Inorganic Compounds with Unusual Properties

7 Redox Properties of Polymetallic Systems THOMAS J. MEYER

Downloaded by UNIV OF ARIZONA on January 21, 2013 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/ba-1976-0150.ch007

University of North Carolina, Chapel Hill, N. C. 27514

The redox properties of three classes of polymetallic systems were studied. In compounds with strong metal-metal bonds, multiple oxidation state properties are found for metal clusters and in compounds where bridging ligands reinforce the metal-metal bond. Compounds with weak interactions between metal ion sites have electronic and chemical properties that are essentially those of isolated monomeric complexes. Electronic interactions between metal centers, electrostatic effects, and statistical effects affect reduction potential values. In systems more complicated than dimers, there are ambiguities about the site of oxidation. In mixed -valence ions, intervalence transfer bands appear; their energies and intensities are functions of both bridging and non-bridging ligand effects. When metal-metal interactions across a bridging ligand are sufficiently strong, the system is delocalized and chemical and electronic properties are significantly modified. ecent w o r k has l e d to the synthesis of a v a r i e t y of c o m p o u n d s i n w h i c h m e t a l atoms or ions are h e l d i n close p r o x i m i t y b y c h e m i c a l linkages. T h e s e p o l y m e t a l l i c c o m p o u n d s represent a n e w class of m a t e rials that h a v e d i s t i n c t i v e c h e m i c a l a n d p h y s i c a l properties, a n d i n some systems the p r o p e r t i e s c a n be v a r i e d s y s t e m a t i c a l l y b y c h e m i c a l synthesis. T h e c o m p o u n d s are of interest because of possible c o o p e r a t i v e

chemical

a n d electronic interactions b e t w e e n the c h e m i c a l l y l i n k e d m e t a l centers. I n the f u t u r e i t m a y p r o v e p o s s i b l e :

(a)

to create s o l i d state m a t e r i a l s

that h a v e c o n t r o l l a b l e , a n d p e r h a p s u n u s u a l , 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 erties; ( b ) to p r e p a r e p o l y m e r i c c o m p l e x e s w h i c h i n s o l u t i o n h a v e p r o p erties that are i n t e r m e d i a t e b e t w e e n those of s o l i d state materials a n d those of s i m p l e m o n o m e r i c complexes; a n d ( c ) to devise c h e m i c a l systems i n w h i c h cooperative c h e m i c a l interactions l e a d to net, m u l t i p l e - e l e c t r o n r e d o x processes, or to s i m u l t a n e o u s , t w o - or m o r e site reactions. 73 In Inorganic Compounds with Unusual Properties; King, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

74

INORGANIC

COMPOUNDS

WITH

UNUSUAL

PROPERTIES

M y i n t e n t i o n is to d e v e l o p , as s y s t e m a t i c a l l y as possible, t h e r e d o x properties

of

polymetallic

systems

i n solution.

A n understanding

of

r e d o x p r o p e r t i e s a n d of m e t a l - m e t a l interactions is essential i n o r d e r to e x p l o i t p o l y m e t a l l i c systems. findings

M o s t of the examples

are b a s e d

on

the

of m y o w n r e s e a r c h g r o u p w i t h t h r e e different classes of c o m

p o u n d s w h i c h differ i n the n a t u r e a n d / o r the extent of the m e t a l - m e t a l interaction. Strong, Direct Metal-Metal

Bonding

A m e t a l - m e t a l b o n d has a p r o f o u n d effect o n the p r o p e r t i e s of t h e


l i n k e d m e t a l centers

(I).

