a r d m a t t e r . For k, t h e q u a n t i t y o f most d i r e c t c o n c e r n h e r e , the r e l e ­ vant expression i s k = (-2R^sin 0)/(l-2R^cos θ + R ) .

(3)

Equations 1 - 3 a l l o w one t o o b t a i n from r e f l e c t i o n data d e t a i l e d absorptive information that i s i d e n t i c a l t o t h a t w h i c h r e s u l t s from d i r e c t a b s o r p t i o n measure­ ments. Moreover, t h i s a n a l y s i s g i v e s a b s o l u t e i n t e n ­ s i t i e s (molar e x t i n c t i o n c o e f f i c i e n t s ) d i r e c t l y , and o b t a i n i n g such d a t a from d i r e c t a b s o r p t i o n s t u d i e s e n t a i l s the o f t e n d i f f i c u l t measurement o f sample t h i c k n e s s on v e r y t h i The o n l y a s p e c t h a t p r e v e n t s i t s a p p l i c a t i o n from b e i n g t o t a l l y r o u t i n e i s the f a c t t h a t the i n t e g r a t i o n o f E q u a t i o n 2 e x t e n d s over the complete f r e q u e n c y range from 0 t o », w h i l e one i s o f c o u r s e l i m i t e d e x p e r i m e n t a l l y t o some f i n i t e s p e c t r a l r e g i o n . The method o f c o r r e c t i n g f o r phase c o n t r i b u t i o n s from o u t s i d e the e x p e r i m e n t a l range f o l l o w e d i n our l a b o r a t o r y i n v o l v e s p l a c i n g an e f f e c ­ t i v e t r a n s i t i o n i n the vacuum u l t r a v i o l e t , the r e g i o n most l i k e l y t o make the most s i g n i f i c a n t c o n t r i b u t i o n i n s t u d i e s such as a r e d e a l t w i t h h e r e . The p r o p e r t i e s o f t h i s t r a n s i t i o n a r e t h e n v a r i e d u n t i l the d e r i v e d spectrum f u l f i l l s some p r e d e t e r m i n e d c r i t e r i o n - most commonly, t h a t € s h o u l d be e q u a l t o z e r o i n r e g i o n s where the c r y s t a l i s known t o be t r a n s p a r e n t . The e f f e c t i v e t r a n s i t i o n thus a r r i v e d a t i s then used t o compute the o p t i c a l c o n s t a n t s t h r o u g h o u t the r e g i o n o f interest. T h i s p r o c e d u r e has the e f f e c t o f i n t r o ­ d u c i n g the p r o p e r phase c o r r e c t i o n i n the r e g i o n where i t can be v e r i f i e d and t h e n u s i n g an a p p r o p r i a t e wave­ l e n g t h dependency i n a p p l y i n g t h i s c o r r e c t i o n t o the r e m a i n d e r o f the spectrum. The a p p l i c a t i o n o f t h i s p r o c e d u r e i s i l l u s t r a t e d i n F i g u r e 2. I t has i n gen­ e r a l provided very s a t i s f a c t o r y r e s u l t s - e s p e c i a l l y f o r bands o f a t l e a s t moderate i n t e n s i t y , w h i c h make a major c o n t r i b u t i o n t o the phase i n the v i c i n i t y o f the f r e q u e n c i e s a t w h i c h they o c c u r and thus r e q u i r e r e l a ­ t i v e l y s m a l l phase c o r r e c t i o n s , and f o r bands t h a t f a l l w e l l w i t h i n the e x p e r i m e n t a l r e g i o n . Unless noted otherwise, c r y s t a l a b s o r p t i o n s p e c t r a d i s c u s s e d i n the f o l l o w i n g s e c t i o n s o f t h i s paper have been o b t a i n e d v i a the a p p l i c a t i o n o f K r a m e r s - K r o n i g a n a l y s i s t o the a p p r o p r i a t e r e f l e c t i o n s p e c t r a .

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

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WAVELENGTH m μ 600

500

400

300

2 50 60 50 40

ο: * 30 20 10

16

20

24 28 FREQUENCY kK

Molecular Crystals Figure 1. Reflection coefficient (R), index of refraction (n), and extinction coefficient (k) for a moderately intense absorber (1)

600

500

WAVELENGTH m μ 400 300

250

1.10 .90

70 m 2 Χ w

.50

.30

.10

-·

1 0

15

20

25 30 FREQUENCY

35 kK

40 45 Molecular Crystals

Figure 2. Kramers-Kronig absorption spectrum in various stages of refine­ ment: ( ) curve obtained by using only experimental data and straightHne high and low energy approximations from Réf. 1; ( ) partially refined curve; ( ) final curve obtained by varying effective ultraviolet reflectivity to obtain zero extinction coefficients in long wavelength region (1).

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

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The

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IONS

Glyoximates

N i c k e l d i m e t h y l g l y o x i m e (NiDMG) i t s e l f p r o v i d e s a c l a s s i c example o f the k i n d o f phenomena t h a t have a t t r a c t e d a t t e n t i o n t o the compounds b e i n g d i s c u s s e d here. Those f a m i l i a r w i t h the d e t e r m i n a t i o n o f n i c k e l as the d i m e t h y l g l y o x i m a t e w i l l r e c a l l the r e d f l o c c u l e n t p r e c i p i t a t e that i s obtained i n t h i s procedure. T h i s m a t e r i a l w i l l d i s s o l v e i n c h l o r o f o r m , however, and does so t o g i v e a p a l e y e l l o w s o l u t i o n . The d i f ­ f e r e n c e between the s o l u t i o n spectrum and t h a t o f the s o l i d becomes even more s t r i k i n g when one examines a s i n g l e c r y s t a l o f NiDMG, w h i c h has. a dark body c o l o r and d i s p l a y s a g r e e n m e t a l l i c l u s t r o u s sheen i n r e ­ flection. T h i s sheen p o l a r i z e r , i s foun n e e d l e , or s t a c k i n g , a x i s o f the c r y s t a l . The d i f f e r e n c e s between the s o l u t i o n and s o l i d s p e c t r a o f NiDMG a r e shown i n a q u a n t i t a t i v e manner by F i g u r e 3. Here the s o l u t i o n s p e c t r u m has been o b t a i n ­ ed i n a s u i t a b l e g l a s s - f o r m i n g s o l v e n t . On c o o l i n g t o l i q u i d n i t r o g e n t e m p e r a t u r e s , a g l a s s forms and s i m u l ­ t a n e o u s l y a f i n e l y d i s p e r s e d p r e c i p i t a t e o f NiDMG a p p e a r s , a l l o w i n g one t o compare r a t h e r p r e c i s e l y the s o l u t i o n and s o l i d - s t a t e s p e c t r a ( 3 ) . The most o b v i ­ ous, but not s i n g u l a r , new f e a t u r e i n the s o l i d - s t a t e s p e c t r u m i s the s t r o n g new band a t 17.8 kK. It is c l e a r l y t h i s g r e e n band, combined w i t h o t h e r bands i n the b l u e , t h a t g i v e s the r e d body c o l o r t o NiDMG, and i t i s t h i s band whose c h a r a c t e r i z a t i o n we seek. The p o l a r i z a t i o n o f the g r e e n band o f NiDMG i s d i s p l a y e d i n F i g u r e 4, w h i c h r e p o r t s the p o l a r i z e d a b s o r p t i o n s p e c t r a o b t a i n e d f o r a t h i n f i l m o f NiDMG p r e p a r e d by vacuum s u b l i m a t i o n o n t o a q u a r t z p l a t e . I t can be seen t h a t t h i s band i s p o l a r i z e d e x c l u s i v e l y p e r p e n d i c u l a r t o the m o l e c u l a r p l a n e s , or p a r a l l e l t o the s t a c k i n g a x i s , w h i c h i s c o n s i s t e n t w i t h the q u a l i ­ t a t i v e o b s e r v a t i o n s on the r e f l e c t i v i t y o f the c r y s t a l n o t e d i n the remarks i n t r o d u c i n g t h i s s e c t i o n and the q u a n t i t a t i v e r e f l e c t i o n s p e c t r a r e p o r t e d by Anex and K r i s t (3). I n an e f f o r t t o e s t a b l i s h the s i n g l e - m o l e c u l e o r i g i n o f the g r e e n band o f NiDMG a s e r i e s o f n i c k e l g l y o x i m a t e s has been s t u d i e d , c o n s i s t i n g o f NiDMG i t s e l f , two forms o f n i c k e l e t h y l m e t h y l g l y o x i m e ( a NiEMG and β-NiEMG), and n i c k e l heptoxime ( N i H e p t ) . β-NiEMG and NiHept p o s s e s s t h e s t a c k i n g s t r u c t u r e c h a r a c t e r i s t i c o f NiDMG, w h i l e α-NiEMG, w h i c h t u r n s out t o be a c r u c i a l compound i n t h i s s t u d y , p o s s e s s e s a n o n - s t a c k e d s t r u c t u r e . The c r y s t a l s p e c t r a o f

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

19.

ANEx

Complexes

with

Extended

Chains

700

500

ι ι ι 11—ι—ι

283 WAVELENGTH my 400 300 1

1

250

1—;

1—

FREQUENCY kK Journal of the American Chemical Society

Figure

3.

Glass ( ) and solution ( of nickel dimethylglyoxime

) absorption (3)

FREQUENCY"kK Journal of the American Chemical Society

Figure nickel vector planes,

4. Polarized absorption spectra of an oriented film of dimethylglyoxime. The || curve obtained when electric of the incident light vibrates parallel to the molecular and ± obtained when it vibrates perpendicular to the molecular planes (3).

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

spectra

284

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

α-NiEMG a p p r o x i m a t e r a t h e r c l o s e l y those you would e x p e c t f r o m i t s s o l u t i o n s p e c t r u m and an o r i e n t e d gas m o d e l f o r the c r y s t a l (3). The i n t e r m e t a l l i c s p a c i n g s i n NiDMG, β-NiEMG, and N i H e p t a r e 3.25, 3.4, and 3.60 A, r e s p e c t i v e l y , and t h u s t h i s s t u d y c a n be v i e w e d a s an e x p e r i m e n t i n w h i c h t h e d i s t a n c e s b e t w e e n t h e complexes i n the s t a c k s are s u c c e s s i v e l y increased un­ t i l i n α - N i E M G we r e a c h t h e p o i n t w h e r e we o b t a i n a s p e c t r u m t h a t i s e s s e n t i a l l y c h a r a c t e r i s t i c o f the free complex. F i g u r e 5 shows t h e o u t - o f - p l a n e s p e c t r u m o f e a c h o f t h e c r y s t a l s i n t h i s s e r i e s and d e m o n s t r a t e s r a t h e r c o n v i n c i n g l y t h a t as t h e c o m p l e x e s w i t h i n t h e s t a c k s are s e p a r a t e d the c h a r a c t e r i s t i c s o l i d - s t a t e band o f NiDMG r e t r e a t s i n t 23.0 kK i n α - N i E M G s o l u t i o n a b s o r p t i o n s observed f o r these complexes ( a l l t h r e e c o m p l e x e s h a v e e s s e n t i a l l y t h e same s o l u t i o n absorption spectrum). I f one a c c e p t s G r a y and Ballh a u s e n s a s s i g n m e n t (4) o f the f i r s t a l l o w e d solution bands of the c y a n i d e s to m e t a l - t o - l i g a n d c h a r g e - t r a n s ­ f e r t r a n s i t i o n s , one may c o n c l u d e by a n a l o g y t h a t the c r u c i a l o u t - o f - p l a n e b a n d o f α - N i E M G may be r e p r e s e n t e d o r b i t a l l y as f

3d 2 z

N i

>

c

r* +

l 7

c 4p 2

z

,

N i

(4)

w h e r e Wj r e f e r s t o t h e a p p r o p r i a t e l i n e a r c o m b i n a t i o n o f a n t i - b o n d i n g l i g a n d ττ-orbitals and c ^ and c are mixing coefficients. I n t h e G r a y and B a l l h a u s e n p i c ­ t u r e , t h e n , one w o u l d h a v e i n s o l u t i o n a s i t u a t i o n w h e r e c\ » c g and be d e a l i n g , a s n o t e d a b o v e , w i t h an essentially metal-to-ligand transition. A n e x and K r i s t (3) h a v e a r g u e d t h a t one c a n t h e n r a t i o n a l i z e g r o w t h i n i n t e n s i t y , r e d s h i f t , and s h a r p e n i n g o f the c r y s t a l t r a n s i t i o n r e l a t i v e to i t s s o l u t i o n counter­ p a r t i n t e r m s o f t h e s o l i d - s t a t e p e r t u r b a t i o n , whose n a t u r e was n o t s p e c i f i e d , l o w e r i n g t h e 4 p orbital in e n e r g y t o t h e p o i n t w h e r e c-i « C 2 « One w o u l d t h e n be dealing with e s s e n t i a l l y a d 2 > p t r a n s i t i o n con­ f i n e d t o the m e t a l . E x p l a n a t i o n s f o r the development o f the g r e e n b a n d o f NiDMG t h a t i n v o l v e s p e c i f i c h y p o t h e s e s c o n ­ c e r n i n g the n a t u r e o f the s o l i d - s t a t e p e r t u r b a t i o n s have been proposed (5,6). These can most f r u i t f u l l y be d i s c u s s e d a f t e r t h e r e l a t e d s y s t e m s t o be dealt w i t h here have been reviewed. 2

z

z

z

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

19.

The

ANEx

Complexes

1

Magnus

with

Extended

Cliains

285

Salts

The q u a l i t a t i v e o b s e r v a t i o n o f t h e c o l o r c h a n g e s i n v o l v e d i n t h e f o r m a t i o n o f Magnus green s a l t , [ P t ( N H 5 ) 4 ] [ P t C l ^ ] , make i t a p p e a r t h a t t h e p h e n o m e n a observed there p a r a l l e l rather c l o s e l y those d e s c r i b e d p r e v i o u s l y f o r NiDMG. One t h u s f i n d s t h a t i f a n aqueous K o P t C l 4 s o l u t i o n , which i s r e d , i s combined w i t h a c o l o r l e s s aqueous Pt ( N H ^ ^ C l o s o l u t i o n , a g r e e n p r e c i p i t a t e o f M a g n u s g r e e n s a l t (MGS) i s o b t a i n e d . A c r y s t a l c o l o r d i f f e r e n t than t h a t o f e i t h e r o f t h e components m i g h t t h u s l e a d one t o assume t h a t once a g a i n o n e h a s some "new a b s o r p t i o n band a p p e a r i n g i n the v i s i b l e p a r t o f t h e spectrum. Day and coworkers (7) a n d , more r e c e n t l y sured thev i s i b l e absorptio c r y s t a l s a n d shown t h e m t o b e p e r t u r b e d P t C l 2 | 2 spec t r a , and n o t t o p o s s e s s any bands t h a t c o u l d n o t be t r a c e d t o those found i n t h e a n i o n i c s p e c i e s i n t h e KgPtClij. c r y s t a l and i t s aqueous s o l u t i o n . The g r e e n c o l o r o f t h e MGS c r y s t a l r e s u l t s f r o m a r e d s h i f t a n d i n t e n s i f i c a t i o n o f t h e P t C l i j " b a n d s i n MGS l e a d i n g t o a "window" i n t h e g r e e n r e g i o n o f t h e s p e c t r u m , r a t h e r than from t h e development o f an a d d i t i o n a l band i n t h e v i s i b l e p a r t o f t h e spectrum. (KgPtC^ also has a c r y s t a l s t r u c t u r e i n which t h e complex s p e c i e s are stacked, b u t here the metal-metal s e p a r a t i o n i s 4.13 a n d o n e may e x p e c t t h a t s t r o n g i n t e r c o m p l e x i n t e r a c t i o n s w i l l n o t be m a n i f e s t e d - an assumption t h a t i sborn o u t by t h e reasonably c l o s e correspondence o f t h e s o l u t i o n and c r y s t a l a b s o r p t i o n f o r t h i s system.) A c o n n e c t i o n between t h e o p t i c a l phenomena o b s e r v e d i n MGS-type s a l t s and t h o s e d i s c u s s e d p r e v i o u s l y f o r n i c k e l g l y o x i m a t e s was t o b e c o m e a p p a r e n t , h o w e v e r . Day jet a l . c a r r i e d o u t a s e r i e s o f p a r t i c u l a r l y s i g n i f i e a n T ~ e x p e r ime n t s, a n a l o g o u s t o t h o s e d e s c r i b e d i n the p r e c e d i n g s e c t i o n on t h e g l y o x i m a t e s , i n which t h e s p a c i n g b e t w e e n c o m p l e x e s i n t h e MGS c r y s t a l was i n creased by modifying t h e nature o f the ligands i n a manner e x p e c t e d t o a f f e c t t h e " s i n g l e - m o l e c u l e " s p e c t r a i n a minimal fashion (7 £). MGS-type s a l t s were t h u s p r e p a r e d i n w h i c h t h e NH-j l i g a n d s o f P t ( N H - j ) w e r e r e p l a c e d b y C H 3 - N H 2 (Me-MGS) a n d CH-z-Ct^-NiL (Et-MGS), r e s p e c t i v e l y . T h e MGS a n d Me-MGS c r y s t a l s h a v e t h e same m e t a l - m e t a l s p a c i n g ( 3 . 2 5 Â 1 , w h i l e i n Et-MGS t h i s p a r a m e t e r h a s a v a l u e o f 3 . 4 A ( 5 ) . T h e v i s i b l e s p e c t r a o f MGS a n d Me-MGS a r e v e r y s i m i l a r , a n d a s o n e m i g h t e x p e c t , t h e Et-MGS s p e c t r a a r e i n t e r m e d i a t e b e t w e e n t h o s e o f MGS a n d K ^ P t C l ^ ( 7 , 9 ) . Of 1

1

11

2

5

2 +

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

286

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

e q u a l i m p o r t a n c e , D a y et; a l . a l s o c a r r i e d o u t d i f f u s e r e f l e c t i o n s t u d i e s o n t h e s e same s y s t e m s , a n d f o u n d a s t r o n g c o r r e l a t i o n between m e t a l - m e t a l d i s t a n c e and the l o c a t i o n o f a s t r o n g u l t r a v i o l e t t r a n s i t i o n :

Compound

Metal-Metal Distance

P o s i t i o n o f uv T r a n s i t i o n (Diffuse Reflectance)

MGS

3.25 S

3^.5 kK

Me-MGS

3.25

34.5

Et-MGS

3.4

40

K PtCl^

4.1

2

* Actual

absorption

t o blue

of this

(11).

I t was t h u s s u g g e s t e d t h a t t h e e f f e c t s o b s e r v e d i n t h e v i s i b l e spectrum o f t h e Magnus s a l t s c o u l d be u n d e r stood i n terms o f t h e v i s i b l e bands borrowing intens i t y from t h e s t r o n g m e t a l - m e t a l s e p a r a t i o n dependent u l t r a v i o l e t band. I n such a s i t u a t i o n , the c l o s e r the uv b a n d was t o v i s i b l e t r a n s i t i o n s , t h e g r e a t e r i t s i n f l u e n c e on them c o u l d be e x p e c t e d t o be. Further, t h e f a c t t h a t t h e i n t e n s i t y p e r t u r b a t i o n s were s t r o n g er f o r t h e o u t - o f - p l a n e bands than t h e i n - p l a n e , l e d t o t h e c o n c l u s i o n t h a t t h e p e r t u r b i n g band must have out-of-plane polarization. T h e s e o b s e r v a t i o n s and t h e c o n c l u s i o n s drawn f r o m them l e d d i r e c t l y t o t h e s t u d y o f t h e a l l o w e d bands i n t h e same s e r i e s o f t h r e e M a g n u s s a l t s i n our laboratories (10). The r e s u l t s were s t r i k i n g l y i n a c c o r d w i t h t h e s u g g e s t i o n s made b y D a y et> a l . , a s may b e s e e n i n F i g u r e 6, w h i c h p r e s e n t s t h e c r u c i a l p o r t i o n s of the out-of-plane absorption spectra obtained by Kramers-Kronig a n a l y s i s o f the corresponding reflection spectra. The s t r o n g u l t r a v i o l e t a b s o r p t i o n band i s f o u n d t o o c c u r a t 3 4 . 5 , 3 4 . 4 , a n d 3 9 - 9 kK r e s p e c t i v e l y f o r MGS, Me-MGS a n d Et-MGS, i n e x c e l l e n t a g r e e ment w i t h t h e r e s u l t s o f d i f f u s e r e f l e c t i o n . I t was a l s o p o s s i b l e t o develop a q u a n t i t a t i v e c o r r e l a t i o n between t h e p o s i t i o n and i n t e n s i t y o f t h e s t r o n g u l t r a v i o l e t band i n these systems and t h e c o r r e s p o n d i n g outof -plane v i s i b l e i n t e n s i t y . S t r o n g e v i d e n c e was t h u s p r o v i d e d f o r t h e p r o p o s a l t h a t i n t e r a c t i o n between t h e e x c i t e d s t a t e s r e s p o n s i b l e f o r t h e v i s i b l e bands and that o f the u l t r a v i o l e t t r a n s i t i o n plays a substantial role i n the perturbation of thev i s i b l e spectra. 1

1

1

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

19.

Complexes

ANEx

400

with

Extended

WAVELENGTH πψ 500

Chains

600

16

12

25

2 FREQUENC Journal of the American Chemical Society

Figure 5. Portions of out-of-plane absorption spectra for four nickel dimethylglyoximates ob­ tained by direct absorption studies (3)

WAVELENGTH 300

30 Journal of Chemical Physics

Figure 6. Kramers-Kronig M G S (A), Me-MGS (B)

y

250

35 40 F R E Q U E N C Y kK

absorption curves for and Et-MGS (C) (10)

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

288

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

The b a n d s shown i n F i g u r e 6 w e r e t h o u g h t b y b o t h Day a n d c o w o r k e r s a n d o u r g r o u p t o be c o r r e l a t e d w i t h t h e s o l u t i o n b a n d o f K ^ P t C l i j . t h a t p e a k s a t 46.3 kK w i t h a maximum m o l a r e x t i n c t i o n c o e f f i c i e n t o f 1 0 2 3 0 crrf-Hd"" (11). T h i s t r a n s i t i o n would c l e a r l y h a v e u n d e r g o n e a n i n t e n s i f i c a t i o n i n t h e MGS crystal, but i n e s t i m a t i n g the magnitude o f t h i s e f f e c t one must f i r s t o b s e r v e t h a t t h e e - v a l u e s r e p o r t e d i n F i g ­ u r e 6 must be d i v i d e d by 3 f o r c o m p a r i s o n t o s o l u t i o n v a l u e s and, s e c o n d l y , n o t e t h a t a t r u e comparison of i n t e n s i t i e s r e q u i r e s i n t e g r a t i o n s o v e r t h e b a n d s . Re­ c e n t s t u d i e s (12) i n d i c a t e t h a t t h i s band i n t e n s i f i e s b y a f a c t o r o f a p p r o x i m a t e l y 2.5 i n g o i n g f r o m ^ P t C l ^ t o MGS. Thus, a l t h o u g h the b e h a v i o r o f the u l t r a v i o ­ l e t band i n the Magnus the v i s i b l e band o i n one c r u c i a l f a c t o r : i t d o e s n o t show t h e d r a m a t i c i n t e n s i f i c a t i o n o b s e r v e d f o r t h e g r e e n b a n d o f NiDMG i n going from e s s e n t i a l l y i n f i n i t e metal-metal sepa­ r a t i o n t o one o f 3.25 This intensity behavior i s i n a c c o r d w i t h t h e p i c t u r e d e v e l o p e d f o r NiDMG, i f one r e c a l l s t h a t i n Magnus s a l t s one i s d e a l i n g w i t h l i g a n d s t h a t do n o t p o s s e s s u n s a t u r a t i o n . In Equation 4, t h e r e f o r e , c-^ w o u l d become z e r o f o r MGS and r e l a t e d c o m p o u n d s a n d t h e t r a n s i t i o n w o u l d be e s s e n t i a l l y d —> ρ at a l l stages of s o l i d - s t a t e perturbation. H e n c e one w o u l d n o t e x p e c t i n t h i s c a s e t o s e e a d r a ­ m a t i c d r o p i n i n t e n s i t y a s one moves t o t h e " s i n g l e molecule spectrum. T h i s i n t e r p r e t a t i o n o f the Magnus salt spectra o f c o u r s e c a r r i e s s t r o n g i m p l i c a t i o n s f o r t h e 46.3 kK s o l u t i o n band o f KpPtClij.: i t should, i f t h i s p i c t u r e i s c o r r e c t , a t l e a s t h a v e a c o m p o n e n t t h a t c a n be a s s o c i a t e d w i t h a p l a t i n u m i o n 5 d 2 —> 6p promotion. S i n c e a t t h e t i m e t h i s p r o p o s a l was made t h e 46.3 kK a b s o r p t i o n was g e n e r a l l y t h o u g h t t o be l i g a n d - t o - m e t a l c h a r g e - t r a n s f e r i n c h a r a c t e r (4,11), a s t u d y aimed a t c h a r a c t e r i z i n g the h i g h - e n e r g y a l l o w e d bands i n K ^ P t C l ^ a n d r e l a t e d compounds was undertaken. 1

3

1

1 1

1

z

Tetrachloroplatinate(II)

Ion

and

Related

Systems

F i g u r e J shows t h e s o l u t i o n s p e c t r a o f K p P t C l 4 and K P d C l 4 . I t w i l l be n o t e d t h a t t h e 46.3 kK b a n d o f KpPtCl2| h a s r o u g h l y t h e same e and, e x c e p t f o r i t s s t r u c t u r e , i s v e r y s i m i l a r i n a p p e a r a n c e t o t h e 35-7 kK KpPdCl/j. b a n d . I t was t h u s r a t h e r g e n e r a l l y a c c e p t e d (11) t h a t t h e s e two b a n d s w e r e c o r r e l a t e d w i t h one a n o t h e r , t h a t t h e 46.3 kK P t C l ^ " b a n d was strongly 2

m

a

x

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

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Complexes

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289

r e d s h i f t e d when Pd was s u b s t i t u t e d f o r P t , a n d t h a t t h e s e c o n d P d C l ^ " " band had s i m i l a r l y r e d - s h i f t e d from i t s vacuum-ultraviolet location i n PtCl^ "". Moreover, the whole a r r a y o f s t r o n g q u a r t z u l t r a v i o l e t bands i n t h e s e i o n s was t a k e n a s b e i n g c h a r g e - t r a n s f e r i n nature. I n o r d e r t o examine the v a l i d i t y o f t h i s p i c t u r e and, more p a r t i c u l a r l y , t o e x p l o r e t h e p o s s i b i l i t y o f a d —> ρ t r a n s i t i o n b e i n g i n v o l v e d i n t h e 46.5 kK P t C l i j " band, t h e r e f l e c t i o n s p e c t r a o f ^ P d C l i i and ^ P t C l i j . w e r e o b t a i n e d a n d a r e shown i n F i g u r e s 8 a n d 9. I t i s c l e a r t h a t K2PtCl4 c o n t a i n s a s t r o n g h i g h e n e r g y o u t - o f - p l a n e t r a n s i t i o n , w h i l e KoPdCl4 d i s p l a y s no a n a l o g o u s a b s o r p t i o n . I f one m a k e s t h e r e a s o n a b l e assumption t h a t comparabl transition i P t C l i j . ' and P d C l ^ p l a n e P t C l 4 " band jus opposit l y t h o u g h t t o be t h e c a s e - i t s h i f t s t o h i g h e r , n o t p d c l l o w e r , f r e q u e n c i e s on m o v i n g f r o m P t C ^ " 4 · E x a m i n a t i o n o f t h e d —> ρ t r a n s i t i o n s o f P t and Pt and t h e c o m p a r a b l e p a l l a d i u m s p e c i e s r e v e a l s a b l u e s h i f t s i m i l a r t o t h a t i n f e r r e d f o r the t e t r a c h l o r o i o n s a s one m o v e s f r o m p l a t i n u m t o p a l l a d i u m (11), a c o r r e l a t i o n that lends a d d i t i o n a l p l a u s a b i l i t y t o t h e d —> ρ assignment i n PtCl4 ~. One may a l s o n o t e t h a t a t l e a s t two p r e d i c t i o n s f l o w f r o m t h e f o r e ­ going discussion: 1) S i n c e i n ammonia t h e e l e c t r o n s a n a l o g o u s t o those thought to p a r t i c i p a t e i n the lowestenergy l i g a n d - t o - m e t a l c h a r g e - t r a n s f e r band i n P t C l i . 2 " a r e t i e d up i n b o n d i n g , one w o u l d expect t h i s band t o o c c u r a t c o n s i d e r a b l y _ higher energies i n Pt(NH^)4 than i n PtCl4 ", w h i l e t h e 5 d 2 —> 6p~ t r a n s i t i o n , b e i n g con­ f i n e d t o t h e m e t a l , s h o u l d be f o u n d i n r o u g h l y t h e same s p e c t r a l r e g i o n f o r b o t h i o n s . 2) S i n c e t h e s t r o n g o u t - o f - p l a n e t r a n s i t i o n i n t h e s o l u t i o n s p e c t r u m o f P t C l 4 " u n d e r g o e s an a p p r o x i m a t e l y 12 kK s h i f t i n MGS, i t m i g h t be e x p e c t e d t h a t t h e d —> ρ t r a n s i t i o n hypothe­ s i z e d t o o c c u r i n the vacuum u l t r a v i o l e t i n t h e f r e e P d C l 4 ~ i o n c o u l d be s h i f t e d i n t o t h e quartz u l t r a v i o l e t i n the s p e c t r a o f the p a l l a d i u m a n a l o g o f MGS ([PdiNIU)^][PdCl^]). One i n f a c t o b s e r v e s a s t r o n g t r a n s i t i o n i n a q u e o u s s o l u t i o n s o f P ^ N H ^ ^ C l g (11) and r e c e n t c r y s t a l s t u d i e s (15) h a v e shown t h i s t r a n s i t i o n t o have an o u t - o f - p l a n e p o l a r i z a t i o n . L i k e w i s e , the p a l l a d i u m Magnus s a l t shows a m o d e r a t e l y strong outof-plane absorption peaking i n r e f l e c t i o n at 2

2

2

2

2

2

t

o

+

2

?

2 +

z

2

2

f

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

290

EXTENDED INTERACTIONS BETWEEN METAL IONS WAVELENGTH

500 490 30t

2

t

mjj

250

30

20 Figure K PtCl,

300

200

40

F R E Q U E N C

7. (

Solution absorption ) and K PdCh ( 2

t

spectra (in 2M HCl) of ) (data from Réf. II)

Journal of the American Chemical Society

Figure 8. Polarized single-crystal reflection spectra of K PdCl, . The II curve obtained when electric vector of the incident light vibrates parallel to the molecular planes; 1 curve obtained when it vibrates perpendicular to the molecular planes (11). 2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

(

19.

ANEX

Complexes

with

Extended

Chains

291

45.8 kK (14). The f i n d i n g s f o r P t ( N H 2 ) 4 C l are p a r t i c u l a r l y s i g n i f i c a n t r e l a t i v e t o t h e 5a 2 —> 6p assignment, since for Pt(NHj)4 i t i s d i f f i c u l t to p r o p o s e an a l t e r n a t i v e a s s i g n m e n t f o r an o u t - o f - p l a n e b a n d o c c u r r i n g i n t h e 45-50 kK r e g i o n . 2

2 +

Characterization

o f the E x c i t e d S t a t e

H a v i n g r e a c h e d t h e p o i n t w h e r e one h a s developed a r e a s o n a b l y good p i c t u r e o f the s i n g l e - m o l e c u l e o r i g i n o f the c r u c i a l s o l i d - s t a t e a b s o r p t i o n bands i n t h e s e s y s t e m s , t h e n e x t l o g i c a l s t e p i s t o s e e k knowledge r e g a r d i n g the nature of the e x c i t e d s t a t e s a s s o c i a t e d w i t h these bands. I n p a r t i c u l a r , one w o u l d like to distinguis a c t e r i z e the t r a n s i t i o m o l e c u l a r one t h a t i s h i g h l y p e r t u r b e d b y i t s e n v i r o n ment a n d t h o s e t h a t w o u l d i n v o k e v a r y i n g d e g r e e s o f d e r e a l i z a t i o n i n at l e a s t the e x c i t e d s t a t e . As has been p o i n t e d out p r e v i o u s l y (3), mixed c r y s t a l e x p e r i m e n t s o f t h e g e n e r a l t y p e done by Banks a n d B a r n u m (15) and B a s u , Cook, and B e l f o r d (16) c o u l d h a v e a n i m p o r t a n t b e a r i n g on t h i s p r o b l e m . Unf o r t u n a t e l y , t h i s e a r l i e r w o r k on t h e g l y o x i m a t e s , w h i c h was c a r r i e d o u t on c o l l o i d a l s u s p e n s i o n s and NaCl p e l l e t s , appears t o have y i e l d e d s p e c t r a t h a t were n o t p a r t i c u l a r l y w e l l r e s o l v e d . I n any c a s e , these studies r e s u l t e d i n c o n t r a d i c t o r y f i n d i n g s , i n t h a t B a n k s and Barnum r e p o r t e d a s i n g l e i n t e r m e d i a t e band l y i n g between t h o s e o f the pure components i n v a r i o u s b i n a r y m i x t u r e s o f t h e N i ( I I ) , P t ( I I ) , and Pd(II) dimethyIglyoximates, while Basu et a l . reported a s e r i e s of i n t e r m e d i a t e bands i n such s i t u a t i o n s . In l i g h t o f the above o b s e r v a t i o n s , a s e r i e s o f m i x e d - c r y s t a l s t u d i e s i n v o l v i n g s i n g l e - c r y s t a l meas u r e m e n t s was u n d e r t a k e n i n o u r l a b o r a t o r i e s . The r e s u l t s o f one o f t h e s e i n v e s t i g a t i o n s was recently r e p o r t e d (14). I n t h i s work "mixed" Magnus salts of t h e f o r m [ P t ( N H j ) ^ ] [ P d C ^ ] and [ P d ( N H ) 4 ] [ P t C ^ ] were p r e p a r e d and s t u d i e d v i a t h e t e c h n i q u e s o f s p e c u l a r r e f l e c t i o n spectroscopy. The o u t - o f - p l a n e spectrum f o r [Pd(NH-5)4][PtCl4l i s shown i n F i g u r e 10, w h e r e t h e o u t - o f - p l a n e s p e c t r a o f MGS a n d i t s p l a t i n u m a n a l o g a r e a l s o shown f o r c o m p a r i s o n . I t w i l l be n o t e d t h a t t h e m i x e d - c r y s t a l s p e c t r u m p o s s e s s e s a s i n g l e band w i t h a w i d t h c o m p a r a b l e t o t h o s e o f t h e two "pure" s a l t s l o c a t e d a l m o s t midway b e t w e e n them. It also shows an i n t e n s i t y i n t e r m e d i a t e b e t w e e n t h o s e o f t h e pure s a l t s . ( A l l t h r e e c r y s t a l s have e s s e n t i a l l y the same u n i t - c e l l p a r a m e t e r s (17). Thus, the m e t a l - m e t a l T

3

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

292

EXTENDED

600

500

20

INTERACTIONS

BETWEEN

METAL

IONS

WAVELENGTH 300

30 F R E Q U E N C Y kK Journal of the American Chemical Society

Figure

9. Pohrized single-crystal reflection and 1 are of same significance k

WAVELENGTH 400 30Ç

30 FREQUENCY

20

spectra of K^tCl,,. as in Figure 8.

mn 25Ç

40

kK

Chemical Physics Letters

Figure 10. Reflection spectra when electric vector of the incident radiation vibrates parallel to the out-of-plane direction of single crystals of [Pt(NH )J [PtClJ ( ), [Pd(NH )A [PtClJ ( ), and [Pd(NH<),] [PdClJ ( · · · ) (14) 3

it

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

The

19.

ANEX

Complexes

with

Extended

293

Chains

s p a c i n g and m o l a r c o n c e n t r a t i o n s a r e a p p r o x i m a t e l y t h e same f o r e a c h s a l t . ) Perhaps t h e most s u r p r i s i n g f a c t d e v e l o p e d i n t h i s i n v e s t i g a t i o n , however, i s t h a t t h e o u t - o f - p l a n e spectrum o f the second mixed Magnus salt s t u d i e d , [Pt(NH-z)i. ] [ P d C l ^ ] , c o r r e s p o n d s c l o s e l y enough t o t h a t shown i n F i g u r e 10 f o r [ P d ( N H - x ) 2 i ] [ P t C l j i ] t h a t i t c a n b e r e p r e s e n t e d b y t h e same c u r v e . These f i n d i n g s c e r t a i n l y p r o v i d e t e l j i n g evidence for a strongly delocalized excited state. It i sin f a c t as i f a l l t h a t matters f o r t h i s t r a n s i t i o n i s t o h a v e t h e m e t a l i o n s l i n e d up a t t h e a p p r o p r i a t e s p a c ­ i n g , w i t h the l i g a n d s , beyond t h i s r o l e , simply a c t i n g as s p e c t a t o r s . As w i l l be n o t e d l a t e r , however, t h e s i t u a t i o n h a s t o b e somewhat m o r e c o m p l e x t h a n t h i s . T h e 3 4 . 5 kK MG o n l y one a p p a r e n t i q u a r t z u l t r a v i o l e t r e g i o n and, i n p a r t i c u l a r , no o u t of-plane t r a n s i t i o n s corresponding to the d 2 — > p bands o f K ^ P t C l i i and P ^ N H ^ H C l o a r e observed i n t h i s r e g i o n f o r MGS. * I n view o r t h e h i g h degree o f del o c a l i z a t i o n now known t o e x i s t i n t h e s e s y s t e m s , t h e most r e a s o n a b l e i n t e r p r e t a t i o n o f t h i s f i n d i n g would a p p e a r t o b e t h a t t h e 3 4 . 5 kK MGS-band h a s i t s p a r e n t ­ age i n b o t h t h e c a t i o n i c and a n i o n i c d — > ρ t r a n ­ s i t i o n s , b o t h h a v i n g b e e n r e d - s h i f t e d s h a r p l y i n MGS. For t h i s reason, c u r r e n t p r a c t i c e i n our l a b o r a t o r y i s t o use the t o t a l c o n c e n t r a t i o n o f complex s p e c i e s p r e s e n t , b o t h a n i o n i c a n d c a t i o n i c , i n c o m p u t i n g ev a l u e s f o r MGS-type s y s t e m s . R e c o g n i t i o n i s t h u s made of the f a c t that both metal-containing species are t h o u g h t t o be c o n t r i b u t i n g t o t h e a b s o r p t i v e p r o c e s s i n t h e Magnus salts. The e - v a l u e s and o t h e r i n t e n ­ s i t y measures - such as i n t e g r a t e d i n t e n s i t i e s o b t a i n e d f o r the Magnus s a l t s a r e t h e n d i r e c t l y com­ p a r a b l e t o t h o s e c o m p u t e d i n t h e u s u a l way f o r t h e component i o n s as " i s o l a t e d species. I t w i l l be s e e n t h a t t h i s manner o f c o m p u t i n g c o n c e n t r a t i o n s r e s u l t s i n concentration values that are j u s t twice those b a s e d o n t h e number o f f o r m u l a u n i t s o f t h e M a g n u s s a l t i n q u e s t i o n p e r l i t e r , w h i c h was t h e b a s i s o f t h e c o n c e n t r a t i o n s used and l i s t e d i n R e f e r e n c e 10. 1

z

z

1

f

1 1

f

*The emphasis h e r e i s finds l i t t l e evidence t r a n s i t i o n s o f MGS o r other than the c r u c i a l t h a t the ground s t a t e **Analogous

comments

on t h e e x c i t e d s t a t e , s i n c e o n e i n , f o r instance, the d — > d t r a n s i t i o n s o f t h e NiDMG c r y s t a l g r e e n b a n d t o make o n e f e e l i s extensively delocalized.

apply

to other

Magnus-type

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

salts.

