Bioinorganic ChemistryâII

7 Nonadiabatic Electron Transfer in Oxidation-Reduction Reactions HENRY TAUBE

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Department of Chemistry, Stanford University, Stanford, Calif. 94305

Several criteria are applied in searching for evidence of nonadiabatic electron transfer in oxidation-reducton reac tions. There is direct evidence for a nonadiabatic factor only for some self-exchange reactions of Mn(CNR) and Fe(Rphen) where R is a bulky group, although the factor also may affect rates for some f-electron couples. Refinement of the criteria may show that the nonadiabatic factor decreases the rates in other cases. One approach is to study the nonadiabatic regime by using electron transfer from Ru(II) to Co(III) in an intramolecular mode. Obser vations on such systems and on the intensities of the inter valence bands in Ru(II)-Ru(III) mixed valence complexes offer clues to the extent of coupling by the bridging groups. 2+,+

6

3+,2+

3

TPhe measurement of the rate and determination of the rate law (1,2) for the self-exchange process Fe ' mark an important stage in the study of electron transfer in oxidation-reduction reactions. Dodson's results attracted a great deal of attention to thefield,stimulating other experimental work and, by providing some definite data at a critical time, also stimulating discussion of the mechanism of the electron transfer process (see, for example, discussion reported in Ref. 3). In retrospect, the development of the subject could as well have been based on studies of orthodox oxidation-reduction reactions of simple chemistry. However, until some specific proposals about the mechanism of electron transfer 3+

2+

aq

had been m a d e , t h e r e w a s l i t t l e i n c e n t i v e for m e a s u r i n g the rates o f t h e o r d i n a r y reactions. T h e self-exchange reactions c a m e to the fore n o t o n l y b e c a u s e t h e y w e r e c a r r i e d b y the m o m e n t u m of interest w h i c h t h e n p r e v a i l e d i n a p p l y i n g artificial r a d i o a c t i v i t y to p r o b l e m s i n c h e m i s t r y b u t also b e c a u s e the e l e m e n t of s y m m e t r y simplifies the u n d e r s t a n d i n g of t h e observations. 127

In Bioinorganic Chemistry—II; Raymond, K.; Advances in Chemistry; American Chemical Society: Washington, DC, 1977.

128

BIOINORGANIC

CHEMISTRY

W i t h the demonstration that a n activated complex for the F e

a q

II

3 +

»

2 +

self-exchange contains o n e o f e a c h o f t h e reactants ( I ) a n d t h a t there are o t h e r a c t i v a t e d complexes

( 2 ) w h i c h c o n t a i n , i n a d d i t i o n , anions

s u c h as O H " o r X ' , p r o d u c t i v e d i s c u s s i o n o f the e l e c t r o n transfer processes i n terms o f m o l e c u l a r m o d e l s

began.

Q u i t e e a r l y (4),

attention was

d i r e c t e d to t h e e n e r g y b a r r i e r to e l e c t r o n transfer w h i c h is i m p o s e d b y the F r a n c k - C o n d o n restriction. B u t w h e n a molecular m o d e l for the activated complex

f o r e l e c t r o n transfer is p r o p o s e d ,

a d i s t a n c e of a p -

p r o a c h f o r t h e r e a c t a n t ions needs to b e specified, a n d a t once t h e q u e s Downloaded by UNIV OF OKLAHOMA on August 17, 2013 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/ba-1977-0162.ch007

t i o n of t h e " c o n d u c t i v i t y " o f t h e m a t t e r i n t e r v e n i n g b e t w e e n o x i d a n t a n d r e d u c t a n t arises. I n t h e discussions of t h e l a r g e a m o u n t of e x p e r i m e n t a l d a t a w h i c h has b e e n a c c u m u l a t e d d u r i n g t h e past t w o decades, m o r e a t t e n t i o n has p r o b a b l y b e e n d e v o t e d to this aspect of the e l e c t r o n transfer process t h a n to t h e F r a n c k - C o n d o n b a r r i e r . T h i s is s o m e w h a t i r o n i c a l i n v i e w of t h e c o n c l u s i o n that w i l l b e r e a c h e d that i n f e w of t h e systems s t u d i e d u n t i l n o w a r e differences i n l i g a n d " c o n d u c t i v i t y " u s e f u l i n u n d e r s t a n d i n g differences

i n r e a c t i o n rates.

T h i s a p p l i e s also to systems i n

w h i c h oxidation—reduction i n v o l v e s e l e c t r o n transfer o v e r l a r g e distances, w h e r e t h e t e m p t a t i o n to ascribe rate differences to t h e c o n d u c t i v i t y of l i g a n d s has b e e n p a r t i c u l a r l y difficult to resist. T h e first i n t i m a t i o n s of r e m o t e e l e c t r o n transfer i n a r t i f i c i a l systems w e r e r e p o r t e d i n 1955 ( 5 ) , b u t c o n v i n c i n g e v i d e n c e f o r r e m o t e attack w a s n o t p r o v i d e d u n t i l 1966 (6,7).

I n b i o c h e m i c a l systems, e l e c t r o n transfer over m a n y b o n d lengths

is a p a r t i c u l a r l y a p p e a l i n g i d e a , a n d i t m a y t u r n o u t to b e i m p o r t a n t as well.

E l e c t r o n t r a n s p o r t o v e r large distances i n proteins w a s p r o p o s e d

m a n y years a g o b y S z e n t - G y o r g i (8,9),

a n d he may w e l l have been pre-

c e d e d b y others w h o suggested t h e p o s s i b i l i t y seriously. I n fact, s t r o n g e v i d e n c e f o r e l e c t r o n transfer over l a r g e distances has b e e n o b t a i n e d b y D e V a u l t a n d C h a n g e (10) i n e x c i t e d states of systems r e l a t e d t o t h e photosynthetic cycle. O n e p u r p o s e of this p a p e r is to e x a m i n e t h e e v i d e n c e t h a t t h e rates of o x i d a t i o n - r e d u c t i o n reactions are r e l a t e d to t h e c o n d u c t i v i t y of t h e m e d i u m separating the oxidant a n d reductant.