W i t h a metal-metal bond:

(a)

absorption

b a n d s are present w h i c h c a n b e assigned to transitions b e t w e e n b o n d i n g a n d a n t i b o n d i n g m e t a l - m e t a l o r b i t a l s (2, 3, 4);

(b)

multiple oxidation

state properties c a n a p p e a r that are b a s e d o n the m e t a l - m e t a l b o n d

or

b o n d s ; a n d ( c ) c h e m i c a l p r o p e r t i e s are s t r o n g l y m o d i f i e d . A s a n e x a m p l e of the latter, H u g h e y a n d B o c k d e m o n s t r a t e d that

[(7r-C Hr,)Mo(CO) ]2 s

and

3

induced homolytic

fission

related

(Reaction

b y flash p h o t o l y s i s

compounds

undergo

the m o n o m e r i c

1);

fragments

are p r o d u c e d react r a p i d l y w i t h a v a r i e t y of substrates u n d e r i n w h i c h t h e p a r e n t c o m p o u n d is u n r e a c t i v e (4,

[ ( π - 0 , Η ) M o ( C O ),] ο Γ>

hv — lOMO^AT

that

conditions

6).

2 G r - C H ) M o (CO) 5

sec'

1

(5) light-

5

(1)

8

1

R e d o x processes i n r e l a t i v e l y s i m p l e m e t a l - m e t a l b o n d s l e a d to a breakdown solvent

i n p r i m a r y structure

a n d R e a c t i o n 2b

(9)1.

R e a c t i o n 2a (7,

[see

However, 2e

reversible

•2U-C H )Fe(CO) S 5

[(ir-C H )Fe(C0) ] — s

2

5

+

2

2

+

where

8)

electron

5

2

2

2e"

(

7 r

n o n a q u e o u s solvents (CO)]

4

(10,11)

b y b r i d g i n g (1). revealed

5

2

reinforce

V o l t a m m e t r i c experiments

that the

( F i g u r e 1)

(2b)

-C H )Fe(CO) 5

cluster systems

and [ ( ^ C H ) F e S ] 5

5

(10)

4

5

and

[(^C H5)FeS]4 5

".

5

5

5

5

5

5

4

2 +

/ °/+ /

4

5

and

[(ΤΓ-0 Η )-

(13)

and

(12)

5

FeS] -[(7r-C H )FeS] (PF6)-[(7r-C H5)FeS] (PF6) 4

5

remain intact

T h e s t r u c t u r a l details of the c o m p o u n d s

[(7r-C H )Fe(CO)]4-[(7r-C H )Fe(CO)]4(PFe) 5

in

[(7r-C H )Fe-

i n s e v e r a l different m o l e c u l a r o x i d a t i o n states : [ ( 7 r - C H ) F e ( C O ) ] 3 + / 2 + / + / 0 /

=

(2a)

+

S

c a n o c c u r i n m e t a l clusters a n d i n c o m p o u n d s w h e r e l i g a n d s the m e t a l - m e t a l b o n d

S

transfer

4

2

n e t i c field M o s s b a u e r d a t a f o r [ ( ^ C H ) F e ( C O ) ] ( P F e ) 5

5

4

5

(14)

5

mag

are c o n

sistent w i t h a m o d e l i n w h i c h r e d o x properties are c a r r i e d , at least i n p a r t ,

In Inorganic Compounds with Unusual Properties; King, R.; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

7.

Polymetallic Systems

MEYER

75

Inorganic Chemistry

Figure 1. Structure of the [(TT-CsH^FeiCO)'], cluster unit (14)


b y delocalized m e t a l - m e t a l bonding a n d antibonding orbitals. T h e clusters a p p e a r to u n d e r g o f a c i l e e l e c t r o n transfer C h e m i c a l l y reversible

electron

(15).

transfer processes h a v e

also

been

r e p o r t e d for l i g a n d - b r i d g e d complexes of C r , M o , W , M n , F e , R u , C o , a n d N i b y D e s s y a n d co-workers i n 1,2-dimethoxyethane

who used electrochemical

for di-ter£-phosphine-bridged d e r i v a t i v e s of C5H )Fe(CO)] (Ph P(CH )3PPh ) 5

2

2

2

2

2 + / + / 0

techniques

S i m i l a r b e h a v i o r was

(16, 17, 18, 19).

reported

[(7r-C H )Fe(CO)2]2; 5

(20,

21).