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Comment s h o u l d a l s o be made a t t h i s p o i n t on t h e d i f f e r e n c e s i n t h e l e v e l s o f r e f l e c t i v i t y shown b y t h e MGS c u r v e s o f F i g u r e 2 o f R e f e r e n c e 10 a n d F i g ­ u r e 10 o f t h e c u r r e n t p u b l i c a t i o n . The i m p r o v e m e n t shown i n t h e l a t e r m e a s u r e m e n t s p r i m a r i l y r e f l e c t s i m p r o v e d sample s i z e and q u a l i t y r e s u l t i n g f r o m r e c e n t l y - d e v e l o p e d t e c h n i q u e s o f sample p r e p a r a t i o n . The i n c r e a s e d r e f l e c t i v i t i e s f o r MGS i n t h e m s e l v e s would l e a d t o i n c r e a s e d e-values f o r the KramersK r o n i g a b s o r p t i o n s p e c t r a t h a t r e s u l t f r o m them. This e f f e c t i s , however, o p p o s i t e t o t h a t r e s u l t i n g from the change i n the b a s i s o f computing the c r y s t a l con­ c e n t r a t i o n n o t e d a b o v e , a n d one t h u s o b t a i n s roughly t h e same % f o r the d — > ρ t r a n s i t i o n s of K PtCl4 and P t ( N H ) 4 C l an (12) increased widtn of approximate 2.5-fold increase i n i n t e g r a t e d i n t e n s i t y i n MGS a l l u d e d t o i n an e a r l i e r s e c t i o n o f t h i s p a p e r . A n o t h e r g r o u p o f s u b s t a n c e s whose s t u d y i s p a r ­ t i c u l a r l y i n f o r m a t i v e c o n c e r n i n g the e x t e n t o f del o c a l i z a t i o n i n the c r u c i a l e x c i t e d s t a t e i n c r y s t a l s o f the type being d e a l t w i t h here are the barium s a l t s o f t h e t e t r a c y a n o c o m p l e x e s o f N i ( I I ) , P d ( I I ) , and Pt(II). T h e s e compounds, w h i c h f o r m s i n g l e c r y s t a l s h a v i n g the g e n e r a l f o r m u l a BaM(CN)i|-4H 0, p r e s e n t a s i t u a t i o n t h a t i n many r e s p e c t s p a r a l l e l s t h a t d e ­ s c r i b e d p r e v i o u s l y f o r NiDMG. O n c e a g a i n one h a s a s t a c k i n g o f the complexes i n the s o l i d s t a t e - w i t h c o n s e q u e n t " m e t a l c h a i n " f o r m a t i o n - and t h e develop­ ment o f a s t r o n g , r e l a t i v e l y l o w - e n e r g y band t h a t has no o b v i o u s c o u n t e r p a r t i n t h e f r e e i o n s ( 5 * 1 8 , 1 9 ) . The " s o l i d - s t a t e " b a n d s h e r e a r e a g a i n t h o u g E t z o h a v e a s t h e i r f r e e - i o n p a r e n t a g e r e l a t i v e l y weak b a n d s i n the " c h a r g e - t r a n s f e r " r e g i o n o f the s o l u t i o n absorp­ t i o n spectrum (18). One o f t h e " ~ T n t e r e s t i n g a s p e c t s o f t h e s o l i d s t a t e s t u d i e s on t h e s e c o m p o u n d s i s t h e f a c t t h a t one c a n p r e p a r e m i x e d c r y s t a l s o f t h e m i n w h i c h two dif­ f e r e n t c o m p l e x s p e c i e s a p p e a r i n t h e same c r y s t a l . One can, f o r i n s t a n c e , grow c r y s t a l s o f t h e f o l l o w i n g t y p e : a

5

x

p

2

2

Ba[Ni(CN) ] [Pt(CN) l ^ -4H o' 4

x

4

1

x

2

f

(5)

^Isomorphism o f these c r y s t a l s w i t h the s i n g l e - a n i o n c r y s t a l s h a s n o t b e e n a s c e r t a i n e d by d i r e c t e x p e r i m e n t , but i s i n f e r r e d from the g r a d u a l gradation of o p t i c a l p r o p e r t i e s a s one v a r i e s t h e c o m p o s i t i o n o f t h e m i x e d crystals.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

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295

f

While the mixed-Magnus s t u d i e s had the a d v a n t a g e o f d e f i n i t e i o n i c d i s t r i b u t i o n i n t h e m e t a l s t a c k s (one knows t h a t i n M a g n u s s a l t s t h e a n i o n i c and c a t i o n i c s p e c i e s a l t e r n a t e a l o n g t h e c h a i n ) , one i s l i m i t e d , a t l e a s t w i t h the c r y s t a l s prepared t o date, t o a 1:1 r a t i o o f t h e two m e t a l s i n v o l v e d . In the p r e s e n t s i t u a t i o n t h e a d v a n t a g e s and d i s a d v a n t a g e s a r e j u s t r e v e r s e d r e l a t i v e t o the Magnus salts: one c a n v a r y composition, but the sequencing a l o n g the c h a i n i s n o l o n g e r known. I f one p r e p a r e s c r y s t a l s w i t h a c o m p o s i t i o n corresponding to χ i n Equation 5 having a value of approximately 0.5, the s p e c t r a l r e s u l t s are c o m p l e t e l y a n a l o g o u s t o t h o s e o b s e r v e d f o r the mixed-Magnus c a s e : a s i n g l e b a n d whos between t h o s e o f BaNi(CN) (18). The u n i q u e r e s u l t s o f t h i s s t u d y a r e , h o w e v e r , i l l u s t r a t e d b y F i g u r e 11, where t h e r e i s p l o t t e d t h e p o s i t i o n o f the o u t - o f - p l a n e r e f l e c t i o n band as a function of P t ( C N H ~ content f o r a series of B a P t ( C N ) h - B a N i ( CN ) 4 m i x e d c r y s t a l s i n w h i c h t h e Pt(CN)4^~ c o n t e n t i s v a r i e d over r a t h e r wide l i m i t s . R e g a r d l e s s o f t h e a m o u n t o f P t ( C N ) 4 ~ t h a t was intro­ d u c e d i n t o t h e B a N i ( C N ) 4 c r y s t a l , o n l y one r e f l e c t i o n b a n d was f o u n d , a n d f o r e v e n t h e l o w e s t l e v e l s o f c o n c e n t r a t i o n o f one s p e c i e s i n t h e p r e s e n c e o f t h e o t h e r , t h i s b a n d was a s s u m i n g " i n t e r m e d i a t e " c h a r a c ­ teristics. T h e b e h a v i o r shown i n F i g u r e 11, which i s p a r a l ­ l e l e d by o t h e r s p e c t r a l c h a r a c t e r i s t i c s o f t h e bands i n question, presents very convincing evidence f o r e x t e n s i v e d e r e a l i z a t i o n i n the e x c i t e d s t a t e respon­ s i b l e f o r the s t r o n g o u t - o f - p l a n e band i n t h e s e c r y s ­ tals. The i m p l i c a t i o n h e r e i s t h a t i f , a s a p p e a r s t o b e t h e c a s e , t h e e f f e c t s o f s u b s t i t u t i o n became a p p a r ­ e n t a f t e r 5% " i m p u r i t y " i s i n t r o d u c e d , t h e n one w o u l d h a v e t o h a v e a v e r a g i n g o v e r a r a t h e r l a r g e number o f c e n t e r s i n the r e l e v a n t e x c i t e d s t a t e . Otherwise, one m i g h t e x p e c t t h e d e v e l o p m e n t o f one o r more " i n t e r ­ m e d i a t e " bands, a l o n g w i t h a band c o r r e s p o n d i n g t o the m a j o r component, i n the e a r l i e r s t a g e s o f s u b s t i t u ­ f

1

2

2

tion

(5,16).

General Discussion The o v e r a l l p i c t u r e t h a t e m e r g e s h e r e i s one o f a s t r o n g p a r a l l e l i s m i n t h e o p t i c a l phenomena o b s e r v e d f o r a l l of the c l o s e l y stacked c r y s t a l s d i s c u s s e d . Thus, i n the g l y o x i m a t e s , the Magnus s a l t s , and t h e 1

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

296

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METAL

28

Figure 11. a function

Variation of position of the out-of-plane reflectivity of BaPt(CN), · 4Η*0 content in mixed crystals BatNifCNhhlPtfCNhl,.* · 411 JO. t

maximum as of the form

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

19.

ANEX

Complexes

with

Extended

Chains

297

c y a n i d e s , one f i n d s f o r t h e m e t a l - s e p a r a t i o n d e p e n d e n t o u t - o f - p l a n e band: 1) S i m i l a r b e h a v i o r w i t h r e s p e c t t o r e l a t i v e p o s i t i o n as a f u n c t i o n o f the c e n t r a l m e t a l ion involved (l4,l6,l8). 2) S i m i l a r b e h a v i o r wTtYTrespect t o i n c r e a s i n g metal-metal separation (3,10,l8). 3) S i m i l a r b e h a v i o r when m i x e c T c r y s t a l s a r e studied (l4,15 l8).* These o b s e r v a t i o n s p r o v i d e a c o n v i n c i n g case f o r t h e p r o p o s i t i o n t h a t one i s d e a l i n g w i t h b a s i c a l l y t h e same k i n d o f t r a n s i t i o n i n e a c h i n s t a n c e . I f one a c c e p t s the v a r i o u s arguments p r e s e n t e d i n the f o r e ­ g o i n g s e c t i o n s , t h i s t r a n s i t i o n becomes i d e n t i f i e d as a highly perturbed A s was r e m a r k e Magnus s a l t s t u d i e s , i t i s t e m p t i n g t o v i e w t h i s as a s i t u a t i o n i n w h i c h one i s e s s e n t i a l l y i n v o l v e d w i t h a c h a i n o f i n t e r a c t i n g m e t a l atoms and t o d e e m p h a s i z e the r o l e of l i g a n d s i n attempting t o understand the phenomena b e i n g d e a l t w i t h . T h a t t h i s w o u l d be an o v e r s i m p l i f i c a t i o n b e c o m e s c l e a r when one r e c o g n i z e s , f o r i n s t a n c e , t h a t Magnus green s a l t , with a Pt-Pt d i s t a n c e o f 3.25 A (17). has i t s s t r o n g o u t - o f - p l a n e a b s o r p t i o n l o c a t e d aT; 34.5 kK ( 1 0 ) , w h i l e t h e compar­ able absorption i n B a P t ( C N ) h · 4 H 0 , which possesses a P t - P t s e p a r a t i o n o f 3.27 fi, o c c u r s a t 22.6 kK ( l 8 , 1 9 ) . S i m i l a r l y , the s t r o n g o u t - o f - p l a n e a b s o r p t i o n band~Tor β-NiEMG o c c u r s a t 20.5 kK (3) a n d t h e a n a l o g o u s b a n d i n B a N i ( C N ) 4 - 4 H 0 i s o b s e r v e d a t 27.2 kK ( l o ) , i n spite of approximately equal metal-metal separations (3,21). F i n a l l y , one may n o t e that the orthorïïombic f o r m o f N i ( I I ) N - m e t h y l s a l i c y l a l d i m i n a t e , w h o s e s t r u c t u r e i s s i m i l a r t o NiDMG a n d h a s a m e t a l m e t a l s p a c i n g o f 3.30 A ( 2 2 ) , shows no b a n d analogous t o t h e NiDMG g r e e n b a n d tïïroughout t h e v i s i b l e a n d q u a r t z u l t r a v i o l e t (23). Thus, the n a t u r e o f the l i g a n d a p p e a r s t o p l a y an i m p o r t a n t r o l e i n t h e e n e r g e t i c s and e v e n t h e f o r m a t i o n o f t h e o u t - o f - p l a n e band of s p e c i a l i n t e r e s t here. The s p e c i f i c n a t u r e o f t h e s o l i d s t a t e p e r t u r b a t i o n o p e r a t i v e i n t h e s e s y s t e m s i s as y e t n o t det e r m i n e d w i t h any c e r t a i n t y . E a r l y s p e c u l a t i o n s on 5

1

1

p

2

* P r e l i m i n a r y s t u d i e s on s i n g l e - c r y s t a l s o f t h e m i x e d d i m e t h y l g l y o x i m a t e s (20) show t h a t t h e b e h a v i o r h e r e p a r a l l e l s t h a t found T o r the mixed Magnus s a l t s and t h e P t ( I I ) and N i ( I I ) c y a n i d e m i x e d c r y s t a l w i t h χ e q u a l a p p r o x i m a t e l y 0.5 i n E q u a t i o n 5. f

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

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t h e e l e c t r o n i c s t r u c t u r e o f t h e s e compounds (£4) i n v o k e d band f o r m a t i o n i n v o l v i n g the h i g h e s t T i l l e d d 2 and t h e l o w e s t empty p o r b i t a l s on t h e m e t a l i o n s m a k i n g up t h e c h a i n s , a n d some o f t h e l a t e s t w o r k i n t h i s a r e a c o n t i n u e s t h e s e i d e a s (25). Alternate pro­ p o s a l s ( 5 , 6 , 1 6 , 2 6 ) h a v e b e e n made i n w h i c h e l e c t r o ­ s t a t i c p e r t u r b a t i o n s , e x c i t o n i c i n t e r a c t i o n s , and c h a r g e - t r a n s f e r e f f e c t s a r e i n v o k e d , a n d i n some c a s e s the d — > ρ assignment t o a g r e a t e r or l e s s e r extent challenged. The e x p e r i m e n t a l i n f o r m a t i o n now on h a n d makes i t u n l i k e l y t h a t an e l e m e n t a r y a p p l i c a t i o n o f any o f t h e s e a p p r o a c h e s w i l l a d e q u a t e l y a c c o u n t f o r a l l t h e o b s e r v a t i o n s , a l t h o u g h e a c h o f them c a n be d i s c u s s e d i n terms o f a p e r t u r b e d d > ρ transition. F o r i n s t a n c e , band (25) r a t e d 2 b a n d s and a n d c a t i o n i c s p e c i e s , a n d t h u s m o r e t h a n one e l e c t r o n ic transition. E v e n i f one i n v o k e s l a c k o f s p e c t r o ­ s c o p i c r e s o l v i n g power as t h e r e a s o n f o r t h e f a i l u r e t o o b s e r v e m o r e t h a n one t r a n s i t i o n , s u c h a r g u m e n t s w i l l not apply t o the m i x e d - c r y s t a l experiments. S i m i l a r l y , i n a d d i t i o n t o h a v i n g t o i n v o k e i n a some­ w h a t a d h o c m a n n e r some e x t r e m e l y l a r g e second-order e f f e c t s ' t o e x p l a i n the i n t e n s i t y b e h a v i o r i n the g l y o x i m a t e s and c y a n i d e s , i t does n o t a p p e a r t h a t exciton theory w i l l , at least i n a straight-forward manner, a c c o u n t f o r t h e m i x e d - c r y s t a l r e s u l t s . Com­ ments s i m i l a r t o those r e g a r d i n g the e x c i t o n mechanism apply to that of charge-transfer. The f o r e g o i n g r e m a r k s a r e n o t m e a n t t o i m p l y t h a t e x c i t o n e f f e c t s , c h a r g e - t r a n s f e r , and/or band forma­ t i o n w i l l not f i g u r e i n the f i n a l e x p l a n a t i o n o f the o p t i c a l p r o p e r t i e s o f these systems. At this point a l l one c a n s a y i s t h a t n o n e o f t h e m o b v i o u s l y a n d c l e a r l y e x p l a i n s t h e o b s e r v a t i o n s , and more s o p h i s t i ­ c a t e d a p p r o a c h e s a p p e a r t o be r e q u i r e d . W h e t h e r one or a c o m b i n a t i o n o f the o l d approaches, o r a t o t a l l y new one, w i l l p r o v e a d e q u a t e r e m a i n s t o b e s e e n . In a n y c a s e , a s u f f i c i e n t amount o f e x p e r i m e n t a l e v i d e n c e now e x i s t s t o p r o v i d e r a t h e r c r i t i c a l q u a l i t a t i v e a n d q u a n t i t a t i v e t e s t s f o r any p r o p o s a l s t h a t a r e p u t forward. z

z

2

Acknowledgment : The a u t h o r w i s h e s t o a c k n o w l e d g e t h e e f f o r t s o f t h e many u n d e r g r a d u a t e s , g r a d u a t e s t u d e n t s , a n d p o s t ­ d o c t o r a l a s s o c i a t e s who h a v e b e e n i n l a r g e p a r t r e s p o n s i b l e f o r the account t h a t has been g i v e n here.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

19. ANEx

Complexes

with

Extended

Chains

1

299

A number o f t h e s e w o r k e r s names a p p e a r i n t h e r e f e r e n c e s t h a t have been c i t e d . O t h e r s , who made c o n t r i b u t i o n s t o t h e development o f t h e equipment and t e c h n i q u e s u s e d i n t h i s work i n t h e c o n t e x t o f o t h e r types o f studies, a l s o deserve a share o f the c r e d i t . W o r t h y o f s p e c i a l n o t e among t h i s l a t t e r g r o u p a r e s e v e r a l o f my e a r l y g r a d u a t e s t u d e n t s : D r s . Α . V . F r a t i n i , S. C. N e e l y , J . B e r n s t e i n , a n d C. J . E c k h a r d t .

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

EXTENDED INTERACTIONS BETWEEN

300

M E T A L IONS

Literature Cited 1. 2.

Anex, B. G., M o l . C r y s t . (1966) 1, 1. S t e w a r t , R. F., D a v i d s o n , N., J. Chem. Phys.

3.

Anex, B. G., Krist, F. K., J. Amer. Chem. Soc.

4.

Gray, H. B., B a l l h a u s e n , C. J., J. Amer. Chem.

(1965) 39, 255.

5. 6.

(1967)

89,

(1963)

Soc.

6114. 85, 26Ο.

Day, P., I n o r g . Chim. A c t a , Rev. (1969) 3, 8l. O h a s h i , Y., H a n a z a k i , I., Nagakura, S., I n o r g . Chem. (197Ο) 9, 2551. 7. Day, P., O r c h a r d , A. F., Thomson, A. J., Williams, R. J. 1973. 8. M a r t i n , D. S., Jr., Rush, R. Μ., K r o e n i n g , R. F., Fanwick, P. Ε . , I n o r g . Chem. (1973) 12, 9.

10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

301.

Day, P., O r c h a r d , A. F., Thomson, A. J., Williams, R. J. P., J. Chem. Phys. (1965) 4 3 , 3763.

Anex, B. G., Ross, M. E., Hedgcock, M. W., J. Chem. Phys. (1967) 46, 1090. Anex, B. G., T a k e u c h i , N., J. Amer. Chem. Soc., accepted for publication. Anex, B. G., F o s t e r , S. I., F u c a l o r o , A. F., u n p u b l i s h e d work. Anex, B. G., F o s t e r , S. I., u n p u b l i s h e d work. Anex, B. G., F o s t e r , S. I., F u c a l o r o , A. F., Chem. Phys. Lett. (1973) 18, 126. Banks, C. V., Barnum, D. W., J. Amer. Chem. Soc.

(1958) 80, 4767.

Basu, G., Cook, G. M., Belford, R. L., I n o r g . Chem. (1964) 3, 1361. Miller, J. R., J. Chem. Soc. (1961), 4452. Anex, B. G., Musselman, R. L., u n p u b l i s h e d work. M o n c u i t , C., P o u l e t , H., J. Phys. Radium (1962) 23,

353.

22. 23. 24. 25.

Anex, B. G., N a r a i n , G., u n p u b l i s h e d work. L a r s e n , F. K., Hazell, R. G., Rasmussen, S. Ε., A c t a Chem. Scand. (1969) 23, 6 1 . F e r g u s o n , J., J. Chem. Phys. (1961) 34, 611. Anex, B. G., F u r t a d o , L. T., u n p u b l i s h e d work. R u n d l e , R. E., J. Phys. Chem. (1957) 6l, 45. I n t e r r a n t e , L. V., Messmer, R. P., I n o r g . Chem.

26.

Zahner, J. C., D r i c k a m e r , H. G., J. Chem. Phys.

(1971) 10, (1960)

33,

1174. 1625.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

20 T h e Single Crystal Polarized Reflectance Spectra and Electronic Structures o f Rh(CO) acac and 2

THOMAS A . DESSENT a n d RICHARD D u k e U n i v e r s i t y , D u r h a m , N . C . 27706 SALLY

M.

A.

Ir(CO) acac 2

PALMER

HORNER

M e r e d i t h C o l l e g e , R a l e i g h , N . C . 27602

Introduction 8

Among s q u a r e planar d c o m p l e x e s it m i g h t be e x pected that those o f the lowest formal metal valence (in t h e a b s e n c e o f "partial oxidation"(1)) w o u l d exhibit t h e most striking evidence o f extended metal-metal interaction. This prospect was b o r n e o u t in 1966 b y t h e w o r k o f Pitt a n d others (2), who first documented t h e anisotropic electrical conductivity of dicarbonyl acetylacetonato-rhodium(I) and -iridium(I). As illustrated in Figure 1, t h e s e c o m p o u n d s crystallize ( a s d o many o f their a n a l o g s discussed in this Symposium) with the planar molecules stacked parallel t o e a c h other in chains a n d short m e t a l - m e t a l d i s t a n c e s . The compounds w e r e first p r e p a r e d b y Bonati a n d Wilkinson (3), and t h e crystal structure work by B a i l e y , et al (4), h a s shown t h e two structures t o be i s o m o r p h o u s , with a 3.26Å M-M separation for t h e r h o d i u m compound and a 3.20Å separation f o r the iridium c o m p o u n d . The s p a c e g r o u p is P1 with Ζ = 2. The molecular sym­ metry is C , although t h e site s y m m e t r y is, of c o u r s e , only C. Pitt found the dc electrical conductivity of single crystals t o be 100 t i m e s greater along t h e M-M direction than perpendicular to it for t h e r h o d i u m com­ p o u n d a n d 500 t i m e s greater for the iridium compound (2). He s u g g e s t e d t h a t we might try t o d e t e r m i n e t h e polarized single crystal electronic spectra as an adjunct to the electrical m e a s u r e m e n t s , b u t it was obvious a t a glance, from the lustrous, metallic appear­ ance o f the crystals, that absorptivities in excess of 10 were involved, which would preclude consideration of single crystal absorption techniques. However, t h e s u c c e s s o f Anex (5) in deriving polarized single cry­ stal absorption s p e c t r a from specular reflectance mea­ surements s u g g e s t e d t h a t this m i g h t be a useful tech2v

1

3

301

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nique t o t r y i n t h i s i n s t a n c e . We h a v e c o n s t r u c t e d e q u i p m e n t f o r t h i s p u r p o s e a n d t h e r e s u l t s o f o u r mea­ s u r e m e n t s on t h e s e r h o d i u m a n d i r i d i u m c o m p l e x e s f o r m the b a s i s f o r t h i s p a p e r . Single C r y s t a l Micro f o r the Cary-14R

Specular

Reflectance

Attachment

F i g u r e 2A shows a s c h e m a t i c d i a g r a m o f o u r p o l a r ­ i z e d micro s p e c u l a r r e f l e c t a n c e attachment f o rthe Cary l 4 R spectrophotometer. We c h o s e t o b a s e t h e e q u i p m e n t on a h i g h q u a l i t y s p e c t r o p h o t o m e t e r rather t h a n s t a r t i n g " f r o m s c r a t c h " , i n o r d e r t o t a k e .advan­ tage o f the i n h e r e n t l o n g term s t a b i l i t y o f the double beam i n s t r u m e n t a n cromator. The a t t a c h m e n compartment o f t h e Cary ; t s not a t t a c h e d t o the compartment, b u t i s p l a c e d w i t h i n i t and a l i g n e d by screws v e r t i c a l l y and h o r i z o n t a l l y . Once a l i g n e d , t h e a c c e s s o r y c a n b e t a k e n ou t a n d r e p l a c e d w i t h o u t f u r t h e r alignment. As shown i n F i g u r e 2A, t h e m o n o c h r o m a t i c l i g h t beam e n t e r s f r o m t h e s i g n a l g e n e r a t o r c o m p a r t m e n t t h r o u g h a c a l c i t e Glan-Thompson p o l a r i z e r (P) i n the l e f t w a l l o f sample compartment, passes t h r o u g h a q u a r t z s u b s t r a t e beam s p l i t t e r ( B S ) a n d i s f o c u s e d o n t h e s u r f a c e o f t h e s a m p l e ( S ) b y a 36X C a s s e g r a i n m i c r o s c o p e o b j e c t i v e ( 0 ) . The s a m p l e i s a l i g n e d b y a g o n i o m e t e r ( G ; S O a s t o r e f l e c t t h e beam b a c k through the o b j e c t i v e . The l i g h t t h u s i m p i n g e s o n t h e c r y s t a l at nominally normal i n c i d e n c e . The r e f l e c t e d i n t e n s i t y s t r i k e s t h e beam s p l i t t e r a n d t h a t w h i c h i s r e f l e c t e d i s d i r e c t e d b y p l a n e m i r r o r s ( M l , M2, M3) b a c k i n t o t h e normal path f o r the i n s t r u m e n t and thus t o the d e t e c ­ tor. C o n c u r r e n t l y t h e r e f e r e n c e beam i s a t t e n u a t e d b y n e u t r a l d e n s i t y f i l t e r s and an i r i s diaphram so as t o b a l a n c e t h e beams w i t h i n t h e r a n g e o f t h e s l i d e w i r e . In o r d e r t o a v o i d back r e f l e c t i o n from the c e n t r a l m i r r o r o f the Cassegrain o p t i c s , the o b j e c t i v e i s used i n an o f f a x i s o r i e n t a t i o n , as i l l u s t r a t e d i n F i g u r e 2B. The d e s i g n o f t h i s e q u i p m e n t i s v e r y s i m i l a r t o t h a t u s e d p r e v i o u s l y b y Anex (5_) e x c e p t t h a t i t i s a n e a s i l y i n t e r c h a n g e a b l e a c c e s s o r y t o an e x i s t i n g i n ­ strument. I t i s d e s c r i b e d more c o m p l e t e l y e l s e w h e r e (6). The

Electronic

Structure

of Planar

d^ Complexes

For a planar C d^ complex the o r i e n t a t i o n o f the axes g i v e n i n F i g u r e 1 l e a d s t o the h i g h e s t f i l l e d d o r b i t a l b e i n g t h e d 2 _ 2 (&i i n C p ) a n d t h e l o w e s t u n f i l l e d , the d (ο "ΐη C ) · T h i s o r i e n t a t i o n em2 v

Y

x

y

2

Y

2 v

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

DESSENT

E T AL.

Single

Crystal

Spectra

Β Figure 2. Single crystal microspecular reflectance attachment for the Cary 14R spectrophotometer. (A) Incident ( ) and reflected ( ) light paths. (B) Orientation of light path with respect to objective axis.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

304

EXTENDED

INTERACTIONS

BETWEEN METAL

IONS

p h a s i z e s the importance of the metal-metal i n t e r a c t i o n a n d i s t h e same a s t h a t u s e d b y M a r t i n (7_) r e c e n t l y i n h i s a n a l y s i s o f the s p e c t r u m o f the a n a l o g o u s Pt(en)Cl2. I n s u c h a m o l e c u l a r s y s t e m one m i g h t e x p e c t t o o b s e r v e s e v e r a l t y p e s o f t r a n s i t i o n s , i n c l u d i n g ones l o c a l i z e d on t h e m e t a l , o t h e r s i n v o l v i n g t r a n s f e r o f e l e c t r o n d e n s i t y f r o m the m e t a l t o the l i g a n d s and v i s a v e r s a , a n d s t i l l o t h e r s l o c a l i z e d on t h e l i g a n d s . Some o f t h e s e a r e i l l u s t r a t e d i n F i g u r e 3 a l o n g w i t h t h e symmetry p r o p e r t i e s o f the e x c i t e d s t a t e s and the molec u l a r a x i s a l o n g w h i c h e a c h w o u l d be a l l o w e d . The g r o u n d s t a t e i s o f c o u r s e ^A^ i n t h e s e d i a m a g n e t i c compounds. Results

and

Discussio

Solutions of Rh(C0) Ir(C0)oaca y f a i n t l y c o l o r e d , i n marked c o n t r a s t t o the l u s t e r o u s l y opaque s o l i d s . The s p e c t r a o f t h e two compounds i n c h l o r o f o r m s o l u t i o n a r e shown i n F i g u r e 4 . The r e s o l ution i s not very i n s p i r i n g , but i n both spectra e s s e n t i a l l y t h e same p a t t e r n i s o b s e r v e d ; t h a t i s , t h r e e s t r o n g bands, e a c h accompanied by a weaker shoulder. A l t h o u g h i t i s p o s s i b l e t h a t some l i g a n d l i g a n d b a n d s a r e i n v o l v e d h e r e — p a r t i c u l a r l y i n t h e 33 kK r e g i o n { & ) — t h e s e b a n d s m i g h t o t h e r w i s e l o g i c a l l y be a s s i g n e d t o e i t h e r d - d o r c h a r g e t r a n s f e r t r a n s i tions. I n terms o f metal l o c a l i z e d t r a n s i t i o n s the t h r e e s t r o n g e r b a n d s w o u l d be t h e t h r e e s p i n - a l l o w e d , symmetry-a11owed bands and the s h o u l d e r s , t h e s p i n forbidden counterparts. H o w e v e r , o b s e r v a t i o n s on P t ( l l ) complexes would s u g g e s t t h a t the s p i n - f o r b i d d e n b a n d s s h o u l d be e x p e c t e d a t c o n s i d e r a b l y l o w e r e n e r g y than those observed here. C h a r g e t r a n s f e r , on t h e o t h e r h a n d , a p p e a r s a much more l i k e l y e x p l a n a t i o n — a t l e a s t f o r t h e more p r o m i n e n t f e a t u r e s a t 3 0 . 5 and 3 9 . 0 kK f o r t h e r h o d i u m c o m p l e x a n d 2 9 . 2 and 39.2 kK f o r the i r i d i u m complex. ( T h e r e g i o n a r o u n d 33 kK l i k e l y i n v o l v e s an acac t r a n s i t i o n , though the d i f f e r e n c e i n a b s o r p t i v i t y a n d b a n d s h a p e b e t w e e n t h e two compounds may i n d i c a t e a t h i r d p r o m i n e n t c h a r g e t r a n s f e r t r a n s i t i o n here a l s o . ) The l a r g e r s i z e o f t h e i r i d i u m would p r e d i c t lower energy charge t r a n s f e r than i n t h e r h o d i u m a n a l o g , w h e r e a s t h e d-d t r a n s i t i o n s w o u l d be e x p e c t e d a t h i g h e r e n e r g y . Thus, the r e l a t i v e l y low e n e r g y o f the i r i d i u m bands f a v o r s the charge t r a n s f e r assignment. These t e n t a t i v e a s s i g n ments a r e s u m m a r i z e d i n T a b l e 1. B e f o r e we c o m p l e t e d o u r s p e c u l a r r e f l e c t a n c e a c c e s s o r y we a t t e m p t e d t o q u a n t i f y t h e i n t e n s e a b s o r p 2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

20.

DESSENT

E T AL.

Single

Crystal

Spectra

Excited-state Symmetry

One-electron Orbitale

3,l

d

Λ

V d

3,1

+ d

3.1ο

x y

B

* w'Ca,) χ -y d -w (a ) χ -y 2

Figure 3. Oneelectron transitions in M(CO) acac. + applies only to sin­

2

2

a

2

° 2 2 χ -y -

2 v

forbidde

* d xy

xz

Polarizat in C

n

d • d yz xy d

305

-

forbiddei

2

2

**
«• t * ( b ) 0

2

2

χ -y

xy D

d

« < V * xy forbidden

^(b ) * d 2

y

x y

Rh(CO) acac

20

2

lr(CO) acac Solution Spectra in Chloroform 2

15

S X «Ο

10

20 Figure 4.

Solution

spectra of

M(CO) acac 2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

306

EXTENDED

2

M Energy

SOLUTION

= Rh

(kK)

27.0 30.5 32.0 33.8 35.5 39.0

M e

max

500 4000

500 2000 800 4500

BETWEEN

METAL

IONS

1

TABLE M(C0) acac

INTERACTIONS

Energy

SPECTRA

= Ir

(kK)

Assignment

ε max

500 3000 200 2500 200 3000

26.5 29.2 31.9 33.8 36.3 39.2

d-KL

d-Λπ d-*d d-feLn d->d d-Λιπ

t l o n o f t h e s e compound to the m e t a l l i c l u s t e r m u l l s and KBr p e l l e t s . The m u l l s p e c t r a w e r e o f r a t h e r p o o r q u a l i t y and, a l t h o u g h the KBr d i s k s d i d , i n f a c t , r e v e a l a s t r o n g v i s i b l e b a n d i n b o t h compounds b e t w e e n 14 a n d 20 kK, t h e r e s u l t s f r o m r u n t o r u n w e r e n o t r e p o r d u c i b l e and gave e v i d e n c e o f d e c o m p o s i t i o n d u r i n g p r e p a r a t i o n — p a r t i c u l a r when t h e s a m p l e s w e r e g r o u n d i n the p r e s e n c e o f oxygen. Furthermore, the p o l a r i z a t i o n p r o p e r t i e s o f t h e s o l i d s t a t e band o r bands were o f course not a v a i l a b l e from these n o n o r i e n t e d samples. F r o m m a t e r i a l g e n e r o u s l y s u p p l i e d b y P i t t we w e r e a b l e t o g r o w h i g h q u a l i t y s i n g l e c r y s t a l s o f b o t h com­ p o u n d s w i t h f a c e s o f c a . 1 mm s q u a r e . The rhodium c r y s t a l s were grown by s l o w s u b l i m a t i o n a t r e d u c e d p r e s s u r e and the i r i d i u m c r y s t a l s , by s l o w e v a p o r a t i o n of acetone s o l u t i o n s . I n F i g u r e 5 i s shown t h e p o l a r i z e d s p e c u l a r r e ­ f l e c t a n c e e l e c t r o n i c spectrum of Rh(C0) acac measured i n the C a r y l4R. The m o s t p r o m i n e n t f e a t u r e i s t h e i n t e n s e ζ - p o l a r i z e d b a n d a t 19.1 kK. I n F i g u r e 6 we see t h e a n a l o g o u s d a t a f o r the i r i d i u m compound. Again the most p r o m i n e n t f e a t u r e o f the s p e c t r u m i s t h e z p o l a r i z e d band i n the v i s i b l e . I t i s a t lower energy than i n the rhodium a n a l o g — s o low i n f a c t t h a t i t i s the l o w e s t e n e r g y band o b s e r v a b l e w i t h i n the range o f t h e GaAS d e t e c t o r . The d a t a w e r e o b t a i n e d b y s u b s t r a c t i n g t h e a p p a r ­ e n t a b s o r p t i o n o f the i n s t r u m e n t w i t h the r e f e r e n c e m i r r o r i n the sample p o s i t i o n ( A f ) from t h a t w i t h the c r y s t a l i n p o s i t i o n ( A y t ) , w i t h a l l o t h e r c o n ­ d i t i o n s t h e same. The r e f l e c t a n c e R o f t h e c r y s t a l i s then g i v e n by 2

r e

c r

R

c r y s t

(a))

= R

r 8 f

(u))xl0"

S

( A

cryst-A

r e f

)

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

( l

)

DESSENT

Single

E T AL.

Crystal

Spectra

Rh(CO) acac 2

Specular Reflectance ζ •20

X

y

oc

.10

0 Figure

5.

40 Polarized

single

30 crystal specular Rh(CO) acac

20 reflectance

spectrum

of

2

lr(CO) acac 2

Specular Reflectance xy

10

—

1

40

30

kK Figure

6.

Polarized

single

crystal specular Ir(CO) acac

1

20 reflectance

spectrum

of

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

308

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

R f ( u ) ) was d e t e r m i n e d b y P l a n a r O p t i c s o v e r t h e r a n g e 5-50 kK. D a t a p o i n t s were t a k e n m a n u a l l y f r o m the C a r y r e c o r d e r c h a r t a n d f e d i n t o a PDP-15 c o m p u t e r w h i c h c a l c u l a t e d the R t ( u > ) and p l o t t e d r e f l e c t a n c e v s . e n e r g y s p e c t r a ( p r o g r a m s PSRC, S R I , P I X R a n d P L O T R ) . A v a l u e s u s e d w e r e t h e a v e r a g e o f 3-5 r u n s on d i f f e r e n t crystals. The s t a n d a r d d e v i a t i o n o f t h e r a w d a t a was n o r m a l l y < 5$ e x c e p t i n t h e h i g h e n e r g y r e g i o n ( < 4 0 k K ) . The r e f l e c t a n c e d a t a w e r e u s e d a s i n p u t t o a p r o ­ g r a m (KRAMER) w h i c h c a l c u l a t e d t h e p h a s e s h i f t a n d from t h i s t h e o t h e r n e c e s s a r y o p t i c a l c o n s t a n t s n, k, a n d e,. The m e t h o d s u s e d a r e s i m i l a r t o t h o s e r e p o r t e d b y Anex (5_) a n d b y S t e r n ( £ ) a n d o t h e r s a n d a m o u n t t o a Kramers-Kronig analysis. In such a c a l c u l a t i o n the phase s h i f t θ depend o f a) f r o m -°° t o + » r e f l e c t a n c e a t e n e r g i e s h i g h e r and l o w e r t h a n the r e g i o n o f o b s e r v a t i o n ( 1 1 - 4 0 kK) i s n e c e s s a r y . Sev­ e r a l p r o c e d u r e s h a v e b e e n u s e d t o make t h i s e x t r a p o ­ l a t i o n , i n c l u d i n g s e t t i n g the r e f l e c t a n c e o u t s i d e the measurement i n t e r v a l e q u a l t o an a d j u s t a b l e c o n s t a n t (10) o r an e x p o n e n t i a l f u n c t i o n ( £ ) , o r g e n e r a t i n g a dummy b a n d i n one o r b o t h o f t h e o u t l y i n g r e g i o n s (5.). We f o u n d a m o d i f i c a t i o n o f S t e r n ' s e x p o n e n t i a l m e t h o d (£) m o s t s a t i s f a c t o r y . I n t h i s method r e

c r v s

R

= R

(ω/ω·^

x

u> <

R = R (o^/m)*

(2a)

1

ω > u)

2

(2b)

2

w h e r e R^ a n d m r e f e r t o t h e r e f l e c t a n c e a n d e n e r g y a t t h e l o w e r l i m i t o f t h e m e a s u r e m e n t i n t e r v a l , a n d R2 and t o the c o r r e s p o n d i n g v a l u e s a t the upper l i m i t . The v a l u e s o f t h e p h a s e s h i f t f o r ω b e l o w t h e m e a s u r e ­ ment i n t e r v a l ( θ·^), i n t h e i n t e r v a l ( θ ) , a n d a b o v e i t (Θ3) are then 2

R(U) )

œ

0

1

,

ω

,

Μ-Ο)

2

°

π

R

Γ 2 ω

0

= ~

J

u 3

) _ 0

l 2

, π

n

0

+

ω

ω

*

ι

p π

1

1

0

η=0

(®ι/<»ο) (2η

2η+1

i f

+

(o )

'tnRUJ-taRUJ

2

,

2 ^ -

0)Q

I

θ

ω

l

o

*2 RU ) 0

.

n

a

^

(3b)

(

3

a

)

0)

^

~ o w

α>2 + «)0

. a ^ π n

=

"o

( t

Q

(

2

n

/ u

+

^

) 2 n + 1

χ )

2

(3c)

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

20.

DESSENT

ET

AL.

Single

Crystal

Spectra

309

From e q u a t i o n (3) v a l u e s o f θ = Θ + θ 2 + θ 3 were c a l c u ­ l a t e d u s i n g v a l u e s o f the p a r a m e t e r s ρ and q e m p i r i ­ c a l l y chosen a c c o r d i n g t o the f o l l o w i n g c r i t e r i a : a ) θ i s p o s i t i v e on t h e i n t e r v a l m < ω < u^; b ) t h e c a l c u l a t e d e x t i n c t i o n c o e f f i c i e n t , k, i s n e a r l y z e r o i n t h e n e i g h b o r h o o d o f u)i ; ( c ) the s m a l l e s t p o s s i b l e v a l u e s o f ρ and q a r e u s e d t o o b t a i n the c o n d i t i o n s i n ( a ) and ( b ) . C r i t e r i o n ( b ) was e s t a b l i s h e d b e c a u s e i t was assumed t h a t the complexes s t u d i e d are n e a r l y t r a n s p a r e n t i n t h e n e i g h b o r h o o d o f m ( c a . 11 k K ) . C r i t e r i o n ( c ) was e s t a b l i s h e d so t h a t the c a l c u l a t e d r e s u l t s would have t h e l e a s t p o s s i b l e d e p e n d e n c e on t h e e m p i r i c a l p a r a ­ meters. No w o r k a b l e c r i t e r i o n c o u l d be e s t a b l i s h e d f o r c h o o s i n g the h i g h - f r e q u e n c parameter It p e c t e d t h a t the complexe a b s o r p t i o n bands beyon high-frequency , uj ( c a . 45 k K ) , a n d i t was i m p o s s i b l e t o g u e s s t h e m a g n i ­ t u d e o f t h e p h a s e c h a n g e w h i c h w o u l d be i m p a r t e d by these bands. S i n c e the c a l c u l a t e d v a l u e of θ near c o u l d be w i d e l y v a r i e d , d e p e n d i n g on t h e v a l u e q c h o s e n , n o t much c o n f i d e n c e c a n be p l a c e d i n t h e mag­ n i t u d e o f θ a b o v e 3 0 t o 35 kK. The s h a p e o f t h e c u r v e was n o t c h a n g e d t o a n a p p r e c i a b l e e x t e n t , h o w e v e r , b y w i d e v a r i a t i o n o f q; t h e r e f o r e t h e g e n e r a l f e a t u r e s o f the c a l c u l a t e d s p e c t r a are v a l i d . Once s u i t a b l e v a l u e s o f θ w e r e e s t a b l i s h e d , t h e r e f r a c t i v e i n d e x , n, a n d t h e e x t i n c t i o n c o e f f i c i e n t , k, were c a l c u l a t e d , and l a s t l y , v a l u e s o f the m o l a r a b s o r p t i v i t y , e, w e r e c o m p u t e d f r o m k b y t h e f o l l o w i n g expression: 1

evcu;

- 2.303M

K

J

w h e r e OD i s t h e f r e q u e n c y i n cm"" and M i s the c o n c e n ­ t r a t i o n o f the c r y s t a l l i n e complex i n moles p e r l i t e r . F i g u r e 7 shows t h e d e r i v e d m o l a r a b s o r p t i v i t y o f Rh(C0)2acac. F o u r b a n d s a r e e v i d e n t , t h e most p r o m i ­ nent b e i n g a t 19.1 kK w i t h a n e o f 1 5 x 1 0 · ^ , a l l o w e d , a s n o t e d b e f o r e , o n l y a l o n g the m e t a l - m e t a l a x i s ( z ) . Of p a r t i c u l a r i n t e r e s t a l s o a r e t h e two l o w e r e n e r g y b a n d s a t Ik.6 a n d 15Λ kK, one x y , t h e o t h e r ζ p o l a r i z e d . A l t h o u g h , as Anex ( l l j has p o i n t e d o u t , s p u r i o u s bands are o f t e n observed adjacent to regions of high r e f l e c ­ t i v i t y , i t i s i n t e r e s t i n g t o note i n t h i s case a band i s o b s e r v e d i n the xy p o l a r i z a t i o n even though t h e r e i s no a d j a c e n t r e g i o n o f h i g h r e f l e c t i v i t y i n t h a t o r i e n t a t i o n of the p o l a r i z e r . The f o u r t h b a n d i s a l s o 1

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

310

EXTENDED

INTERACTIONS B E T W E E N

METAL

IONS

xy

p o l a r i z e d a n d i s s e e n a t 32.5 kK. F i g u r e 8 shows t h e c o r r e s p o n d i n g r e s u l t s f o r t h e iridium analog. In t h i s spectrum o n l y the s t r o n g zp o l a r i z e d b a n d a t 17.6 kK seems t o be w e l l d e f i n e d . T h i s i s a l m o s t c e r t a i n l y a n a l o g o u s t o t h e 19.1 b a n d o f the rhodium c r y s t a l . Because o f the low-energy l i m i t o f t h e d e t e c t o r i t was n o t p o s s i b l e t o c o v e r t h e r e g i o n w h e r e w e a k e r b a n d s c o r r e s p o n d i n g t o t h e 14.6 a n d 15.4 kK b a n d s o f t h e r h o d i u m compound w e r e o b s e r v e d . In a s s i g n i n g these s p e c t r a , e i t h e r a m o l e c u l a r o r b i t a l band model o r an e x c i t o n model i s p o s s i b l e . Both of t h e s e models a r e c o n s i d e r e d i n d e t a i l i n o t h e r p a p e r s i n t h i s symposium w i t h r e f e r e n c e t o c l o s e l y a n a logous "square p l a n a r " complexes. O u r own c o n s i d e r a t i o n of these p o s s i b i l i t i e led first t estimat the gap w h i c h m i g h c u l a r o r b i t a l band empty p„ e n e r g i e s were c a l c u l a t e d by a m o d i f i e d W o l f s b e r g H e l m h o l t z scheme. I n the absence o f a v a i l a b l e molec u l a r o r b i t a l o v e r l a p i n t e g r a l s , t h e o v e r l a p s were b a s e d on a t o m i c o r b i t a l s . These r e l a t i v e l y c r u d e c a l c u l a t i o n s l e a d t o e s t i m a t e d band w i d t h s f o r the r h o d i u m d 2 and p b a n d s o f 6 a n d 50 kK, r e s p e c t i v e l y , w i t h a b a n d g a p o f 26 kK. F o r t h e i r i d i u m compound t h e p b a n d w i d t h i s e s t i m a t e d t o be 13$ s m a l l e r , b u t t h e d 2 b a n d w i d t h , 50$ g r e a t e r , l e a d i n g t o a n a p p r e c i a b l y s m a l l e r band gap t h a n i n the r h o d i u m complex. This would p r e d i c t l o w e r t r a n s i t i o n e n e r g y and g r e a t e r c o n d u c t i v i t y f o r t h e i r i d i u m compound, i n a g r e e m e n t w i t h observation. A l t h o u g h the s i z e o f the e s t i m a t e d band g a p s i s r e a s o n a b l e c o m p a r e d t o t h e 19.1 a n d 17.6 kK e n e r g i e s o f t h e p r o m i n e n t b a n d s , t h i s m o d e l o f f e r s no e x p l a n a t i o n f o r the l o w e r e n e r g y bands o b s e r v e d i n the r h o d i u m compound. Presuming t h a t these bands a r e not a r t i f a c t u a l , t h i s i s a s e r i o u s drawback t o the s i m p l e MO m o d e l . The a b o v e c o n s i d e r a t i o n s l e d us t o e x p l o r e t h e p r e d i c t i o n s o f an e x c i t o n model. M a r t i n (7) h a s r e c e n t l y p r e s e n t e d arguments f o r a s s i g n i n g the s o l i d s t a t e bands i n Pt(en)Cl2 t o b o t h F r e n k e l and i o n i c exciton transitions. A s s u m i n g t h e same b a s i c d i p o l a r m o d e l (7)* one s h o u l d e x p e c t i n t h e s e r h o d i u m ( l ) a n d i r i d i u m f l ) complexes a l s o t h a t F r e n k e l e x c i t o n s t a t e s b a s e d on z - p o l a r i z e d b a n d s i n t h e i s o l a t e d m o l e c u l e s h o u l d be s h i f t e d c a . 12 kK t o l o w e r e n e r g y . The a b s o r p t i v i t i e s o f the p r o m i n e n t z - p o l a r i z e d bands o f c a . 15x10-3 a r e j u s t t h r e e t i m e s t h o s e o f t h e s o l u t i o n s p e c t r a l t r a n s i t i o n s a t 30.5 a n d 29.2 kK f o r r h o d i u m and iridium, respectively. This i s the r e q u i r e d r e l a t i o n s h i p between the i n t e n s i t i e s o f the z - p o l a r i z e d z

z

z

z

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

20.

DESSENT E T A L .

Single

Crystal

Spectra

311

Rh(CO) acac 2

Derived Molar Absorptivity

Figure

7.

Motor

absorptivity

of Rh(CO) acac 2

derived

from

specular

reflectance

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

312

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

s p e c t r u m and t h o s e o f the s o l u t i o n s p e c t r u m , and s u p p o r t s the a s s i g n m e n t o f these bands t o F r e n k e l ex­ c i t o n s b a s e d on t h e d 2 _ 2 ( a n ) -> L (b ) transitions i n the i s o l a t e d m o l e c u l e s , w i t h Dav^dov s p l i t t i n g s o f 11.4 and 11.6, respectively. T u r n i n g t o the weaker t r a n s i t i o n s i n the rhodium c r y s t a l , we m i g h t c o n s i d e r f i r s t t h o s e a t h i g h e r e n e r g y t h a n t h e 19.1 kK b a n d . T h o s e a t 32.5 a n d ca_. 40 kK a p p e a r most l i k e l y t o be u n p e r t u r b e d m o l e c u l a r transi­ tions. The t a i l on t h e h i g h e n e r g y s i d e o f t h e 19.I kK b a n d c o u l d c o n c e i v a b l y be due t o s p i n - f o r b i d d e n d-d t r a n s i t i o n s w h i c h g a i n i n t e n s i t y by s p i n o r b i t c o u p l i n g and e x c i t e d s t a t e m i x i n g w i t h the e x c i t o n band. The two b a n d s w h i c h a p p e a r a t 1 4 . 6 kK ( z ) a n d 15.4 kK ( x y ) a r e more d i f f i c u l p o l a r i z e d b a n d may a l s e x c i t o n s t a t e , b a s e d p e r h a p s on t h e d -> L transition a t 27.Ο kK. However, s i n c e a Davydov s p l i t t i n g can o n l y i n c r e a s e the e n e r g y o f an x y - p o l a r i z e d t r a n s i t i o n , t h e b a n d a t 15.4 kK c a n n o t be t h e r e s u l t o f a F r e n k e l state. On t h e o t h e r h a n d , i t c o u l d be due t o a n i o n i c e x c i t o n s t a t e s i m i l a r t o those d e s c r i b e d by M a r t i n (7_) for Pt(en)Cl2. S u c h s t a t e s i n v o l v e e x c i t a t i o n o f an e l e c t r o n from a f i l l e d d or d o r b i t a l on one m o l e ­ c u l e t o an empty d £ o r b i t a l on a n a d j a c e n t molecule. One o f t h e s e e x c i t a t i o n s ( d -> d * ) i s y - p o l a r i z e d a n d the o t h e r , x - p o l a r i z e d . I f this transition is derived f r o m t h e 32.0 kK d-d b a n d i n t h e s o l u t i o n s p e c t r u m , t h e o b s e r v e d s h i f t i s 16.6 kK, w h i c h i s w i t h i n reasonable bounds. The a s s i g n m e n t s o f t h e c r y s t a l s p e c t r a a r e summarized i n Table 2. A l t h o u g h the p o s s i b i l i t y o f c o n f i r m i n g a d d i t i o n a l i o n i c e x c i t o n s t a t e s as d e s c r i b e d above i s a p p e a l i n g , and the f l e x i b i l i t y o f the e x c i t o n model o f f e r s r e a ­ s o n a b l e e x p l a n a t i o n s f o r a l l the f e a t u r e s o f these s p e c t r a , i f f u r t h e r i n v e s t i g a t i o n s h o u l d show t h a t t h e l o w e r e n e r g y b a n d s i n R h ( C 0 ) a c a c a r e ( a s Anex ( l l _ ) has s u g g e s t e d ) a r t i f a c t u a l , the a p p a r e n t advantage o f the e x c i t o n model o v e r the m o l e c u l a r o r b i t a l model would be c o n s i d e r a b l y diminished. x

y

1

x ?

y z

y

x

z

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

20.

Single

DESSENT E T AL.

Crystal

313

Spectra

TABLE 2 M ( C 0 ) a c a c SINGLE CRYSTAL ABSORPTIVITY (DERIVED) 2

E n e r g y (kK)

e

max

( z )

e

max<

x y )

M = Rh 14.6

1000

15.4 19.1 24

720

14000 1000

n

3500

32.5

d-xl P r e n k e l e x c i t o n d-*d I o n i c e x c i t o n d-4L P r e n k e l e x c i t o n d-κί S p i n f o r b i d d e n ( ? ) d->L n

1000

40 M =I r 17.6 22

1500

1400

d-*d S p i n ?

2000

d-»L

4000

25 35

5000

w

forbidden(?)

(?)

Literature Cited 1. 2. 3. 4. 5. 6. 7.

Krogman, Κ., et. al., in paper no. o f this Symposium d i s c u s s "partially oxidized" compounds. Pitt, C. G., et. al., J. Amer. Chem. S o c . (1966) 88,

4288.

B o n a t i , F. and Wilkinson, G., J. Chem. S o c . (London)(1964) 3156. B a i l e y , Ν. Α., et. al., Chem. Comm. (1967) 104l. Anex, B. G., M o l . Cryst. (1966) 1, 1. D e s s e n t , T. A. and P a l m e r , R. Α., t o be p u b l i s h e d . M a r t i n , D. S., et. al., J. Amer. Chem. S o c . (1971) 93, 5433.

8. 9. 10. 11.

C o t t o n , F. Α., Harris, C. B. and W i s e , J. J., I n o r g . Chem. (_967) 6, 909. S t e r n , F., Solid S t a t e Phys. (1963) 15, 338. S c h a t z , P. Ν., et. al., J. Chem. Phys. (1963) 38, 2658.

Anex, B. G.. R o s s , M. E. and Hedgcock, J. Chem. Phys. (1967) 46, 1090.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

21 Directed Synthesis o f Linear Chain Metal Complexes with Well-Defined Cooperative Properties R. ADERJAN, D. BAUMANN, H. BREER, H. ENDRES, W. GITZEL, H. J. KELLER, R.

LORENTZ,

W.

MORONI,

M.

MEGNAMISI-BÉLOMBÉ,

D.

NÖTHE,

and

H. H. RUPP A n o r g a n i s c h - C h e m i s c h e s Institut der Universität H e i d e l b e r g , D 6900 H e i d e l b e r g 1, I m N e u e n h e i m e r Feld 7, GFR

Abstract A s i m p l e band model deduced from t h e one-electron metal functions split by an electric field of the ligands c a n be successfully u s e d as a guideline in the directed synthesis of linear chain transition metal complexes with strong intermolecular interactions. One-dimensional metallic behaviour c a n be f o u n d for instance in planar transition m e t a l compounds w i t h 8 d-electrons b u t o n l y if small a n d strongly π-electron accepting equatorial ligands are used. Suitable ligands are carbonyl and isonitrile groups in connec­ tion with iridium(I) and r h o d i u m ( I ) ions. I.r., H-n.m.r., 195Pt-n.m.r., e.s.r., u.v. and 193Ir-Mößbauer spectra show that increasing increasing and increasing electron donating properties of the ligands considerably decrease the strength of the intermolecular metal interactions. A planar transition m e t a l complex w i t h less t h a n 8 d-electrons p e r m e t a l s h o u l d be far better suited for the formation of 1d-metals. But b e c a u s e o f t h e strong L e w i s acid activity of these compounds t h e donating solvent m o l e c u l e s occupy the axial positions of the central m e t a l atom and h e n c e prevent the self-association necessary for the building up o f a linear chain. Preparative procedures to o v e r c o m e t h e s e p r o b l e m s a r e p r o p o s e d . The structure and c h e m i c a l properties o f one o f t h e obtained "mixed valence" solids which contains a linear I-3-chain additionally is especially interesting since two n o n d e generate conduction spines are running closely parallel t o e a c h o t h e r t h r o u g h t h e lattice. 1

I.

Introduction Very

recently

i t was

shown b y

a.c.

and

d.c.

314

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

con-

21.

ADERJAN ET AL.

Linear

Chain

315

Complexes

d u c t i v i t y m e a s u r e m e n t s a s w e l l a s by o p t i c a l , X - r a y a n d n e u t r o n s c a t t e r i n g e x p e r i m e n t s t h a t a t l e a s t one o f t h e many l i n e a r c h a i n m e t a l c o m p l e x e s - a p l a t i n u m c o m p l e x o f s t o i c h i o m e t r y K [ P t ( C N ) 2 | ] B r 3 · 3H2O - b e haves l i k e a o n e - d i m e n s i o n a l m e t a l a t room t e m p e r a t u r e (1). While the e v i d e n c e f o r a l d - m e t a l l i c s t a t e i n t h i s "mixed v a l e n c e " p l a t i n u m s y s t e m a t room temperat u r e i s o v e r w h e l m i n g and w i d e l y a c c e p t e d , t h e i n t e r p r e t a t i o n o f the phase t r a n s i t i o n s which o c c u r a t l o wer t e m p e r a t u r e s a n d w h i c h d e s t r o y t h e l d - m e t a l l i c s t a t e seems t o be c o n t r o v e r s i a l . T h i s c o n t r o v e r s y mot i v a t e s the search f o r other l d - m e t a l l i c t r a n s i t i o n metal complexes. In order to study the aptitude of l i n e a r metal c h a i n compounds t o must be d e v e l o p e d t s u i t e d m o l e c u l e s f o r b u i l d i n g up l i n e a r l d - m e t a l l i c c h a i n s . T h i s problem w i t h r e s p e c t to the h i g h l y cond u c t i n g o r g a n i c c h a r g e t r a n s f e r s a l t s o f t h e "TCNQ" t y p e was r e c e n t l y d i s c u s s e d b y H e e g e r a n d G a r i t o ( 2 ) a n d w i l l be o u t l i n e d i n t h e f o l l o w i n g s e c t i o n s u s i n g p l a n a r m e t a l complexes as l a t t i c e e l e m e n t s . These planned syntheses give r i s e to the f o l l o w i n g questions: "Which k i n d o f m o l e c u l a r p a r a m e t e r s o f t h e i n d i v i d u a l e l e m e n t s g o v e r n t h e p r o p e r t i e s o f t h e s e s o l i d s ? " and "What s p e c i a l r u l e s c a n be s e t up t o p r e p a r e s u i t a b l e compounds i n a s y s t e m a t i c w a y ? " I n o r d e r t o f i n d s u c h r u l e s f o r the s y n t h e s i s of t a i l o r - c u t molecules, the e l e c t r o n i c s t r u c t u r e o f p l a n a r t r a n s i t i o n m e t a l comp l e x e s , which c r y s t a l l i z e p r e f e r a b l y i n columnar s t a c k s w i t h d i r e c t m e t a l c o n t a c t s , h a s t o be i n v e s t i gated . 2

II.

Electronic

structure

of suitable

0 #

metal

complexes

T h e i n t e r m o l e c u l a r c o n t a c t s i n t h e known l i n e a r c h a i n m e t a l compounds w i t h d i r e c t m e t a l - m e t a l b o n d s a r e b r o u g h t a b o u t m a i n l y by t h e c e n t r a l m e t a l s d-elect r o n s . Though most o f t h e c o m p l e x e s have c o v a l e n t l y bonded l i g a n d s an o v e r l a p o f d - f u n c t i o n s o f t h e c e n t r a l metal ions with a p p r o p r i a t e l i g a n d f u n c t i o n s can be o m i t t e d i n a f i r s t a p p r o x i m a t i o n , t o k e e p t h e m o d e l as c l e a r as p o s s i b l e . A v e r y s i m p l e and p e r h a p s o v e r s i m p l i f i e d p u r e c r y s t a l f i e l d a p p r o a c h c a n be u s e d t o e x p l a i n t h e e l e c t r o n i c s t r u c t u r e o f t h e s o l i d s . The c e n t r a l m e t a l i o n s o f p l a n a r t r a n s i t i o n m e t a l comp l e x e s r e s i d i n g i n columnar s t a c k s a r e exposed t o a tetragonally distorted octahedral ligand f i e l d (fig.l) e x e r t e d by t h e f o u r e q u a t o r i a l l i g a n d s a n d t h e two a d j a c e n t m e t a l i o n s i n t h e a x i a l p o s i t i o n s . The d e g r e e

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

EXTENDED

INTERACTIONS

BETWEEN

METAL

Figure 1. Energies of d-electrons in a onedimensional linear chain d -configurated transition metal complex 8

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

21.

Linear

ADERJAN ET AL.

Chain

317

Complexes

o f t e t r a g o n a l d i s t o r t i o n depends on t h e r e l a t i v e " s t r e n g t h " o f t h e a x i a l and e q u a t o r i a l l i g a n d g r o u p s . E q u a l s t r e n g t h o f a x i a l and e q u a t o r i a l l i g a n d s r e s u l t s i n an " o c t a h e d r a l " f i e l d w i t h d e g e n e r a t e d 2 - a n d d 2_ 2 - l e v e l s . I f intermolecular i n t e r a c t i o n s are i n c l u d e d bands a r e o b t a i n e d the w i d t h o f which depends u p o n t h e d e g r e e o f o v e r l a p b e t w e e n t h e one e l e c t r o n d f u n c t i o n s . The b e s t i n t e r m o l e c u l a r o v e r l a p i s a s s u m e d between the d 2 - f u n c t i o n s , w h i l e the o r b i t a l s locali­ zed m a i n l y i n the complex p l a n e are l e s s s u i t e d f o r i n t e r a c t i o n s and t h e r e f o r e g i v e v e r y s m a l l b a n d s . P a r ­ t i c i p a t i o n o f l i g a n d o r b i t a l s c o u l d be a d d e d e a s i l y i f n e c e s s a r y but would not g i v e a q u a l i t a t i v e l y d i f f e r e n t p i c t u r e . T h i s model c o u l d i n c o n t r a s t t o e a r l i e r pro­ posed energy l e v e l quate d e s c r i p t i o n o s o l i d s , b e c a u s e an a p p r e c i a b l e m i x i n g o f m e t a l s- and p - s t a t e s seems f a i r l y u n l i k e l y w i t h r e s p e c t t o t h e l a r g e e n e r g y d i f f e r e n c e s o f a b o u t 15 eV i n v o l v e d ( 5 ) . What k i n d o f c o n s e q u e n c e s c a n be d r a w n f r o m t h i s scheme? T h i s q u e s t i o n i s f i r s t a n s w e r e d i n c o n t e x t w i t h d ° - c o m p l e x e s . M o s t o f t h e known l i n e a r m e t a l c h a i n compounds w i t h d i r e c t m e t a l - m e t a l c o n t a c t s c o n ­ t a i n metal ions with 8 d-electrons (4). In t h i s case, a l l the bands a r e f i l l e d w i t h e l e c t r o n s w i t h the ex­ c e p t i o n o f the d 2 „ 2 - b a n d . T h i s model p r e d i c t s i f the t e t r a g o n a l d i s t o r t i o n i s a p p r e c i a b l e and t h e e n e r g y gap Λ Ε i s q u i t e l a r g e , a s e m i c o n d u c t i n g b e h a v i o u r a n d o n l y weak i n t e r m o l e c u l a r i n t e r a c t i o n s . On t h e o t h e r hand the model c o u l d e x p l a i n l d - m e t a l l i c b e h a v i o u r o f d ° - c o m p o u n d s as w e l l . S t r o n g l y e l e c t r o n a c c e p t i n g l i ­ gands w i l l d e c r e a s e the energy s e p a r a t i o n between the d 2a n d d 2 - y 2 - o r b i t a l s a n d t h e e n e r g y gap w o u l d v a n i s h i n a f o r m a l l y " o c t a h e d r a l " f i e l d . The former c a s e i s t h a t what i s a c t u a l l y f o u n d i n m o s t known l i ­ n e a r c h a i n d ° - m e t a l compounds. But t h i s model c l e a r l y h i n t s a t t h e s t r e n g t h e n i n g o f t h e p r e d i c t e d weak i n t e r ­ a c t i o n s i n d°-complexes with l a r g e Δ Ε . T h i s g o a l c o u l d be a c h i e v e d i f i t i s p o s s i b l e t o : i) d i m i n i s h t h e e n e r g y gap b e t w e e n t h e d 2 and d 2 - y 2 - b a n d s by u s i n g s u i t a b l e e l e c t r o n a c c e p t i n g ligands, ii) o x i d i z e t r a n s i t i o n metal complexes c r y s t a l l i z i n g i n c o l u m n a r s t a c k s , so t h e f i l l e d d 2 - b a n d w i l l be d e p o p u l a t e d a n d a p a r t i a l l y o c c u p i e d b a n d r e ­ s u l t s no m a t t e r how large ΔΕ i s , i i i ) s y n t h e s i z e l i n e a r c h a i n m e t a l complexes w i t h l e s s than 8 d-electrons per metal s i t e . x

y

z

x

z

y

x

z

x

z

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

318

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

III. tion

C h o i c e o f S p e c t r o s c o p i c Methods f o r I d e n t i f i c a of Co-operative Phenomena A f t e r h a v i n g s e t up r u l e s f o r s e l e c t i n g s u i t a b l e compounds t h e r e r e m a i n s t h e q u e s t i o n o f how t o i d e n t i fy the v a r i a t i o n of i n t e r m o l e c u l a r i n t e r a c t i o n s i n diff e r e n t c o m p l e x e s on a q u a n t i t a t i v e b a s i s . I t i s n e c e s s a r y t o f i n d s p e c t r o s c o p i c methods w h i c h a r e a b l e t o i d e n t i f y i n t e r m o l e c u l a r i n t e r a c t i o n s and t o c l a s s i f y t h e i r s t r e n g t h . The m e t h o d s s h o u l d w o r k o n p o l y c r y s t a l l i n e samples s i n c e the p r e p a r a t i o n o f l a r g e s i n g l e c r y s t a l s i s often very difficult. In g e n e r a l , the u s u a l l y s u c c e s s f u l s p e c t r o s c o p i c m e t h o d s : o p t i c a l , e . s . r . , n.m.r., e . g . c a n be u s e d t o i d e n t i f y c o l l e c t i v e e l e c t r o n b e h a v i o u r . E s p e c i a l l y obvious are the r e s u l t (M-M d i s t a n c e s ) an a n d n e a r i n f r a r e d r e g i o n . The i n t e r m o l e c u l a r i n t e r a c t i o n s c a u s e an i n t e n s e e l e c t r o n i c t r a n s i t i o n , w h i c h i n s i n g l e c r y s t a l s i s l i n e a r l y p o l a r i z e d p a r a l l e l to the d i r e c t i o n o f t h e m e t a l c h a i n s and o f t e n a p p e a r s as a s t r i k i n g a n i s o t r o p i c m e t a l l i c l u s t r e i n c a s e s where strong i n t e r a c t i o n s take place (1). T h e r e a r e two m e t h o d s n a m e l y n.m.r. a n d Moftbauer s p e c t r o s c o p y , w h i c h c a n and h a v e b e e n u s e d t o m e a s u r e e l e c t r o n d e n s i t y at the p o s i t i o n o f metal n u c l e i i n the c h a i n a n d s o s h o u l d be a b l e t o i d e n t i f y co-operat i v e e l e c t r o n i c i n t e r a c t i o n s . Results of these two methods a r e l a t e r d i s c u s s e d i n c o n n e c t i o n w i t h t h e irid i u m a n d p l a t i n u m c o m p o u n d s . O t h e r m e t h o d s ; e . s . r . ar^d n e u t r o n s c a t t e r i n g e x p e r i m e n t s h a v e b e e n u s e d by d i f f e r e n t groups t o examine the c o l l e c t i v e s t a t e i n l i n e a r c h a i n compounds. IV.

Choice

of

suitable transition

metal

compounds

What we h a d t o do f i r s t o f a l l was t o s y n t h e s i z e l i n e a r c h a i n m e t a l compounds w i t h m o d e r a t e i n t e r m o l e c u l a r i n t e r a c t i o n s , to vary t h e i r molecular c o n s t i t u t i o n s y s t e m a t i c a l l y and t o i n v e s t i g a t e t h e i r c h a n g i n g s o l i d s t a t e p r o p e r t i e s by s p e c t r o s c o p i c m e t h o d s . I t i s r e a s o n a b l e t o s t a r t w i t h known l i n e a r c h a i n t r a n s i t i o n m e t a l c o m p l e x e s , t h e s t r u c t u r e s o f w h i c h a r e p r o v e d by X - r a y a n a l y s i s . On t h e s e s e l e c t e d s p e c i e s a s y s t e m a t i c v a r i a t i o n o f l i g a n d p r o p e r t i e s by p r e p a r a t i v e chemical m e t h o d s c o u l d be a c h i e v e d w i t h o u t m a j o r s t r u c t u r a l c h a n g e s . The p r o p e r t i e s o f d ° - c o m p o u n d s w e r e i n v e s t i g a t e d f i r s t l y . The f o l l o w i n g t y p e s o f c o m p l e x e s w i t h l i n e a r m e t a l c h a i n s a n d 8 d - e l e c t r o n s p e r c e n t r a l met a l i o n a r e known:

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

21.

Linear

ADERJAN ET AL.

Chain

319

Complexes

1

T e t r a h a l o g e n o m e t a l a n i o n s o f t h e t y p e [M X i | ] n w i t h X = C l " , B r ~ , CN" l i k e [ P t ( C N ) i | ] 2 - ( 3 ) . 2 Complex-cations o f the typé[M L ^ ] w i t h L a monod e n t a t e n e u t r a l d o n o r l i k e NH3 or i s o n i t r i l e . The c a t i o n [Rh(CNR)i|] i s a s i m p l e example f o r t h i s g r o u p o f compounds ( 6 , 7 ) . 3 In m i x i n g both types o f complexes s t a c k s w i t h a l t e r n a t i n g c a t i o n s and a n i o n s a r e o b t a i n e d (Magnus G r e e n S a l t e.g.) (4, 8). 4 Planar d i c a r b o n y l m e t a l ( I ) s p e c i e s c o n t a i n i n g the m e t a l s r h o d i u m a n d i r i d i u m . To t h i s g r o u p b e l o n g t h e numerous d e r i v a t i v e s o f (Pentan-2,4-dionato) d i c a r b o n y l i r i d i u m ( I ) and r h o d i u m ( I ) (9, 10, 11) . A s i m p l e compound o f t h i s t y p e i s I r ( C 0 ) ^ C l ( 1 2 ) 5 Bis (oc,ft-diondioximato)metal(II 6 Bis(maleodinitriledithiolato)metal(II (13). O t h e r known s p e c i a l e x a m p l e s o f compounds w i t h c o l u m n a r s t r u c t u r e and d ^ - c e n t r a l m e t a l - i o n s a r e c l a s s i f i e d i n t o one o f t h e a b o v e g r o u p s . T h e r e a r e o n l y a few compounds known e a r l i e r w h i c h c o u l d be c l a s s i f i e d a s " m i x e d v a l e n c e " compounds w i t h d i r e c t m e t a l - m e t a l i n t e r a c t i o n s and a r e as f o l lows : The m o s t f a m o u s e x a m p l e s a r e t h e " m i x e d v a l e n c e " s o l i d s d e r i v e d from the p l a t i n u m ( I I ) complex i o n s n

+

+

1

[Pt(CN)2|]2- and

[Pt^OlOç] " 2

(3) .

H a l o g e n o c a r b o n y l i r i d i u m a n d p o s s i b l y r h o d i u m comp o u n d s s y n t h e s i z e d f i r s t l y by M a l a t e s t a ( 1 4 ) . P a r t i a l l y o x i d i z e d Bis (c*,&-diondioximato)metal(II) compounds c o n t a i n i n g a d d i t i o n a l l i n e a r i o d i n e c h a i n s (15).

V.

Systematic

Variation

of Ligand

Parameters

There are s e v e r a l well-known examples o f the var i a t i o n of metal-metal i n t e r a c t i o n s with the kind of l i g a n d . The compounds K 2 P t C l i | a n d i t s d e r i v a t i v e s may be u s e d a s a s i m p l e e x a m p l e o f t h e c h a n g e s w h i c h c a n be d o n e o n t h i s t y p e o f c o m p l e x e s a n d what s o l i d s t a t e c o n s e q u e n c e s w i l l be e x p e c t e d . The i n t e r m o l e c u l a r i n t e r a c t i o n s b e t w e e n t h e p l a t i n u m atoms a r e a l m o s t zero i n s o l i d I ^ P t C l i j . A l a r g e e n e r g y gap Δ Ε c a u s e s solid s t a t e p r o p e r t i e s t o show no s i g n o f c o l l e c t i v e e l e c ­ t r o n behaviour. T h e r e f o r e , the i n t e r m o l e c u l a r platinump l a t i n u m d i s t a n c e i s q u i t e l a r g e ( 4 , 13 A) ( 1 6 ) . How­ e v e r , as p o i n t e d out i n t h e d e s c r i b e d c r y s t a l f i e l d m o d e l , t h e i n t e r m e t a l l i c i n t e r a c t i o n s c a n be e n h a n c e d h e a v i l y by u s i n g more e l e c t r o n w i t h d r a w i n g l i g a n d s than c h l o r i n e ; e s p e c i a l l y π-acceptors ( e . g. cyano-groups).

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

320

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

I n a s i m p l e e n e r g y scheme t h i s r e s u l t s i n a lowering of the energy of the d 2-y2-band d i m i n i s h i n g the energy g a p . A c t u a l l y t h e P t - P t - d i s t a n c e s i n [Pt(CN)i|] compounds a r e c o n s i d e r a b l y d e c r e a s e d a n d some o f t h e s o l i d s t a t e p r o p e r t i e s o f compounds o f t h i s t y p e c l e a r l y show t h e e f f e c t o f a n i s o t r o p i c intermolecular i n t e r a c t i o n s ( 3 ) . Comparison of d i f f e r e n t s o l i d M [ P t ( C N ) i | ] o r M* [ P t ( CN ) 4 ] s p e c i e s p o i n t s o u t t h a t a t l e a s t one more i m p o r t a n t p a r a m e t e r b e s i d e s t h e " e l e c t r o n i c " f a c t o r h a s t o be c o n s i d e r e d , t h e s t e r i c r e q u i r e m e n t s o f t h e l a t t i c e e l e m e n t s . T h i s i s p r o v e d by the d é p e n d a n c e o f i n t e r m o l e c u l a r m e t a l i n t e r a c t i o n s on h y d r a t i o n sphere around the c a t i o n s (3)» F u r t h e r m o r e t h e i n t e r a c t i o n c a n be e n h a n c e d by adding e l e c t r o s t a t i ments i n s t e a d o f r e p u l s i o s t a c k s of P t C l j i . Magnus Green S a l t with a l t e r n a t i n g [Pt(NH3) ] a n d P t C l / j " u n i t s and P t - P t d i s t a n c e s of 3.25 A shows c o n s i d e r a b l y e n h a n c e d i n t e r m o l e c u l a r i n t e r a c t i o n s w i t h a n e n e r g y gap ΔΕ o f a b o u t 0.7 eV (17). B u t t h e s e compounds a r e n o t w e l l s u i t e d f o r o u r s p e c i a l purpose. In o r d e r to f i n d a r e l a t i o n between t h e e l e c t r o n a c c e p t i n g p r o p e r t i e s -of a l i g a n d a n d i t s s t e r i c a l r e q u i r e m e n t s on one s i d e a n d t h e d e g r e e o f i n t e r m o l e c u l a r e l e c t r o n e x c h a n g e on t h e o t h e r t h e l i ­ g a n d s h a v e t o be v a r i e d s y s t e m a t i c a l l y o v e r a w i d e r a n g e b u t i n s m a l l s t e p s . F o r t h i s r e a s o n we p r e p a r e d and i n v e s t i g a t e d a g r o u p o f i s o n i t r i l e d e r i v a t i v e s t h e t y p e [Pt (CNR)i|] [ P t C l i j ] w i t h R = a r y l a n d v a r i e d t h e l i g a n d s s y s t e m a t i c a l l y by u s i n g d i f f e r e n t s u b s t i ­ t u t e d i s o n i t r i l e s (18) . As e x p e c t e d on t h e b a s i s o f t h e s i m p l e m o d e l , e l e c t r o n w i t h d r a w i n g g r o u p s had an e f f e c t of strengthening the m e t a l - m e t a l i n t e r a c t i o n s and hence t h e c o - o p e r a t i v e p r o p e r t i e s w h i l e e l e c t r o n d o n a t i n g s u b s t i t u e n t s on t h e l i g a n d a r e w e a k e n i n g t h e s e i n t e r a c t i o n s . T h i s i s e a s i l y shown by studying t h e o p t i c a l p r o p e r t i e s o f t h e compounds. Complexes with strongly e l e c t r o n accepting groups, decrease of AE, a b s o r b s t r o n g l y i n the r e d p a r t o f t h e v i s i b l e s p e c t r u m and a r e deep b l u e o r v i o l e t i n t h e s o l i d s t a t e . Complexes w i t h d o n a t i n g i s o n i t r i l e groups a r e a b s o r b i n g i n t h e b l u e p a r t and a p p e a r y e l l o w f o r t h i s r e a s o n . By u s i n g b u l k y i s o n i t r i l e l i g a n d s t h e inter­ a c t i o n s b e t w e e n t h e m e t a l i o n s a r e d e c r e a s e d as w e l l but t h i s e f f e c t i s m i n o r compared t o the " e l e c t r o n i c " e f f e c t . Complexes o f t h i s t y p e , w i t h b u l k y and d o n a t i n g l i g a n d s , l o o k y e l l o w g e n e r a l l y w h i l e the ones w i t h s m a l l a c c e p t i n g l i g a n d s l o o k b l u e , show a t y p i c a l c o p ­ p e r - l i k e m e t a l l i c l u s t r e which i s l i n e a r l y p o l a r i z e d x

2

2

2 -

2 +

1

2

i |

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

21.

Linear

ADERJAN ET AL.

Chain

Complexes

321

p a r a l l e l to the needle a x i s of the c r y s t a l s (18). The same s y s t e m a t i c change i n s o l i d s t a t e p r o p e r t i e s w i t h the v a r i a t i o n i n e q u a t o r i a l l i g a n d s can be observed i n planar chelatodicarbonylrhodium(I) and i r i d i u m ( I ) compounds ( g r o u p 4) some o f w h i c h c r i s t a l l i z e i n c o l u m n a r s t a c k s ( 9 ) . The a n i s o t r o p i c p h y s i c a l p r o p e r t i e s of these s o l i d s r e s u l t i n g from intermolecul a r i n t e r a c t i o n s , w h i c h w e r e the s u b j e c t o f i n t e n s e i n v e s t i g a t i o n s s e v e r a l y e a r s ago ( 1 9 ) , c a n be v a r i e d by c h e m i c a l s y n t h e s i s . C o m p l e x e s o f t h i s t y p e , e. g . o f s t o i c h i o m e t r y I r ( C 0 ) 2 ( L - L ) , c a n be o b t a i n e d e a s i l y by s u b s t i t u t i n g one c a r b o n y l g r o u p a n d a c h l o r i d e l i g a n d i n I r ( C O ) - j C l . The s y n t h e s i s p r o c e e d s e s p e c i a l l y w e l l i f the t r i c a r b o n y l c h l o r o i r i d i u m ( I ) i s r e a c t e d with the " T r i t o n B" ( t r i p h e n y l b e n z y l a m m o n i u the l i g a n d (10). Some c o n c l u s i o n s u p o n t h e i n t e r m o l e c u l a r i n t e r a c t i o n s c a n be d r a w n by l o o k i n g a t t h e i r o p t i c a l a b s o r p t i o n s p e c t r a . The i n t e r a c t i o n s a r e d i m i n i s h e d w i t h i n c r e a s i n g s i z e o f t h e l i g a n d s a n d by s u b s t i t u t i n g d o n a t i n g g r o u p s and t h e absorption i s s h i f t e d t o s h o r t e r wave l e n g t h s . The e l e c t r i c c o n d u c t i v i t y i s d e c r e a s e d d r a s t i c a l l y i n compounds w i t h l a r g e o r g a n i c d y e b a s e s a s l i g a n d s ( 2 0 ) . I n some c a s e s w i t h v e r y l a r g e l i g a n d s e v e n m o d i f i c a t i o n c h a n g e s c a n be o b s e r v e d , w h i c h i n d i c a t e t h a t the i n t e r a c t i o n s are not very s t r o n g i n t h e s e s o l i d s and t h a t t h e l a t t i c e s t r u c t u r e i s d e t e r m i n e d by t h e l a r g e o r g a n i c l i g a n d m o l e c u l e s . The conc l u s i o n t h a t one o f t h e m o d i f i c a t i o n s crystallizes w i t h o u t e s s e n t i a l i n t e r m o l e c u l a r i n t e r a c t i o n s i s drawn from the d i f f e r e n c e s i n the o p t i c a l s p e c t r a of these s y s t e m s . The o p t i c a l a b s o r p t i o n s p e c t r a o f t h e two mod i f i c a t i o n s a r e q u i t e d i f f e r e n t . One o f t h e m shows a b r o a d band i n the " r e d " which i s t o t a l l y absent i n t h e s e c o n d m o d i f i c a t i o n , w h i c h no l o n g e r h a s s i g n s o f c o l l e c t i v e e l e c t r o n b e h a v i o u r (11) . We a r e i n t h e p r o c e s s of completing the X-ray s t r u c t u r e determinat i o n of the d i f f e r e n t m o d i f i c a t i o n s . The Môfèbauer e f f e c t i s a m o r e s e n s i t i v e s p e c t r o s c o p i c probe f o r the i d e n t i f i c a t i o n o f intermolecular i n t e r a c t i o n s and t h e more a c c u r a t e c l a s s i f i c a t i o n o f t h e i r s t r e n g t h . I t works q u i t e w e l l w i t h i r i d i u m (193i ) d shown by o u r r e s u l t s , t h e i s o m e r s h i f t s as w e l l as t h e q u a d r u p o l e s p l i t t i n g s d e p e n d s t r o n g l y on t h e s t r e n g t h o f t h e i n t e r m o l e c u l a r i n t e r a c t i o n s a l o n g the m e t a l c h a i n s ( 2 1 , 22). T h i s i s summarized i n f i g u r e 2 w h i c h shows t h a t a v e r y c o n s i s t e n t l i s t o f s h i f t s i s t obtained. Negative isomer s h i f t s , r e l a t i v e t o i r i d i u m m e t a l as t h e s t a n d a r d , a r e f o u n d i n s o l i d s r

a

n

a

s

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

322

Figure 2. hidentate,

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

Ir-Mossbauer shifts of Ir(L-L)(CO) and IrL'L(CO) compounds (l^L — uninegative, L = neutral, unidentate U = unidentate, uninegative ligand. Arrows connect different modifications of one compound). 193

2

t

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

21.

ADERJAN ET A L .

Linear

Chain

Complexes

323

w h i c h give c l e a r e v i d e n c e to strong intermolecular i n ­ t e r a c t i o n s i n t h e s o l i d s t a t e . The compounds w i t h on­ l y weak m e t a l - m e t a l i n t e r a c t i o n s show p o s i t i v e s h i f t s and q u i t e l a r g e q u a d r u p o l e s p l i t t i n g s . E s p e c i a l l y i n t e r e s t i n g a r e t h e d a t a o f t h e s e com­ p l e x e s w h i c h c r y s t a l l i z e i n two m o d i f i c a t i o n s , one of w h i c h o n l y c o n t a i n s c o l u m n a r s t a c k s . As e x p e c t e d , t h e i s o m e r s h i f t s o f t h e m o d i f i c a t i o n w i t h o u t any s i g n o f co-operative e l e c t r o n behaviour are l a r g e r than those o f the deeply c o l o r e d form c r y s t a l l i z i n g i n columnar s t a c k s . The q u a d r u p o l e s p l i t t i n g s a r e s m a l l e r f o r t h e compounds w i t h s t r o n g i n t e r m o l e c u l a r i n t e r a c t i o n s growing l a r g e r with d e c r e a s i n g i n f l u e n c e of the a x i a l l i g a n d s a s p r e d i c t e d by t h e s i m p l e c r y s t a l f i e l d mo­ d e l . By c o m p a r i n g t h Moftbaue result with thos f other spectroscopi bauer e f f e c t i s a w e l i r i d i u m complexes t o c l a s s i f y i n t e r m o l e c u l a r i n t e r a c ­ t i o n s i n l i n e a r chain metal complexes. A s i m i l a r r e l a t i o n between s o l i d s t a t e p r o p e r t i e s o n one s i d e a n d s t e r i c a l a n d e l e c t r o n i c l i g a n d p a r a m e ­ t e r s o n t h e o t h e r h a n d c a n be f o u n d i n t h e "dioximato" c o m p l e x s e r i e s . N u m e r o u s c o m p l e x e s o f d " i o n s w i t h these l i g a n d s were d e s c r i b e d i n t h e p a s t (4, 2 3 ) . E s p e c i a l l y s t r o n g i n t e r a c t i o n s i n these s o l i d s are found i n the Bis(1,2-benzoquinonedioximato)metal(II) s p e c i e s the l i g a n d o f w h i c h i s r e l a t i v e l y s m a l l and i s c a p a b l e of d e l o c a l i z i n g metal d-electrons (24). These examples prove t h a t even d°-ions i n l i n e a r m e t a l c h a i n complexes are capable o f s t r o n g i n t e r a c ­ t i o n s g i v i n g r i s e t o c o - o p e r a t i v e e l e c t r o n b e h a v i o u r as p o i n t e d o u t by i n v e s t i g a t i o n s o f t h e i r o p t i c a l s p e c t r a . E s p e c i a l l y t h o s e c a n d i d a t e s w h i c h have o n l y s m a l l and strongly e l e c t r o n accepting ligands are capable of v e r y s t r o n g i n t e r a c t i o n s a n d show c o l l e c t i v e e l e c t r o n b e h a v i o u r i n one d i m e n s i o n s u c h a s h i g h c o n d u c t i v i t y and l o n g w a v e l e n g t h a b s o r p t i o n s . T h e r e f o r e , i t seems p o s s i b l e to s y n t h e s i z e ld-metals even with ions" i f a p p r o p r i a t e l i g a n d s are used. I r ( C Q ^ C l which con­ s i d e r e d i n o u r o p i n i o n t o be a p u r e d ° - c o m p o u n d , i s a n i m p r e s s i v e e x a m p l e t h a t t h e e n e r g y gap ΔΕ c a n be r e ­ d u c e d v e r y much by u s i n g e l e c t r o n a c c e p t i n g g r o u p s a n d t h a t t h e d . c . - c o n d u c t i v i t y c a n be r a i s e d by u s i n g small ligands to almost m e t a l i c behaviour. Nevertheless the i n t e r m e t a l l i c i n t e r a c t i o n s i n a l l o f t h e s e d" t r a n s i t i o n m e t a l complexes a r e not s t r o n g enough t o overcome t h e s t e r i c a l p r o b l e m s and a d.c.-conductivity corresponding to a m e t a l l i c state has not b e e n a c h i e v e d . E s p e c i a l l y , i f u s i n g l a r g e and p o l a r i z e a b l e o r g a n i c dye m o l e c u l e s w h i c h a r e necessary

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

324

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

i n t h e s e a r c h f o r a " L i t t l e m o d e l compound" ( 2 5 , 26) the m o b i l i t y o f charge c a r r i e s i s decreased considera b l y . T h e r e f o r e we t r i e d t o s y n t h e s i z e new "mixed v a l e n c e " s o l i d s by o x i d i z i n g d ^ - c o m p o u n d s . VI.