[(w-

5

The multiple oxida-

t i o n state b e h a v i o r i n l i g a n d - b r i d g e d systems also seems to arise b e c a u s e of b o n d i n g or a n t i b o n d i n g m e t a l - m e t a l o r b i t a l s . a n d f r o z e n s o l u t i o n E P R d a t a for the o n c e - o x i d i z e d Fe(CO)] (m-Ph PCH=CHPPh ) 2

2

2

Fe- - -

( F i g u r e 2)

F o r example, f o r m of

ESCA

[(7r-C H )5

indicate that

5

oxidation

-Fe

Ph Ph"" ^ ç — ç /

Figure 2. Proposed structure of [(Tr-C H )Fe (CO)] (cis-Ph PCH= CHPPh )

^Ph

5

2

5

2

2

+

occurs at i r o n a n d that the u n p a i r e d e l e c t r o n resides i n a m e t a l - m e t a l o r b i t a l (22, 23). (CO)SR]

2

+ / 0

occurs f r o m

F r o m s t r u c t u r a l studies of the system

, Connelly and D a h l concluded an antibonding

metal-metal bond

(24).

F e - F e orbital

[(7r-C H )Fe5

that one-electron that also

gives a

H o w e v e r , i n m a n y of these systems,

i n f o r m a t i o n is n e e d e d a b o u t the electronic structure.

5

oxidation partial detailed

I t is c o n c e i v a b l e

that i n s o m e cases t h e o b s e r v e d r e d o x b e h a v i o r is c a r r i e d b y

orbitals

w h i c h are l a r g e l y l i g a n d b a s e d , a n d i n r e l a t e d systems i t is n o t a l w a y s clear w h e t h e r the m e t a l - m e t a l i n t e r a c t i o n occurs p r i m a r i l y t h r o u g h s p a c e or t h r o u g h a b r i d g i n g l i g a n d (see

below).


76

INORGANIC

COMPOUNDS WITH

UNUSUAL

PROPERTIES

T h e l i g a n d - b r i d g e d systems a r e a t t r a c t i v e i n terms o f r e d o x

proper

ties because of c h e m i c a l v e r s a t i l i t y a n d t h e p o s s i b i l i t y of p r e p a r i n g polymeric compounds.

T h e c o m p o u n d s are also of interest e l e c t r o n i c a l l y

since, f o r a g i v e n m e t a l , t h e o r b i t a l c h a r a c t e r of t h e m e t a l - m e t a l i n t e r a c t i o n c a n b e v a r i e d . F o r e x a m p l e , t h e ions Ph PCH=CHPPh ) 2

2

a n d d -d?

+

[(7r-C H )Fe(CO)] (cis5

and [ ( 7 r - C H 5 ) F e ( C O ) S C H ] 2 5

3

+

2

5

are f o r m a l l y d - d 6

7

cases, r e s p e c t i v e l y , b u t i n b o t h ions there is a p a r t i a l m e t a l -

5

metal bond.


Weak Interactions between Metals through a Bridging

Ligand

W h e n m e t a l - m e t a l interactions t h r o u g h a c o n n e c t i n g l i g a n d b r i d g e are w e a k , t h e m e t a l centers h a v e t h e e l e c t r o n i c a n d c h e m i c a l p r o p e r t i e s of i s o l a t e d , m o n o m e r i c

c o m p l e x e s except f o r c e r t a i n s p e c i a l effects.

If

a p p r o p r i a t e m o n o m e r i c c o m p l e x e s u n d e r g o r e v e r s i b l e e l e c t r o n transfer, a r e l a t e d l i g a n d - b r i d g e d system w i l l u n d e r g o a series of e l e c t r o n transfer steps i n w h i c h e a c h of t h e m e t a l sites i n t u r n u n d e r g o e s o x i d a t i o n o r Figure 3. Structure of the Ι,Γ-polyferrocenes. η = 0: Biferrocene (FcFc), η = I : Ι,Γ-terferrocene (Fc-Fc-Fc) and η = 2: Ι,Γ-quatreferrocene (Fc-Fc-Fc-Fc) 9

reduction. 3), Fc)

Examples

are k n o w n

for the

Ι,Ι'-polyferrocenes

(C H5)Fe(C H4-C5H )Fe(C H4-C5H )Fe(C H5) 5

5

3 + / 2 V V 0

]