Preparation

of

"Mixed V a l e n c e "

Solids

The r e s u l t s o f o x i d a t i o n r e a c t i o n s on d ^ l i n e a r c h a i n m e t a l c o m p l e x e s w e r e s t u d i e d t h o r o u g h l y Jbr t h e tetrakis(cyano)platinum(II)complexes a n d some o x a l a t o d e r i v a t i v e s . The f i r s t i n v e s t i g a t i o n s w e r e c a r r i e d o u t more t h a n 100 y e a r s ago a n d t h e r e s u l t s o f a l l o f t h e w o r k a r e w e l l known a n d a r e s u m m a r i s e d (3) » One i m p o r t a n t q u e s t i o n r e m a i n s t o be a n s w e r e d : "Why does the o x i d a t i o n u m ( I I ) compounds not p r o c e e d t o "mixed v a l e n c e " o n e - d i m e n s i o n a l m e t a l s ? " T h i s c a n be a n s w e r e d by i n v e s t i g a t i n g t h e a s s o c i a t i o n r e a c t i o n s i n s o l u t i o n , which lead to l i n e a r chains. This " p i l i n g u p " - r e a c t i o n i s e s s e n t i a l l y a donora c c e p t o r a s s o c i a t i o n between a square p l a n a r platin u m ( I I ) d o n o r and a s o l v a t e d p l a t i n u m ( I V ) a c c e p t o r . The r e a c t i o n i s p r e v e n t e d i f s o l v e n t m o l e c u l e s o c c u p y the " a x i a l p o s i t i o n s " i n the s t r o n g l y a c c e p t i n g platin u m ( I V ) m o i e t y , w h i c h c a n n o t be r e m o v e d by d o n a t i n g p l a t i n u m ( I I ) s p e c i e s . A chain i s obtained i f the a x i a l l y b o u n d s o l v e n t m o l e c u l e s c a n be s u b s t i t u t e d by t h e d o n a t i n g p l a t i n u m ( I I ) s p e c i e s , t h a t means, i f t h e following r e a c t i o n s proceed to a s s o c i a t e s : [CI · P t I V ( C N ) 4 · H 0 ] + [H 0 · P t ( H ) ( C N ) 4 · H 0] 1=* 2

2

2

[H 0 2

· Pt^

1 1

) (CN)

2

I V

· Pt (CN)2j

4

[H 0-Pt(H)(CN)4.Pt 2

[H 0-PtH(CN)i|.Pt

I V

2

I V

· H 0] "

(CN)4.H 0] " 2

(CN)

i |

2

+ H 0

2

2

+

2

CI";

+ [ H 0 · P t ( CN ) 4 * H 0 ] 2

2

2

.PtII(CN)i -H 0] " + 2H 0; 4

1 |

2

2

T h i s t y p e o f r e a c t i o n does t a k e p l a c e i n aqueous s o l u t i o n i n t h e c a s e o f [Pt (CN)ij] ~ and [ P t ( C 0 j j ) ] " i o n s b u t i s p r e v e n t e d i n t h e same s o l v e n t when u s i n g comp l e x e s o f the type P t C l ^ " o r [Pt ( N H j ) 4] , b e c a u s e e i t h e r the r e d u c e d form o f the o x i d i z i n g agent (mostl y h a l o g e n i d e i o n s ) or the s o l v e n t m o l e c u l e s are bonded v e r y c o v a l e n t l y o n t o t h e a x i a l p o s i t i o n s o f t h e platinum(IV) acceptor. I f t h i s assumption i s c o r r e c t , t h e o x i d a t i o n s h o u l d p r o c e e d t o " m i x e d v a l e n c e " comp o u n d s o n l y i f t h e r e a c t i o n c a n be c a r r i e d o u t i n n o n d o n a t i n g o x i d i z i n g s o l v e n t s . The medium c o u l d be h a l f concentrated s u l f u r i c a c i d . The r e d K P t C l j | , t h e y e l low P t ( N H - C H - C H - N H ) C l a n d t h e g r e e n Magnus S a l t c a n be o x i d i z e d t o d e e p l y c o l o u r e d compounds w i t h a 2

2

2

2

2

2 +

2

1

2

2

2

2

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

Â

21.

ADERjAN E T A L .

Linear

Chain

Complexes

325

remarkable red m e t a l l i c l u s t r e i n h a l f concentrated s u l f u r i c a c i d . C h e m i c a l and p h y s i c a l e v i d e n c e clearly p r o v e s t h a t t h e s e compounds a r e m i x e d v a l e n c e solids. S i n c e t h e r e a c t i o n i s a h e t e r o g e n o u s one a n d f o r t h i s r e a s o n o n l y p o w d e r s c o u l d be o b t a i n e d ( 2 7 ) t h i s procedure i s not a p p r o p r i a t e i n the s y n t h e s i s of l d - m e t a l l i c s i n g l e c r y s t a l s . We a r e s t i l l l o o k i n g f o r o t h e r b e t t e r s u i t e d s o l v e n t s and o x i d i z i n g a g e n t s . On t h e o t h e r h a n d we h a v e c o n t i n u e d o u r search f o r "mixed v a l e n c e " s o l i d s w i t h c e n t r a l m e t a l s o t h e r than platinum. In order to obtain p a r t i a l l y o x i d i z e d s o l i d s o n l y c o - o r d i n a t i o n compounds, t h e l i g a n d s o f w h i c h s h o u l d be s t a b l e a g a i n s t o x i d a t i o n by h a l o g e n s o r s i m i l a r s u b s t a n c e s c o u l d be u s e d a s s t a r t i n g m a t e r i a l s . As shown by gands are c h e m i c a l l n a r s t a c k s . " M i x e d v a l e n c e " s o l i d s i s o l a t e d by o x i d a t i o n o f B i s ( l , 2 - d i p h e n y l g l y o x i m a t o ) - n i c k e l ( I I ) and palladium(II) h a v e b e e n known f o r a l o n g t i m e b u t were not r e c o g n i z e d as s t r o n g l y i n t e r a c t i n g m e t a l c h a i n s . In f a c t the c r y s t a l s c o n t a i n stacks of pancaked metal c o m p l e x e s ( w i t h a n a v e r a g e o x i d a t i o n number o f 2.33 f o r t h e c e n t r a l m e t a l i o n ) and l i n e a r c h a i n s o f I j ~ u n i t s p a r a l l e l to these s t a c k s . A s t r u c t u r e very s i m i l a r t o t h e one f o u n d i n t h e b l u e s t a r c h - i o d i n e a d d u c t s was p r o p o s e d . The e s s e n t i a l d i f f e r e n c e i s t h a t b o t h s t a c k s c o u l d show c o l l e c t i v e e l e c t r o n b e h a v i o u r . This i s very i n t e r e s t i n g with respect to the p r e d i c t i o n t h a t m o b i l e e l e c t r o n s on two p a r a l l e l n o n - d e g e n e r a t e conduction spines could i n t e r a c t with each other i n a m a n n e r p r o p o s e d by L i t t l e f o r an e x c i t o n i c h i g h - t e m p e r a t u r e s u p e r c o n d u c t o r ( 2 8 ) . A s i m i l a r s o l i d c o u l d be o b t a i n e d by o x i d i z i n g b i s ( 1 , 2 - b e n z o q u i n o n e d i o x i m a t e ) p a l l a d i u m ( I I ) and n i c k e l ( I I ) i n d i c h l o r o b e n z e n e to mixed v a l e n c e s p e c i e s which d i f f e r i n t h e i r s o l i d s t a t e p r o p e r t i e s c o n s i d e r a b l y from t h e i r u n o x i d i z e d parent c o m p o u n d s . The r e s u l t s o f X - r a y a n a l y s i s s o f a r s u g g e s t t h a t l i n e a r m e t a l and i o d i n e c h a i n s r u n p a r a l l e l t h r o u g h t h e l a t t i c e o f t e t r a g o n a l s y m m e t r y a s shown s c h e m a t i c a l l y i n f i g u r e 3. F u r t h e r m o r e we h a v e u s e d a n o t h e r w e l l - k n o w n t y p e o f r e a c t i o n t o come t o m i x e d v a l e n c e s o l i d s : t h e s o c a l l e d o x i d a t i v e a d d i t i o n r e a c t i o n s . This type of r e a c t i o n c o u l d p o s s i b l y be u s e d f o r p i l i n g up l i n e a r met a l c h a i n s by a d d i n g a t e t r a k i s ( a r y l i s o n i t r i l e ) r h o dium(III) species to a t e t r a k i s ( a r y l i s o n i t r i l e ) rhod i u m ( I ) compound i n t h e f o l l o w i n g m a n n e r :

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

326

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

Figure 3. Schematic of two non-degenerate conduction spines in the lattice of bis(1,2-benzoquinonedioximato)iodonickel (II, IV)

^7 ^7 ^7

RNCRNC "

Rh I

CNR CNR

I RNC RNC

Rh

CNR • CNR

I Figure 4. Structure "mixed valence'' solid methylphenylisonitrile

proposed for the diiodotetrakis(2fi-di)rhodium(I III ). y

RNC RNC

I Rh

I

•CNR CNR

I RNC RNC

Rh

CNR CNR

I RNC RNC

I Rh I

I

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

- CNR • CNR

Figure

5.

Molecular structure cation in

of the μ-iodobis [tetrakis(phenylisonitrilejcobaltfll)] diiodotetrakis(phenylisonitrile)cobalt(II)

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

+

328

EXTENDED

2[Rh(CNR)Z|]+ +

INTERACTIONS

BETWEEN

METAL

IONS

+

[Rh(CNR)4l ] *==a 2

[I-Rh(CNR) -Rh(CNR)4-I]2+ i|

+

[Rh(CNR)i|]

[I-Rh(CNR) -Rh(CNR)i -Rh(CNR)i|-I]3 i |

+

+

|

By m i x i n g t h e 2 , 6 - d i m e t h y l p h e n y l i s o n i t r i l e r h o d i u m ( I ) and r h o d i u m ( I I I ) d e r i v a t i v e s i n c h l o r o f o r m a s u d d e n change o f c o l o r i s observed t u r n i n g from a y e l l o w - r e d t o d a r k b l u e . From t h i s s o l u t i o n a v i o l e t s o l i d w i t h a s t r i k i n g l i n e a r l y p o l a r i z e d m e t a l l i c l u s t r e c a n be o b ­ t a i n e d . W h e t h e r t h e compound c o n t a i n s d i r e c t l y b o u n d m e t a l - m e t a l c h a i n s o r an i o d i n e b r i d g e d M - I - M - I l a t t i c e c a n be d e c i d e d o n l y a f t e r t h e X - r a y determina­ t i o n has b e e n c o m p l e t e d . The p r e l i m i n a r y s p e c t r o s c o p i c d a t a i n d i c a t e an i o d i d e - b r i d g e cated schematicall F i n a l l y we w o u l d l i k e t o p r e s e n t j u s t one r e s u l t w h i c h we o b t a i n e d o n o u r s e a r c h f o r l i n e a r c h a i n com­ p o u n d s c o n t a i n i n g d 7 - m e t a l i o n s . A t y p i c a l and v e r y stable i o n with t h i s e l e c t r o n i c c o n f i g u r a t i o n i s co­ b a l t ( I I ) . The e a r l i e r r e p o r t o f M a l a t e s t a t h a t d i i o d o t e t r a k i s ( a r y l i s o n i t r i l e ) c o b a l t ( I I ) compounds c a n be i s o l a t e d i n two c r y s t a l l i n e m o d i f i c a t i o n s o f w h i c h one i s d i a m a g n e t i c a n d shows a r e m a r k a b l e m e t a l l i c l u s t r e (29) p r o m p t e d us t o d e t e r m i n e t h e e x a c t s t r u c t u r e o f one o f t h e s e compounds ( 3 0 ) . T h e m o l e c u l a r s t r u c t u r e o f t h e t e t r a k i s ( p h e n y l i s o n i t r i l e ) c o b a l t ( I I ) i s shown i n f i g u r e 5 which i s i n f a c t μ-iodo-bis [ i o d o t e t r a k i s ( p h e n y l i s o n i t r i l e ) c o b a l t ( I I ) ] i o d i d e . The l i n e a r l y p o ­ l a r i z e d o p t i c a l absorption of the s o l i d corresponds to a t r a n s i t i o n a l o n g t h e I - Co - I - Co - I h e a v y a t o m chains a l l o f which r u n p a r a l l e l t o each o t h e r . T h o u g h a p u r e l i n e a r c h a i n d^-compound w o u l d be i d e a l f o r our purposes t h e s y n t h e s i s o f such s o l i d s seems t o b e v e r y d i f f i c u l t b e c a u s e o f t h e s t r o n g L e w i s a c i d a c t i v i t y o f t h e s e compounds. Acknowledgements T h i s work was s u p p o r t e d by D e u t s c h e F o r s c h u n g s g e m e i n s c h a f t and Fonds d e r Chemischen I n d u s t r i e .

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

ADERJAN E T A L .

21.

Linear

Chain

Complexes

329

"Literature Cited," Zeller, H.R., A d v . Solid S t a t e P h y s . , ( 1 9 7 3 ) , Vol. X I I I 2 Garito, A.F., Heeger, A.J., (preprint) 3 K r o g m a n n , K., Angew. Chem., ( 1 9 6 9 ) , 8 1 , 10 4 Thomas, T.W., Underhill, A . E . , Chem. S o c . R e v . , 1

( 1 9 7 2 ) , 1, 99

5

Gitsel,

W.,

Keller,

Z.Naturforsch., 6 7

8 9 10

H.J.,

Lorentz,

R., Rupp, H.H.,

( 1 9 7 3 ) , 28b, 161

Dart, J.W., Lloyd, M.K., M a s o n , R., M c C l e v e r t y , J . A . , J.C.S. Dalton, ( 1 9 7 3 ) , 2039 Baumann, D., (1974),Dissertation, Univ.Heidelberg Mehran, F., Scott, B.A., Phys. Rev. Lett., (1973), 31, 99 Bailey, N.A., Coates, E.U., Robertson, G.B., Bonati, F . , U g o , R., Chem. Commun., ( 1 9 6 7 ) , 1041 Aderjan, R., Breer, H., Keller, H . J . , R u p p , H.H.,

( 1 9 7 3 ) , 28b, 164

Z.Naturforsch., 11

Aderjan, R., Keller, 28b, 500 12 Hieber, W., Lagally,

H.J.,

Z.Naturforsch.,

(1973),

H., M a y r , Α . , Z . A n o r g .

Allg.

Chem., ( 1 9 4 1 ) , 2 4 6 , 138

13 B e n s o n , R . E . , U.S. Patent, ( 1 9 6 6 ) , 3-255-195 14 Malatesta, L . , Canziani, F . , J. Inorg. Nucl. Chem.,

(1961), 19, 81

15

Keller,

H.J.,

Seibold,

K., J. Amer.

Chem. Soc.,

(1971), 93, 1309 Dickinson, G.E., J. Amer. Chem. Soc., ( 1 9 2 2 ) , 44, 2404 17 Fritz, H.P., Keller, H.J., Z.Naturforsch., (1965), 20b, 1145 18 Lorentz, R., ( 1 9 7 4 ) , Dissertation, Univ.Heidelberg 19 Monteith, K.L., Ballard, L . F . , Pitts, C.G., Klein, B.K., Slifkin, L.M., C o l l m a n , J . P . , Solid S t a t e

16

Comm., ( 1 9 6 8 ) , 6, 301 20

Aderjan, R., Keller, H . J . , R u p p , H.H., (submitted for publication) 21 Aderjan, R., ( 1 9 7 3 ) , Dissertation, Univ.Heidelberg 22 Aderjan, R., Keller, H . J . , R u p p , H.H., W a g n e r , F . , W a g n e r , U., (to be published) 23 B a n k s , C.V., B a r n u m , D.W., J. Amer. Chem. S o c . ,

( 1 9 5 8 ) , 80, 3579

24 E n d r e s , Η . , Keller, H.J., Megnamisi-Bélombé, M o r o n i , W., Nöthe, D., Inorg. Nucl. Chem.

( 1 9 7 4 ) , 10, 467

25 26

Μ., Lett.,

Little, W.A., P h y s . R e v . , ( 1 9 6 4 ) , A 135, 1416 Jagubskii, E . B . , Khidekel, M.L., R u s s . Chem. R e v . ,

( 1 9 7 2 ) , 41, 1011

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

330

27

EXTENDED

Gitzel,

W.,

Z.Naturforsch., 28 29

Keller,

INTERACTIONS B E T W E E N

H . J . , Rupp, H.H.,

J.

Seibold,

IONS

K.,

( 1 9 7 2 ) , 27b, 365

Little, W.A., (private communication) Malatesta, L., Isocyanide Complexes o f

(1969),

METAL

Wiley,

Metals,

London

30 Baumann, D., E n d r e s , Η . , Keller, J . C . S . Chem. Comm., ( 1 9 7 3 ) , 853

H . J . , W e i s s , J.,

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

22 Magnus Green Salt Solid Solutions Containing M i x e d Valence Platinum Chains: An Approach to 1-D Metals B. A. S C O T T , R. MEHRAN, a n d B. D . SILVERMAN IBM T. J. W a t s o n Research Center, Y o r k t o w n Heights, N.Y. 10598

M. A. RATNER N e w Y o r k U n i v e r s i t y , New York,

.Y.

Introduction The c l a s s o f i n o r g a n i c s o l i d s composed o f the partially o x i d i z e d platinum chain s a l t s are o f s p e c i a l i n t e r e s t to participants in t h i s Symposium, as they represent perhaps the most extended type o f i n t e r a c t i o n r e a l i z a b l e in the 1-D systems: that l e a d i n g t o f r e e c a r r i e r s and m e t a l l i c - l i k e c o n d u c t i v i t y . Such systems are t y p i f i e d by the compound K Pt(CN) Br .3H O(KCP). 2

4

0.3

2

In terms o f the known formal o x i d a t i o n s t a t e s o f platinum (+2,+4) the stoichiometry o f KCP can be w r i t t e n K [Pt(II) Pt(IV) (CN) ]Br .3H O. However, it is c l e a r 2

0.85

0.15

4

0.3

2

1

from Professor Krogmann's c r y s t a l structure f o r t h i s compound that all P t - s i t e s are equivalent and separated by the extremely short i n t e r m e t a l l i c d i s t a n c e o f 2.88 Å along the [001] s t a c k i n g a x i s . Thus, strong metal-metal i n t e r a c t i o n i n v o l v i n g the 5d 2 z

2

o r b i t a l s , which are the highest l y i n g occupied states , along the c-axis permits the system t o a t t a i n a f r a c t i o n a l , o r partially o x i d i z e d valence o f +2.3. The x-ray s t r u c t u r e shows that the charge compensating Br ions are l o c a t e d in the i n t e r s t i c e s between the [Pt(CN) ] s t a c k s . The i n t e r s t i c e s a l s o contain s i t e s 4

f o r the alkali ions and water molecules. Table I , taken i n p a r t from recent reviews , shows some of the considerable number o f known partially o x i d i z e d platinum chain s a l t s . All o f these s a l t s are composed o f tetracyano- o r b i s o x a l a t o - p l a t i n u m complexes. In the s t r u c t u r e s of these com3,4

3

pounds

the common f e a t u r e is the presence o f [Pt(CN) ] o r

Pt(Ox)

stacks comprised of c l o s e l y spaced Pt ions with

2

4

t i o n numbers between 2.26 and 2.40.

oxida-

Charge compensation occurs

through the presence o f halide ions in the s t r u c t u r e , or vacanci e s on the a l k a l i - c a t i o n s i t e s . 2.3

For KCP the o x i d a t i o n number o f

corresponds to 1.7 e l e c t r o n s per platinum; i.e., = 5/6 filling

331 In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

332

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

of the 5d 2 band. This c o n d i t i o n leads to m e t a l l i c - l i k e ζ e l e c t r i c a l c o n d u c t i v i t y near room temperature. The e l e c t r i c a l p r o p e r t i e s of KCP, as w e l l as the r e s u l t s of a l a r g e number of s o l i d s t a t e measurements, are reviewed by Dr. Z e l l e r i n t h i s Symposium volume and w i l l not be discussed here. Rather, our primary concern w i l l be the phase diagram of KCP and r e l a t e d sys­ tems shown i n Table I. Why do these compounds occur with s t o i ­ chiometrics corresponding to 1.6-1.74 5d 2 electrons/platinum? z

Is there a s p e c i a l s t a b i l i t y i n the degree of band f i l l i n g a s s o c i ­ ated with these e l e c t r o n concentrations? Answers to these ques­ t i o n s are important to s o l i d s t a t e chemists, p r o v i d i n g the r u l e s necessary to design high c o n d u c t i v i t y i n t o new 1-D metal chain systems. In d i s c u s s i n g ou h understandin th stoichiom e t r y of the 1-D m e t a l l i c - t y p plexes, i t i s u s e f u l to consider the o v e r s i m p l i f i e d "binary" phase diagram of the aqueous K P t ( I I ) ( C N ) ^ - K P t ( I V ) ( C N ) ^ B r s y s ­ tem at room temperature, as shown i n F i g . 1. Here the d i v a l e n t platinum complex i s denoted by II and the t e t r a v a l e n t complex designated IV. KCP occurs at a mole f r a c t i o n , x»0.15 i n t h i s system, corresponding to n ( d 2 ) = l . 7 and the o x i d a t i o n s t a t e 2.3. 2

2

2

z

For χ < 0.15 compound II and KCP c r y s t a l l i z e out of s o l u t i o n , whereas f o r χ > 0.15 KCÇ and IV c r y s t a l l i z e on evaporation of the s o l v e n t . A l l of the known p a r t i a l l y o x i d i z e d platinum s a l t s can be represented on a phase diagram such as F i g . 1. For example, the b i s o x a l a t o s a l t s of Table I can be designated on a phase diagram i n which II - ( c a t i o n ) P t ( I I ) ( O ^ O ^ and 2

IV = " P t ( I V ) ( C 0 ) 2

platinum. The dized compound 0.20. We term system " gi x

m

a

c

M

4

f f 2

,

i . e . , a pseudo-compound of t e t r a v a l e n t

s p e c i f i c composition at which the p a r t i a l l y o x i appears on the diagram ranges between χ = 0.13 to the exact composition of the phase f o r a given > because there appears to be a s p e c i f i c minimum,

or "magic" e l e c t r o n concentration at which the 1-D m e t a l l i c phase i s s t a b l e . The reasons f o r t h i s are not yet c l e a r , but t h e o r i e s 3 have been proposed and others w i l l be discussed i n t h i s as w e l l as subsequent papers i n the Symposium. In the present paper we wish to i n d i c a t e an a l t e r n a t i v e ap­ proach to the 1-D m e t a l l i c s t a t e which attempts to reach i n a way which we b e l i e v e provides some clues to the s t o i c h i o metry of the p a r t i a l l y o x i d i z e d platinum chain compounds. This approach i s i l l u s t r a t e d i n F i g . 2, d e p i c t i n g a h y p o t h e t i c a l phase diagram between a platinum(II) and a platinum(IV) compound. We r e q u i r e that II be an i n s u l a t i n g phase with a s t a c k i n g of P t ( I I ) ο

ions at a separation < 3.3 A, and f u r t h e r that the s t r u c t u r e of II contain vacant i n t e r s t i t i a l s i t e s f o r the i n t r o d u c t i o n of a

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

seoir E TAL.

22.

Magnus

Green

Salt

333

TABLE I P a r t i a l l y Oxidized Chain Compounds

K [Pt(CN) ]Cl 2

4

K [Pt(CN) ]Br 2

4

Pt-Pt Distance

-3H O

+2.32

2.880 A

0.16

1.68

-3H O

+2.30

2.887

0.15

1.70

-7H O

+2.28

2.985

0.14

1.72

Mg[Pt(CN) ]Cl 4

K

1 7 4

H

1 6 0

+2.40

2.80

0.20

1.60

L i

+2.36

2.81

0.18

1.64

K

+2.38

2.81

0.19

1.62

+2.36

2.82

0.18

1.64

0 e 2 8

+2.32

2.85

0.16

1.68

2

2

2

z

[Pt(CN) ]-1.8H 0 4

1 6 2

2

[Pt(C O ) ].2H O 2

1.64

4

[ P t ( C

2

2

0

)

]

2 4 2 -

6 H

2

0

[Pt(C 0 ) ].xH 0 2

(NH ) 4

Ba

0 3 2

0 e 3 ( )

n(d 2)

O x i d a t i o n Number of Platinum

Complex

0 8 4

1 6 4

4

2

2

[Pt(C 0 ) ].H 0 2

4

2

2

[Pt(C O ) ].4H O 2

4

2

2

n ( d 2 ) = number of 5d 2 e l e c t r o n s per platinum z

z

χ • mole f r a c t i o n composition parameter i n pseudo-binary P t ( I I ) - P t ( I V ) system. (See text) KCP«K Pt(CN) Br ·3Η 0 2

4

0 3

2

KCP

π +

KCP +TSL

KCP

0

π

0.15

1.0

0.5

is:

KgPtiCN^

K Pt(CN) Br 2

4

2

Figure 1. Oversimplified "binary" phase diagram between II = K Pt(CN), and IV = K Pt(CN)., Br in water at room temperature 2

t

£

t

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

EXTENDED

334

INTERACTIONS

BETWEEN

METAL

IONS

charge compensating ion i n t o the l a t t i c e . Thus, we wish to create a s o l i d s o l u t i o n system through the i n t r o d u c t i o n of P t ( I V ) com­ plexes i n t o the stacks"* with concomitant charge compensation i n the i n t e r s t i t i a l s i t e s . By analogy with the inorganic semicon­ ductor s i l i c o n doped with aluminum, we might expect II to t r a n s ­ form as follows with i n c r e a s i n g χ : I I ( i n s u l a t o r ) -> p-type semi­ conductor -* degenerate semiconductor - 1-D metal. We contend that the f i n a l transformation w i l l occur ( p o s s i b l y abruptly) at x = gi moment we neglect the i n f l u e n c e of d i s o r d e r , tf

x

e

m

a

F

o

r

ft

t n e

c

the very l i k e l y p o s s i b i l i t y of one or more m i s c i b i l i t y gaps (6 5 0 i n F i g . 2) and other perturbations which may turn o f f the m e t a l l i c conduction (e.g. P e i r e l s d i s t o r t i o n ) , and confine our­ selves instead to the f i r s t order problem: What are the s o l i d state factors s t a b i l i z i n e

f o r g e n e r a l i t y , a f i n i t e homogeneity range, γ * 0, f o r the 1-D metal phase i n F i g . 2. F i g . 2 would transform i n t o the s p e c i a l case of F i g . 1 i f γ = 0 and χ . = δ = 0.15. magic In the present study we have chosen II as Magnus green s a l t (MGS), P t ( N H ) P t C l , and IV = K P t ( C N ) Y (Y - CI,Br) and K«PtCl . The t e t r a g o n a l Magnus s a l t s t r u c t u r e i s shown i n F i g . 3. 1

3

4

4

2

4

2

6

II

I t i s c h a r a c t e r i z e d by the s t a c k i n g of square co-planar cations and P t C l " anions at a 3.24 4

Pt(NH^)

4

A separation along the c r y s -

t a l l o g r a p h i c c - a x i s , and the r e l a t i v e l y long i n t e r c h a i n Pt-Pt ° 6 distance of 6.35 A along the <110> d i r e c t i o n . Previous studies 7-9 suggested that the e l e c t r i c a l and o p t i c a l p r o p e r t i e s of MGS could be described by M i l l e r ' s model i n which f i l l e d 5d 2 metal ζ

bands are separated by £ 0.6 eV from empty 6p bands. Recent band c a l c u l a t i o n s ^ , c o n d u c t i v i t y ' " ^ and i n f r a r e d measurements ^ suggest that the 5d 2-6p gap i s ^ 4.5 eV and that the measured 8 9 ïl 12 —6 —2 —1 —1 conductivities, ' ' ' which range from 10 to 10 Ω -cm , 14 may be impurity dominated. In our EPR studies of MGS we have suggested that the s i g n a l i s due to a s e l f - t r a p p e d 5d 2 hole whose energy l e v e l may l i e ^ 0.6 eV above the top of the 5d 2 z

1

1 1

1

Z

z

z

valence bands. Such states are formally equivalent to those i n ­ troduced by the s u b s t i t u t i o n of P t ( I I I ) f o r P t ( I I ) i n the P t ( I I ) l a t t i c e and are introduced i n t o MGS due to the presence of Pt(IV) complexes during the preparation of the compound. This s i t u a ­ t i o n i s depicted i n the band model of F i g . 4, which may be con­ s i d e r e d our s t a r t i n g point and equivalent to a s o l i d s o l u t i o n 14 -4 with a very small value of χ (-10 ) i n the phase diagram of F i g . 2. In the present work we have found i t p o s s i b l e to i n ­ crease χ to f a i r l y l a r g e values and report the e f f e c t of such

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

22.

SCOTT

ET

Magnus

AL.

Green

Salt

335

β /-ID METAL

Br

—

ID

METAL + 12

1 SOLID ' Ι § SOLUTIONS ρ - 8 —

• wÊÊÊÊÊÊË Χ

magic

Figure 2. Hypothetical phase diagram between a Pt(H) and Pt(IV) compound illustrating the approach taken here for achiev­ ing the 1-D metallic state. Parameter δ is width of two-phase region, and γ is assumed homogeneity range for the 1-D metallic phase. In principle, many intermediate phases all separated by small immiscibility gaps could exist in such a system.

Figure PtCh, ' 2

3. Stacking ofPt(NH,) and ions in Magnus' green salt (6) /t

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2+

336

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

systematic v a r i a t i o n s i n the t o t a l e l e c t r o n concentration on the p h y s i c a l p r o p e r t i e s of the system. I t i s our contention, then, that there w i l l be a s p e c i a l χ » χ . f o r which MGS w i l l magic behave as a 1-D "metal". The approach to x sys­ tem, and i t s i m p l i c a t i o n s f o r the e n t i r e c l a s s of p a r t i a l l y o x i ­ d i z e d metal chain compounds, i s thus the main subject of t h i s work. r

i n

t n e

M

G

S

m a g i c

M a t e r i a l s Preparation A l l platinum s a l t s used i n t h i s study were prepared from 99.99% pure platinum metal by standard conversion techniques as 1 5

described by B r a u e r .

Samples of K P t C l 2

I ^ P t C l ^ and

6 >

e

K P t ( C N ) 3 H 0 were a l s 2

4

2

Mass.) f o r purposes of comparison. The r e s u l t s reported h e r e i n were found to be independent of m a t e r i a l s source. The Magnus s a l t s o l i d s o l u t i o n s (MGS(SS)) were prepared ac­ cording to the f o l l o w i n g r e a c t i o n scheme (1)

(a)Pt(II)(NH ) 3

+ + 4

+ (b)Pt(II)Cl

= 4

+ (c)Pt(IV)L Y 4

= 2

+ MGS(SS),

where the tetrammine platinum(II) complex was present as the CI or CIO. s a l t , K P t C l , was the anion source, and the molar r a t i o 4 2 4 (a):(b) = 1. S u c c e s s f u l i n c o r p o r a t i o n of higher valence Pt i n t o MGS was found to occur_un
4

2

4

r e a c t i o n , ( i i ) maintaining the s o l u t i o n concentrations i n the range 0.01-0.1 M with (c) < 0.5(a) = 0.5(b) and f i n a l l y , ( i i i ) r a p i d l y mixing the c a t i o n and doped-anion s o l u t i o n s . The r e s u l t i n g MGS(SS) i s metastable as i t i s not the expected phase under e q u i l i b r i u m c o n d i t i o n s . This i s because a complex redox e q u i l i b r i u m occurs during MGS p r e c i p i t a t i o n . For example, i f P t C l ~ i s e l i m i n a t e d from r e a c t i o n (1) and L = Y = C l , we f i n d 4

(2)

Ρ^(ΝΗ ) 3

4

+ PtCl

= 6

+

[Pt(NH ) Cl ]PtCl 3

4

2

4

Such r e a c t i o n s have been e x t e n s i v e l y analyzed i n the past where the platinum ( I I ) - ( I V ) redox p a i r are both present as anions (or c a t i o n s ) , and the r e s u l t i n g s u b s t i t u t i o n r e a c t i o n s are known to 1

occur r a p i d l y i n the presence of h a l i d e i o n . ^ For t h i s reason we have c a r r i e d out r e a c t i o n (1) i n the presence as w e l l as abscence of excess C l " , but t h i s had no d i s c e r n i b l e i n f l u e n c e on the degree of s o l i d s o l u t i o n formation. The point to be made, however, i s that a redox r e a c t i o n between the c a t i o n and p l a t i ­ num (IV) "dopant" i s a l s o occuring during MGS(SS) preparation un­ der the conditions of our experiments. I f such experiments are

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

22.

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Salt

337

c a r r i e d out under c o n d i t i o n s r e q u i r e d f o r the growth of s i n g l e c r y s t a l s - f o r example, by slow d i f f u s i o n of the components as i n a g e l t h e redox r e a c t i o n s have gone to completion p r i o r to c r y s t a l growth and "doping" proceeds under e q u i l i b r i u m c o n d i t i o n s , -4 y i e l d i n g only small concentration (x £ 10 ) of platinum(III) centers. For these reasons we are convinced that the i n i t i a l formation of h a l i d e bridged chains of the type P t ( I I ) . . . Y - P t ( I V ) L , - Y . . . P t ( I I ) , as i n the P t ( I I ) - c a t a l y z e d Pt(IV) 16 substitution reaction , i s e s s e n t i a l to the doping of MGS with high concentrations of P t ( I I I ) c e n t e r s . Experimental Results Provided that the MGS(SS) are f u l f i l l e d of s i n g l e phase s o l i d s o l u t i o n s ranging i n c o l o r from deep green f o r the undoped (x • 0) Magnus s a l t to blue f o r the most p a r t i a l l y o x i d i z e d (SS) phase. The m a t e r i a l s were then c h a r a c t e r i z e d by s e v e r a l techniques i n c l u d i n g chemical a n a l y s i s , e l e c t r o n s p i n r e s onance, x-ray d i f f r a c t i o n , IR spectroscopy and e l e c t r i c a l conduct i v i t y measurements. S o l i d S o l u t i o n Stoichiometry. Using G u i n i e r x-ray d i f f r a c t i o n techniques i t was e s t a b l i s h e d that the products of r e a c t i o n (1) were s i n g l e phase m a t e r i a l s having the s t r u c t u r e of Magnus green s a l t . This was done by searching f o r secondary phases on x-ray f i l m which had been overexposed to the primary MGS(SS) d i f f r a c t i o n pattern. The s e n s i t i v i t y of t h i s technique was found to be considerably b e t t e r than 1 mole% of second phase contamination as determined by a n a l y z i n g standard mixtures of MGS with other P t ( I I ) and Pt(IV) complexes. Chemical a n a l y s i s of the p r e c i p i t a t e d MGS(SS) powders f o r the case of K P t ( I V ) ( C N ^ B ^ dopant (oxidant) permits d i r e c t measurement of the degree of p a r t i a l o x i d a t i o n s i n c e both carbon and bromine determinations can be compared. This i s shown i n F i g . 5, where an e s s e n t i a l l y 1:1 c o r r e l a t i o n of incorporated platinum (based on the carbon a n a l y s i s ) to incorporated bromine i s observed. This suggests that Pt i s f o r m a l l y present as P t ( I I I ) , s i n c e t h i s would r e q u i r e compensation by j u s t one Br . Complete chemical a n a l y s i s of the products, moreover, i n d i c a t e s that the MGS i s P t C l " d e f i c i e n t . S i m i l a r r e s u l t s were obtained f o r K P t ( C N ) C l o x i d a t i o n of MGS, r e s u l t i n g i n the f o l l o w i n g formul a t i o n f o r the s o l i d s o l u t i o n s : 1

2

2

(3)

4

2

Pt(II)(NH ) (Pt(II)Cl ) 3

where X - B r " , C l " .

4

4

1 - q

(Pt(III)(CN) ) Y , 4

q

q

S i m i l a r l y , f o r doping (oxidation) with I ^ P t C ^

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

338

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METAL

ANION 6 p I

IONS

z

CATION 6 p

z

~4.5eV

Figure

I

I

1

1

4.

Schematic extrinsic

band structure MGS

y

1 /

ο

30 ο

-

/

ο

\y

Ο

- κ -

Ρ °

° y O

0

1

j

-

^

1 ,

I

1

I

I

10 20 30 MOLES (χ ΙΟ ) Br" INTRODUCED

40

3

Figure 5. Number of moles of Pt vs. Br introduced into MGS by solid solution formation (and partial oxidation) with K Pt(CN) Br . Potassium concen­ tration is 3 ± 2 mmoles for all samples shown. The two straight lines are the results expected for Pt:Br = 1:1 or 1:2. 2

/f

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

for

22.

seoir E TAL.

(4)

Magnus

Green

Salt

339

Pt(II)(NH ) (Pt(II)Cl ) 3

4

4

χ

(Pt(III)Cl > Cl . 4

q

q

The maximum values of q obtained f o r the v a r i o u s systems i s shown i n Table I I . I t should be noted that the parameter q as used i n the formulae f o r MGS(SS) systems i s a measure o f the de­ gree of s u b s t i t u t i o n of MGS a n i o n i c s i t e s by P t ( I I I ) . I t i s of course r e l a t e d , but not e q u i v a l e n t t o , the composition parameter χ of F i g s . 1 and 2. For comparison, the p a r t i a l l y o x i d i z e d Pt(+2.3) s t a t e occurs at χ = 0.15 i n F i g s . 1 and 2 and would be equivalent to q = 0.6 i n Formulae (3) and (4) i f an "averaged" o x i d a t i o n s t a t e were a p p r o p r i a t e f o r the MGS s o l i d s o l u t i o n s . This p o i n t w i l l be considered again i n the subsequent d i s c u s s i o n . I f the top of the 5d 2 valence bands are mostly of a n i o n i c z

( P t ( I I ) C l ~ ) c h a r a c t e r , as shown i n F i g . 4, then the 5d 2 hole i s 4

z

l o c a t e d on the P t C l (5)

4

group

Pt (II) ( N H ) ( P t (II) C l 3

4

4

) ^ (Pt (II) (CN) ) (Pt ( I I I ) C l ) Y . 4

4

q

q

E.P.R. powder data f o r s o l i d s o l u t i o n s of the type (4) gave the f o l l o w i n g g-tensor parameters: g., = 1.94 and g = 2.50. 14 These values are i d e n t i c a l to those measured f o r MGS(SS) i n which K P t ( C N ) C l or K P t ( C N ) B r i s the oxidant; Thus, EPR suggests that the P t ( I I I ) s i t e i s independent of o x i d i z i n g agent. Whether the 5d 2 hole r e s i d e s p r i m a r i l y on ( P t C l ) or P t ( N H ) 2

4

2

2

4

2

g

4

3

4

groups depends on the r e l a t i v e energies of the c a t i o n and anion bands, of course, and we choose the former s i t e to be c o n s i s t e n t with the assignments i n M i l l e r ' s o r i g i n a l model.^ Our formulation of Magnus s a l t s o l i d s o l u t i o n systems places charge compensating h a l i d e ions i n i n t e r s t i t i a l s i t e s i n the s t r u c t u r e . Our main evidence f o r t h i s c o n s i s t s of i n f r a r e d data and x-ray d i f f r a c t i o n measurements of the l a t t i c e constants of MGS(SS). The f a r i n f r a r e d spectrum of the K P t ( C N ) B r - o x i d i z e d 2

4

2

MGS product with q = 0.2 shows no evidence f o r a Pt-Br s t r e t c h band. Such a band would be expected f o r s u b s t i t u t i o n along the chain, or f o r charge compensation through replacement of one of the l i g a n d s w i t h i n the Pt square c o o r d i n a t i o n plane (e.g., r e - _^ placement of NH ). The changes i n i n f r a r e d p a t t e r n above 700 cm 3

with i n c r e a s i n g q values are shown i n F i g . 6. These s p e c t r a were taken f o r constant s o l i d c o n c e n t r a t i o n i n the KBr matrix, and show only the i n t r o d u c t i o n of one new band due to the C Ξ Ν 1

s t r e t c h i n g vibrâtion_near 2200 cm" . This i s c o n s i s t e n t with the replacement of P t C l ~ anions i n the MGS s t r u c t u r e with Pt(CN) ~~ 4

4

as determined by chemical a n a l y s i s . Unit c e l l data as a f u n c t i o n o f q f o r the K P t ( C N ) B r 2

4

2

oxi-

dized s o l i d s o l u t i o n s , shown i n F i g . 7, i n d i c a t e s expansion of

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

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METAL

TABLE I I Solid Solution Limits Dopant ( O x i d i z i n g Agent)

K Ptci 2

0.03

6

K Pt(CN) Cl 2

4

K Pt(CN) Br 2

4

0.08

2

0.20

2

See Eqs. ( 4 ) , ( 5 ) . WAVELENGTH (MICRONS) 3

3000

4

5

6

2000

7

1500

8

9

1200 cm

Figure

6. Infrared KgPtfCN^Brj.

10

II

1000 900

12

13

800

14

15

700

-I

spectra for MGS(SS) formed by partial oxidation Composition parameter q is defined in Equation 5.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

with

IONS

22.

seoir E TAL.

the MGS the C

q

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Green

Salt

341

c e l l along the t e t r a g o n a l a - a x i s and c o n t r a c t i o n along Q

stacking d i r e c t i o n .

Expansion

transverse to the l i n e a r

chains i s p l a u s i b l e f o r Br" i n t e r s t i t i a l s combined with the i n ­ t r o d u c t i o n of P t ( C N ) groups i n a n i o n i c l a t t i c e s i t e s , whereas 4

c o n t r a c t i o n along C i s c o n s i s t e n t with the attendant i n t r o d u c ­ t i o n of the s m a l l e r P t ( I I I ) ions i n the c h a i n s . In t h i s connec­ t i o n i t i s r e l e v a n t that a l l the p a r t i a l l y o x i d i z e d Pt s a l t s shown i n Table I have very short Pt-Pt spacings. Thus, our ob­ s e r v a t i o n of a decrease i n C f o r MGS(SS) i s e n t i r e l y expected Q

q

with p a r t i a l o x i d a t i o n ( i n c r e a s i n g q) o f t h i s phase. However, u n l i k e the m e t a l l i c - l i k e s a l t s o f Table I , we expect two d i f f e r ­ ent types of platinum s i t e s along the c h a i n s : a m a j o r i t y con­ s i s t i n g o f the P t ( I I ) f a c t , the expected d i s o r d e t h i s arrangement i s c l e a r l y observed i n the x-ray d i f f r a c t i o n as a l o s s and broadening of (00il) r e f l e c t i o n s with i n c r e a s i n g q. E l e c t r i c a l and O p t i c a l P r o p e r t i e s . The formation o f mixed valence s o l i d s o l u t i o n s through the i n t r o d u c t i o n of P t ( I I I ) has profound e f f e c t s on the p r o p e r t i e s of MGS. Measurements of the d.c. e l e c t r i c a l c o n d u c t i v i t y of powder compactions shown i n Table I I I show a four order of magnitude change i n σ as the com­ p o s i t i o n was changed from o s t e n s i b l y pure MGS (undoped, q = 0) to a s o l i d s o l u t i o n c o n t a i n i n g the maximum degree of s u b s t i t u t i o n , q = 0.20. Moreover, the h o l e i o n i z a t i o n e n e r g i e s , E determined &9

from the a c t i v a t i o n energies f o r e l e c t r i c a l conduction become s m a l l e r with i n c r e a s i n g q. Although the measurements were per­ formed on compacted powders, these general trends are c l e a r l y evident and q u i t e s i m i l a r to the behavior of doped three-dimen­ s i o n a l semi-conductors as the impurity concentrations are i n ­ creased · The o p t i c a l spectrum i n the near i n f r a r e d r e g i o n a l s o r e ­ v e a l s some i n t e r e s t i n g f e a t u r e s . These measurements were c a r r i e d out on samples d i s p e r s e d i n KBr p e l l e t s . F i g . 8 d e p i c t s the spec­ trum f o r two d i f f e r e n t "pure" (q = 0) MGS samples. Sample A was measured i n two separate pressings of the same p e l l e t to d e t e r ­ mine the v a r i a t i o n i n o p t i c a l d e n s i t y to be expected due to v a r ­ i a t i o n s i n sample preparatory technique. The s p e c t r a were mea­ sured with no r e f e r e n c e i n the standard compartment of the spec­ trometer. We a s c r i b e the absorption r i s e i n the 1-2.4μ r e g i o n of the spectrum mostly to s c a t t e r i n g , as there do not appear to be any d i s t i n c t bands i n t h i s range. On the other hand, F i g . 9 shows the a b s o r p t i o n i n the MGS(SS) system, prepared with K P t ( C N ) B r oxidant, f o r the range of compositions q = 0.05 to 2

4

2

0.2. The new band appearing i n t h i s frequency r e g i o n i s i n t e n s e , and d i r e c t l y p r o p o r t i o n a l to the c o n c e n t r a t i o n of P t ( I I I ) centers i n the sample as determined by chemical a n a l y s i s based on carbon

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

342

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METAL

IONS

9.04

5 10 MOLE % Pt INTRODUCED

Figure 7. Lattice constants vs. mole % platinum introduced for MGS(SS) prepared by K Pt(CN)j,Br . Constants obtained by least-squares refinement of Guinier camera powder data. 2

3.00|

1

Q00 4000 1

1

1

1

8000

1

1

1

1

1

1

12000

1

1

ι

1

16000

r

1

1

20000

1

24000

WAVELENGTH, Â Figure 8. Spectra of nominally pure MGS in the near IR. Curves A correspond to spectra obtained in two separate pressings of the same pellet; curve Β obtained from a separate MGS preparation made up to the same concentration in pellet as A.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2

22.