4

5

4

(bipy) ClRu(pyz)Ru(bipy) (pyz)RuCl(bipy) 2

bipy =

5

( 2 5 ) , a n d for ligand-bridged 2

2,2'-bipyridine)

( 2 6 , 2 7 , 28).

complexes 2

7 + / 6 + / 5 + / 4 +

(Figure [Fc-Fc-

3 + / 2 + / + / 0

of r u t h e n i u m ,

(pyz =

pyrazine,

R e d u c t i o n potentials w i l l

e l e c t r o n i c effects ( r e s o n a n c e a n d i n d u c t i v e )

i f t h e y are large

reflect enough,

a n d they w i l l b e affected b y s i m p l e electrostatic effects. A s a n e x a m p l e of t h e latter, i n t h e c o m p l e x :

t h e m e t a l centers are e l e c t r o n i c a l l y i s o l a t e d , a n d y e t t h e y are o x i d i z e d at s l i g h t l y h i g h e r potentials t h a n r e l a t e d m o n o m e r i c c o m p l e x e s b e c a u s e of t h e h i g h e r c h a r g e o n t h e d i m e r ( 2 9 , 30). I f there is s y m m e t r y , s t a t i s t i c a l effects a p p e a r .

T o m , Creutz, and

T a u b e n o t e d that i n t h e e q u i l i b r i u m i n R e a c t i o n 3 ( w h e r e 4 , 4 ' - b i p y

=

4 , 4 ' - b i p y r i d i n e ) , t h e m i x e d - v a l e n c e i o n is f a v o r e d b y a statistical f a c t o r of 4 e v e n i n t h e absence of other effects (31).

W h e n o n e compares t h e


7.

Polymetallic

MEYER

77

Systems

(NH3)5Ru(4,4'-bipy)Ru(NH3)5

+

6 +

( N H ) R u (4,4'-bipy) R u ( N H ) 3

5

3

*± 2 ( N H ) R u ( 4 , 4 ' - b i p y ) R u ( N H ) 3

5

3

5

5

4 +

(3)

5 +

r e d u c t i o n potentials for the h a l f - r e a c t i o n s 4 a n d 5 ( w h e r e F c represents ferrocene a n d a f e r r o c e n y l g r o u p ) , t h e b i f e r r o c e n e c o u p l e d i s f a v o r e d b y a statistical factor of 2 or 0.018 V [(RT/nF)

(Fc-Fc) In 2 =

since there are t w o w a y s of f o r m i n g ( F c - F c ) — F c - F c a n d F c - F c +

Fc


0.018] (25).

+

+ e -> Fc

+

(Fc-Fc)

+

(4) (5)

+ e -» F c - F c

+

is

+ / 0

F o r systems m o r e c o m p l i c a t e d t h a n d i m e r s , there are a m b i g u i t i e s r e g a r d i n g the site of o x i d a t i o n . F o r e x a m p l e , i n the Ι,Ι'-polyferrocenes, Fc

n

(n — 3, 4 ) ( F i g u r e 3 ) , a n d i n l i g a n d - b r i d g e d r u t h e n i u m complexes,

(bipy) ClRu(pyz)[Ru(bipy) pyz]nRuCl(bipy) 2

2

2

( n — 1-4)

( 2 n + 2 ) +

(25,

26, 2 7 ) , there are c h e m i c a l l y different sites. O x i d a t i o n gives a series of o x i d a t i o n state isomers w h i c h differ i n t h e site of o x i d a t i o n ( 2 5 ) . example, for ( l , l ' - t e r f e r r o c e n e ) isomers

( F c - F c - F c and F c - F c - F c ) +

a n d one

+

(Fc-Fc -Fc).

non-equivalent

differences

between

dominant.