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ET

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AL.

Green

Salt

343

(CN) content. Excluding the q = 0.05 samples, whose chemical a n a l y s i s i s l e a s t r e l i a b l e , we f i n d an absorption c o e f f i c i e n t α = ( 6 . 0 + 0 . 3 ) χ 10 1/mol-cm, assuming the absorption i s en­ t i r e l y p o l a r i z e d along the c-axis of the c r y s t a l l i t e s ( i . e . , α » α ). (|

χ

In a d d i t i o n to the intense absorption peaking near 1.3μ, another important feature of F i g . 9 i s the gradual s h i f t of ab­ s o r p t i o n edge to lower energies with i n c r e a s i n g q. This i s sug­ gested by the general increase i n o p t i c a l d e n s i t y at 2.4μ as q i n c r e a s e s . Although we can not o b t a i n an accurate absorption edge from the p e l l e t measurements, t h i s q u a l i t a t i v e trend i s con­ s i s t e n t with the lowering of a c t i v a t i o n energy f o r e l e c t r i c a l conduction with q as shown i n Table I I I . I t i s r e l e v a n t to point out that e a r l y measurements of the IR spectrum of o s t e n s i b l the near IR region shown i n F i g . 9. Moreover, Collman e t . a l . reported photocurrent i n MGS with e x c i t a t i o n between 0.74-2.5μ. 13 Other measurements, i n c l u d i n g those on much purer MGS samples , 18 cast doubt on the presence of t h i s absorption, but i t i s c l e a r that samples prepared i n the presence of Pt(IV) complexes d e f i ­ n i t e l y c o n t a i n the broad IR peak. We have i n f a c t found the iden­ t i c a l IR band i n MGS(SS) samples produced by K P t ( C N ) C l and K P t C l ^ o x i d a t i o n . However, we i n t e r p r e t the absorption as due to t r a n s i t i o n s from the 5d 2 valence band to the 5d 2 bound ζ ζ s t a t e s , r a t h e r than to an interband t r a n s i t i o n . ' 2

4

2

2

?

8

Discussion I t i s worthwhile to review some of the p r o p e r t i e s of the mixed valence MGS s o l i d s o l u t i o n s before examining t h e i r r e ­ l a t i o n s h i p to the c l a s s of p a r t i a l l y o x i d i z e d platjinum s a l t s of Table I such as KCP. P r e v i o u s l y , we found from EPR measure14 ments that " e x t r i n s i c " MGS c r y s t a l s u s u a l l y c o n t a i n small con­ c e n t r a t i o n s of bound 5d 2 holes ( P t ( I I I ) ) l y i n g ^ 0.6 eV above the 5d 2 o r b i t a l s . The P t ( I I I ) holes whose occurence i s due to the presence of Pt(IV) complexes during sample p r e p a r a t i o n , have a f a i r l y wide extent-over at l e a s t s e v e r a l metal chain l a t t i c e 14 sites . In these s t u d i e s we have been able to s i g n i f i c a n t l y i n ­ crease the concentration of P t ( I I I ) centers by p a r t i a l o x i d a t i o n with Pt(IV) complexes, r e s u l t i n g i n as many as one out of ten P t ( I I ) chain s i t e s replaced by P t ( I I I ) f o r the case of o x i d a t i o n with K P t ( C N ) B r (Table I I , q - 0.2). Concomitant with the i n ­ c r e a s i n g P t ( I I I ) concentration i s an increase i n d.c. c o n d u c t i v i t y and the appearance of a strong IR t r a n s i t i o n due to 5d 2 band to z

2

4

2

z

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

EXTENDED INTERACTIONS

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BETWEEN

METAL

TABLE I I I E l e c t r i c a l Data on Powder Compactions ((IV)

a

= K Pt(CN) Br ) 2

R T

4

, (ohm-cm)"

2

Ε

1

q

, eV a'

0.0

3.7 χ 1 0 ~

7

0.66

0.12

2.5 χ 1 0 "

4

0.27

0.20

6.3 χ 1 0 ~

3

0.15

>gl 4000

ι

ι 8000

ι

ι

I

ι

12000

16000

1

1 20000

1 — -J 24000

W/VELENGTH, Â

Figure 9. Near IR spectra for MGS(SS) prepared from Br . Composition parameter q defined in Equation results were obtained for partial oxidation of MGS with CL and K PtCl . 2

z

K Pt(CN) Similar K>Pt(CN) É

5.

r

r

r>

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

22.

SCOTT

ET

Magnus

AL.

5d 2 h o l e e x c i t a t i o n . z

Green

Salt

345

A c t i v a t i o n energies obtained from the d.c.

c o n d u c t i v i t y measurements i n d i c a t e a decrease i n Ε , the h o l e i o n ­ i z a t i o n energy. In what f o l l o w s we w i l l consider the p o t e n t i a l the MGS system has f o r e x h i b i t i n g a t r a n s i t i o n to a "1-D metal" at dopant l e v e l s higher than we have been p r e s e n t l y able to achieve and f u r t h e r i m p l i c a t i o n s concerning the s t a b i l i z a t i o n of known 1-D metals such as KCP at s p e c i a l s t o i c h i o m e t r i e s . By c o n s i d e r i n g the low h o l e c o n c e n t r a t i o n l i m i t , one can more c l e a r l y understand j u s t what might be r e s p o n s i b l e f o r a t r a n s i t i o n to m e t a l l i c behavior at higher hole c o n c e n t r a t i o n . I f we e n v i s i o n a f u l l band (the MGS l i m i t ) and introduce a few l o c a l c a r r i e r s by s u b s t i t u t i o n , then the c a r r i e r , i n t h i s case a d 2 h o l e , w i l l not ζ * be f r e e l y mobile, but w i l l i n s t e a d be r a t h e r t i g h t l y bound to i t s site. This b i n d i n g ma between the P t ( I I I ) hol s u l t i n the formation of an e x c i t o n i c or quasibound p a r t i c l e - h o l e 19 s t a t e r a t h e r than i n a f r e e c a r r i e r . This coulomb p a r t i c l e - h o l e a t t r a c t i o n can however be reduced i n s e v e r a l ways. As the concen­ t r a t i o n of dopant and d e n s i t y o f c a r r i e r s i n c r e a s e s , the s i n g l e c a r r i e r p o t e n t i a l r e s u l t i n g from the charge compensators w i l l tend to be screened. This mechanism, analogous to the Mott mechanism i n higher dimensional i s o t r o p i c conductors, should l e a d to a semi­ conductor-metal t r a n s i t i o n . A l s o , with an i n c r e a s e i n doping, the d e n s i t y of charge compensators which an i n d i v i d u a l d 2 hole sees w i l l become e f f e c t i v e l y constant, i . e . independent of the s i t e on which the hole f i n d s i t s e l f . This w i l l occur s i n c e the o v e r l a p ­ ping coulomb t a i l s from the impurity or compensator d i s t r i b u t i o n w i l l average out the one e l e c t r o n (hole) p o t e n t i a l f l u c t u a t i o n s more e f f e c t i v e l y at high c o n c e n t r a t i o n s . For the case of the KCP l i m i t , there are ^ 0.3 h a l i d e ions f o r each P t . We suggest that f o r KCP i t i s t h i s s t o i c h i o m e t r i c r a t i o , corresponding to the composition g ^ 0.15 i n our phase diagram ( F i g . 1) at which the l o c a l e x c i t o n s t a t e goes over to the m e t a l l i c s t a t e . The f a c t that t h i s r a t i o occurs c o n s i d e r a b l y below a P t ( I I I ) / P t ( I I ) r a t i o of u n i t y comes about because the h o l e s , w h i l e l o c a l i z e d , are i n extended e i g e n s t a t e s which reach s e v e r a l Pt s i t e s along the c h a i n . Such i n t e r p r e t a t i o n can be i n f e r r e d from the EPR spec­ t r u m * ^ as w e l l as the l a t t i c e r e g u l a r i t y . This i n d i c a t e s that the hole-anion i n t e r a c t i o n reduced by h o l e - h o l e i n t e r a c t i o n s and smoothed due to i n c r e a s e d h a l i d e c o n c e n t r a t i o n could r e s u l t , at s u f f i c i e n t l y high dopant l e v e l , i n a s i n g l e hole wave f u n c t i o n with a coherence length s u f f i c i e n t l y l a r g e to r e s u l t i n m e t a l l i c l i k e behavior. I t i s i n t e r e s t i n g to note that f o l l o w i n g M o t t s o r i g i n a l a r ­ gument, the dopant d e n s i t y ^ 0.X5 i s i n near agreement with 9

z

x

m

2

a

i

c

1

1 , 3

1

x

m

a

g

i

c

1

the o r i g i n a l Mott c r i t e r i o n f o r the t r a n s i t i o n . Following Mott s o r i g i n a l argument, based on Thomas-Fermi screening of the

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

346

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

c a r r i e r s , the semiconductor -> metal t r a n s i t i o n f o r impurity con­ d u c t i o n i n a three dimensional system should occur approximately when n ^ /

3

a^ *υ .25.

As Mott and Twose p o i n t o u t , ^

1

a sharp break

i n the c o n d u c t i v i t y of both η-type and p-type germanium occurs i n 1/3 f a c t f o r n^ a^ ^ .3. In these expressions, i s the c a r r i e r d e n s i t y and a^ i s the e f f e c t i v e s i t e r a d i u s . I f we take a as 1/3 one-half of the Pt-Pt spacing along the c h a i n , then n<, a^ *\* .3 at the KCP s t o i c h i o m e t r y ; thus our suggestion of the r o l e of the c r i t i c a l stoichiometry i s i n accord with the o r i g i n a l Mott c r i t e r i o n f o r the t r a n s i t i o n . The dopant d e n s i t y , however, c a l c u ­ l a t e d i n t h i s manner must be taken with some r e s e r v a t i o n . F i r s t , the short distances tha ion-Pt s i t e s e p a r a t i o n macroscopic d i e l e c t r i c constant. Second, the extreme anisotropy of the system as w e l l as the f a c t that the h a l i d e ions are l o c a t e d o f f the Ρt chain w i l l introduce s i g n i f i c a n t m o d i f i c a t i o n s i n t o any screening c a l c u l a t i o n . I t i s however i n t e r e s t i n g ( i f not f o r t u i ­ tous) that the value one obtains from the Mott type estimate i s so c l o s e to what i s observed. At the maximum s o l i d s o l u t i o n l i m i t found i n our MGS system, q = 0.2, the holes s t i l l appear to be bound so that t r a n s p o r t i s an a c t i v a t e d process. I t i s t h e r e f o r e evident that at t h i s com­ p o s i t i o n , which i s e q u i v a l e n t to χ = q/4 = 0.05 i n the t e r m i n o l ­ ogy of the phase diagrams of Figures 1 and 2, the doping i s s t i l l not high enough to p r e c i p i t a t e an e l e c t r o n i c phase t r a n s i t i o n i n ­ to the 1-D m e t a l l i c s t a t e , i . e . χ i s s t i l l l e s s than χ . for ' magic t h i s system. With i n c r e a s i n g χ we can expect such a t r a n s i t i o n to occur f o r MGS. KCP and the remaining p a r t i a l l y o x i d i z e d metal­ l i c phases of Table I have c r y s t a l l i z e d already at t h i s s t o i c h i o m ­ x e t r y and as can be appreciated from the ranges m a ^ c 2? 0· 13-0.20 i n t h i s t a b l e , the p r e c i s e value of χ , w i l l be very system dependent. Regardless of the s p e c i f i c e l e c t r o n i c model r e q u i r e d to un­ derstand the stoichiometry of the 1-D m e t a l l i c Pt s a l t s , we may ask why KCP (and s i m i l a r compounds of Table I) do not show a range of s o l i d s o l u t i o n s from χ = 0 to χ . . That i s , why the magic d i f f e r e n c e between MGS, which i s f o l l o w i n g the phase diagram of x F i g . 2, with s o l i d s o l u t i o n s between χ = 0 to χ = 0.05 < m a g i c f and the m e t a l l i c p a r t i a l l y o x i d i z e d systems of F i g . 1? Taking KCP as an example, there appear to be two main reasons f o r the d i f f e r 8

o

ence:

the r a t h e r l a r g e metal-metal d i s t a n c e (3.50 A) i n the

end

member K P t ( C N ) ^ · 3 Η 0 , and the apparent l a c k of a l a r g e enough i n ­ 2

2

t e r s t i t i a l s i t e f o r the s u b s t i t u t i o n of a charge compensating ha-

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

22.

SCOTT

ET

Magnus

AL.

Green

Salt

347

Z

lide ion. This l a s t requirement may not be a necessary c o n d i ­ t i o n f o r mixed valence s o l i d s o l u t i o n formation, s i n c e a l k a l i c a t i o n vacancies can f u l f i l l the l a t t e r r o l e . However, a r e l a ­ t i v e l y short Pt-Pt d i s t a n c e , as found i n MGS, may be a necessary c o n d i t i o n f o r the s t a b i l i z a t i o n of the t r i v a l e n t P t ( I I I ) s t a t e , s i n c e EPR i n d i c a t e s that even at very low concentrations these s t a t e s extend over s e v e r a l l a t t i c e s i t e s . L a t t i c e energy may a l s o play an important r o l e . There i s considerable evidence from o r 23 ganic systems that separate s t a c k i n g ("segregated" stacks) of c a t i o n s and anions does not provide h i g h Madelung energy. Thus, segregated stack-type end members (II) such as I^Pt (CN) *3H 0 may be considerably d e s t a b i l i z e d by (SS) formation, whereas the MGStype of c r y s t a l s are not, due to the extremely high l a t t i c e ener4

2

gies of a l t e r n a t e c a t i o reasons the two phase system of F i g . 1 i s more s t a b l e i n the cyanoplatinates than the s o l i d s o l u t i o n s of F i g . 2, and the par­ t i a l l y o x i d i z e d form occurs only when χ , i s reached. magic Since we have yet to completely transform MGS i n t o a 1-D met­ a l , i t i s r e l e v a n t to i n q u i r e whether i t i s p o s s i b l e to complete the process of continuously i n t r o d u c i n g 5d 2 h o l e s , with charge compensation, i n t o the l a t t i c e u n t i l g^ reached. Our ex­ perimental work suggests that s o l u t i o n redox r e a c t i o n s are the l i m i t i n g f e a t u r e of the s u b s t i t u t i o n process, and not c r y s t a l chemical f a c t o r s . For example, the s o l i d s o l u t i o n region i s l a r ­ ger i n MGS p a r t i a l l y o x i d i z e d with K P t ( C N ) B r (q « 0.20) than with K P t ( C N ) C l (q = 0.08). T h i s i s the reverse of the extent J

z

x

i s

m a

c

2

2

4

4

2

2

of_(SS) expected on the b a s i s of c r y s t a l chemical f a c t o r s , s i n c e CI i s smaller than Br . However, the greater r a t e of K Pt(CN) Br 2

4

2

vs. K P t ( C N ) C l 2

4

4

2

1

r e d u c t i o n by P t i N H ^ " "

suggests

that the s o l u t i o n redox r e a c t i o n i s the important feature to be c o n t r o l l e d i f more extensive s o l i d s o l u t i o n i s to be a t t a i n e d . There are s e v e r a l other compounds i n which mixed valence s o l ­ i d s o l u t i o n formation may be p o s s i b l e . Among these are d i c h l o r o (ethylenediamine) p l a t i n u m ( I I ) , P t ( e n ) C l , i n which n e u t r a l 2

P t ( e n ) C l u n i t s stack with an i n t e r m e t a l l i c s e p a r a t i o n of ° 25 9

3.39

A.

The a l k a l i n e e a r t h cyanoplatinates form a more i n t e r ­ 3

e s t i n g group, s i n c e many of them c r y s t a l l i z e ^ with P t ( I I ) - P t ( I I ) ο

separations below 3.3 A. In p a r t i c u l a r , MgPt(CN) ·7Η 0 e x h i b i t s at Pt-Pt d i s t a n c e of 3.16 A and the s t r u c t u r e appears c l o s e l y r e ­ l a t e d to that of the 1-D m e t a l l i c MgPt(CN) Cl * 7 H 0 , according to Krogmann. ^ Thus, there e x i s t s the strong p o s s i b i l i t y of ex­ t e n s i v e s o l i d s o l u t i o n i n t h i s system between χ = 0 and 4

4

2g

2

2

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

348

X

EXTENDED

magic

=

1 Ζ

°· * (

δ =

°>

F i

2

8- ) ·

INTERACTIONS

BETWEEN

METAL

IONS

The Pt s i t e equivalence present

i n these s t r u c t u r e s a l s o makes them a t t r a c t i v e , s i n c e the i n e q u i v ­ alence of c a t i o n and anion s i t e s i n MGS (hence strongest mixing between a l t e r n a t e i d e n t i c a l s i t e s ) may u l t i m a t e l y be the f a c t o r l i m i t i n g the attainment o f g^ * that system. x

n

m a

c

Acknowledgements The authors wish to thank Drs. F. Kaufman and J . B. Torrance f o r p r e l i m i n a r y IR data and many u s e f u l d i s c u s s i o n s . We are g r a t e f u l to Mr. A. R. Taranko f o r the o p t i c a l data, and Mr. F. Dacol f o r help with the e l e c t r i c a l measurements. We a r e a l s o indebted to Mr. B. Olson and Mr. B. L. G i l b e r t f o r t h e i r t i r e l e s s e f f o r t s i n working out the chemical a n a l y t i c a l proce­ dures . L i t e r a t u r e Cited 1. 2. 3. 4. 5.

6. 7.

8.

9. 10. 11. 12. 13. 14.

Krogmann, K. and Hansen, H. D., Z e i t s . anorg. allg. Chem., (1968), 358, 67. Mehran, F. and S c o t t , Β. Α., Phys. Rev. L e t t . , (1973), 31, 1347. Krogmann, Κ., Angew. Chem., I n t . Ed., (1969), 8, 35. Thomas, T. W. and U n d e r h i l l , A. E., Chem. Soc. Revs., (1972), 1, 99. Such a solid s o l u t i o n should i d e a l l y occur with the e l i m i n a ­ t i o n of the trans l i g a n d s of the Pt(IV) complex, as will be discussed subsequently. A t o j i , Μ., Richardson, J.W. and Rundle, R. E., J.A.C.S., (1957), 79, 3017. M i l l e r , J. R., J. Chem. Soc., London 1965, 713; Day, P., Orchard, A. F., Thomson, A. J . and W i l l i a m s , R. J. P., J . Chem. Phys., (1965), 42, 1973. Collman, J . P., B a l l a r d , L. F., Monteith, L. K., Pitt, C. G. and Slifkin, L., in I n t e r n a t i o n a l Symposium on Decomposition o f Organometallic Compounds to Refractory Ceramics, Metals, and Metal A l l o y s , e d i t e d by K. S. Mazdiyasni (Univ. o f Dayton Research I n s t i t u t e , Dayton, Ohio, 1968), pp. 269-283. Gomm, P. S., Thomas, T. W. and U n d e r h i l l , A. E., J. Chem. Soc. A 1971, 2154. I n t e r r a n t e , L. V. and Messmer, R. P., Inorg. Chem., (1971), 10, 1174. I n t e r r a n t e , L. V., J. Chem. Soc., Chem. Commun., (1972), 6, 302. Rao, C. N. R. and Bhat, S. Ν., Inorg. Nucl. Chem. L e t t . , (1969), 5, 531. Fishman, E. and I n t e r r a n t e , L. V., Inorg. Chem., (1972), 11, 1722. Mehran, F and S c o t t , Β. Α., Phys. Rev. L e t t . , (1973), 31, 99.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

22. 15. 16. 17. 18. 19.

SCOTT E T A L .

Magnus

Green

Salt

349

Brauer, G., Handbuch der Präparaten Anorganishen Chemie, V o l . 2, Ferdinand Enke V e r l a g , S t u t t g a r t , 1962. Mason, W. Roy, Coor. Chem. Revs., (1972), 7, 241. I n t e r r a n t e , L. V., and Bundy, F. P., Inorg. Chem., (1971), 10, 1169. Day, P., Orchard, A. F., Thomson, A. J . and W i l l i a m s , A.J.P., J . Chem. Phys., (1965), 43, 3764. I f the h o l e becomes l o c a l i z e d on the P t - c h a i n in the vicinity of an o f f - c h a i n charge compensator, a n o n - a x i a l l y symm e t r i c EPR s i g n a l would be expected, contrary to that ob­ served.14 However, our approach i s q u a l i t a t i v e and so we choose the present p i c t u r e o f a quasi-bound particle-hole s t a t e r a t h e r than the s e l f - t r a p p e d h o l e model in our discussion. For a review o f th Revs. Mod. P h y s i c s Mott, N.F. and Twose, W. D., Adv. P h y s i c s , (1961), 10, 107. Monfort, F., B u l l . Soc. Roy. Sci. Liége, (1942), 11, 567. Metzger, R. Μ., J. Chem. Phys., (1972), 57, 1876, 2218. M i l l e r , J. R., J. Chem. Soc., (1961), 4552. M a r t i n , Don S, Hunter, LeRoy D., Kroening, R. and Coley, Ronald F., J. Am. Chem. Soc., (1971), 93, 5433. Krogmann, K. and Ringwald, G., Z e i t s . Naturforschg., (1968), 23b, 1112. 14

20. 21. 22. 23. 24. 25. 26.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

23 Studies o n Some 1-D Metal G r o u p VIII Complexes K.

KROGMANN

Institut für A n o r g a n i s c h e C h e m i e H . P.

GESERICH

Institut für a n g e w a n d t e Physik, Universität K a r l s r u h e , D 7500 K a r l s r u h e 1, G e r m a n y

The a b b r e v i a t i o n "TCNQ compounds" stands f o r a group of organic substances with s t r o n g l y a n i s t r o p i c behavior i n conductivity, reflectivity, and other p r o p e r t i e s . A s i m i l a r abbreviation f o r an i n o r g a n i c group of compounds with s i m i l a r p r o p e r t i e s i s still l a c k i n g besides "KCP" which stands f o r only one compound. I am tempted t o propose TCMP (= t e t r a coordinated metal plane) f o r t h i s group of square planar complex compounds. In both cases, the l e t t e r s give a short hand notation of the e s s e n t i a l s t r u c t u r a l u n i t s , and t h e i r s i m i l a r i t y r e f l e c t s the similarity of t h e i r properties. There are two types of TCMP compounds, the s t o i c h i o m e t r i c ones with c e n t r a l atoms i n i n t e g r a l o x i d a t i o n s t a t e s , and the partially o x i d i z e d (po-TCMP) compounds. The latter are known to behave like 1-D m e t a l s , whereas the former are more or l e s s semiconducting. Of the po-TCMP, three s t r u c t u r e s have been studied t o some d e t a i l : K [Pt(CN)4] X . 3H 0 (1). Mg [Pt(C 0 ) ] · 6 H0 ( 2 ), Ir(C0) Cl ( 3 ) They all contain columns of TCMP u n i t s stacked i n a staggered manner to minimize i n t e r l i g a n d r e p u l s i o n . The p a r t i a l o x i d a t i o n makes the complex columns l e s s negative than they were without o x i d a t i o n . In the first s t r u c t u r e ( a l s o named KCP), this p a r t i a l p o s i t i v e charge per complex i s allowed for by i n c o r p o r a t i n g the necessary amount of h a l i d e ions (CI- or Br-) at s i t e s apart from the complexes, and occupied up to 64 percent ( 1 ). The second s t r u c t u r e a t t a i n s e l e c t r o n e u t r a l i t y by a defect of c a t i o n s , which way seems t o be favoured by all d i o x a l a t o p l a t i n a t e s and some I r compounds ( 2 ). The third s t r u c t u r e ( 3 ) i s supposed t o contain mainly neutral u n i t s without atoms or ions l i n k i n g the d i f f e r e n t columns. The c r y s t a l s of all these m a t e r i a l s tend to grow as t h i n needles i n one d i r e c t i o n o n l y , making it difficult to obtain c r y s t a l s big enough f o r p h y s i c a l mearurements, which i s e s p e c i a l l y true f o r the compound l a s t mentioned with VAN DER WAALS forces only a c t i n g between the c h a i n s . We supposed Ir(+I) to be a promising c e n t r a l atom f o r po-TCMP compounds because it has l e s s e f f e c t i v e charge than 2

0.82

2

4

2

0.3

2

2.9

2

l.l

350

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

23.

1-D Metal

KROGMANN AND GESERicH

351

Complexes

Pt(+II) i n i t s 5 d c o n f i g u r a t i o n , and we found a number of s a l t s c o n t a i n i n g the I r ( C 0 ) C l u n i t , which are o f the 1-D metal type ( 4 ). A l i s t of compounds i s given i n t a b l e I, showing no great v a r i a t i o n i n distances except i n the l a s t case, though there are rather large d i f f e r e n c e s i n the degree of p a r t i a l o x i d a t i o n (DPG). The r e s u l t s are based on p r e l i m i n a r y s i n g l e c r y s t a l ( H , K ) or r e l i a b l e GUINIER powder data. Some of these s a l t s have been 8

2

2

+

Table I. Columnar s t r u c t u r e s with Ir compound

(4)

o x i d a t i o n number

I r ( C 0 ) 93 CI (H 0) .38 I r ( C 0 ) C l 2

3

c§-

0

2

2

"

58

(As( h) y P

4

;;

0 e 6 2

+

+1.07 +1.62 +1.42

2.85 2.86 2.86

+1.45 +1.38

2.86 2.86 2.86 2.86 2.86 2.78

? ?

X

Κ

distance

?

(8)

Ba +L50 Irfc0) Cl Ir(C0) acetat p r e v i o u s l y prepared e i t h e r by high pressure CO treatment ( 5 ) or formic a c i d r e f l u x i n g (6, 7) of K I r C l 5 ( s i m i l a r compounds), but analyses reported and s t r u c t u r e s derived from them d i d not lead t o the c o r r e c t i n t e r p r e t a t i o n . I t i s i n t e r e s t i n g t o note, that the I r ( C 0 ) C l s a l t s are s i m i l a r t o the dioxa1 a t o p ! a t i n a t e s in t h e i r c a t i o n defect s t r u c t u r e s as well as i n the tendency of the f r e e a c i d t o from s t r o n g l y absorbing blue coloured s o l u t i o n s , which may contain polymers l i k e the Pt a c i d ( 8 ). D i o x a l a t o p l a t i n a t e s of the 1-D type are a"Ele t o c r y s t a l l i z e in many d i f f e r e n t phases with no detectable d i f f e r e n c e i n composition. We reported three Mg s a l t m o d i f i c a t i o n s ( 2 ), but f o r potassium, the s i t u a t i o n i s more complicated, as there are at l e a s t f i v e d i f f e r e n t powder p a t t e r n s . Recent s i n g l e c r y s t a l growth and x - r a y studies r e v e a l e d , that one of the powder diagrams we named "Phase A" can be obtained from three c r y s t a l s t r u c t u r e s d i f f e r i n g i n t h e i r s u p e r s t r u c t u r e s , which do not show up i n the powder photographs. We can imagine several reasons f o r super­ s t r u c t u r e s . One already mentioned i s the staggered stacking of ligands leading t o a l i g a n d s u p e r s t r u c t u r e . There may a l s o be a l a t t i c e period connected with the composition or the degree of p a r t i a l o x i d a t i o n (DPO), which may be c a l l e d a PEIERLS super­ s t r u c t u r e according t o the theory o f 1-D metals ( 10 ). Such a l a t t i c e d i s t o r t i o n was shown f o r KCP ( 11,^2 ) t o "Behave l i k e a soft v i b r a t i o n a l mode instead of a s t a t i c s u p e r s t r u c t u r e . The e v a l u a t i o n of the d i f f u s e s c a t t e r i n g e f f e c t s lead t o s h i f t s of about 0.01 Λ i n the Pt p o s i t i o n s along the metal c h a i n . A r e c i p r o c a l l a t t i c e vector due t o a PEIERLS d i s t o r t i o n period c 11

2

2

2

2

2

o r

2

p

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

x

352

EXTENDED INTERACTIONS BETWEEN METAL IONS

should be r e l a t e d to the r e c i p r o c a l metal s u b i a t t i c e vector c ^ * C

P

X

=

C

M

X

*

(DP0)/2

.

The corresponding superstructure period c c = c · 2/(DPQ) , p

M

p

i s then given by

p

which i s no simple integer m u l t i p l e of CM in most cases. Neutron and d i f f u s e x - r a y s c a t t e r i n g proved such an " a p e r i o d i c " but dynamic superstructure f o r KCP ( 11,12 ). On the other hand, the superTâttices we observed ( t a b l e II) are s t a t i c , as we see BRAGG d i f f r a c t i o n s p o t s , and " a p e r i o d i c " , but with s t i l l d i f f e r e n t p e r i o d s , at l e a s t in the f i r s t two phases A and Α . α β Λ

r

Superstructure periods in Κ substructure

(c )

t r i c l i n i c AQ triclinic A monoclinic Α

2.84 8 2.84 8 2.84 8

M

p

γ

Pt(C 0 )

· 4 H 0 , Phase "A"

superstructure ligand (q_) s h i f t e d metal 4.00 χ 4.00 6.00

6.63 χ c 9.35

(c$)

M

m

The PEIERLS period may be c a l c u l a t e d from the composition : Cp = 5.56 χ CM f o r a l l phase A m o d i f i c a t i o n s , which does not c o i n c i d e with any of the observed s u p e r s t r u c t u r e s . P r e l i m i n a r y r e s u l t s show that the displacements of the heavy atoms from the i d e a l p o s i t i o n s are transverse instead of l o n g i t u d i n a l as f o r KCP. The amplitude of the " s h i f t wave" i s about 0.2 8 ( F i g . 1). There i s a l s o the case of a h e l i c a l "wave" in the Ir compound l a s t mentioned i n t a b l e I, where the radius of the h e l i x i s 0.7 8 , and the period i s 9.6 χ c^. Many TCMP compounds show a m e t a l l i c l u s t e r , which leads t o the assumption of a 1-Dmetal system. P o l a r i z e d specular reflectance spectra c l e a r l y d i s t i n g u i s h between 1-D metal and semiconductor systems (po- and no-TCMP), as the comparison of KCP and Ir(C0)2 shows. The l a t t e r i s a no-TCMP compound with Ir(+1). In F i g . 2, the KCP s t r o n g l y r e f l e c t s f o r photon energies l e s s than 2.0 eV even i n the FIR, i f the l i g h t i s p o l a r i z e d p a r a l l e l to the chain d i r e c t i o n . The Ir(+1) complex with completely f i l l e d valence s h e l l d i s p l a y s a steep maximum i n r e f l e c t i v i t y up t o 60 % at 2.0 eV a l s o , but q u i t e normal values of about 10 % are found i n the IR r e g i o n . The substance i s , there f o r e , no 1-D m e t a l , although i t i s of s i m i l a r appearance as KCP (13). a

c

a

c

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

Figure

1.

Shifts

of heavy

atoms

from

ideal positions

in some 1-D

metal

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

phases

354

EXTENDED INTERACTIONS BETWEEN M E T A L IONS

îι Ο

Q ι

i ι

ι

1 Q

u a

to

s 8

to §· 13

X

.3

«2

.a.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

23.

KROGMANN

AND

GESERiCH

1-D Metal

Complexes

355

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

13.

Krogmann, K . , and Hausen, H . D . , Z.Anorg.Allg.Chem. (1968) 358, 67-81. Krogmann, Κ., Z.Anorg.Allg.Chem. (1968) 358, 97-110. Krogmann, Κ., B i n d e r , W., and Hausen, H.D., Angew.Chem. I n t e r n a t . E d i t . (1968) 7, 812. Z i e l k e , H.J., D i s s e r t a t i o n Karlsruhe 1973. M a l a t e s t a , L., and C a n z i a n i , F., J . I n o r g . N u c l . C h e m . (1961) 18, 81. Chernaev, I . I . , and Novozhenyuk, Z . M . , Russ.J.Inorg.Chem. (1966) 11, 1004. C l e a r e , M.J., and Griffith, W.P., J.Chem.Soc. A (1970) 2788. Krogmann, Κ., and Dodel, P . , Chem.Berichte (1966) 9 9 , 3408-3418. Krogmann, K . , Angew.Chem.Internat.Edit. (1969) 8 , 35-42. P e i e r l s , R . E . , "Quantum Theory c f Solids", p. 108, Clarendon P r e s s , Oxford (1955). Comes, R . , Lambert, Μ., Launois, Η . , and Zeller, H . R . , Physical Review Β (1973) 8, 571. Renker, B . , R i e t s c h e l , H., P i n t s c h o v i u s , L., G l ä s e r , W., Brüesch, P . , Kuse, D . , and R i c e , M.J., P h y s . R e v . L e t t . (1973) 30, 1144. W a g n e r , Η., D i s s e r t a t i o n Karlsruhe 1974.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

24 Some Comments o n the Electronic Structure o f K r o g m a n n Salts and the Stability of Pt AARON

N.

BLOCH*

and

R.

BRUCE

WEISMAN

2.3

†

†

T h e Johns H o p k i n s U n i v e r s i t y , B a l t i m o r e , Md. 21218

I.

Introduction

S a l t s of the d i v a l e n t tetracyanoplatinateo r dioxaloplatinate i o n s typically d i s p l a y c r y s t a l s t r u c t u r e s in which the p l a n a r comp l e x anions a r e stacked t o form l i n e a r c h a i n s , separated by comp a r a t i v e l y wide channels c o n t a i n i n g counterions and water molec u l e s (1). From s o l u t i o n s o f the complex which have been partially o x i d i z e d , c r y s t a l s can be grown having s i m i l a r s t r u c t u r e s but cons i d e r a b l y reduced metal-metal d i s t a n c e s (1) and a formal platinum valence (1) c l o s e t o 2.3+. Nominally, these a r e "mixed-valence" compounds, but t h e term is probably m i s l e a d i n g inasmuch as X-ray (1), i n f r a r e d (2,3), and p h o t o e l e c t r o n (4) s p e c t r o s c o p i c s t u d i e s show the o x i d a t i o n s t a t e s o f all the metal atoms t o be e q u a l . Rather, t h e r e is ample evidence (1,5) f o r t h e formation o f a well-defined quasi-one-dimensional band s t r u c t u r e , i n c l u d i n g a partially filled conduction band o f a p p r e c i a b l e width. Hence, w h i l e their d i v a l e n t parent compounds a r e electrical i n s u l a t o r s (1), the partially o x i d i z e d "Krogmann salts" d i s p l a y l a r g e electrical (5,6) and o p t i c a l (7) c o n d u c t i v i t i e s along the c h a i n a x i s , and r e c e n t l y have e x c i t e d widespread i n t e r e s t as prototypes for the study o f one-dimensional conductors. From this p o i n t o f view, our understanding o f the p h y s i c s o f these m a t e r i a l s is by now w e l l developed (5,8-10). T h e i r theoretical chemistry, however, has r e c e i v e d l e s s a t t e n t i o n , and s e v e r a l fundamental questions remain outstanding, (a) Why do these unique s t r u c t u r e s occur in o n l y two chemical forms (1), the f i l l e d - b a n d (Pt2+) i n s u l a t o r s and a s e r i e s o f conductors whose conduction-band occupations fall in a narrow range near f i v e - s i x t h s filling (Pt 2.3+)? (b) Why do the i n t e r m e t a l l i c spacings (1) v a r y among the former group o f compounds over t h e enormous range 3.09-3.60Å, w h i l e those in t h e latter group a r e much s h o r t e r and r e s t r i c t e d t o the range * A l f r e d P. Sloan Foundation F e l l o w Present address: Department o f Chemistry, U n i v e r s i t y o f Chicago, Chicago, I l l i n o i s 60637

356 In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

24.

BLOCH

AND

WEISMAN

Krogmann

Salts

357

2.88-2.98A f a r the cyanides and 2.80-2.85A f o r the o x a l a t e s ? (c) F i n a l l y , why do cyanide and o x a l a t e , alone among common l i g a n d s t o platinum, form concourais o f t h i s c l a s s ? The f i n a l d i s p o s i t i o n o f such q u e s t i o n s must o f course await d e t a i l e d c a l c u l a t i o n s o f t h e e l e c t r o n i c s t r u c t u r e o f the m a t e r i a l s . Pending these, however, enough experimental i n f o r m a t i o n i s a v a i l a b l e t o guide a p r e l i i n i n a r y i n q u i r y i n t o the b a s i c p r i n c i p l e s involved. Such a c o n t r i b u t i o n has a l r e a d y been made by Krogmann (1), who proposed a p l a u s i b l e answer t o (a) and (b) based on simp l e one-dimensional band-structure arguments. I n t h i s a r t i c l e we c o n s i d e r t h e s e arguments i n more d e t a i l . We f i n d , s u r p r i s i n g l y , t h a t w h i l e they probably do answer q u e s t i o n (b), they do not r e s o l v e (a). We suggest i n s t e a d t h a t the f r a c t i o n a l v a l e n c e i s a c o v a l e n t molecular e f f e c t , and represents the optimum v a l u e f o r s t a b i l i t y o f the comple a n i o i t s e l f A tentativ t (c) follows d i r e c t l y . The paper i s d i v i d e parts approach i s formulated w i t h i n the framework o f c u r r e n t e m p i r i c a l evidence. We conclude t h a t t h e arguments o f Krogmann (1) a r e t o be c a s t i n terms o f the c o n t r i b u t i o n o f a s i n g l e f i v e - s i x t h s - f i l l e d one-dimensional conduction band t o the i n t e r n a l energy o f the c r y s t a l . As an i l l u s t r a t i o n , t h i s c a l c u l a t i o n i s performed i n S e c t i o n I I I u s i n g a simple t i g h t - b i n d i n g model. We f i n d t h a t t h e bands t r u c t u r e energy gained from o x i d a t i o n o f the Krogmann-salt s t r u c t u r e t o P t 2.3+ i s much t o o s m a l l t o p r o v i d e a r e a l i s t i c answer t o q u e s t i o n (a). S e c t i o n IV i n q u i r e s whether t h i s c o n c l u s i o n i s a l t e r e d s i g n i f i c a n t l y by improving upon the simple t i g h t - b i n d i n g model. Appealing once more t o e m p i r i c a l evidence, we f i n d t h a t n e i t h e r t h e t i g h t - b i n d i n g l i m i t nor the f r e e - e l e c t r o n model r e c e n t l y proposed by Z e l l e r (10) i s f u l l y c o n s i s t e n t w i t h experiment. A simple intermediate treatment c o r r e c t s t h i s d e f i c i e n c y , but s t i l l does not account f o r the s t a b i l i t y o f the f i v e - s i x t h s - f i l l e d band. We t h e r e f o r e suggest i n S e c t i o n V t h a t t h e e f f e c t must r e s i d e i n the c o v a l e n t i n t e r n a l energy o f the ccmplex anion, and b r i e f l y d i s c u s s the chemical p h y s i c s o f a system i n which t h e v a l e n c e can be t r e a t e d a s a continuous v a r i a t i o n a l parameter. We note t h a t t h e most s t a b l e o x i d a t i o n s t a t e o f a molecule need not be an i n t e g e r , and t h a t i n p r a c t i c e i t can be a t t a i n e d o n l y i n unique m o l e c u l a r c r y s t a l s t r u c t u r e s , such as those o f the Krogmann s a l t s , i n which the e f f e c t i v e t o t a l charge o f the molecule i s not quantized. The exi s t e n c e o f two s t a b l e o x i d a t i o n s t a t e s w i t h the same g e n e r a l c r y s t a l s t r u c t u r e has e l e c t r o c h e m i c a l i m p l i c a t i o n s as w e l l , and S e c t i o n VI p r e s e n t s a p r e l i m i n a r y experimental account o f the electrochemic a l o x i d a t i o n o f a s i n g l e c r y s t a l o f K2Pt(CN)4*2H20 t o a s o l i d state galvanic c e l l . I n S e c t i o n V I I we summarize our c o n c l u s i o n s . II.

General Remarks

Wë s h a l l f i n d i t u s e f u l a t the o u t s e t t o review b r i e f l y t h e s a l i e n t p h y s i c s o f the m a t e r i a l s . The Krogmann s a l t s represent a

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

358

EXTENDED INTERACTIONS BETWEEN M E T A L IONS

p a r t i c u l a r l y i n t e r e s t i n g c l a s s o f model systems i n which t h e r o l e s of i n s t a b i l i t i e s (11), f l u c t u a t i o n s (9), and d i s o r d e r (8,12) i n the p h y s i c s o f the one-dimensional e l e c t r o n gas can be s t u d i e d d i r e c t l y . The m e t a l l i c s t a t e i n one dimension i s i n h e r e n t l y u n s t a b l e (11), and the b e a u t i f u l experiments o f Z e l l e r and c o workers (10) have shown t h a t t h e p l a t i n u m c h a i n s do undergo a P e i e r l s d i s t o r t i o n (11) w i t h d e c r e a s i n g temperature. The phase t r a n s i t i o n i s , however, incomplete (13), probably owing t o t h e i n t r i n s i c s t r u c t u r a l d i s o r d e r (1) o f the systems. The s t a t i c potent i a l f l u c t u a t i o n s a s s o c i a t e d w i t h random occupantion o f the count e r i o n and water s i t e s tend t o suppress the t r a n s i t i o n , r e d u c i n g the m e a n - f i e l d t r a n s i t i o n temperature and p l a c i n g weakly l o c a l i z e d e l e c t r o n i c s t a t e s i n the P e i e r l s energy gap (12,14). Indeed, we have argued elsewhere (8,12,15) t h a t c e r t a i n f e a t u r e s o f the e l e c t r o n t r a n s p o r t and low-frequenc d i e l e c t r i t b understood i n terms o f from a f i n i t e d e n s i t y o For the p r e s e n t d i s c u s s i o n o f chemical s t a b i l i t y , however, the d i s o r d e r i s o f scant importance. The p o t e n t i a l f l u c t u a t i o n s r e q u i r e d t o e x p l a i n t h e observed t r a n s p o r t phenomena (8,12,15) a r e i n t h i s case but a s m a l l f r a c t i o n o f t h e unperturbed bandwidth (14), and t h e œnfigurational entropy a s s o c i a t e d w i t h the p a r t i a l l y occupied i n t e r c h a i n s i t e s make a c o n t r i b u t i o n o f but o r d e r kT t o the t o t a l f r e e energy p e r molecule (16). The e l e c t r o n i c entropy o f t h e p a r t i a l l y f i l l e d conduction band i s a n a l l e r s t i l l (16). We conclude t h a t the s t a b i l i t y o f P t 2.3+ r e s i d e s i n the i n t e r n a l energy o f the c r y s t a l , and t h a t t h i s i s n e g l i g i b l y d i f f e r e n t from t h a t o f a h y p o t h e t i c a l analogous ordered system. We w r i t e t h i s energy s c h e m a t i c a l l y a s : U

=

U

C

+

U

B

+

U

R

+

U

M

( 1 )

where U i s t h e t o t a l c o v a l e n t b i n d i n g energy o f the complex anion, 1% t h e band-structure s t a b i l i z a t i o n o f the c h a i n s , U R t h e r e p u l s i v e energy between adjacent p l a n a r complexes on a c h a i n , and % t h e remainder o f the Madelung energy o f the c r y s t a l . Admittedly, these d i s t i n c t i o n s a r e t o some e x t e n t a r b i t r a r y — t h e f i r s t and second terms, f o r example, a r e never f u l l y s e p a r a b l e — b u t we s h a l l f i n d them t o be o f conceptual v a l u e . Now, we can immediately d i s m i s s on e m p i r i c a l grounds t h e p o s s i b i l i t y t h a t the number o f conduction e l e c t r o n s p e r molecule, Z, and i n t e r m e t a l l i c spacing, R, i n t h e P t 2.3+ Krogmann s a l t s a r e determined p r i m a r i l y by Among the dozens o f compounds i n t h e c l a s s , s t r u c t u r a l and chemical d i f f e r e n c e s which presumably l e a d t o s u b s t a n t i a l d i f f e r e n c e s i n Madelung energy produce o n l y minor d i f f e r e n c e s i n Ζ and R (1). From those o f a t y p i c a l example such a s K2Pt(CN)4Bro.3*3H20, these parameters change but l i t t l e when CI i s s u b s t i t u t e d f o r Br; when halogen i s excluded a l t o g e t h e r i n f a v o r o f a d e f i c i e n c y o f c a t i o n s ; when o x a l a t e i s s u b s t i t u t e d f o r cyanide; when any a l k a l i o r a l k a l i n e e a r t h metal i s s u b s t i t u t e d f o r K; when c

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

24.

BLOCH

AND

Krogmann

WEISMAN

Salts

359

the water content i s changed; o r when the gross c r y s t a l s t r u c t u r e i t s e l f changes from t e t r a g o n a l t o t r i c l i n i c , m o n œ l i n i c , o r o r t h o hcmbic ( 1 ) . I t i s as though t h e i n t e r c h a i n s p e c i e s had been l e f t t o a d j u s t themselves t o dimensions and chemical composition which a r e e s s e n t i a l l y f i x e d by s t r o n g i n t r a c h a i n f o r c e s . Toward an e l u c i d a t i o n o f these f o r c e s , Krogmann ( 1 ) has suggested i n e f f e c t t h a t the s t a b i l i t y o f t h e o x i d a t i o n s t a t e Pt 2.3+ ( Z = 1 . 7 ) a r i s e s frcm an o p t i m i z a t i o n o f the band-structure term 1%. Such an e f f e c t would represent a rough one-dimensional analog o f the well-known Hume-Rothery r u l e s ( 1 7 ) f o r the composit i o n of alloys. We s h a l l c a s t these arguments i n a simple mathematical form. As a preamble, we observe t h a t i n c a l c u l a t i n g 1% we a r e d e a l i n g w i t h a s i n g l e conduction band, not degerate w i t h any other band a t t h e Fermi l e v e l . T h i as a probe o f band occupancy l y i n g divergence o f the one-diinensional e l e c t r o n i c response f u n c t i o n s a t wavenumber q=2kp, where kp i s the Fermi wavenumber ( 1 8 ) . Where the conduction e l e c t r o n s a r e shared by two one-dimensional bands which o v e r l a p i n energy a t the Fermi l e v e l , the Fermi "surf a c e " c o n s i s t s i n g e n e r a l o f two d i s t i n c t v a l u e s o f k ; o n l y when such degeneracy i s absent does kp assume the s i n g l e v a l u e TCZ/2R. Now, i t i s found (13) experimentally t h a t w i t h the s m a l l v a r i a t i o n s i n Ζ which do occur among the d i f f e r e n t Krogmann s a l t s ( i n f e r r e d from t h e i r s t o i c h i c m e t r i e s ), the p e r i o d o f the d i s t o r t i o n a l s o v a r i e s , and always corresponds p r e c i s e l y t o a reduced wavenumber o f πΖ/R. We conclude t h a t o n l y one hand i n t h e system i s p a r t i a l l y occupied. F

III.

Band-Structure Energy ϋ" i n the T i g h t - B i n d i n g Model ρ

F o r s i m p l i c i t y o f i l l u s t r a t i o n , we t r e a t t h i s band i n i t i a l l y i n t h e o n e - e l e c t r o n t i g h t - b i n d i n g l i m i t ( 1 9 ) , and d e f e r t o the next s e c t i o n a d i s c u s s i o n o f the adequacy o f t h i s approximation and the r o l e s o f h y b r i d i z a t i o n and the P e i e r l s d i s t o r t i o n . L e t E° be the energy o f the r e l e v a n t molecular o r b i t a l |i>, centered on s i t e i o f the c h a i n , i n the presence o f the Madelung f i e l d but the absence of any i n t e r a c t i o n between adjacent molecules on t h e c h a i n . When t h e s t a t e |i> i s perturbed by the l a t t i c e p o t e n t i a l o f the c h a i n , l jïi j ' 9Y t ^ o n e - e l e c t r o n conduction-band s t a t e o f waveSumœr k i s : H

=

v

t

h

e

e n e r

o

f

= E° - α - 2(3 cos kR

(2)

Here - a i s t h e band s h i f t , and-β the c h a r g e - t r a n s f e r i n t e ­ g r a l . As u s u a l , o v e r l a p i n t e g r a l s S o f the form have been ignored. C l e a r l y , E° c o n t a i n s l o c a l c o n t r i b u t i o n s t o OQ and U M - T O e v a l u a t e % , we i n t e g r a t e the q u a n t i t y E - E ° over a l l occupied s t a t e s |k> o f the band: k

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

EXTENDED INTERACTIONS BETWEEN M E T A L IONS

360

U

= - Ζα -

B

ψ

sin

φ

(3)

We note t h a t both α and β a r e f u n c t i o n s o f R and Z, and t h a t the p r i n c i p a l dependence o f β on these parameters i s s u r e l y expo­ n e n t i a l . Taking t h e r a d i a l screening constant f o r t h e wavefunction I i> t o have roughly the S l a t e r form (20), we w r i t e t h i s dependence a s

β - 3 exp [ - ( y - n Z ) R ]

(4)

o

where βο, μ, and η « μ a r e constants. I n c o n t r a s t , the v a r i a t i o n o f t h e s m a l l (19) energy α w i t h each o f t h e parameters R and Ζ i s a t most a paver-law dependence, comparable w i t h those o f t y p i c a l c o n t r i b u t i o n s t o the Madelung energy U . T h i s o b s e r v a t i o n has s p e c i a l s i g n i f i c a n c e f o r t h e f i l l e d band case Z=2, where U Under these circumstances % r a t h e r than U . We thereby account f o r t h e wide v a r i a t i o n i n R among the d i v a l e n t parent cxmpounds (1), i n p a r t i a l answer t o q u e s t i o n (b). With p a r t i a l o x i d a t i o n o f these m a t e r i a l s t o form Krogmann s a l t s , t h e s t r o n g l y R- and Z - dépendait second term i n Equation 3 e n t e r s U , and the s i t u a t i o n i s a l t e r e d d r a s t i c a l l y . On t h e b a s i s o f o u r d i s c u s s i o n i n t h e preceding s e c t i o n , i t i s n o t unreasonable t o regard t h e i n t e r c h a i n channels a s a "charge s i n k " , capable o f accormodating a s many counterions a s are necessary t o balance any Ζ i n t h e range o f i n t e r e s t . To t h e extent t h a t t h i s i s t r u e , t h e c h a i n s c o n s t i t u t e a c h e m i c a l l y unique system i n which the number o f bonding e l e c t r o n s may be regarded a s a v a r i a t i o n a l parameter. I f , as suggested by Krogmann (1), Ζ i s determined by U , then f o r g i v e n R the observed Ζ must correspond t o a minimum i n 1%: M

B

B

B

R

s i n -γ)

(5)

When Equations 4 and 5 a r e combined, a t r a n s c e n d e n t a l equa­ t i o n i s obtained r e l a t i n g R and Z. A t y p i c a l s o l u t i o n i s shown i n F i g u r e 1. Here we have simply determined μ and η a c c o r d i n g t o S l a t e r ' s r u l e s (20), and adjusted t h e r ^ t i o βς/α s o as t o reproduce the experimental v a l u e Z=1.7 f o r R=2.88A. More e l a b o r a t e f i t t i n g procedures a r e c e r t a i n l y a v a i l a b l e , but we have found no p h y s i c a l ­ l y reasonable c h o i c e o f parameters which s u b s t a n t i a l l y a l t e r s Figure 1 o r our f i n a l conclusions. Under t h e c o n d i t i o n imposed by Equation 5, we a r e now i n a p o s i t i o n t o c a l c u l a t e U from Equations 3 and 4. The r e s u l t i s B

π Λν - V ~ a

7 + ù

sin(ïïZ/2) (π/2)cos (πΖ/2) + (3£ηβ/9Ζ) sin(πΖ/2)

l 0 j

Equation 6 i s p l o t t e d i n F i g u r e 2, which represents t h e v a r i a t i o n o f U w i t h R under t h e c o n s t r a i n t t h a t each R, Ζ i s s e l f c o n s i s t e n t l y t o be r e a d j u s t e d so a s t o minimize U . Q u a l i t a t i v e l y , the c a l c u l a t i o n i s c o n s i s t e n t w i t h t h e B

B

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

BLOCH

AND

WEiSMAN

Krogmann

Salts

Figure 1. Variation of conduction-band occupation, Z, with lattice constant, R, for a one-dimensional tight-binding system under the constraint of Equation 5

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

EXTENDED

INTERACTIONS

BETWEEN

METAL

R(A)

,2.0 I

I

I.I

2.6 2.7 I

I

12

2.85

2.8 i

1.3

1.4

l

2.88

29

l

1.5

1.6

ι

1.7

1.8

1.9

2.0

Ζ

Figure 2. Variation of band-structure stabilization U with R and Ζ for a one-dimensional tight-binding system under the constraint of Equation 5. At the experimental values Ζ — 1.7 and R = 2.88 A , U (Z)-Ui*(2) is much too small to account for the stability of Pt 2.3+. B

B

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

IONS

24.

BLOCH

AND

WEISMAN

Krogmann

363

Salts

d e s c r i p t i o n o f f e r e d by Krogmann (1). F o r l a r g e R, we have Z=2 and ϋβ/α independent o f R. With decreasing R, the bandwidth 4β i s increased, u n t i l the h i g h e s t - l y i n g s t a t e s i n the band become a n t i bonding w i t h r e s p e c t t o E ° . The band-structure energy 1% now f a v o r s p a r t i a l o x i d a t i o n o f the chains, and the parameters Ζ and R a r e coupled according t o F i g u r e 1. With p r o g r e s s i v e o x i d a t i o n and shrinkage o f the l a t t i c e , energy 1% i s gained a s shewn i n F i g u r e 2. U l t i m a t e l y , however, the compression and hence the o x i d a t i o n must o f course be l i m i t e d by the r e p u l s i v e term U R i n Equation 1, which r i s e s r a p i d l y f o r R s m a l l and decreasing. The r e s u l t i s a minimum i n the sum U + U a t seme s h o r t R and f i x e d o x i d a t i o n s t a t e 1
R

B

b

B

IV.

Beyond the T i g h t - B i n d i n g

j^proximation

How h e a v i l y model-dependent a r e the r e s u l t s o f the preceding s e c t i o n ? L e t u s examine some p h y s i c a l l y p l a u s i b l e departures frcm the simple t i g h t - b i n d i n g approximation considered t h e r e . E x t r a s t a b i l i t y i s c e r t a i n l y c o n f e r r e d , f o r example, by the i n c i p i e n t (9,21) P e i e r l s gap i n the d e n s i t y o f s t a t e s ; the c o n t r i b u ­ t i o n t o U B + U R , however, w i l l o n l y be o f the order o f the mean-field t r a n s i t i o n temperature (18,21) o f a few hundred degrees K e l v i n . F u r t h e r , t h i s c o n t r i b u t i o n appears a t a l l f i l l i n g s Z<2, and i s l a r g e s t f o r Z=l (11). In e f f e c t , then, the P e i e r l s c o n t r i b u t i o n simply aggravates the t r e n d o f F i g u r e 2. Nor i s d i s t o r t i o n o f the t i g h t - b i n d i n g bands through h y b r i d i ­ z a t i o n o f the b a s i s molecular o r b i t a l s l i k e l y t o p l a y an important r o l e . I t i s easy t o shew t h a t the s-d h y b r i d i z a t i o n suggested by Z e l l e r (10) has no e f f e c t upon the c e n t e r o f the conduction band, but lowers the energy o f the remaining s t a t e s , w i t h the l a r g e s t

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

EXTENDED INTERACTIONS

364

BETWEEN M E T A L IONS

e f f e c t coning a t the t o p and bottom extremes o f the band. Compared w i t h t h e symmetric band o f the p r e v i o u s s e c t i o n , the r e s u l t i s a compression o f the t o p h a l f o f the band on t h e energy s c a l e and an e l o n g a t i o n o f the bottom h a l f . T h i s amounts t o a f u r t h e r d e s t a b i l i z a t i o n o f P t 2.3+ r e l a t i v e t o P t 2+. The p-d mixing suggested by Krogmann (1) has the o p p o s i t e e f f e c t : i t i s synmetry-forbidden a t t h e c e n t e r and edges o f the B r i l l o u i n zone and s t r o n g e s t a t t h e c e n t e r o f t h e band. Thus i t reduces t h e d e n s i t y o f s t a t e s i n t h e top h a l f o f the band and does tend t o h e l p s t a b i l i z e P t 2.3+. T h i s e f f e c t i s p a r t i a l l y c a n c e l l e d by the s-d h y b r i d i z a t i o n , however, and we have s a t i s f i e d o u r s e l v e s t h a t the 5dz -6s and 5dz -6p energy d i f f e r e n c e s deduced from s p e c t r o s c o p i c (22) and t h e o r e t i c a l (23) s t u d i e s o f the f r e e complex a r e i n any case too l a r g e f o r such mix­ i n g t o p l a y a s i g n i f i c a n t r o l e i n the s t r u c t u r e o f the conduction band. W i t h i n t h e standar appears l i t t l e room f o Pt 2.3+. A t t h i s p o i n t i t i s n a t u r a l t o ask t o what extent t h i s approximation i t s e l f i s v a l i d f o r the Krogmann s a l t s . There i s s t r o n g experimental evidence t h a t i t i s not. Consider, f o r exaitple, t h e o p t i c a l r e f l e c t i v i t y (5,7,10), which shows a w e l l - d e f i n e d plasma edge near 2 eV, p o l a r i z e d along t h e c h a i n a x i s and r e s p o n s i b l e f o r the c h a r a c t e r i s t i c coppery l u s ­ t e r o f the Krogmann s a l t s . I n a quasi-one-dimensional conductor, the plasma frequency i s -(25): 2

2

(7)

oo

where Ν i s the number o f c h a i n s p e r u n i t c r o s s - s e c t i o n a l a r e a , v the Fermi v e l o c i t y , and the background d i e l e c t r i c constant. For the Krogmann s a l t s , w i t h ε^-2.2, the Fermi v e l o c i t y deduced i n t h i s manner (24) would correspond i n t i g h t - b i n d i n g theory t o an unreasonably l a r g e bandwidth o f c a . 8.5 eV. Indeed v i s v e r y c l o s e t o i t s f r e e - e l e c t r o n v a l u e (24). T h i s circumstance has l e d Z e l l e r (10) t o suggest t h a t due t o a f o r t u i t o u s amount o f s-d mixing, the e n t i r e conduction band i n the Krogmann s a l t s has n e a r l y the f r e e - e l e c t r o n form, w i t h an e f f e c t i v e mass m* o f u n i t y . I n our view t h e r e i s l i t t l e founda­ t i o n f o r t h i s p r o p o s a l , e i t h e r i n theory o r i n experiment. One does not achieve a s p a t i a l l y uniform charge d i s t r i b u t i o n through s u p e r p o s i t i o n o f t h e r a p i d l y o s c i l l a t i n g 5d and 6s wavefunctions, nor does one escape the t i g h t - b i n d i n g l i m i t simply by a d j u s t i n g the geometries o f the b a s i s o r b i t a l s . To put the matter another way, the n e a r l y - f r e e - e l e c t r o n r e p r e s e n t a t i o n i s fundamentally a statement about t h e e f f e c t i v e n e s s o f the s c r e e n i n g o f the atomic pseudopotential, and not about the degree o f c o n f i g u r a t i o n i n t e r ­ a c t i o n between atomic o r b i t a l s . In l i g h t o f these o b j e c t i o n s i t i s not s u r p r i s i n g t o f i n d the n e a r l y - f r e e - e l e c t r o n model i n c o n s i s t e n t w i t h experiment. For example, the t h e r m o e l e c t r i c power i s s m a l l and p o s i t i v e (6,26-28), F

F

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

24.

BLOCH AND

Krogmann

WEiSMAN

Salts

365

i n d i c a t i n g t h a t i n t h e v i c i n i t y o f t h e Fermi l e v e l t h e band i s h o l e - l i k e r a t h e r than e l e c t r o n - l i k e . I n o t h e r words, although t h e f i r s t d e r i v a t i v e yp o f the e l e c t r o n i c d i s p e r s i o n curve has approx­ i m a t e l y the f r e e - e l e c t r o n v a l u e a t t h e Fermi l e v e l , the second d e r i v a t i v e has t h e wrong s i g n . F u r t h e r , i n a n e a r l y - f r e e - e l e c t r o n system w i t h a f i v e - s i x t h s f i l l e d conduction band and m*/hT=l, i t i s r e a d i l y v e r i f i e d t h a t t h e t h r e s h o l d f o r the f i r s t i n t e r b a n d o p t i c a l t r a n s i t i o n should occur a t o r above 23,000 cm" . No such t h r e s h o l d o c c u r s i n t h e observed o p t i c a l spectrum (7,24,29) : indeed, the o n l y d i s t i n c t f e a t u r e found i n t h e e n t i r e v i s i b l e and n e a r - u l t r a v i o l e t r e g i o n above the plasma edge i s t h e weak plasmon a b s o r p t i o n (24,25,29) c e n t e r e d a t 15,800 cm~l. T h i s a b s o r p t i o n has been s t u d i e d i n d e t a i l by one o f us and c o l l a b o r a t o r s (24,25). We found t h a t f o r a quasi-one-dimens i o n a l s t r u c t u r e i t doe i s appreciable only i n which t h e bandwidthW i s comparable w i t h , o r l e s s than, ηωρ. We con c l u d e t h a t the n e a r l y - f r e e - e l e c t r o n model i s i n c o n s i s t e n t w i t h t h e observed o p t i c a l p r o p e r t i e s o f the m a t e r i a l . The f a i l u r e o f both t h e f r e e - e l e c t r o n and t i g h t - b i n d i n g models suggests t h e need f o r an i n t e r m e d i a t e d e s c r i p t i o n w i t h i n which the observed plasma frequency and t h e r m o e l e c t r i c power can be r e c o n ­ c i l e d w i t h t h e upper l i m i t on W i n f e r r e d from t h e plasmon absorp­ t i o n . Fcward such a d e s c r i p t i o n , we note t h a t f o r a band as narrow as W~2 eV, the d e p a r t u r e s from the simple t i g h t - b i n d i n g model a r e u n l i k e l y t o be severe. T h i s encourages us t o r e p r e s e n t t h e c o r r e c ­ t i o n s simply by r e t a i n i n g t h e b a s i c t i g h t - b i n d i n g formalism, but i n c l u d i n g the o v e r l a p i n t e g r a l s S= which were n e g l e c t e d i n t h e standard Equation 2. I n l i g h t o f the r e c e n t molecular o r b i t a l c a l c u l a t i o n s o f I n t e r r a n t e and Messmer (23), such n e g l e c t appears u n j u s t i f i e d . T h i s e x t e n s i o n o f the t i g h t - b i n d i n g approximation i s t h e p r e c i s e analog o f the Wheland e x t e n s i o n o f the simple Hiïckel approximation i n o n e - e l e c t r o n π-molecular o r b i t a l t h e o r y (30). With i n c l u s i o n o f o v e r l a p , the o n e - e l e c t r o n e n e r g i e s o f Equa­ t i o n 2 a r e r e p l a c e d by 1

w - τ?° α+2β cos kR \ " 1+2S c o s kR

/m

E

( 8 }

w i t h group v e l o c i t y _ k

(β-aS) (2R s i n kR) (1+2S

cos

kR)

( 9 )

2

Now, i n a quasi-one-dimensional m e t a l , the s i g n o f the thermo­ e l e c t r i c power i s determined by the l o g a r i t h m i c d e r i v a t i v e o f the d e n s i t y o f the s t a t e s p(E) w i t h r e s p e c t t o energy, e v a l u a t e d a t the Fermi l e v e l : 31np 3E

=

Ep

1 8£ 3k ρ 3k 3E Ep

^

Π 3k

\>

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

,10,

366

EXTENDED INTERACTIONS BETWEEN METAL IONS

S u b s t i t u t i n g (9) i n t o (10) and assuming 3 » a S , we f i n d t h a t f o r Z=1.7 the t h e r m o e l e c t r i c power i s p o s i t i v e f o r s m a l l S, and c r o s s e s z e r o near S=0.37. From the v e r y s n a i l p o s i t i v e v a l u e s observed, we estimate S~0.35. Then u s i n g Equations 7 and 9 and the observed plasma frequency (7,24,29) o f 15,800 cm" , we have 3-aS~0.32 eV. The bandwidth 1

W - i S f c f U 2.5 eV 1-4S

(11)

Z

i s t h e n comparable w i t h Ru)p. The o b s e r v a t i o n o f a w e l l - d e f i n e d plasmon a b s o r p t i o n (24,29) suggests t h a t the t r u e v a l u e o f W i s somewhat s m a l l e r s t i l l , but t h e o v e r a l l agreement w i t h experiment i s good enough t o demonstrate t h a t a minimal improvement upon t h e simplest tight-binding reasonably a c c u r a t e r e p r e s e n t a t i o Krogmann s a l t s . I n accounting f o r t h e s t a b i l i t y o f P t 2.3+, however, the im­ proved d e s c r i p t i o n f a r e s no b e t t e r than the o l d . Indeed, the bands t r u c t u r e term U now tends t o d r i v e P t 2.3+ l e s s s t a b l e than P t 2+. I n t e g r a t i n g Equation 8 (31), r e c a l l i n g t h a t S s c a l e s roughly as 3, and proceeding as i n the p r e v i o u s s e c t i o n , we f i n d t h a t w i t h t h e same v a l u e s o f μ and η, ϋβ f o r Z=1.7 i s p o s i t i v e w i t h r e s p e c t t o Z=2 by about 0.1 eV. T h i s r e s u l t i s not s u b s t a n t i a l l y a l t e r e d f o r any p h y s i c a l l y reasonable c h o i c e s o f μ and η. Improving the a c c u r ­ acy o f our r e p r e s e n t a t i o n o f the conduction band has simply r e ­ a f f i r m e d our c o n c l u s i o n t h a t the s t a b i l i t y o f P t 2.3+ i s not a s s o c i a t e d w i t h the k i n e t i c energy o f the conduction e l e c t r o n s . B

V.

The Covalent Energy

U

c

I n the p r e c e d i n g s e c t i o n s we have argued t h a t t h e v a r i a t i o n s i n 1 % H % H J i n Equation 1 a r e t o o s n a i l , and p o s s i b l y o f the wrong s i g n , t o be r e s p o n s i b l e f o r the p e c u l i a r s t a b i l i t y o f P t 2.3+. The remaining term i s the c o v a l e n t b i n d i n g energy UQ o f the complex c y a n o p l a t i n a t e i o n i t s e l f , and we now suggest t h a t t h i s term accounts f o r the observed e f f e c t . I t i s w e l l known t h a t i n square-planar complexes o f d^ t r a n s ­ i t i o n metals the occupied molecular o r b i t a l s uppermost i n energy a r e weakly antibonding. The complex i s n e v e r t h e l e s s s t a b l e because the r e p u l s i v e c o n t r i b u t i o n from these o r b i t a l s t o U i s more than compensated by the a t t r a c t i v e c o n t r i b u t i o n from o t h e r o r b i t a l s , p r i n c i p a l l y the s t r o n g l y σ-bonding a^g, b^g, and e (23,32). Let us c o n s i d e r the o x i d a t i o n o f such a complex under OTrffitlons where, as i n t h e Krogmann s a l t s t r u c t u r e , the (time-averaged) occupation Ζ o f a h i g h - l y i n g o r b i t a l may be regarded as a continuous r a t h e r than a d i s c r e t e v a r i a b l e . S i n c e the removal o f an i n f i n i t e s i m a l amount o f charge from the d complex lowers the occupation o f an antibonding o r b i t a l , i t c e r t a i n l y tends t o s t a b i l i z e the complex. With p r o g r e s s i v e R

c

u

8

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

24.

BLOCH

AND

WEiSMAN

Krogmann

Salts

367

o x i d a t i o n , however, the nany-electron wavefunction i s r e a d j u s t e d and the e f f e c t i v e one-electron e n e r g y - l e v e l scheme renormalized s o as t o accommodate the change i n elœtron-elecrtron i n t e r a c t i o n and screening o f the c o r e p o t e n t i a l . I n p a r t i c u l a r , the h i g h - l y i n g o r b i t a l s must drop i n energy, r e f l e c t i n g the i n c r e a s i n g d i f f i c u l t y o f f u r t h e r o x i d a t i o n ; e v e n t u a l l y the h i g h e s t occupied o r b i t a l becomes bonding, and the o x i d i z e d s p e c i e s d i s p l a y s s u b s t a n t i a l electron a f f i n i t y . I n the case o f square-planar platinum complexes, t h i s has apparently happened by the time one f u l l e l e c t r o n i s removed. Hence P t 3+ i s unstable, and four-coordinated P t 4+ does not occur i n the square-planar geometry. Viewed from t h i s p e r s p e c t i v e , t h e u s u a l P t 2+ i n the f r e e complex does not represent a t r u e minimum i n Ut a s a f u n c t i o n o f Z, a s the presence o f f i l l e d antibonding o r b i t a l s a t t e s t s . Rather be a t t a i n e d under the c o n s t r a i n t i s l i f t e d , the r e a l minimum occurs a t t h a t f r a c t i o n a l o x i d a t i o n s t a t e f o r which the energy o f the p a r t i a l l y occupied o r b i t a l passes through zero. C l e a r l y , t h i s occurs somewhere between P t 2+ and P t 3+, and we suggest t h a t i n the c y a n o p l a t i n a t e i o n i t o c c u r s c l o s e t o P t 2.3+. I t i s h a r d l y necessary t o emphasize t h a t t o t e s t t h i s conjec­ t u r e r e q u i r e s more d e t a i l e d c l a c u l a t i o n s than have been presented here. Nevertheless, we a r e impressed a t t h i s p r e l i m i n a r y stage by i t s p l a u s i b i l i t y and by the l a c k o f a reasonable a l t e r n a t i v e , u n l i k e the weak (or non-existent) minimum i n U B + U R , the minimum i n UQ o c c u r s a t a s h a r p l y d e f i n e d value o f Z, and presumably r e p r e ­ sents a s u b s t a n t i a l g a i n i n the b i n d i n g energy o f the complex. The e x i s t e n c e o f such a minimum a t a f r a c t i o n a l valence i s not o f course unique t o c y a n o p l a t i n a t e , but probably o c c u r s f o r any mole­ c u l e whose ground s t a t e i n c l u d e s f i l l e d antibonding o r b i t a l s . The r e a l i z a t i o n o f the minimum i n p r a c t i c e , however, r e q u i r e s a unique s t r u c t u r e , such a s t h a t o f the Krogmann s a l t s , i n which c o v a l e n t , m e t a l l i c , and i o n i c bonding c o e x i s t . VI. Electrochemical Oxidation o f C r y s t a l l i n e K Pt(CN), t o a S o l i d State Galvanic C e l l 2

Before concluding, we d i g r e s s t o remark t h a t the e x i s t e n c e o f two s t a b l e o x i d a t i o n s t a t e s i n the same general c r y s t a l s t r u c t u r e has i n t e r e s t i n g p r a c t i c a l r a m i f i c a t i o n s . I n p a r t i c u l a r , t h e m a t e r i a l s are known t o e x h i b i t i o n i c c o n d u c t i v i t y . I t has been shown by Gcmm and U n d e r h i l l (33), and independently by others (28,34), t h a t s u f f i c i e n t l y strong e l e c t r i c f i e l d s reduce the Krogmann s a l t K2Pt(CN)4Bro.3*2.3H20 t o a compound o f d i v a l e n t platinum. We have found t h a t , c o n t r a r y t o an a s s e r t i o n by Gcmm and u n d e r h i l l (33), the r e v e r s e r e a c t i o n can a l s o be induced: hydrated s i n g l e c r y s t a l s o f KjPt(CN)4 are e a s i l y and r e v e r s i b l y o x i d i z e d by a p p l i c a t i o n o f modest e l e c t r i c f i e l d s along the needle a x i s , u s i n g mercury o r s i l v e r - p a s t e e l e c t r o d e s . A f t e r the f i e l d i s r a i s e d

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above a t h r e s h o l d o f c a . 100 V/cm, t h e r e s i s t a n c e f a l l s a b r u p t l y by two o r d e r s o f magnitude, and a dark, rapper-colored area forms a t t h e anode and spreads toward t h e cathode, where vigorous e v o l u ­ t i o n o f hydrogen gas i s observed. When t h e p o l a r i t y i s r e v e r s e d the r e a c t i o n i s r e v e r s e d a l s o , and t h e sharp boundary between t h e two phases recedes u n t i l t h e o r i g i n a l white c r y s t a l i s recovered. A t constant temperature, humidity, and a p p l i e d v o l t a g e , t h e c u r r e n t does n o t d i m i n i s h a s t h e r e a c t i o n proceeds t o completion. The (hydrated) product c r y s t a l d i s p l a y s t h e coppery l u s t u r e and d i c h r o i s m c h a r a c t e r i s t i c o f Krogmann s a l t s (1), and has shrunk t o about 90% o f i t s former l e n g t h ; t h i s corresponds t o the t y p i c a l d i f f e r e n c e i n t h e l a t t i c e spacing R between Krogmann s a l t s and t h e i r d i v a l e n t parent ocmpounds (1). By monitoring t h e i n f r a r e d t r a n s m i s s i o n spectrum o f the c r y s ­ t a l a s t h e r e a c t i o n proceeds Krogmann s a l t , and t h a s t a t e s present i n measurable q u a n t i t y . We have accumulated e v i dence t h a t t h e r e a c t i o n proceeds v i a a p r o t o n - t r a n s f e r - a n d r e d u c t i o n mechanism s i m i l a r t o t h a t suggested by Lecrone and P e r l s t e i n (35) f o r the e l e c t r o c h e m i c a l r e d u c t i o n o f mixed-valence o x a l o p l a t i n a t e systems, and we a s s i g n t o the product t h e p r e v i o u s ­ l y unreported formula. K2Pt (CN) 4 (OH) ·ηΗ2θ, where x~0.3. I f t h e a p p l i e d f i e l d i s removed b e f o r e the r e a c t i o n i s com­ p l e t e , a p o t e n t i a l d i f f e r e n c e e x i s t s between t h e o x i d i z e d and unoxidized s e c t i o n s , and t h e p a r t i a l l y converted c r y s t a l a c t s as a galvanic c e l l . The Krogmann-salt r e g i o n i s t h e anode. Because o f the h i g h i n t e r n a l r e s i s t a n c e a s s o c i a t e d w i t h t h e K2Pt (CN) 4 s e c t i o n , t h i s m i n i a t u r e s o l i d - s t a t e b a t t e r y f u n c t i o n s as a c u r r e n t r a t h e r than a v o l t a g e source. As i n t h e i n i t i a l o x i d a t i o n , t h e c u r r e n t i s q u i t e s e n s i t i v e t o water vapor i n t h e surrounding atmosphere, and i s i n f a c t a measure o f r e l a t i v e humidity. Near 100% humidity and 22°C, we f i n d t y p i c a l c u r r e n t d e n s i t i e s o f c a . 5 ma/cm and an o p e n - c i r c u i t p o t e n t i a l o f 1.35 V. χ

2

VII.

Summary and Conclusions

We summarize o u r c o n c l u s i o n s a s f o l l o w s . The p a r t i a l l y o x i d i z e d Krogmann s a l t s a r e c h a r a c t e r i z e d by a s i n g l e f i v e - s i x t h s f i l l e d conduction band which i s adequately d e s c r i b e d n e i t h e r by the n e a r l y - f r e e - e l e c t r o n model n o r by c o n v e n t i o n a l t i g h t - b i n d i n g theory. Instead, a reasonable intermediate r e p r e s e n t a t i o n , c o n s i s t e n t w i t h experiment, i s obtained by extending t h e t i g h t - b i n d i n g formalism t o i n c l u d e t h e e f f e c t s o f o r b i t a l o v e r l a p between neighboring molecules. The e l e c t r o n i c k i n e t i c energy o f such a band cannot account f o r t h e s t a b i l i t y o f t h e f i v e - s i x t h s f i l l e d bard (Pt 2.3+). Rather, we suggest t h a t t h e appearance o f t h i s phase i s a molecu­ l a r e f f e c t , r e p r e s e n t i n g the mijiimization o f t h e c o v a l e n t b i n d i n g energy o f t h e complex anion w i t h r e s p e c t t o t h e band occupation Z. Such an e f f e c t can o n l y occur, o f course, i n an unusual c r y s t a l

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AND

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WEiSMAN

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s t r u c t u r e i n which t h e o x i d a t i o n s t a t e o f the canplex i s continuously adjustable. Once Ζ i s f i x e d i n t h i s manner, the i n t e r m e t a l l i c spacing R i s c l o s e l y c o n t r o l l e d by the band energy U as suggested by Krogmann (1) and d e p i c t e d i n F i g u r e 1. I n c o n t r a s t , R i n the P t 2+ parent compounds i s determined by the Madelung energy and hence v a r i e s w i d e l y frcm ccmpound t o compound. Thus we answer questions (a) and (b) posed i n the I n t r o d u c t i o n . In response t o ( c ) , we suppose t h a t o n l y i n the o x a l o - and cyancrp l a t i n a t e complexes i s the balance between σ-bonding and π-"backbonding" such t h a t t h e 5 a i g ( d z ) o r b i t a l l i e s h i g h enough i n energy t o form a band which can be o x i d i z e d t o Z=1.7. F o r example, t h i s band must not i n t e r s e c t any o f the f l a t bands d e r i v e d frcm the o t h e r h i g h - l y i n g occupied o r b i t a l s (23) above the Z=1.7 Fermi l e v e l . I f these c o n c l u s i o n p a r t i a l l y o x i d i z e d Krogman which, by a c c i d e n t , a s e t o f r a t h e r e x a c t i n g e n e r g e t i c and s t r u c t u r a l c r i t e r i a a r e simultaneously s a t i s f i e d . I f t h i s i s so, then the s y n t h e t i c search f o r analogs i s l i k e l y t o be f r u s t r a t i n g . F i n a l l y , we remark t h a t these c o n s i d e r a t i o n s probably do not apply t o most o f the o r g a n i c TCNQ s a l t s (12,27,36), whose s t r u c ­ t u r a l and p h y s i c a l p r o p e r t i e s a r e i n some r e s p e c t s s i m i l a r t o those o f the Krogmann s a l t s . Here packing c o n s i d e r a t i o n s seem t o p r e ­ c l u d e any adjustment o f the c o u n t e r i o n p o p u l a t i o n so as t o minimize U , and the s m a l l v a l u e o f W s e v e r e l y l i m i t s the i n f l u e n c e o f ϋρ. A p a r t i a l exception i s the newly developed c l a s s o f o r g a n i c semimetals such as TTF-TCNQ (36), i n which t h e conduction e l e c t r o n s a r e d i s t r i b u t e d between two d i f f e r e n t s p e c i e s o f conducting c h a i n s . I n these, the b e s t o r g a n i c conductors known, t h e r e i s r e a l promise o f a d j u s t i n g Z, and hence the e l e c t r i c a l p r o p e r t i e s , through chemical c o n t r o l o f U (36). B

2

c

c

Literature Cited 1. Krogmann, Κ., Angew. Chem. I n t e r n a t . Ed., (1969), 8, 35, and references therein. 2. Evstaf'eva, Ο. Ν., Russ. Jour. Inorg. Chem., (1966), 11, 711. 3. Rousseau, D. L., B u t l e r , Μ. Α., Guggenheim, H. J . , Weisman, R. B. and Bloch, Α. Ν., Phys. Rev. B, 10 ( i n p r e s s ) . 4. B u t l e r , Μ. Α., Wertheim, Κ., Rousseau, D. L., and Hüfner, S., Chem. Phys. L e t t . , (1972), 13, 473. 5. Z e l l e r , H. R., Adv. S o l . S t . Phys. X I I I , (1973), and r e f e r e n c e s therein. 6. Minot, M. J . and P e r l s t e i n , J. H., Phys. Rev. L e t t . , (1971), 26, 371. 7. Bernasconi, J . , Brüesch, P., Kuse, D., and Z e l l e r , H. R., Brown-Boveri Res. Report KLR-73-05 (Brown-Boveri Res. Center, Baden, S w i t z e r l a n d , 1973). 8. B l o c h , Α. Ν., Weisman, R. B., and Varma, C. M., Phys. Rev. L e t t . , (1972), 28, 753.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

370

EXTENDED

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9. Lee, P. Α., R i c e , T. M., and Anderson, P. W., Phys. Rev. L e t t . , (1973), 31, 462. 10. Z e l l e r , H. R., this volume, and r e f e r e n c e s t h e r e i n . 11. P e i e r l s , R. Ε . , "Quantum Theory o f Solids", pp. 108 ff, Clarendon P r e s s , Oxford, 1955. 12. B l o c h , Α . Ν., in Masuda, K. and Silver, Μ., ed., "Energy and Charge T r a n s f e r in Organic Semiconductors", p . 159, Plenum P r e s s , New York, 1974, and r e f e r e n c e s t h e r e i n . 13. Z e l l e r , H. R., p r i v a t e communication. 14. Sen, P. and Varma, C. Μ., Bull. Am. Phys. Soc., (1974), 19, 49. 15. B l o c h , A. N. and Varma, C. M., J. P h y s i c s C., (1973), 6, 1849. 16. Weisman, R. B., unpublished work. 17. See,forexample, Heine, V. and Weaire, D., Sol. S t . Phys., (1970) 24, 250. 18. See,forexample Comm., (1973), 13, 697 19. See,forexample, Friedel, J. in Ziman, J. M., ed., "The P h y s i c s o f Metals I : Electrons", pp. 340 ff, Cambridge U n i v e r s i t y P r e s s , Cambridge, 1969. 20. See,forexample, E y r i n g , H., Walter, J., K i m b a l l , G. E . , "Quantum Chemistry", pp. 162-3, Wiley, New York, 1944. 21. Lee, P. Α., R i c e , T. M., and Anderson, P. W., S o l . S t . Comm., (1974), 14, 703. 22. Piepho, S. Β., Schatz, P. Ν., and McCaffery, A. J., J. Am. Chem. Soc., (1969), 91, 5994. 23. I n t e r r a n t e , L. V. and Messmer, R. P., this volume. 24. W i l l i a m s , P. F., B u t l e r , Μ . Α., Rousseau, D. L., and B l o c h , A. N., Phys. Rev. B, (1974), 10, 1109. 25. W i l l i a m s , P. F. and B l o c h , A. N., Phys. Rev. B, (1974), 10, 1097. 26. McKenzie, J. W., Wu, C., and Bube, R. Η., A p p l . Phys. L e t t . , (1972), 21, 1. 27. Schegolev, I . F., P h y s i c a S t a t . Solidi, (1972), A12, 9, and references therein. 28. Minot, M. J., (Ph.D. T h e s i s , The Johns Hopkins University, 1973). The experiments o f References 26 and 28 show t h a t t h e s m a l l n e g a t i v e thermopower r e p o r t e d by Kuse and Z e l l e r [ S o l . S t . Comm., (1972), 11, 355] is probably t h e result o f u s i n g partially dehy­ drated c r y s t a l s . 29. Wagner, H., G e s e r i c h , H. P., B a l t z , R. V., and Krogmann, Κ., S o l . S t . Comm., (1973), 13, 659. 30. See,forexample, Daudel, R., LeFebvre, R., and Moser, C., "Quantum Chemistry: Methods and Applications", pp. 67 ff, I n t e r s c i e n c e , New York, 1959. 31. Gradshteyn, I . and R y z h i k , I . Μ., "Tables o f I n t e g r a l s , S e r i e s , and Products", p. 148, Academic P r e s s , New York, 1965. 32. See,forexample, Ballhausen, C. J, "Ligand Field Theory", McGraw-Hill, New York, 1962. 33. Gomm, P. S. and Underhill, Α. Ε . , Chem. Comm., 1971, 511; J . Chem. (Dalton), 1972, 34. 34. Würfel, P., Hausen, H. D., Krogmann, Κ., and Stampfl, R.,

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WEISMAN

Krogmann

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Phys. S t a t . S o l . , A10, (1972), 537. 35. Lecrone, F. and P e r l s t e i n , Chem. Comm., 1972, 75. 36. Bloch, Α. Ν., Cowan, D. O., and Poehler, T. O., in Masuda, K. and S i l v e r , Μ., Ed., "Energy and Charge T r a n s f e r i n Organic Semi­ conductors", p. 167, Plenum P r e s s , New York, 1974, and r e f e r e n c e s therein. Acknowledgments We a r e g r a t e f u l t o the M v a n c e d Research P r o j e c t s Agency o f the Department o f Defense and t o the N a t i o n a l Science Foundation f o r p a r t i a l support o f t h i s r e s e a r c h .