It w a s e s t i m a t e d t h a t i n s o l u t i o n the free e n e r g y

between F c - F c - F c +

isomers m a y be l a r g e a n d a s i n g l e i s o m e r m a y

m

be

difference

a n d F c - F c - F c is ~ 0.12 V ( 2 5 ) a n d t h a t b e t w e e n

+

+

+

(NH ) Ru (pyz)Ru Cl(bipy) 5

isomer

D e p e n d i n g o n differences i n l i g a n d e n v i r o n m e n t s , e n e r g y

+

3

For

there are t w o e n e r g e t i c a l l y e q u i v a l e n t

+

n

is ~ 0.30 V (29,30).

2

4 +

and

(NH ) Ru 8

B

I I

(pyz)Ru a(bipy)2 m

4 f

I f a n o r b i t a l p a t h w a y exists b e t w e e n m e t a l centers,

t h e different isomers are accessible b y t h e r m a l - a n d l i g h t - i n d u c e d i n t r a m o l e c u l a r e l e c t r o n transfer processes. It is i m p o r t a n t to r e a l i z e that the assignment of o x i d a t i o n

states

b a s e d o n s o l u t i o n i n f o r m a t i o n m a y not a p p l y to the s o l i d state. A l t h o u g h a mixed valence ion like ( b i p y ) C l R u ( p y z ) R u C l ( b i p y ) 2

2

3 +

m a y be favored

i n s o l u t i o n , there is no guarantee t h a t it is f a v o r e d i n the s o l i d state o v e r a s t o i c h i o m e t r i c m i x t u r e of the t w o adjacent ions RuCl(bipy)

2

2 +

and

(bipy) ClRu(pyz)RuCl(bipy) 2

2

(bipy) ClRu(pyz)2

4 +

.

I n m i x e d - v a l e n c e complexes, w e a k metal—metal i n t e r a c t i o n s l e a d t o intervalence regions (25). (NH )5Ru 3

n i

transfer

(IT)

bands,

usually i n the visible or near

IR

I n a n I T t r a n s i t i o n , l i g h t - i n d u c e d e l e c t r o n transfer occurs N Ο

NRu Cl(bipy) n

2

4 +

.«ι (NH ) Ru N 3

5

n

NRu Cl(bipy) **

Ο V ^

m

'

2

/


78

INORGANIC

COMPOUNDS WITH

b e t w e e n m e t a l sites i n different o x i d a t i o n states

UNUSUAL

PROPERTIES

(Reaction

6).

I n the

i m m e d i a t e p r o d u c t of l i g h t - i n d u c e d e l e c t r o n transfer, the m e t a l sites are 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 n d s o l v a t i o n states since n u c l e a r

motion

is s l o w c o m p a r e d w i t h e l e c t r o n m o t i o n ( F r a n c k - C o n d o n p r i n c i p l e ) . The potential energy-configurational H u s h to d e s c r i b e

I T transitions ( 3 2 )

grams s h o w i n g I T 34)

and 8 (33)]

transitions for b o t h s y m m e t r i c a l

3


diagrams used i n F i g u r e 4. [Reactions

by Dia

7

(31,

a n d u n s y m m e t r i c a l ( R e a c t i o n 6 ) m i x e d - v a l e n c e ions are

(NH )5Ru(4,4'-bipy)Ru(NH )5 3

hv -» (NH ) Ru(4,4 -bipy)Ru(NH )5 * (7)

5 +

Fc-Fc given.

coordinate

are p r e s e n t e d

3

r

5

5 +

3

-» Fc -Fc*

+

(8)

+

T h e t r a n s i t i o n i n R e a c t i o n 6 is at h i g h e r e n e r g y t h a n t h a t for a

s y m m e t r i c a l system b e c a u s e the e l e c t r o n transfer process is e n e r g e t i c a l l y unsymmetrical.