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

25 T h e Peierls Transition in One-Dimensional Solids H . R.

ZELLER

a n d P.

BRÜESCH

B r o w n B o v e r i Research Center, C H - 5 4 0 1 B a d e n , S w i t z e r l a n d

In h i s book "Quantum that a one-dimensional metal i s i n h e r e n t l y unstable and will undergo a phase t r a n s i t i o n i n t o a semiconducting s t a t e . Almost simultaneously and independently the theory of t h i s phase transition was worked out by Fröhlich (2). Fröhlich showed that the phase t r a n s i t i o n may r e s u l t i n a low temperature s t a t e which i s not semiconducting but superconducting with t r a n s i t i o n temper a t u r e s not r e s t r i c t e d to the cryogenic range. Recently experimental systems have been studied which undergo a P e i e r l s t r a n s i t i o n and which are p o t e n t i a l candidates f o r the Fröhlich mechanism of s u p e r c o n d u c t i v i t y . In p a r t i c u l a r the t r a n s i t i o n was shown to occur i n K [Pt(CN) ] Br ·3(H O) (KCP) and r e l a t e d s a l t s (3). There is evidence that a P e i e r l s transition a l s o takes p l a c e i n TTF TCNQ (4). In the f o l l o w i n g we will restrict the d i s c u s s i o n to the best understood system, i . e . , K [Pt(CN) ] Br ·3(Η Ο). For a general i n t r o d u c t i o n the reader is r e f e r r e d to reference (3). Before we t u r n to the d i s c u s s i o n of the P e i e r l s instability, there is one p o i n t which should be made in connection with extended metal-metal i n t e r a c t i o n s . At first s i g h t it would seem that due to the small overlap a t i g h t binding model f o r the band s t r u c t u r e should be a very good approximation, i . e . , the c a r r i e r s would be holes i n a d band. I t came as a b i g s u r p r i s e when we discovered that the conduction band is d e f i n i t e l y not a s i n u s o i d a l t i g h t binding band but r a t h e r a p a r a b o l i c f r e e e l e c t r o n band with e f f e c t i v e mass m* = me. Although t h i s sounds h i g h l y implausible at first s i g h t , it can be explained as t y p i c a l d i m e n s i o n a l i t y e f f e c t . Nearly f r e e e l e c t r o n behaviour means n e a r l y constant e l e c t r o n d e n s i t y . As can be seen from F i g . 1 a n e a r l y constant e l e c t r o n d e n s i t y along the strand a x i s can be achieved a t a relatively modest overlap of the wave f u n c t i o n s ( i n t h i s case d -s o r b i t a l s ) . T h i s is not p o s s i b l e i n two or three dimensions. The e s s e n t i a l f e a t u r e s of the argument can a l s o be v i s u a l i s e d as f o l l o w s : In an a r r a y of spheres it is always p o s s i b l e to have them touch i n one l i n e but there is always empty space between the 2

2

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Transition

373

spheres i n a plane or i n 3-d space. Thus as a f u n c t i o n of over­ lap, f r e e e l e c t r o n behaviour i s reached much e a r l i e r i n 1-d systems than i n 2-d and 3-d ones. Of course f r e e e l e c t r o n behaviour i s r e s t r i c t e d to the strand a x i s (5). In the f o l l o w i n g we w i l l d i s c u s s the P e i e r l s - F r o h l i c h t r a n ­ s i t i o n and the p o s s i b i l i t y of high temperature superconductivity based on the F r o h l i c h mechanism. Below the phase t r a n s i t i o n a s i n u s o i d a l d i s t o r t i o n of the strands takes p l a c e such that an energy gap a t the Fermi energy i s created (6). For instance i n a quarter f i l l e d band t h i s d i s t o r t i o n w i l l have the period of four l a t t i c e spacings, s p l i t t i n g the conduction band i n t o a f i l l e d and three empty bands. Above the t r a n s i t i o n temperature a precursor shows up i n the form of a s o f t l a t t i c e v i b r a t i o n which corresponds to the low temperature s t a t i c d i s t o r t i o n . Due to the onedimensional nature o tant. There i s i n genera transformation from a m e t a l l i c i n t o a semiconducting s t a t e . Associated with the s i n u s o i d a l d i s t o r t i o n i s a s i n u s o i d a l charge d e n s i t y wave (CDW). F r o h l i c h had r e a l i z e d that w i t h i n a continuum model the f r e e energy of the system does not depend on the phase of the CDW. This implies that the CDW can be s h i f t e d f r e e l y up and down the strands without any a c t i v a t i o n energy. As i n conventional superconductivity the presence of an energy gap e f f e c t i v e l y i n h i b i t s s c a t t e r i n g processes. Thus the system should behave as a superconductor w i t h the e l e c t r o n s s u r f i n g on the propagating l a t t i c e d i s t o r t i o n ( F i g . 2). The existence of a P e i e r l s d i s t o r t i o n i n KCP was c l e a r l y demonstrated by d i f f u s e x-ray s c a t t e r i n g (7) and i n e l a s t i c neutron s c a t t e r i n g (8) experiments. A l s o the P e i e r l s gap shows up i n the o p t i c a l spectra a t low temperatures a t about 0.2 eV (£). Next we t u r n to the c e n t r a l question whether the P e i e r l s F r o h l i c h t r a n s i t i o n r e a l l y leads to superconductivity. For a conventional BCS superconductor the c o n d u c t i v i t y σ (ω) i s represented by a ό-function a t ω * ο and a peak at energies c o r ­ responding to the breaking of a Cooper p a i r . In an analogous f a s h i o n the i d e a l F r o h l i c h superconductor should e x h i b i t a 6f u n c t i o n a t ω « ο and a peak at ω * Eg, where Eg i s the P e i e r l s gap. F i g u r e 3 shows the experimental r e s u l t on KCP at 40 K obtained from Kramers Kronig a n a l y s i s of r e f l e c t i v i t y data. The peak a t ω * 1600 cm" « 0.2 eV corresponds to e x c i t a t i o n s across the P e i e r l s gap. But instead of a 6-function a t ω * ο F r o h l i c h c o l l e c t i v e mode produces a peak with f i n i t e width centered a t about 2 - 4 meV dependent on sample p e r f e c t i o n . At higher tempe­ r a t u r e s the peak gets broader and disappears around 200°K. In t h e i r fundamental paper Lee, R i c e and Anderson (10) have discussed why no true superconductivity based on the F r o h l i c h mode i s expected. Due to commensurability w i t h the l a t t i c e para­ meter, to random p o t e n t i a l s provided by i m p u r i t i e s or d i s o r d e r and to 3-d coupling the t r a n s l a t i o n a l i n v a r i a n c e i s broken and the CDW i s pinned. This corresponds to a spring constant and e

1

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

374

EXTENDED

INTERACTIONS

BETWEEN

METAL

IONS

OgGD80D8GD2O 2

Iψ I

Φ

f (z) 11^

iff ( l )

" FREE

9

e

ELECTRONS"

Figure 1. Even at a relatively small overlap hybridized d,2-s orbitals ηαϋβ an electron density which does nof depend on z, and tence the sys­ tem is free electron like in the z-direction. The same is true for higher dimensions.

Figure 2. Electron transport by the Frohlich collective mode. As in any in­ sulator the electrons are bound to a periodic potential. In the Peierls Frohlich state the periodic potential is not fixed in space but propagating and able to carry an electric current along the strands.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

25.

ZELLER

A N D BRUESCH

Peierls

Transition

375

hence the c o n d u c t i v i t y peak w i l l be centered not a t ω « ο but a t a small (compared w i t h Eg) but f i n i t e frequency* Lifetime effects which a r e p a r t i c u l a r l y Important a t higher temperature cause a f i n i t e width o f the peak i n s t e a d o f a (S-function. As a con­ sequence i n the r e g i o n o f the t r a n s i t i o n temperature the F r o h l i c h mode may c o n t r i b u t e t o o r even dominate the dc c o n d u c t i v i t y . Whether t h i s i s the case o r not can most e a s i l y and d i r e c t l y be determined from measurements o f R (ω) i n the microwave and f a r i n f r a r e d r e g i o n . Due to the l a r g e e f f e c t i v e mass o f the F r o h l i c h mode i t s o s c i l l a t o r strength i s small and i t can a t most form a narrow peak i n σ (ω) superimposed on the very broad s i n g l e p a r t i c l e conductivity. From what we have learned on the model system KCP i t seems f e a s i b l e t o s y n t h e s i z e systems which e x h i b i t a s u f f i c i e n t l y small pinning f o r c e suc F r o h l i c h mode can be achieved described i n t h i s paper was c a r r i e d out i n c o l l a b o r a t i o n w i t h D. Kuse, H.J. R i c e and S. S t r a s s l e r . We a l s o wish to acknowledge s t i m u l a t i n g d i s c u s s i o n s with P. Fulde and T.M. R i c e . F i g . 2 i s due to L. Niemeyer. References 1. 2. 3. 4.

5. 6. 7. 8.

9. 10.

P e i e r l s R.E., "Quantum Theory o f S o l i d s " (Oxford U n i v e r s i t y Press, London (1955). Fröhlich, Η., Proc. Roy.Soc.,London (1954) A 223, 296. Z e l l e r , H.R., Advances in S o l i d State P h y s i c s , (1973) 13, 31. Coleman, L.B., Cohen, M.J., Sandman, D.J., Yamagishi, F.G., G a r i t o , A.F. and Heeger, A.J., S o l i d State Comm. (1973) 12, 1125. The above arguments a r e due to P. Fulde and S. Strässler. R i c e , M.J. and Strässler, S., S o l i d State Comm. (1973) 13, 125. Comès, R., Lambert, Μ., Launois H. and Z e l l e r , H.R., Phys. Rev. (1973) B8, 571. Renker, B., R i e t s c h e l , H., P i n t s c h o v i u s , L., Gläser, W., Brüesch, P., Kuse, D. and R i c e , M.J., Phys. Rev. Letters(1973) 30, 1144. Brüesch, P. and Z e l l e r , H.R., t o be p u b l i s h e d . Lee, P., R i c e , T.M. and Anderson, P.W., p r e p r i n t .

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

26 T h e Preparation o f and the Anisotropic Dielectric Properties o f R. B .

SAILLANT

K Pt(CN) Br 2

a n d R.

4

C.

·

0.30

3H O 2

JAKLEVIC

F o r d M o t o r C o . , P . O . B o x 2053, D e a r b o r n , Mich. 48121

The purpose o f t h i the composition o f K Pt(CN) Br ·3H O, the chemical methods used t o minimize these v a r i a t i o n s , and a p h y s i c a l method t h a t re­ flects the intrinsic physico-chemical p r o p e r t i e s but is s e n s i t i v e to small m a t e r i a l v a r i a t i o n s . E a r l y attempts t o grow c r y s t a l s o f the mixed-valence plati­ num salt r e s u l t e d in c r y s t a l s which were p i t t e d , e x h i b i t e d trans­ verse s t r i a t i o n s and tended t o grow v e r y r a p i d l y in the needle direction. Cavities and pits had been reported by other workers (1). Neutron a c t i v a t i o n a n a l y s i s o f t y p i c a l c r y s t a l s revealed c h l o r i d e contamination o f the bromide salt. Results o f t h i s study have appeared (2). The c r y s t a l growth a f f o r d s a partition­ ing between c h l o r i d e and bromide which s t r o n g l y favors the c h l o ­ r i d e complex in the s o l i d s t a t e . The s o l u t i o n behavior o f K Pt(CN)4Br ·3H O was examined t o determine if exchange r e a c t i o n s might occur s i n c e P t ( I I ) is known t o c a t a l y z e Pt(IV) s u b s t i t u t i o n r e a c t i o n s ( 3 ) . The e l e c ­ t r o n i c spectrum from 10,000 t o 40,000 cm o f K Pt(CN) was taken and compared t o t h a t o f K Pt(CN) Br , f i g . 1. One maximum was observed f o r each complex i n t h i s r e g i o n . K Pt(CN) exhibited a t r a n s i t i o n a t 35,900 cm , ε = 1528, p r e v i o u s l y i d e n t i f i e d as a metal to l i g a n d charge t r a n s f e r band (4). K P t ( C N ) B r e x h i b i t s a max­ imum a t 29,200 c m , ε = 1280, which is l o c a t e d i n an ε = 0 region o f the K Pt(CN) spectrum. The energy separation o f these two maxima and knowledge o f the e x t i n c t i o n c o e f f i c i e n t s enable one t o o b t a i n an exact r a t i o o f P t ( I I ) / P t ( I V ) and, t h e r e f o r e , fix the stoichiometry o f the mixed-valence complex. When, however, c r y s t a l s o f the mixed-valence complex are d i s s o l v e d i n aqueous s o l u t i o n , f i g . 2, the spectrum i s not t h a t expected from the p r o ­ p o r t i o n a l mixing o f the s t a r t i n g compounds. The c r y s t a l s must be d i s s o l v e d i n 0.5N KBr i n order t o suppress i n t e r f e r e n c e due t o aquation and h y d r o l y s i s products. When the spectrum i s obtained i n 0.5N KBr s o l u t i o n s , the r a t i o o f P t ( I I ) t o Pt(IV) i s d e t e r ­ mined t o be 3.333 ± .004. T h i s c o r r e s p o n d s t o 0.30 bromide atoms per complex i f the platinum (IV) i s present as K P t ( C N ^ B r ^ other 2

4

0.30

2

2

0.30

2

-1

2

2

4

4

2

4

2

-1

2

4

2

-1

2

4

2

376 In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

SAILLANT AND JAKLEVIC

K Pt(CN)f Br . o 2

t

0 S

'

3H 0 g

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

EXTENDED

INTERACTIONS

BETWEEN

K Pt (CN) C1 +0.IN KCl 2

4

2

K Pt (CN) Cl 2

J

4

I

I

2

I

I

I

I

MIN. χ Figure

3. pH

vs. time

for solutions

containing

KfPtfCN^cU without KCl ( — Δ — Δ — Δ — ) with 0.ΙΝ KCl ( — Ο — O — O — )

Mylar tub* for positioning and orientation

B. W. 0. 8-12 GHz

κζ5

Isolator Attenuator

Detector Wavemeter Attenuator

4·Κ

Resonator Crystal ^

Microwave Réflectance Spectrometer Figure

4.

Experimental

setup, B.W.O. oscillator

=

backward

wave

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

METAL

IONS

SAILLANT. AND JAKLEVIC

26.

·

K ?t(CN) Br , t

h

0 i0

3H 0

379

2

methods i n c l u d i n g X-ray f l u o r e s c e n c e suggest the r a t i o o f K:Pt:Br as 2:1:0.30 (5) which g e n e r a l l y supports the bromide concentra­ tion. The h y d r o l y s i s o f the platinum (IV) complex may be followed by observing the change i n pH with time (6). The r a t e and extent o f h y d r o l y s i s of the chloro-complex i s much more r a p i d than the bromide complex and i s shown i n f i g . 3. I t i s p o s s i b l e t o sup­ press h y d r o l y s i s by the a d d i t i o n of the h a l i d e s a l t . The con­ tamination of the bromo-complex by c h l o r i d e may be the r e s u l t o f the increased l a b i l i t y of the c h l o r i d e compound. T h i s i n c r e a s e d l a b i l i t y might p l a y a r o l e i n the growth of the s o l i d phase*which contains square p l a n a r platinum,from the s o l u t i o n phase which contains both square p l a n a r and o c t a h e d r a l platinum complexes. In the f o l l o w i n g X-Pt-X (Pt)

2

n

2

+ X - P t " ^ (Pt) -Pt-X -

(2)

n

(Ptj -Pt-X - + Pt "55i (Pt)l; 2

3

2

n

2

+ X-Pt "

(3)

r e a c t i o n sequence the cyanide l i g a n d s have been omitted f o r clarity. The more l a b i l e the h a l i d e i s i n the l e f t hand s p e c i e s i n r e a c t i o n 3, the more r a p i d l y the s o l i d phase would grow. Thus, the competition o f c h l o r i d e and bromide depends on t h e i r r e s p e c t i v e l a b i l i t i e s a t t h i s step (2). S p e c i a l care must be taken to grow K P t ( C N ) ^ B r Q Β 3 ο * 3H 0 from only the p u r e s t m a t e r i a l s and the s a l t i s prepared from high p u r i t y platinum metal. The best c r y s t a l s are grown from s o l u t i o n s which are t e n t h normal i n KBr and 1 molar i n urea. These c r y s t a l s show no p i t s , no t r a n s v e r s e etch marks and no channels. In a d d i ­ t i o n , the urea a c t s t o r e t a r d the growth along the needle a x i s and the r e s u l t i s l a r g e 0.8 χ 0.8 χ 2 cm c r y s t a l s s u i t a b l e f o r v a r i o u s types of p h y s i c a l measurements (2). The p h y s i c a l measurement used i n t h i s study i n v o l v e s the un­ u s u a l l y high d i e l e c t r i c constant reported f o r K P t ( C N ) i^BrQ Β 3 0 * 3H 0. The c o n d i t i o n f o r resonance of a d i e l e c t r i c i s t h a t the s i z e must be *v* (λο//ε) where λο i s the f r e e space wavelength and ε i s the d i e l e c t r i c constant. Since ε has been reported t o be about 10 (1), m i l l i m e t e r type dimensions are t o be expected a t 10 Hz frequencies. The d i e l e c t r i c resonator i s analogous t o a resonant metal c a v i t y i n t h a t the h i g h l y r e f l e c t i n g d i e l e c t r i c w a l l s serve t o t r a p waves i n the d i e l e c t r i c . However, the s i z e o f the resonant d i e l e c t r i c i s reduced by the f a c t o r ε " / and t h e r e f o r e , f o r l a r g e d i e l e c t r i c constant m a t e r i a l s , the c r y s t a l can be p l a c e d d i r e c t l y i n s i d e a standard microwave waveguide and the resonance observed by f i n d i n g the wavelength a t which the c r y s t a l absorbs power. For an a n i s o t r o p i c m a t e r i a l , with values e\\ and ε± p a r a l l e l to and p e r p e n d i c u l a r to the c - a x i s r e s p e c t i v e l y , a r e c t a n g u l a r 2

2

2

2

3

10

1

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

380

EXTENDED INTERACTIONS BETWEEN M E T A L IONS

p a r a l l e l e p i p e d of dimensions a, b and c w i l l have resonant quencies f given by

fre­

0

v

I cJ

ε„ [ 'a

b

'

+ -Φ

2

(4)

where the i n t e g e r 1 or m > 0 and the i n t e g e r η > 0 and v = 3 χ ΙΟ cm/sec. These modes are TM(H) modes i n which the mag­ n e t i c f i e l d i s everywhere transverse to the c-axis and the e x t e r ­ n a l f i e l d s are magnetic m u l t i p o l e i n c h a r a c t e r . The l i n e s o f e l e c t r i c f i e l d tend to be c i r c u l a r while magnetic l i n e s are per­ p e n d i c u l a r to the boundaries. The TE modes a l s o e x i s t but do not have low frequency resonant modes s i n c e only ε i s i n v o l v e d i n the frequency equation and εχ - 4.0. To d e r i v e Eq. 4, the soc a l l e d o p e n - c i r c u i t boundar p a r a l l e l and p e r p e n d i c u l a a maximum a t the boundary, e t h e i respective perpendicula and p a r a l l e l components are zero. These are the exact opposites of the c o n d i t i o n s imposed a t a metal boundary, and are a p p r o p r i ­ ate f o r a n e a r l y i n f i n i t e d i e l e c t r i c constant m a t e r i a l . Hence Eq. 4 i s only approximate and experience has shown t h a t i t can be o f f by as much as 25% f o r low order modes even when ε > 100 (7). T h e o r e t i c a l arguments show t h a t the d i r e c t i o n o f the e r r o r i s such t h a t observed resonant frequencies are higher than p r e d i c t e d by Eq. 4; hence one would underestimate e\\ or εχ. Higher order modes are expected to obey Eq. 4 more a c c u r a t e l y . An accurate theory f o r a n i s o t r o p i c d i e l e c t r i c resonators does not e x i s t a t present. The microwave measurements were done i n a simple r e f l e c t a n c e spectrometer between 4 and 20°K. The experimental d e t a i l has been presented elsewhere (8). The r e s u l t s provide values f o r ε, and εχ of 3000 and 4.0 r e s p e c t i v e l y which must be considered lower l i m i t s . T h i s method i s very s e n s i t i v e t o i m p u r i t i e s , c r y s t a l imperfection and dehydration, and i s non-destructive. c

1 0

χ

(

Literature Cited 1. 2. 3. 4. 5. 6. 7.

Berenblyum, A.S., Buravov, L . I . , Khidekel, M.D., Shchegolev, I.F. and Yakimov, E.B., JETP (1971), 13, 440. S a i l l a n t , R.B., J a k l e v i c , R.C. and Bedford, C.D., Mat. Res. Bull. (1974), 9, 289. Mason, W.R., Coord. Chem. Rev. (1972), 7, 241. Moreau-Colin, M.L., S t r u c t . Bond. (1972), 10, 167. S a i l l a n t , R.B., J a k l e v i c , R.C. and J a k l e v i c , J . , to be pub­ lished. Chernyaev, I . I . , Babkov, A.V., and Zheligoyskaya, N.N., J . Inorg. Chem. (U.S.S.R.) (1963), 8, 1279. Yee, H.Y., IEEE Trans. on Microwave Theory and Techniques MTT-12 (1965), 256 and references c i t e d t h e r e i n ; Gastine, Μ., C o u r t o i s , L. and Dormann, J.L., IEEE Trans, on Microwave

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

26.

8.

SAILLANT

AND

jAKiEvic

K Pt(CN 2

), Br . t

0 30

- 3H 0 2

381

Theory and Technique MTT-15 (1967), 694. J a k l e v i c , R.C. and Saillant, R.B., t o be p u b l i s h e d in S o l i d State Communications (1974).

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

27 A

SCF-Xα-SW

Investigation o f Solid State

Interactions i n L.

V.

INTERRANTE

Pt(CN)4n-

Complexes

a n d R. P. MESSMER

G e n e r a l E l e c t r i c C o r p o r a t e Research a n d Development, P. O . B o x 8, Schenectady, N . Y . 12301

Introduction Recent work on the compounds, K P t ( C N ) X · n H O , (X = Cl, Br, h e r e a f t e r r e f e r r e d t o as KCP) and some r e l a t e d n o n - s t o i c h i o m e t r i c , mixed-valence platinum and i r i d i u m complexes, has evidenced h i g h l y d i r e c t i o n a l electrical and optical p r o p e r t i e s in the s o l i d s t a t e , c o n s i s t e n t with l a r g e l y one-dimensional i n t e r ­ a c t i o n s among the c o n s t i t u e n t planar complex u n i t s (1). X-ray diffraction s t u d i e s suggest that the primary interatomic i n t e r ­ a c t i o n s a r e w i t h i n the l i n e a r , parallel chains o f c l o s e l y spaced metal atoms which r e s u l t from the columnar s t a c k i n g o f these units (2). The nature o f these interatomic i n t e r a c t i o n s and t h e i r r e l a t i o n s h i p to the unusual s o l i d s t a t e behavior of these systems is not very w e l l understood a t present. I t has been suggested that the i n t e r a c t i o n s o f primary importance a r e those i n v o l v i n g the "d 2 " o r b i t a l s on the metal atoms which overlap in the solid to produce a one-dimensional band of s t a t e s o f a p p r e c i a b l e width (1,2). For the Pt(CN)4n- complexes, an energy l e v e l scheme deduced from s p e c t r a l and magnetic c i r c u l a r dichroism data which p l a c e s the "d " orbital as the highest occupied l e v e l in the free Pt(CN) i o n (3) i s g e n e r a l l y assumed as the appropriate s t a r t i n g p o i n t f o r the d i s c u s s i o n o f the e l e c t r o n i c s t r u c t u r e o f the s o l i d complexes. In the case of the KCP d e r i v a t i v e s , t h i s leads directly t o the c o n c l u s i o n that the Fermi level p o s i t i o n must lie w i t h i n a d like band (Figure 1) ( 4 ) . Recent ESR r e s u l t s lend support to t h i s c o n c l u s i o n regarding the Fermi level p o s i t i o n in KCP (5); however, the assumption that the "d " orbital is a l s o the highest occupied l e v e l in the f r e e Pt(CN) i o n is still open t o question. Indeed, the order o f the d - l i k e o r b i t a l s i n t h i s i o n have been the subject of much controversy i n the past and a t the present time the experimental evidence does not permit any d e f i n i t e conclusions i n t h i s regard (6). 2

4

0.3

2

z

2

z

2-

4

2-

z

2

z

2-

4

382

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

27.

INTERRANTE

Solid State

AND MESSMER

383

Interactions

As p a r t of a d e t a i l e d t h e o r e t i c a l study o f the e l e c t r o n i c s t r u c t u r e of the KCP system we have c a r r i e d out some molecular o r b i t a l c a l c u l a t i o n s u s i n g the s e l f - c o n s i s t e n t - f i e l d - X a - s c a t t e r e d wave (SCF-Xa-SW) method on the PtCCN)^ ~ i o n and a dimer u n i t , [Pt(CN) 2-J arranged i n the c o n f i g u r a t i o n found i n KCP s o l i d (2). The i n t e n t o f these s t u d i e s i s to i n v e s t i g a t e the i n t e r molecular o r b i t a l i n t e r a c t i o n s and, i n g e n e r a l , the transformation i n e l e c t r o n i c s t r u c t u r e that occurs on formation of a molecular s o l i d such as KCP from i t s c o n s t i t u e n t planar complex u n i t s . P r e l i m i n a r y r e s u l t s of the Pt(CN) 2 - c a l c u l a t i o n have been reported elsewhere (7). T h i s paper d e s c r i b e s f u r t h e r d e t a i l s regarding t h i s c a l c u l a t i o n and the r e s u l t s of the study of the dimer u n i t . 2

L F

2 >

ι

U

0°

METAL ORBITALS

W'

.

Pt(CN)

'//////////

SL SL JL SL S_

£- X"

4

K Pt(CN) X -3H 0 2

POSTULATED LIGAND - FIELD SPLITTING OF THE ORBITAL ENERGY LEVELS IN THE Pt (CN ) " ION

4

û3

2

BAND FORMATION IN K PI(CN) X 3* 3 H 0 2

4

0

2

2

4

Figure

1. Previously postulated energy level scheme for Pt(CN)^' the K,Pt(CN)iX . · 3H 0 complexes (cf. Réf. 1 and 4) 9 9

and

£

Method o f C a l c u l a t i o n The SCF-Xa-scattered wave method (8^9) used i n these calculations employs the Χα theory o f S l a t e r (10) and the m u l t i p l e s c a t t e r i n g formalism of Johnson (11) to s o l v e the Χα equations. The Χα theory i s a one-electron method and i s thus s i m i l a r i n t h i s respect t o the more t r a d i t i o n a l Hartree-Fock theory. However, u n l i k e the Hartree-Fock theory which has a non­ l o c a l exchange p o t e n t i a l , the Χα has a l o c a l exchange p o t e n t i a l p r o p o r t i o n a l t o the cube r o o t of the e l e c t r o n i c charge d e n s i t y . The l a t t e r f a c t makes i t p o s s i b l e t o s e t up a m u l t i p l e s c a t t e r i n g formalism f o r a molecule or c l u s t e r of atoms which i s s i m i l a r i n many r e s p e c t s t o the Korringa-Kohn-Rostoker method (12) i n the theory of energy bands i n s o l i d s . In the s c a t t e r e d wave method, the space occupied by a mole­ c u l e i s d i v i d e d i n t o three r e g i o n s . Region I c o n s i s t s o f

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

384

EXTENDED INTERACTIONS BETWEEN M E T A L IONS

s p h e r i c a l volumes l o c a t e d about the centers o f the r e s p e c t i v e atoms, so constructed that the spheres are tangent to one another [or more g e n e r a l l y , are somewhat overlapping (13,14)]. Region I I i s the volume o u t s i d e the atomic spheres and i n s i d e an "outer sphere" which surrounds the molecule and i s centered a t the mole­ c u l a r center; t h i s outer sphere i s constructed to be tangent to the atomic spheres a t the e x t r e m i t i e s of the molecule. Region I I I i s the space o u t s i d e the outer sphere. In the case of i o n i c s p e c i e s , such as the ones considered here, the e l e c t r o s t a t i c p o t e n t i a l provided by the surrounding medium i n s o l i d or s o l u t i o n i s approximated by surrounding the i o n by a sphere [the s o - c a l l e d Watson sphere (15)] w i t h a charge o p p o s i t e to that on the i o n . In each of the atomic spheres a v a l u e of a, the exchange parameter, i s used which has been determined from atomic c a l c u l a t i o n s (16). Th used f o r the Pt(CN) 2 same v a l u e s are employed here f o r the dimer. The outer sphere and Watson sphere r a d i i i n the case of the dimer were both s e t a t 7.828 Bohr. The SCF-Xa-SW method has been a p p l i e d to analogous squareplanar systems (17,18,19) such as P t C l - , P d C l 2 - and [PtClgiCgH^)]", as w e l l as a v a r i e t y or other t r a n s i t i o n metal complexes (20,21) w i t h a great d e a l of success. The o p t i c a l p r o p e r t i e s and, when a v a i l a b l e , x-ray photoemission s p e c t r a i n each case have been found to be i n v e r y good accord w i t h experiment. 2

I|

R e s u l t s and D i s c u s s i o n The r e s u l t s of the c a l c u l a t i o n s on both the P t ( C N ) 2 - and [Pt(CN) 2-] u n i t s are g i v e n i n F i g u r e 2 i n the form of onee l e c t r o n o r b i t a l energy l e v e l diagrams. For the PtiCN)^ ~ i o n there are s e v e r a l f e a t u r e s of the c a l c u l a t i o n which are p a r t i c u l a r l y noteworthy. F i r s t of a l l , c o n t r a r y to previous assumptions, we f i n d extensive admixture o f metal "d" o r b i t a l s w i t h l i g a n d σ and w o r b i t a l s i n many of the molecular o r b i t a l s of the complex suggesting an a p p r e c i a b l y covalent m e t a l - l i g a n d bonding i n t e r a c t i o n . T h i s i s i l l u s t r a t e d i n Table I which l i s t s the p r o p o r t i o n of o r b i t a l charge i n metal and l i g a n d atomic spheres f o r s e v e r a l o f the highest occupied molecular o r b i t a l s . A p p r e c i a b l e metal d - o r b i t a l character i s evident i n the r e l a t i v e l y deep l e , l b g and 4 a bonding o r b i t a l s as w e l l as t h e i r antibonding counterparts, 2 e , 2 b and 5 a a t higher energy. The extensive metal-ligand overlap i n the l b o r b i t a l , f o r example, i s i l l u s t r a t e d i n F i g u r e 3. I|

k

2

g

2

J g

g

l g

g

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

2 g

INTERRANTE AND MESSMER

27.

Figure 2. Partial one-electron (C symmetry) and Pt(CN)f" iv

Solid

State

Interactions

385

molecular orbital energy level diagram for [Pt(CN)> ] (D ) showing all states above-12 eV and the correlation between orbitals f

4h

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

EXTENDED

386

INTERACTIONS

BETWEEN

METAL

IONS

Table I Molecular O r b i t a l Charge D i s t r i b u t i o n For

2Pt(CN)^

% of O r b i t a l Charge Orbital

2 b

2g

2e

Ft

£

Ν

Inter-sphere

Outer Sphere

34.0

10.8

28.4

24.7

2.0

25.9

14.4

31.1

26.9

1.8

0.0

23.9

39.3

35.0

1.8

1.3

27.7

0.0

24.3

36.9

37.0

1.7

72.7

4.2

1.4

21.3

0.4

1.3

24.4

32.1

40.4

1.8

13.0

37.8

19.7

26.6

2.9

65.8

9.5

6.4

17.9

0.3

57.1

13.1

7.6

21.8

0.4

4.3

8.8

58.3

17.6

11.0

2.2

8.4

60.0

17.7

11.8

3.5

17.5

46.2

25.5

7.3

g

5e

u

l b

2u

5 a

lg

2 a

2«

4 e

u

le g l b

2g

4 a

ig

3b. ig 3e

Of these d - l i k e molecular o r b i t a l s the 5 a i s r a t h e r unique i n i t s c o n c e n t r a t i o n of charge on the metal. As i s shown i n F i g u r e 4, t h i s o r b i t a l i s s u b s t a n t i a l l y antibonding with r e s p e c t to the m e t a l - l i g a n d i n t e r a c t i o n and has a high component of metal d c h a r a c t e r w i t h a l a r g e p r o p o r t i o n of the e l e c t r o n i c charge extending out perpendicular to the Pt(CN) ~ plane along the ζ axis. I t a l s o appears that there i s s i g n i f i c a n t s c h a r a c t e r i n t h i s o r b i t a l , as i s suggested by the Pt-C overlap and the general shape of the o r b i t a l . Thus the l i k e l i h o o d of extensive i n t e r ­ a c t i o n among such o r b i t a l s i n a s o l i d such as KCP i s q u i t e evident. Along w i t h the general strong metal-ligand mixing, the upper­ most " d - l i k e " o r b i t a l s are not e n e r g e t i c a l l y separated from the J g

2

z

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

INTERRANTE

AND

MESSMER

Solid

State

Interactions

Figure 3. Contour plot of lb molecular orbital wave function. Amplitude of wave function increases by a factor of 5 with each increase in absolute value of contour label. Sign of labels gives sign of orbital lobes. Interior nodes at various atoms are not shown for clarity. Mg

Figure 4. Contour plot of 5a,g molecular orbital wave function, labeled as in Figure 3 except that each integral increase in value of contour label is a factor of 3 increase in orbital amplitude

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

388

EXTENDED INTERACTIONS

BETWEEN M E T A L IONS

ligand-based o r b i t a l s as i s u s u a l l y supposed but are found i n the same energy r e g i o n . In f a c t , the important d * (+ s) molecular o r b i t a l (the 5 a ) , r a t h e r than being the highest occupied o r b i t a l i n the f r e e i o n , as has been assumed i n most of the s o l i d s t a t e s t u d i e s of the KCP d e r i v a t i v e s , i s found to l i e below three CN" based o r b i t a l s and i s 0.8 eV below the h i g h e s t occupied energy l e v e l . Among the occupied d - l i k e o r b i t a l s the energy order is d y>d y >d * which i s the same as that p r e v i o u s l y proposed by Mason and Oray (22) but u n l i k e that deduced by Piepho, Shatz and McCaffery (PSM) (3) on the b a s i s of s p e c t r a l and magnetic c i r c u l a r d i c h r o i s m data. The same energy l e v e l order was obtained i n our e a r l i e r s t u d i e s of the M C l ^ " (M • Pd,Pt) complexes (18) where again the highest occupied energy l e v e l was the b (d ). The major differences with respec the r e l a t i v e p o s i t i o n 3a (M p +L) o r b i t a l energy l e v e l s which are reversed i n the two types of complexes. This 3 a o r b i t a l i s found to be mainly l o c a l i z e d on the n i t r o g e n atoms of the CN" groups and i s c o n s i d e r a b l y c l o s e r i n energy to the occupied d - l i k e MO's than i n the M C l ^ " case, which should l e a d to r e l a t i v e l y low energy e l e c t r o n i c t r a n s i t i o n s w i t h a p p r e c i a b l e M -*· L charge t r a n s f e r c h a r a c t e r . In a manner analogous to that p r e v i o u s l y reported f o r the MCl^ ~ complexes (18), t r a n s i t i o n s t a t e c a l c u l a t i o n s were c a r r i e d out on the Pt(CN) 2"" i o n l e a d i n g to c a l c u l a t e d e l e c t r o n i c t r a n s i t i o n energies which are i n good agreement w i t h both spec­ t r a l and magnetic c i r c u l a r dichroism experiments as w e l l as p h o t o e l e c t r o n data. The comparison w i t h the s p e c t r a l data i s shown i n Table I I . Following PSM (3), the lowest energy v i s i b l e a b s o r p t i o n band has been assigned to a Ey' s t a t e (using double group n o t a t i o n ) produced by mixing of the u s u a l " s i n g l e t " and " t r i p l e t " e x c i t e d s t a t e s of the P t i C N ) ^ ~ i o n under the i n f l u e n c e of s p i n o r b i t c o u p l i n g ( I t must be emphasized that s p i n o r b i t c o u p l i n g has not been e x p l i c i t l y included i n t h i s c a l c u l a t i o n , o n l y i t s e f f e c t on the number and symmetry of the e x c i t e d s t a t e s has been considered). Here, however, the o r i g i n of t h i s t r a n s i t i o n i s i d e n t i f i e d as the 2b l e v e l r a t h e r than the a ( d 2 ) as was assumed by PSM. Also the shoulder a t 5.1 eV has been assigned to a t r a n s i t i o n from one of the l i g a n d π-levels ( l a ) to the 3 a « Otherwise the a s s i g n ­ ments are i n b a s i c agreement with those proposed p r e v i o u s l y and are e n t i r e l y c o n s i s t e n t with the a v a i l a b l e magnetic c i r c u l a r d i c h r o i s m and s p e c t r a l data. Furthermore, the r e s u l t s are a l s o i n good q u a l i t a t i v e agreement w i t h the photoelectron data f o r the P t ( C N ) ^ 2 - complex (7,23) suggesting t h a t , indeed, a s a t i s f a c t o r y d e s c r i p t i o n f o r the e l e c t r o n i c s t r u c t u r e of t h i s i o n has been obtained. As i s evident from F i g u r e 2, combining two such P t ( C N ) ^ u n i t s a t 2.89 A s e p a r a t i o n i n the staggered c o n f i g u r a t i o n found z

l g

X

x z

Z

2

2 g

2 U

x y

2

2 U

2

z

J|

2

2

l g

2

z

2 u

2

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

(10,700)

39.2

(22,100)

15

1

l a

l a

5 a

5 e

41.1

43.6

50.8

52.4

*

2 b

39.5

2 e

2 e

38.7

38.7

2 b

37.9

3

Transition State Calculation cm" χ 10"

« *

( E

2«

2u

( A

( A

( A

( B

lu

A

)

E

)

2u

lu

)

)

2« 2U, u

2u

lg u>

3 a

3 a

3 a

3 a

(

2u V

( E

(

2u V

2u «» 2u

3 a

3 a

3 a

6 a

lg *

2g -

2g *

2g *

g *

g *

2g *

Transition

)

b

a

e

b

a

a

(

e

2g * u

a

( E

u

( A

V

)

lg - 2u 2u

(E

2 u

g * 2u u>

2g -

)

Mason and Gray*

e

a

°2g

g -

a

a

2 u

(Ε

(

2u

( B

(

)

V

Α

lu

2u V a

&

(E

a

g * 2α ύ, 2ύ>

a

l g * 2u u>

lg *

e

a

Piepho, Shatz and McCaffery c

·

* Several additional transitions (3 symmetry forbidden, singlet and 6 t r i p l e t ) are calculated to occur i n this region. a) Ref. [3]· b) Energies of f u l l y allowed charge transfer transitions are underlined; c) Primes on symmetry labels denote a double group state derived from spin orbit s p l i t t i n g of the appropriate t r i p l e t excited state. Only the resultant symmetry allowed transition states are noted; d) Ref. 122J

46.1

41.3 (^l,900sh)

(1,480)

1

35.8

3

Experimental Absorption Peak Maxima cm" χ 10-3 (extinction coeff)

4

2Table I I . Experimental and T h e o r e t i c a l T r a n s i t i o n Energies f o r T r a n s i t i o n s i n P t ( C N )

EXTENDED

390

INTERACTIONS BETWEEN M E T A L IONS

i n the u n i t c e l l o f the KCP complexes r e s u l t s i n s u b s t a n t i a l i n t e r a c t i o n among s e v e r a l o f the o r b i t a l s l e a d i n g to "bonding" and "antibonding" o r b i t a l p a i r s separated by as much as 2,65 eV, The l a r g e s t i n t e r a c t i o n i s c l e a r l y among the a o r b i t a l s and i n p a r t i c u l a r between the 5 a o r b i t a l s which our c a l c u l a t i o n s i n d i c a t e a r e l a r g e l y based on the metal and have a h i g h concent r a t i o n o f e l e c t r o n d e n s i t y along the metal c h a i n a x i s . In the case o f the 5 a o r b i t a l t h i s interaction i s s u f f i c i e n t to r a i s e an antibonding a o r b i t a l such that i t becomes the highest occupied l e v e l i n the dimer u n i t . Large i n t e r a c t i o n s a r e a l s o i n d i c a t e d among s e v e r a l o f the a l e v e l s - p a r t i c u l a r l y the lowe s t unoccupied 3 a - as w e l l as among c e r t a i n o f the e l e v e l s . In g e n e r a l , w i t h i n each type o f o r b i t a l there seems to be a rough c o r r e l a t i o n between the amount of o r b i t a l i n t e r a c t i o n and the proportion of electro densit th metal would b expected due t o the r e l a t i v e l y staggered arrangement in-plane b and b o r b i t a l s s h o w e s s e n t i a l l y no i n t e r a c t i o n a t t h i s metal-metal s e p a r a t i o n (2.89 A) c a r r y i n g over as e s s e n t i a l l y u n s p l i t b , b p a i r s o f l e v e l s . I n a d d i t i o n t o these d i r e c t o r b i t a l i n t e r a c t i o n s there i s a general r a i s i n g o f the energy o f the o r b i t a l s i n the dimer w i t h respect t o those i n the f r e e i o n , presumably r e f l e c t i n g the increased s h i e l d i n g i n the c l o s e l y spaced dimer u n i t . l

g

l g

J g

x

2

u

2 U

J g

2 g

e

2

These observations have some important i m p l i c a t i o n s w i t h regard to s o l i d s t a t e i n t e r a c t i o n s i n Pt(CN) ~ complexes. In p a r t i c u l a r , the ordering of the energy bands and the p o s i t i o n o f the Fermi l e v e l i n these s o l i d s should be a v e r y s e n s i t i v e f u n c t i o n o f the intermolecular s e p a r a t i o n and i n the case o f the KCP complexes, where t h i s s e p a r a t i o n i s q u i t e short, the h i g h e s t occupied band i s indeed l i k e l y to be a n e a r l y f r e e - e l e c t r o n l i k e d 2 + s band as has been suggested p r e v i o u s l y (24). However, as these r e s u l t s show, i t does not f o l l o w that the d 2 - l i k e o r b i t a l i s a l s o the highest l e v e l i n the f r e e i o n and, indeed, f o r many of the s o l i d P t ( C N ) 2 - complexes which d i s p l a y unusual s o l i d s t a t e s p e c t r a l p r o p e r t i e s buf where the metal-metal separations a r e i n the range 3.63 - 3.09 A (6), the Fermi l e v e l may w e l l l i e w i t h i n q u i t e narrow " d " or d " type bands r a t h e r than a wide d 2 - 1 i k e band. ' More d e f i n i t i v e answers t o these questions and t o the c e n t r a l question regarding the e l e c t r o n i c s t r u c t u r e o f the KCP complexes a r e c u r r e n t l y being sought i n f u r t h e r SCF-Xa-SW c a l c u l a t i o n s on [Pt(CN). ~ ] u n i t s as w e l l as through the use o f band s t r u c t u r e c a l c u l a t i o n methods. n

z

z

|f

, f

x y

x z > y z

z

n

m

Acknowledgment s T h i s r e s e a r c h was supported i n part by the A i r Force O f f i c e of S c i e n t i f i c Research (AFSC), United States A i r Force, under Contract F44620-71-0129.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

27.