T h e product,

(NH ) Ru (pyz)Ru ^l(bipy) 3

n

5

n

is a t h e r m a l l y e q u i l i b r a t e d m i x e d - v a l e n c e b e c a u s e the o x i d a t i o n state c o n f i g u r a t i o n state

(29,

30).

figurations Fc-Fc)

Similar

(in

1:1

v:v

2

2

certain oxidation is at a h i g h e r

3

+

(\

+

Hush

+

hv -» Fc -Fc -Fc

developed a theoretical

+

+

treatment

for

T h e w o r k of H u s h a n d t h a t of R o b i n a n d D a y ( 3 5 ) t h e y relate the p r o p e r t i e s

of

which

disfavored ground

state

I T bands

energy

con (Fc(A

m a x

5.26 k K ) b e c a u s e the t r a n s i

max

t i o n is also e n e r g e t i c a l l y u n s y m m e t r i c a l ( R e a c t i o n 9 )

Fc -Fc-Fc

\

T h e I T b a n d o b s e r v e d for

CH C1 -CH CN)

5.99 k K ) t h a n the I T b a n d for F c - F c

state, is

4

is r e v e r s e d f r o m t h e

effects o c c u r for

of the Ι , Γ - p o l y f e r r o c e n e s .

2 +

excited

2

(25).

* IT

(9) transitions

(32).

is i m p o r t a n t b e c a u s e

to the extent

of

metal-metal

i n t e r a c t i o n a n d to the rate of t h e r m a l e l e c t r o n transfer b e t w e e n m e t a l sites. T h e r e l a t i o n s h i p s are d e p i c t e d d i a g r a m m a t i c a l l y i n F i g u r e 4 w h e r e E p and E

Ul

0

are the energies for the o p t i c a l a n d t h e t h e r m a l

transfer processes, r e s p e c t i v e l y .

electron

O r b i t a l o v e r l a p b e t w e e n m e t a l sites is

t h e o r i g i n of the s p l i t t i n g b e t w e e n surfaces

a n d of the i n t e n s i t y of

IT

bands. Recent

w o r k on dimeric ruthenium complexes demonstrated

that

there is reasonable agreement b e t w e e n e x p e r i m e n t a l d a t a a n d t h e b a n d w i d t h a n d solvent d e p e n d e n c e p r e d i c t d b y H u s h for I T b a n d s

(29,

30,

31 ). T h e w o r k w i t h r u t h e n i u m c o m p l e x e s also r e v e a l e d that the energies a n d intensities of I T b a n d s v a r y s y s t e m a t i c a l l y as a f u n c t i o n of b r i d g i n g a n d n o n b r i d g i n g l i g a n d effects (29, 30,31,

34).

N o I T b a n d was observed


7.

Polymetallic

MEYER

79

Systems

for t h e i o n :

[(NH )5Ru 3

n i

N

0 / - C H

2

C H

2

- Y O

NRu Cl(bipy) ] I I

2

4 +

b e c a u s e t h e o r b i t a l p a t h w a y b e t w e e n m e t a l centers is b l o c k e d b y t h e saturated - C H - C H - linkage. I T bands appear for pyrazine, 4,4-bipyri2

2

d i n e , a n d i r a n s - l , 2 - b i s ( 4 - p y r i d y l ) ethylene as t h e b r i d g i n g l i g a n d s w h e r e there is a n i n t a c t π system.

T h e bands are at h i g h e r energies f o r t h e

l o n g e r b r i d g i n g l i g a n d s ( i n a c e t o n i t r i l e : 10.4 k K f o r p y r a z i n e , ~ 1 4 . 4 k K


for 4 , 4 ' - b i p y r i d i n e , a n d ~ 14.7 k K f o r f r a n s - l , 2 - b i s ( 4 - p y r i d y l ) e t h y l e n e (29, 30) Ru

i n

-Ru

because t h e i n t e r s e c t i o n r e g i o n b e t w e e n n

the R u

I ] L

Ru

m

and

surfaces ( F i g u r e 4 ) a n d therefore t h e energy of t h e I T t r a n s i

t i o n are functions of the d i s t a n c e b e t w e e n t h e m e t a l centers. T h e i n t e n s i t y of t h e b a n d f o r t h e p y r a z i n e - b r i d g e d d i m e r is c o n s i d e r a b l y greater t h a n that f o r t h e other d i m e r s ; this i n d i c a t e s a stronger m e t a l - m e t a l i n t e r a c t i o n .