INTERRANTE AND MESSMER

Solid

State

Interactions

391

Literature Cited 1. Z e l l e r , H. R., Advances in S o l i d State Physics (1973), 13, 31. 2· Krogmann, Κ., Angew. Chem. I n t e r n a t . E d i t . , (1969), 8, 35. 3. Piepho, S. Β., Shatz, P. Ν., and McCaffery, A. J., J. Am. Chem. Soc., (1969), 91, 5994. 4. Minot, M. J . and P e r l s t e i n , J . Η., Phys. Rev. L e t t e r s (1971), 26, 371. 5. Mehran, F. and Scott, Β. Α., Phys. Rev. L e t t e r s (1973), 31, 1347. 6. Moreau-Colin, M. L., S t r u c t . Bond. (1972), 10, 167 and references t h e r e i n . 7. I n t e r r a n t e , L. V. and Messmer, R. P., Chem. Phys. L e t t e r s (1974), 26, 225. 8. S l a t e r , J. C. and Johnson 9. Johnson, Κ. H. and 831. 10. S l a t e r , J. C., "Advances in Quantum Chemistry", V o l . 6, P.O. Loẅdin, ed., p. 1, Academic Press, (1973). 11. Johnson, Κ. Η., "Advances in Quantum Chemistry", V o l . 7, P.O. Loẅdin, ed., p. 143, Academic Press, (1973). 12. Kohn, W. and Rostoker, Ν., Phys. Rev. (1954), 94, 1111. 13. Rösch, N., Klemperer, W. G. and Johnson, Κ. H., Chem. Phys. L e t t e r s (1973), 23, 149. 14. Herman, F., Williams, A. R. and Johnson, Κ. Η., t o be published. 15. Watson, R. E., Phys. Rev. (1958), 111, 1108. 16. Schwarz, Κ., Phys. Rev. (1972), B5, 2466; Schwarz, Κ., t o be published. 17. Messmer, R. P., I n t e r n . J. Quantum Chem. (1973), 7S, 371. 18. Messmer, R. P., I n t e r r a n t e , L. V. and Johnson, Κ. Η., J. Am. Chem. Soc. (1974), 96, 3847. 19. Rösch, Ν., Messmer, R. P. and Johnson, Κ. Η., J. Am. Chem. Soc. (1974), 96, 3855. 20. Norman, J. G., Jr., K o l a r i , H. J., J. C. S. Chem. Comm. (1974) 303. 21. Rösch, N. and Johnson, Κ. Η., Chem. Phys. L e t t e r s (1974), 24, 179. 22. Mason, W. R. and Gray, Η. Β., J. Am. Chem. Soc. (1968), 90, 5721. 23. Bastas, R., Cahen, D., L e s t e r , J . Ε., and Rajaram, J., Chem. Phys. L e t t e r s (1973), 22, 489. 24. Z e l l e r , H. R., t h i s volume

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

28 One-Dimensional and Pseudo-One-Dimensional Molecular Crystals ULRICH IBM

T.

MUELLER-WESTERHOFF

and

FRIEDRICH

HEINRICH

Research L a b o r a t o r y , San Jose, C a l i f . 95193

In t h i s concludin new systems which we are l o o k i n g at i n order to o b t a i n some clarification concerning the P e i e r l s instability and how to circumvent it. Since it is the l a s t t a l k , I a l s o t h i n k that a b r i e f outlook to promising f u t u r e work is in order. F i r s t of all, we want to d i s t i n g u i s h the systems that we are working on and which we want to call Pseudo-one-dimensional Molecular C r y s t a l s , from the one-dimensional square planar complex systems like Krogmann's S a l t and the not so strictly one-dimensional molecular c r y s t a l s of the TTF-TCNQ type. Our systems are composed of e i t h e r mixed valence c a t i o n s of square planar ligand-bridged b i s - t r a n s i t i o n metal c h e l a t e s with acceptor anions such as TCNQ¯ as the counterion or of donor c a t i o n s with l i g a n d bridged b i f u n c t i o n a l t r a n s i t i o n metal complex counterions. In both cases we will d e a l with r a d i c a l i o n s a l t s of charge t r a n s f e r systems. The p a r t i c u l a r l y i n t e r e s t i n g aspect of these compounds lies i n the coexistence of i n t r a m o l e c u l a r as w e l l as i n t e r m o l e c u l a r exchange and e l e c t r o n t r a n s f e r i n t e r a c t i o n s between the t r a n s i t i o n metals i n v o l v e d . These systems are designed to maintain the a n i s o t r o p i c p r o p e r t i e s of one-dimensional molecular c r y s t a l s and have incorporated i n them the first possibility of a v o i d i n g the t r a n s i t i o n to an i n s u l a t i n g P e i e r l s s t a t e . E l i m i n a t i n g the m e t a l - i n s u l a t o r t r a n s i t i o n of h i g h l y conducting compounds is of course equivalent to c r e a t i n g organic and organometallic metals in the a c t u a l sense. However, the molecular design does not e l i m i n a t e the possibility that we may create a true i n s u l a t o r , although from all energetic considerations t h i s would not be very likely. One-dimensional Systems In order to b e t t e r e x p l a i n the reasons f o r our suggesting b i f u n c t i o n a l l i g a n d bridged systems f o r t h i s i n v e s t i g a t i o n , we l i k e to f i r s t take a look at the known i n o r g a n i c and organic one-dimensional molecular c r y s t a l s .

392

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

28.

MUELLER-WESTERHOFF

AND

HEiNRicH

Molecular

Crystals

393

Inorganic L i n e a r Systems Several t a l k s i n t h i s symposium discussed p r o p e r t i e s of a one-dimensional i n o r g a n i c s a l t , which c u r r e n t l y i s very much en vogue: the s o - c a l l e d Krogmann's S a l t ("KCP", K Pt(CN),0.3Br3H 0) i n which during the transformation from the n e u t r a l P t ( I I ) complex by bromine o x i d a t i o n to the mixed valence compound there occurs a s u b s t a n t i a l decrease i n the i n t e r m e t a l l i c ( i n t h i s case equal to the i n t e r p l a n a r ) d i s t a n c e s from 3.2 to 2.88 A, by means of which the overlap between the platinum 5d atomic o r b i t a l s i s i n c r e a s e d . T h i s gives r i s e to m e t a l l i c behaviour i n one dimension along the platinum backbone of the e s s e n t i a l l y one-dimensional molecular c r y s t a l . For s i m i l a r systems l i k e the mixed valence oxalates and o t h e r s , one-dimensional m e t a l l i c behaviour and h i g h a n i s o t r o p r a t i o f optical d othe related p r o p e r t i e s have been e s t a b l i s h e d behaves l i k e a semiconducto (1) 2

2

2

Organic L i n e a r Systems The organic counterpart to KCP i s the c l a s s of TCNQ r a d i c a l anion s a l t s ( 2 ) , which i s c u r r e n t l y being s t u d i e d i n a number of l a b o r a t o r i e s . The unique f e a t u r e of TCNQ s a l t s i s t h a t , due i n p a r t to the p o l a r i z e d charge d i s t r i b u t i o n i n the r a d i c a l anion, Coulomb i n t e r a c t i o n s favor a l i n e a r arrangement w i t h i n the molecular c r y s t a l s . Exceptions are known, e.g. TMPD-TCNQ, i n which the s e p a r a t i o n of the p o s i t i v e charge centers i n the c a t i o n i s i d e n t i c a l to the s e p a r a t i o n of the negative charge centers i n TCNQ anion; t h i s could w e l l be the main reason f o r a D-A-D-A-D-A s t a c k i n g being e n e r g e t i c a l l y favored by coulomb a t t r a c t i o n . While i n charge t r a n s f e r systems the overlap of the π-systems w i l l g e n e r a l l y favor a l i n e a r arrangement, the symmetry of the i n t e r a c t i n g molecular o r b i t a l s i s r a r e l y as s u i t e d f o r the formation of segregated stacks as i n TCNQ r a d i c a l anion s a l t . Due to the r a t h e r d e l i c a t e balance between Coulomb and packing f o r c e s , the molecular symmetry i s d i r e c t l y r e l a t e d to the a n i s o t r o p y o f molecular c r y s t a l s . Two main c l a s s e s of TCNQ s a l t s have to be d i s t i n g u i s h e d : Class C: TCNQ r a d i c a l anion s a l t s of Closed s h e l l c a t i o n s ; w i t h i n t h i s c l a s s , there are such d i v e r s e systems as LiTCNQ, C s (TCNQK, NMP-TCNQ, NEtP-TCNQ, Q-TCNQ, Et NH-TCNQ e t c . C l a s s 0: TCNQ r a d i c a l anion s a l t s of Open s h e l l c a t i o n s , comprising the most I n t e r e s t i n g systems l i k e BFD-(TCNQ)2, TTF-TCNQ, TMPD-TCNQ and o t h e r s . The c a t i o n s i n c l a s s C compounds do not d i r e c t l y p a r t i c i p a t e i n the e l e c t r o n i c conduction process, although i n some of them the q u a l i t i e s of p o l a r i z a b i l i t y and permanent d i p o l e moments have b e a r i n g on the Coulomb p o t e n t i a l along the conductive TCNQ chains· 2

J

3

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

EXTENDED INTERACTIONS BETWEEN M E T A L IONS

394

C l a s s 0 compounds are t r u e charge t r a n s f e r s a l t s of c l o s e d s h e l l donor and acceptor molecules. I t should be kept i n mind, however that a t y p i c a l c l a s s C compound can a l s o be a CT s a l t : L i TCNQ i s a true charge t r a n s f e r s a l t but o f an open s h e l l donor and a c l o s e d s h e l l acceptor, l e a d i n g to a c l o s e d s h e l l donor c a t i o n and an open s h e l l acceptor anion. In C l a s s 0 s a l t s , the coexistence of r a d i c a l c a t i o n s and anions i s the key to high c o n d u c t i v i t i e s , s i n c e i n a d d i t i o n to the p r o p e r t i e s a v a i l a b l e a l s o i n c l a s s C compounds there i s the p o s s i b i l i t y i n donor and acceptor ions f o r both e l e c t r o n and h o l e conduction. In the TCNQ complex of b i s f u l v a l e n e - d i i r o n BFD-(TCNQ) which represents the f i r s t mixed valence complex s a l t of TCNQ, the i n t r a m o l e c u l a r e l e c t r o n t r a n s f e r w i t h i n the F e ( I I ) - F e ( I I I ) systems seems to p l a y an important r o l e . However, here the l i m i t of intermolecula i t i s the overlap betwee T h i s k i n d of l i m i t a t i o n has prompted us to consider mixed valence systems with d i r e c t i n t e r m e t a l l i c bonds as the most promising approach to s t a b l e h i g h l y a n i s o t r o p i c organometallic metals. A

2>

T r a n s i t i o n Metal Complexes and TCNQ S a l t s I t i s a tempting approach, to t r y to combine the two known l i n e a r systems: l i n e a r mixed valence square planar t r a n s i t i o n metal complexes ( f o r which i n the f u r t h e r d i s c u s s i o n we w i l l only consider platinum as the c l a s s i c example) and organic or organometallic acceptors ( f o r which we w i l l take TCNQ as the example of choice - although i n i t s acceptor p r o p e r t i e s i t i s no longer p a r t i c u l a r l y outstanding)· Consider the two approximate spacing parameters of the f o l l o w i n g i l l u s t r a t i o n d e s c r i b i n g a TCNQ stack and a l i n e a r Pt c h a i n : In TCNQ the i n t e r p l a n a r spacing i s about 3.2 A, the overlap i s roughly between the center of the molecule to the center of the m a l o n i t r i l e group of i t s neighbors - g i v i n g us a packing d i s t a n c e o f 4.2 A as a average v a l u e . In KCP the Pt-Pt d i s t a n c e i s 2.88 A. Since we are d e a l i n g i n t h i s case w i t h square planar u n i t s w i t h a t o t a l negative charge of 2.3 u n i t s , i t i s reasonable to expect t h a t due to the l e s s e r coulomb r e p u l s i o n i n a n e u t r a l complex a Pt-Pt d i s t a n c e o f 2.7 to 2.8 À can be obtained, with t h i s number being an important f a c t o r f o r determining the s t o i c h i o m e t r y _ o f square p l a n a r mixed valence systems c o n t a i n i n g stacks of TCNQ~ as the counterions. As shown i n the f o l l o w i n g diagram there i s only the one allowed r a t i o of 2.8/4.2 « 0.67 TCNQ s per Pt i n uniformly stacked s o l i d s . Although s e v e r a l attempts have been made i n other l a b o r a t o r i e s to produce conductive organometallic mixed valence systems of the above k i n d , such approaches cannot be s u c c e s s f u l s i n c e there i s a discrepancy between the Pt-Pt d i s t a n c e of 2.8 Â f o r optimal i n t e r a c t i o n s and the minimal d i s t a n c e (3.2 A) f o r 1

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

28.

MUELLER-WESTERHOFF

TCNQ stack

AND

HEiNRiCH

Molecular

Crystals

395

Pt chain

Pt/TCNQ = 4.2/2.8 = 1.5 ( i . e . : TCNQ/Pt = 0.67)

prganic l i g a n d molecules imposed by r e p u l s i o n f o r c e s . The only way by which i n such compounds s i g n i f i c a n t metal-metal i n t e r a c t i o n s can be obtained i s t o d i s t o r t the molecules from t h e i r planar ground s t a t e and t o create dimeric s p e c i e s . Such s t r u c t u r e s are reminiscent o f the P e i e r l s s t a t e o f one-dimensional systems. A thorough c r y s t a l l o g r a p h i c study (4) of the unsubstituted d i t h i e n e - p l a t i n u m complex showed such dimers. Schematically, the d i s t o r t i o n i s shown i n the f o l l o w i n g diagram.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

396

EXTENDED

INTERACTIONS

BETWEEN

Pt-

I I

3.2

I

*^pt' 2.8

METAL

IONS

r 3.2

Pt-

I I ι

3.2

Pt

I I ι

3.2

Pt

3.2

> t ^

^ P t V

L

I

I

uniform

(no s i g n i f i c a n t Pt-Pt I n t e r a c t i o n s )

D i s t o r t e d (strong p a i r w i s e Pt-Pt I n t e r a c t i o n s )

We consider t h i s d i s t o r t i o n t o be one o f the main reasons f o r the absence o f organometallic t r a n s i t i o n metal complexes o f more than semiconductor q u a l i t i e s (with the BFD system above being the only exception so f a r ) . I t i s reasonably safe t o p r e d i c t that there can be no m e t a l l i c systems based on square planar l i n e a r l y overlapping organometallic t r a n s i t i o n metal complexes. One a l t e r n a t i v e s t r u c t u r e that avoids the s t e r i c problems i s based on the use o f polymeric a s s o c i a t e s o f the (C^H^T1) and (Cgt^EiOg type. For CpTl, a l i n e a r a l t e r n a t i n g Cp and T l arrangement i n the s o l i d i s known and p a r t i a l o x i d a t i o n of such a stack by TCNQ could l e a d t o T 1 ( I ) - T 1 ( I I I ) mixed valence c a t i o n s segregated from stacks o f TCNQ anions. Attempts i n our l a b o r a t o r y to prepare such systems have been u n s u c c e s s f u l . Our approach t o avoid the l i g a n d r e p u l s i o n problems and t o s t i l l u t i l i z e the advantages o f d i r e c t metal-metal bonding through d o r b i t a l s as i n KCP l i e s i n the c o n s t r u c t i o n o f b i f u n c t i o n a l organometallic s p e c i e s , which o f f e r s e v e r a l s u r p r i s i n g advantages. X

2

Pseudo-One-Dimensional Systems The p r i n c i p l e o f our approach l i e s i n the s y n t h e s i s o f two c l a s s e s o f π-ligand bridged b i m e t a l l i c planar t r a n s i t i o n metal complexes. The s i g n i f i c a n t p o i n t i n pursuing the s y n t h e s i s o f these complexes l i e s i n t h e i r unique p o s s i b i l i t y o f avoiding

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

28.

MUELLER-WESTERHOFF AND HEINRICH

Molecular

Crystals

397

the P e i e r l s State. As a matter of f a c t , these compounds have to undergo a d i s t o r t i o n i n order to become m e t a l l i c ! ! Herein l i e s t h e i r p a r t i c u l a r promise. The displacement out of the compound plane should have a n e g l i g b l e i n f l u e n c e on the i n t r a m o l e c u l a r interactions· We are c u r r e n t l y l o o k i n g at two main c l a s s e s of complexes. In the f i r s t c l a s s , t r a n s i t i o n metals are complexed by r i g i d b i f u n c t i o n a l l i g a n d systems. The s y n t h e s i s of such l i g a n d s poses severe d i f f i c u l t i e s , which are understandable t o anyone

f a m i l i a r with the p r e p a r a t i o n of m a c r o c y c l i c compounds. However, we have r e c e n t l y succeeded i n preparing such compounds and complex systems - although these experiments do not as yet allow the p r e p a r a t i o n of l i g a n d s i n u s e f u l q u a n t i t i e s . The second approach i s to use l i g a n d exchange r e a c t i o n s to c o n s t r u c t complexes that e x h i b i t strong Intramolecular interactions. In t h i s approach, we s a c r i f i c e molecular r i g i d i t y f o r s y n t h e t i c s i m p l i c i t y . U s e f u l b r i d g i n g l i g a n d s which are incorporated i n t h i s s y n t h e s i s are of the 2 , 2 - b i p y r i m i d y l type to give us complexes of the f o l l o w i n g general type. f

Both methods produce p l a n a r systems c o n t a i n i n g two metal atoms. For the f o l l o w i n g d i s c u s s i o n , we wish to represent these systems by the n o t a t i o n

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

EXTENDED INTERACTIONS BETWEEN

398 p

p

t

M E T A L IONS

t

which s c h e m a t i c a l l y equals a s i d e view o f these systems. The e n t i r e complex c a r r i e s e i t h e r a p o s i t i v e o r negative p a r t i a l charge, depending on the counterlon i n v o l v e d . The Two Stacking Arrangements There a r e two p r i n c i p a l ways i n which complexes o f t h i s type together l i k e a l i n e a r stack o f counterions can be arranged i n r e g u l a r stacks i n molecular c r y s t a l s and s t i l l f u l l y u t i l i z e the s t a b i l i z a t i o n o f f e r e d by the overlap o f d o r b i t a l s . These two arrangements, termed by us the " s t e p " and " l a d d e r " stackings r e q u i r e q u i t e d i f f e r e n t stoechiometries i n t h e i r mixed valence state. As a s i d e l i n e , i a l s o a few n e u t r a l mixe our l a b o r a t o r y . These, o f course, form a completely d i f f e r e n t c l a s s o f compounds and w i l l be described s e p a r a t e l y .

Pt

Pt Pt

Pt STEP Arrangement — P t

Pt Pt

Pt

Pt

Pt

Pt

Pt

Pt

Pt

Pt

Pt

Pt

Pt

LADDER Arrangement

The STEP Arrangement Considering spacing and stoechiometry f o r a molecular c r y s t a l made up from stacks o f TCNQ r a d i c a l anions and b i f u n c t i o n a l mixed valence c a t i o n s i n the step arrangement, we have only one f r e e parameter that determines the r e l a t i o n between stoechiometry

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

28.

MUELLER-WESTERHOFF

AND

HEiNRicH

Molecular

Crystals

399

and i n t r a m o l e c u l a r Pt-Pt d i s t a n c e s . In both stacks the i n t e r p l a n a r spacing i s 3.2 Â, the TCNQ chain spacing i s 4.2 Â, which has to be matched as i l l u s t r a t e d i n the f o l l o w i n g diagram. For a 1:1 stoechiometry (one Pt per TCNQ), the i n t r a m o l e c u l a r Pt-Pt d i s t a n c e has to be 7.8 A. For n o n - i n t e g r a l stoechiometries, the values i n the f o l l o w i n g t a b l e apply.

Pt-Pt Distance (intramol., a) 4.0 5.0 6.0 7.0 8.0 9.0 10.0

Stacking Distance (c) 5.12 5.92 6.80 7.70 8.62 9.55 10.50

Pt/TCNQ R a t i o 1.64 1.50 1.24 1.10 0.98 0.88 0.80

In order to achieve s i g n i f i c a n t i n t e r m o l e c u l a r metal-metal overlap, the molecules have to d i s t o r t i n the same way as i t was found f o r the d i t h i e n e - P t complex dimer. However, s i n c e the dimers i n t h i s case of b i f u n c t i o n a l systems are connected

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

EXTENDED

400

INTERACTIONS BETWEEN M E T A L IONS

by a conductive π-electron l i g a n d b r i d g e , they a r e i n essence s t a b i l i z i n g the m e t a l l i c s t a t e through t h i s d i s t o r t i o n . As i s evident from the above sketch, we now have systems with i n t r a m o l e c u l a r i n t e r a c t i o n s as w e l l as d i r e c t i n t e r m o l e c u l a r metal-metal c o u p l i n g . The step arrangement appears t o be s t a b l e and not subject t o f u r t h e r d i s t o r t i o n s . The LADDER Arrangement The a l t e r n a t i v e s t a c k i n g arrangement depicted above o f f e r s s i g n i f i c a n t advantages over the step arrangement, s i n c e the r e l a t i o n between packing requirements and i n t r a m o l e c u l a r Pt-Pt d i s t a n c e has been removed. B r i d g i n g l i g a n d s o f any s i z e can be accomodated as long as they do not deviate from p l a n a r i t y . As i s evident from the f o l l o w i n

-Pt-

-Pt-

I

4.2

-Pt-

"Λ

3.2 -Pt-

-Pt-

.it

4.2

-Pt-

-Pt-

the c o n f i n i n g l i m i t a t i o n i n the spacing o f complex u n i t s r e l a t i v e to TCNQ u n i t s leads t o a f i x e d stoechiometry o f (Pt-Pt)/TCNQ = 1.31, which f o r simple TCNQ s a l t s corresponds t o a Pt o x i d a t i o n s t a t e o f 2.66. Again, as i n the step arrangement, a d i s t o r t i o n o f the b i f u n c t i o n a l molecule i s needed to maximize the metal-metal overlap and t o achieve the m e t a l l i c s t a t e :

Pt 3.2

2.8

Pt

I 2.8

3.2

Pt

3.2

Pt

I

2.8

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

28.

MUELLER-WESTERHOFF

AND

HEiNRicH

Molecular

Crystals

401

Although the advantages of t h i s arrangement are obvious, there i s one p o s s i b i l i t y which might lead to a p a i r w i s e " i n phase" d i s t o r t i o n as i l l u s t r a t e d below, which would lead to a much l e s s conductive s t a t e . In t h i s case, the p r o p e r t i e s w i l l be s i m i l a r to the mixed valence BFD-(TCNQ>2 system mentioned e a r l i e r . A p r i o r i , there are no c r i t e r i a to s a f e l y p r e d i c t which of the two d i s t o r t i o n s w i l l occur - the s t a b i l i z e d m e t a l l i c s t a t e or the r e a l P e i e r l s s t a t e . Since extended i n t e r a c t i o n s w i l l s t a b i l i z e the t o t a l system, we would expect the "out of phase" m e t a l l i c s t a t e to be e n e r g e t i c a l l y favored. I t i s important

Pt

11 Pt

Pt I

Pt I

λ Pt !

k to r e a l i z e that the two s t a t e s should not i n t e r c o n v e r t , s i n c e to do so, we would have to break a metal-metal bond, move the metal through the molecular plane and reform a metal-metal bond on the other s i d e . Thus, even i f m e t a l l i c conduction i s not achieved, we are e l i m i n a t i n g the m e t a l - i n s u l a t o r t r a n s i t i o n and are beginning to understand which i n f l u e n c e the d e v i a t i o n s from s t r i c t one-dimensionality have on the p r e s e r v a t i o n of r e g u l a r l y spaced t r a n s i t i o n metal complex systems. Literature Cited 1. 2.

3.

4.

See proceeding papers by K. Krogmann, A. N. Bloch and H. R. Z e l l e r f o r d e t a i l s and r e f e r e n c e on KCP. Coleman, L. Β., Cohen, M. J . , Sandman, D. J . , Yamagishi, F. G., G a r i t o , A. F. and Heeger, A. J . , S o l i d State Commun. (1973) 12, 1125 and F e r r a r i s , J . , Cowan, D. O., Walatka, J . and P e r l s t e i n , J . Η., J . Amer. Chem. Soc. (1973) 95, 948 d e s c r i b e TTF-TCNQ and g i v e r e f e r e n c e to e a r l i e r TCNQ work. Mueller-Westerhoff, U. T. and E i l b r a c h t , P., J . Amer. Chem. Soc. (1972) 94, 9272. Other BFD s a l t s were a l s o prepared by Cowan, D. O. and LeVanda, C., i b i d . (1972) 94, 9271. Browall, K. W., Bursh, T., I n t e r r a n t e , L. V., and Kasper, J . S., Inorg. Chem. (1972) 11, 1800.

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

402

EXTENDED INTERACTIONS BETWEEN M E T A L IONS

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

INDEX

Charge densities, nuclear quadrupole resonance derived Charge transfer Chlorines, anionic Chloro-bridged copper (II) dimers Chromium (III) dimers Chromium doped V 0 Chromium (III) ions Cobalt

A Absorption solid state techniques, single crystal Absorptivity, molar Acetonitrile A c i d activity, Lewis Addition reactions, oxidative Adduct structures, solvent Amine-halide complexes Amine-TaSo complexes 2-Aminoantnracene Anion bridged compounds Anion bridges Anionic chlorines Anisotropy Ising-like magnetic Antiferromagnetic coupling Antiferromagnetic interactions Antiferromagnetic salts Arsenic

343 277 301 309 70 328 325 38 25 2 27 239, 248 236 196 199,346,375 202 210 59,202 241 185 14

2

Complex compounds, square planar .... 350 Complexes directed synthesis of linear chain metal 314 square planar d 301 RMX 154 of tantalum sulfide, organic intercalation 23 Compounds metallic 3 semiconducting 3 TCNQ 350 transitional 3 Computation of field-swept epr spectra 40 Conduction band 372 Conductivity, electrical 341 Cooperative magnetic order 34 Cooperative transitions 36 Copper (II) 94 complexes 109,124 superexchange interactions in 108 dimers 76,84 chloro-bridged 131 Correlation of structural and magnetic properties 118 Corundum 17 Counterion tetraphenylborate 76 Coupling antiferromagnetic 59, 202 exchange 143 ferromagnetic 167 hyperfine 55 spin orbit 224 Covalency effects 159 Covalent energy 366 Cr(urea) -Cr(urea) pairs 59 Crystal chemistry of V 0 , high temperature 16 polarized reflectance spectra, single .... 301 structure of V ( urea ) I 54 studies, mixed291 Crystalline K P t ( C N ) , electrochemical oxidation of 367 Crystals magnetically dilute molecular 34 8

3

Β Band, parabolic free electron Band-structure energy Biscyclopentadienyl titanium (III), linear chain complexes from Bonding contacts, hydrogenBridges, anion Bridging angle hydroxo group ligand interactions through a units, symmetry of the Bronzes, perovskite

372 359 142 85 236 101 121 76 66 81 2

C Cations, shielding between dimeric Cesium metal triiodides Cesium nickel trihalides Chain compounds mixed valence metal partially oxidized single and mixed valency single valence metal transition metal Chain metal complexes Chains, extended metal Chains, linear optical properties of Chalcogenides extended interactions i n transition metal

3

29 304 196 131 94 16 164 12,194

77 182 188 246 333 234 239 236 314 276 194 164 23 1 12

e

3+

e

2

3+

3

6

2

3

4

403

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

404

EXTENDED

Crystals (Continued) pure stoichiometric single rhodium with square complexes, electronic spectra of C s M I transition metal iodides Cubic tungsten bronzes [Cu(diamine)OH]., [Cu(pyrazine) ( N Ô ) ] „ Cyanides Cyanoplatinates

2 306 254 182 2 112 132 298 347

3

2+

3

2

INTERACTIONS

BETWEEN

METAL

IONS

Excited state, delocalized Exciton-exciton transitions Exciton+magnon transition Exciton model Extended interactions in transition metal oxides and chalcogenides Extended metal chains

293 171 169 310

F Fe(bipyridine)CL Fe(morphylyldtc) I Fe ( phenanthroline ) C I , Fermi level Γ. electron density enhancement at the Ferrocene polymers Ferromagnetic coupling Ferromagnetic interaction Ferromagnetism

205 38 205 390 27 72 167 205 218

F i e l d dependent susceptibility Field-swept epr spectra for systems with large interelectronic interactions .... Fields, perturbation formulation to predict transition Formic acid Free electron band, parabolic Free electron model Frequency-shift perturbation Frohlich transition, Peierls-

224

2

D Davydov components of transitions Decylamine-TaS complex Delocalized excited state Delocalized molecular orbitals Destabilization of Pt 2.3+ Dibenzylamine Di-n-butyl form amide Diethylamine Diformylhydrazine Diimine ligands, ferrous chloride polymers of Dilute molecular crystals, magnetically.... Dimeric cations, shielding between Dimers, ehloro-bridged copper (II) Dimers, chromium ( III ) Dioxalatoplatinates Directed synthesis of linear chain metal complexes Directly interacting metal ions 242, 2

246 24 293 66 364 2 2 24 29 205 34 77 131 94 350 314 250

40 41 351 372 365 45 373

G Gas, one dimensional electron Glycinato complex Guaninium complex

Ε

1 276

358 95 130

Eigenfield equation 41 properties of 42 H Electrical conductivity 341 Electrical resistivities of C s N i I 192 Halide-bridged outer-sphere dimers 89 Electrochemical measurements 72 Halide ions 346,350 Electrochemical oxidation of K P t ( CN ) 367 Halide lattices, multisite magnetic Electron interactions in 51 -accepting ligands 323 Halides, hydrated ternary transition metal 238 band, parabolic free 372 Halogen bonding, metal183 density enhancement at the Fermi level 27 Heat capacity 196 gas, one dimensional 358 Heisenberg interaction, isotropic 36 Electroneutrality 350 Hemin, imidazolate bridged polymer of Electronic an iron (III) 221 and magnetic properties 142 Hexaflurophosphate salts 67 spectra 189 Hexaurea-metal ( III ) halide lattices 51 of crystals with square complexes 254 H i g h temperature crystal chemistry of structure of Krogmann salts 356 V O . 16 structure of planar d complexes 302 Hydrated ternary transition metal halides 238 transition 318 Hydrazines, substituted 27 Energy, covalent 366 Hydrogen atoms, hydroxo-bridge 119 Entropy change 196 Hydrogen-bonding contacts 85 E P R spectra for coupled C r ( u r e a ) Hydroxo group, bridging 121 Cr(urea) pairs 59 Hyperfine coupling 55 E P R spectra for systems with large I interelectronic interactions 40 Equation Imidazolate bridged polymer of an properties of eigenfield 42 iron (III) hemin 221 Van Vleck 95, 111 Increased interlayer spacings 31 E S R transitions, singlet-triplet 87 Infinite R M X linear chain complexes .... 142 Exchange coupling 143 Inorganic linear systems 393 Instability, Peierls 392 Exchange paths 36 a

2

4

2

H

8

0

0

3 +

3+

: {

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

405

INDEX

Insulator transition, metal16 Interacting metal ions 242 Interaction, ferromagnetic 205 Interaction, isotropic Heisenberg 36 Interactions antiferromagnetic 241 through a bridging ligand 66 in copper (II) complexes, superexchange 108 in C r ( u r e a ) X 54 in hexaurea-metal ( III ) halide lattices, multisite magnetic 51 interatomic 381 interdimer 128 intermolecular 321 metal-metal 66 in molecular crystals, intermolecular magnetic exchange 34 Ru-Ru 69 systems with large interelectroni in transition metal oxides and chalcogenides, extended 1 weak metal-metal 68 Interatomic interactions 381 Intercalation complexes of tantalum sulfide, organic 23 Intercalation of N-methylform amide 29 Interdimer interactions 128 Interelectronic interactions 40 Interligand repulsion 350 Intermolecular interactions 321 Intermolecular magnetic exchange interactions i n molecular crystals 34 Intermolecular separation 390 Intervalence transfer bands 73 Iodides, C s M I transition metal 182 Ionic states, transitions to 266 Ions, d 323 Ions, directly interacting metal 242, 250 Ions, halide 350 Iridium complexes, platinum and 381 Iron (III) bis(dithiocarbamates) 34 Iron (III) hemin, imidazolate bridged polymer of an 221 Ising-like anisotropy 202 Ising model, one-dimensional 199 Isomer shifts, Môssbauer 215 Isomorphism 294 Isotropic Heisenberg interaction 36 e

3

3

8

Κ K PdCl4 289 K PtBr 257 K PtCl 257,289 Kramers doublet 37 Kramers-Kronig analysis 280 Krogmann salts, electronic structure of .... 356 2

2

4

2

4

L Ladder arrangement Lattices, hexaurea-metal (III) halide .... Lewis acid activity Ligand bridged compounds, polymeric -bridged ruthenium complexes interaction through a bridging

400 51 328 71 68 66

Ligand (Continued) mixing, metal parameters, variation of Ligands, electron accepting Ligands, organic Linear chain complexes from biscyclopentadienyl titanium (III) Linear chain metal complexes, directed synthesis of Linear chain series R M X Linear chains, optical properties of Linear systems, inorganic Linear systems, organic Localized and collective states Lowest-energy allowed electronic transition 3

388 319 323 205 142 314 194 164 393 393 235 276

M

in hexaurea-metal (III) halide lattices, multisite 51 in molecular crystals, intermolecular 34 moments 207 order 37 cooperative 34 properties of chromium ( III ) dimers .... 94 properties, correlation of structural and 118 properties of C s M I transition metal iodides 182 susceptibility 152, 185 and thermal properties of [ (C H ) NH] M X · 2H 0 194 Magnetically dilute molecular crystals .... 34 Magnetization studies 207 Magnon assisted transitions 173 Magnus green salt 261, 285, 319 solid solutions 331 Magnus salts 276 Manganese ( II ) dimers 90 Metal chain compounds, mixed valence 246 chain compounds, single valence 239 chain compounds, transition 236 chains, extended 276 complexes, transition 394 compounds, polymeric, mixed-valence transition 66 -halogen bonding 183 -insulator transition 16 iodides, C s M I transition 182 ions, directly interacting 242, 250 -ligand mixing 388 -metal interactions 66 weak 68 oxides and chalcogenides, extended interactions i n transition 1 plane, tetra coordinated 350 -sulfur bonds 12 systems, triatomic 154 Metallic compounds 3 Metallic Pt salts, 1-D 346 Metalloporphyrins 221 Metals, 1-D 331 N-Methylformamide, intercalation of ... 29 3

3

3

3

2

3

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

406

EXTENDED INTERACTIONS BETWEEN M E T A L IONS

N-Methylformanilide 29 1-Methyl-l-phenylhydrazine 31 Microspecular reflectance attachment ... 302 Mixed-crystal studies 291 M i x e d metal T i ( I I I ) compounds 143 Mixed valence metal chain compounds 234, 246, 331 Mixed valence solids 324 M n ( I I ) in tetrahydrofuran 150 Model, molecular orbital 120 Molar absoiptivity 309 Molar susceptibility 185 Molecular crystals 392 intermolecular magnetic exchange interactions i n 34 magnetically dilute 34 Molecular orbital model 120, 312 Molecular orbitals, delocalized 66 Molecular structure studies 213 Mossbauer isomer shifts 21 Môssbauer spectroscopy 31 Multisite magnetic interactions in hexaurea-metal(III) halide lattices 51

Ν Nickel compounds dimers dimethylglyoxime dimethylglyoximate trihalides, cesium Ν M R spectroscopy Nuclear quadrupole resonance derived charge densities

14 173 77 282 244 188 318

Paramagnetic species Partially oxidized chain compounds Peierls-Frohlich transition Peierls transition i n one-dimensional solids Perovskite bronzes Perovskites Perturbation formulation to predict transition fields Perturbation, frequency-shift Phenanthroline complexes π-exchange Piperidine Planar complex compounds, square Planar d complexes, electronic structure of Planar d systems Platinum chains, mixed-valence 8

Ι,Γ-Polyferrocene compounds Polymer of an iron (III) hemin, imidazolate bridged Polymeric, ligand-bridged compounds Polymeric, mixed-valence transition metal compounds Polymers of diimine ligands Polymers, ferrocene Polymers, ruthenium Polymorphism Properties of eigenfield equation Pseudo-one-dimensional systems Pt(en)Br Pt(en)Cl, Pt(CN) " Pt salts, 1-D metallic Pure stoichiometric single crystals Pyridine-TaS complex 4

Ο One-dimensional behavior electron gas Ising model solids systems Optical properties of linear chains Orbital overlap Order, magnetic Organic intercalation complexes of tantalum sulfide Organic ligands Organic linear systems Outer-sphere dimers, halide-bridged Overlap, orbital Oxalato complex Oxidation of crystalline K P t ( C N ) , electrochemical Oxidative addition reactions Oxyfluorides Oxidized chain compounds, partially 2

23 205 393 89 368 99

4

367 325 7 333

Ρ Packing Palladium (II) Parabolic free electron band Paramagnet

2

2

164 358 199 372 392 164 368 37

37,77 254 372 196

372 2 236 41 45 97 84 24 350

8

2

29

63 333 373

302 277 254 331 71 221 71 66 205 72 72 38 42 396 266 266 388 346 2 24

Q Quadrupole resonance derived charge densities, nuclear

29

R RbMnF Reactions, oxidative addition Reflectance attachment, microspecular... Reflectance spectra, single crystal polarized Reflectance spectrometer Reflection spectroscopy, specular Refluxing, formic acid Repulsion, interligand Resistivity Resonance derived charge densities, nuclear quadrupole R h ( C O ) acac Rhodium crystals Rhodium derivatives R M X systems 154, R u - R u interactions 3

2

3

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

171 325 302 301 379 278 351 350 9 29 301 306 328 164 69

407

INDEX

Ruthenium complexes, ligand-bridged ... Ruthenium polymers

68 72

Salt, Magnus green 285 solid solutions 331 Salts antiferromagnetic 185 1-D metallic Pt 346 electronic structure of Krogmann 356 hexaflurophosphate 67 platinum 336 TCNQ 394 S C F - X a - S W scattered wave method .... 382 Selenocyanate dimers 81 Semiconducting compounds 3 Shielding between oimeric cation Shift perturbation, frequency4 σ-exchange 84 Single crystal absorption techniques 301 Single crystals, pure stoichiometric 2 Single and mixed valency chain compounds 234 Single phase solid solutions 337 Single valence metal chain compounds .... 239 Singlet-triplet E S R transitions 87 Singlet-triplet splitting 101 Solid-state absorption 277 Solid state galvanic cell 367 Solid state interactions in P t ( C N ) " complexes 381 Solids, mixed valence 324 Solids, Peierls transition i n onedimensional 372 Solvent adduct structures 38 Spacings, increased interlayer 31 Spectra for coupled C r ( urea ) - C r ( urea ) pairs, E P R 59 of crystals with square complexes, electronic 254 electronic 189 isolated-molecule 277 Pt Br 264 single crystal polarized reflectance .... 301 Spectrometer, reflectance 379 Spectroscopic and magnetic properties of C s M I transition metal iodides 182 Spectroscopic methods 318 Spectroscopy, specular reflection 278 Specular reflection spectroscopy 278 Spin density 38 Spin Hamiltonian parameters 160 Spin orbit coupling 224 Spin wave 167 Splitting, singlet-triplet 101 Square complexes, spectra of crystals with 254 Square planar complex compounds . 3 0 1 , 3 5 0 Stacking arrangement 398 Step arrangement 398 Structural and magnetic properties, correlation of 118 Substituted hydrazines 27 4

6

2

G

2

3

3 +

n

6

3+

Sulfur bonds, metal12 Superconductivity 24 Superexchange 101,164 interactions i n copper (II) complexes 108 Susceptibility field dependent 224 and magnetization studies 207 molar 185 temperature variation of the inverse .... 126 Symmetry of bridging units 81 Synthesis of linear chain metal complexes, 314

Tantalum sulfide, organic intercalation complexes of

23

T C N Q compounds 350 T C N Q salts 394 Temperature crystal chemistry of V 0 , high 16 Temperature, transition 38 Temperature variation of the inverse susceptibility 126 Ternary transition metal halides, hydrated 238 Tetrachloroplatinate(II) 288 Tetrahydrofuran, M n ( I I ) in 150 Tetrakis(phenylisonitrile) cobalt (II) 328 Tetraphenylborate counterion 76 Thermal properties of [ ( C H ) N H ] MX · 2H 0 194 Thiocyanate dimers 81 Tight-binding model 359 Titanium (III) 142 Titanium-zinc complexes 145 Ti(urea) 63 Transfer bands, intervalence 73 Transition electronic 318 exciton+magnon 169 fields, perturbation formulation to predict 41 lowest-energy allowed electronic 276 metal chalcogenides 12 metal complexes 394 metal compounds 66, 236 metal-insulator 16 metal oxides and chalcogenides 1 in one-dimensional solids, Peierls 372 temperature 38 Transitions cooperative 36 exciton-exciton 171 to ionic states 266 magnon assisted 173 singlet-triplet E S R 87 Triatomic metal systems 154 Triplet E S R transitions, singlet87 Triplet splitting, singlet101 Tungsten bronzes, cubic 2 Tungsten (VI) oxide 2 Two-phase complexes 29 2

3

3

3

3

2

e

3 +

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

408

EXTENDED

2

BETWEEN

METAL

IONS

W

V Vanadium (IV) oxide Van der Waals forces Van Vleck equation Variation of ligand parameters V 0 , high temperature crystal chemistry of V ( u r e a ) I , crystal structure of

INTERACTIONS

5 23 95, 111 319

Wave, spin Weak metal-metal interactions

167 68

Ζ

: i

6

3

16 54

Zinc complex Zinc complexes, titanium-

In Extended Interactions between Metal Ions; Interrante, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1974.

143 145