Ru (H)-Ru (lll) a

a

RuaOlO-RuaOl)

A

Ο UJ Ζ

CONFIGURATIONAL COORDINATE

Ru (ll)-Ru (IH) b

a

Ru^llO-RuJII)

Β

Figure 4. Potential energy-configurational coordinate diagrams for symmetrical (A) and unsymmetncal (B) cases. IT transi tions are indicated by the arrows.


80

INORGANIC

COMPOUNDS

WITH

UNUSUAL

PROPERTIES

N o n b r i d g i n g l i g a n d s c a n affect the e n e r g y a n d a p p a r e n t l y also the i n t e n s i t y of I T b a n d s . and ( N H ) 5 R u

(34)

3

i n

I n the d i m e r s

( N ^ H R u ^ P y z J R u ^ N ^ ^

(pyz)Ru X(bipy)2 n

4 +

(29, 30),

5

*

the e n e r g y of the

I T b a n d increases as v a r i a t i o n s i n L or X increase the energy a s y m m e t r y b e t w e e n the t w o ends. F o r the m i x e d - v a l e n c e d i m e r RuCl(bipy)

2

3 +

(bipy) ORu(pyz)2

, E S C A studies d e m o n s t r a t e d t h a t there are discrete R u ( I I )

a n d R u ( I I I ) sites (36).

I n r e c e n t w o r k , C a l l a h a n a n d M e y e r (37)

a n I T b a n d for t h e i o n ( A

1300 n m , c =

m a x

found

450 i n a c e t o n i t r i l e ) w h i c h has

the a p p r o x i m a t e b a n d w i d t h a n d solvent d e p e n d e n c e t h a t w e r e p r e d i c t e d by Hush.

I R d a t a are consistent w i t h l o c a l i z e d valences.

T h e properties


of this i o n differ m a r k e d l y f r o m those of t h e C r e u t z a n d T a u b e (NH ) Ru(pyz)Ru(NH3)5 3

with

5

a much

system

stronger

(33),

5 +

a n d the differences

are

m e t a l - m e t a l i n t e r a c t i o n i n the

ion

consistent

pentaammine

(37).

Strong Interactions between Metals through a Bridging

Ligand

W i t h s t r o n g m e t a l - m e t a l i n t e r a c t i o n s across a b r i d g i n g l i g a n d , t h e v a l e n c e r e d o x o r b i t a l s are d e l o c a l i z e d m o l e c u l a r o r b i t a l s b o t h m e t a l a n d l i g a n d i n c h a r a c t e r . I n m i x e d - v a l e n c e c o m p o u n d s , different, discrete o x i dation

states d o

not exist since the site of

o x i d a t i o n is

delocalized.

S t r o n g l y c o u p l e d systems are l i k e m e t a l - m e t a l b o n d s i n t h a t t h e i r elec t r o n i c a n d c h e m i c a l p r o p e r t i e s are s i g n i f i c a n t l y m o d i f i e d f r o m those of related monomeric

complexes.

As w i t h m e t a l - m e t a l bonds, such

com

p o u n d s c a n h a v e a n extensive m u l t i p l e o x i d a t i o n state c h e m i s t r y b a s e d on delocalized molecular orbitals. S t r o n g c o u p l i n g is e x p e c t e d

w h e n the b r i d g i n g d i s t a n c e is short,

e s p e c i a l l y for s e c o n d - a n d t h i r d - r o w metals a n d for metals i n l o w o x i d a t i o n states w h e r e d o r b i t a l extension is great a n d s t r o n g π o v e r l a p c a n occur.

S e v e r a l o x o - b r i d g e d complexes of r u t h e n i u m ( I I I ) ,

RuX(AA) ] 2

[(AA) XRuO2

( w h e r e A A is 2 , 2 - b i p y r i d i n e or 1,10-phenanthroline

2 +

/

and

X is C I or N 0 ) , h a v e u n u s u a l s p e c t r a l , r e d o x , a n d c h e m i c a l p r o p e r t i e s 2

I n the salt [ ( b i p y ) ( N 0 ) R u O R u ( N 0 ) ( b i p y ) ] ( C 1 0 ) ,

(38).

2

2

2

2

4

2

the

R u - O - R u a n g l e is

Inorganic Compounds with Unusual Properties

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