This survey w i l l then

d e s c r i b e e x p e r i m e n t s n o w i n progress to investigate s y s t e m a t i c a l l y t h e n o n a d i a b a t i c r e g i m e i n o x i d a t i o n - r e d u c t i o n reactions. F i r s t t h e r e l a t i o n s h i p b e t w e e n w h a t has loosely b e e n r e f e r r e d to as t h e c o n d u c t i v i t y of t h e m e d i u m a n d t h e title t e r m , " n o n a d i a b a t i c , " s h o u l d b e defined. F i g u r e 1 shows t h e d o u b l e - w e l l p o t e n t i a l w h i c h is o f t e n u s e d t o r e p r e s e n t t h e e l e c t r o n transfer a c t i n o x i d a t i o n - r e d u c t i o n .

T h e special

case o f a self-exchange process w a s c h o s e n f o r s i m p l i c i t y . ( F o r a f u l l d i s c u s s i o n of t h e issues b e i n g d i s c u s s e d i n r e l a t i o n to t h e p o t e n t i a l energy d i a g r a m , see R e f . 11. R e f . 13 gives a m o r e c o m p a c t

treatment.)

The

implications of the diagram m a y not be immediately obvious, a n d the



7.

Nonadiabatic Electron Transfer

TAUBE

V•>» e

129

/

Figure 1. Potential energy as a function of reaction coordinate for a self-ex change reaction. AE energy barrier for thermal electron transfer (weak cou pling); AE , energy of an intervalence transition which is possible for the system. lf

2

i m p o r t a n t ones w i l l therefore

b e e x p l a i n e d b y reference

to a

specific

system. I f t h e m i n i m u m o n t h e left at A is t a k e n to r e p r e s e n t t h e s y s t e m *Fe(H 0) 2

6

3 +

+ Fe(H 0) 2

6

2 +

, m i n i m u m Β represents t h e final p r o d u c t o f

t h e e l e c t r o n t r a n s f e r act, n a m e l y * F e ( H 0 ) 6 2

2 +

+ Fe(H 0) 2

6

3 +

.

T h e reac

t i o n c o o r d i n a t e is a c o m b i n a t i o n of n u c l e a r m o t i o n s w h i c h results i n t h e m o v e m e n t of t h e e l e c t r o n f r o m F e ( H 0 ) 2

6

2 +

to * F e ( H 0 ) 2

6

3 +

.

F o r present

p u r p o s e s , i t is a r b i t r a r i l y a s s u m e d to b e m a d e u p of t h e b r e a t h i n g f r e q u e n cies f o r t h e w a t e r m o l e c u l e s i n t h e first c o o r d i n a t i o n spheres of t h e t w o r e a c t a n t ions. T h e r e a c t i o n c o o r d i n a t e t a k e n i n t h e d i r e c t i o n t o t h e r i g h t of A t h e n represents t h e m o t i o n o f t h e l i g a n d s a w a y f r o m * F e w i t h those a t t a c h e d to F e

2 +

3 +

i n phase

m o v i n g t o w a r d i t , t h e s e p a r a t i o n of t h e ions

r e m a i n i n g fixed. A t a p o i n t d e f i n e d b y t h e i n t e r s e c t i o n o f t h e t w o c u r v e s , the c o o r d i n a t i o n spheres a b o u t b o t h m e t a l ions are t h e same, a n d t h e e n e r g y o f t h e s y s t e m a r r i v e d a t f r o m A is t h e s a m e as t h a t a r r i v e d a t f r o m B . A t this p o i n t a l o n g t h e r e a c t i o n c o o r d i n a t e , t h e c o n d i t i o n i m p o s e d b y t h e F r a n c k - C o n d o n r e s t r i c t i o n is m e t , b u t i f t h e ions are f a r a p a r t so 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

t h e o r b i t a l s is w e a k

(very


small

130

BIOINORGANIC

CHEMISTRY

II

e n e r g y gap at t h e cross-over p o i n t ) , the p r o b a b i l i t y of passing f r o m t h e state

*Fe(H 0) 2

6

3 +

+ Fe(H 0) 2

6

2 +

to

*Fe(H 0) 2

6

2 +

+ Fe(H 0) 2

6

will

3 +

d e p e n d o n t u n n e l l i n g p r o b a b i l i t y ( h e n c e a r e l a t i o n to the " c o n d u c t a n c e " of the m e d i u m ) .

( I f t u n n e l l i n g p r o b a b i l i t y is i n t e r p r e t e d as the p r o b a

b i l i t y that the system, once i t has the r e q u i s i t e energy, w i l l pass o n to p r o d u c t s , i t has a d i r e c t r e l a t i o n to t h e p r o b a b i l i t y of a d i a b a t i c transfer. M a r c u s (11)

has p o i n t e d o u t t h a t w h e n t u n n e l l i n g p r o b a b i l i t y is c a l c u

l a t e d i n the u s u a l w a y (see for e x a m p l e Ref. 12),

the s i m p l e d i r e c t r e l a

t i o n s h i p is lost. A c c o r d i n g to M a r c u s , the result of this k i n d of c a l c u l a Downloaded by UNIV OF OKLAHOMA on August 17, 2013 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/ba-1977-0162.ch007

t i o n is a r o u g h m e a s u r e of the energy g a p at t h e crossover p o i n t , b u t this is o n l y one of the factors w h i c h affect the p r o b a b i l i t y of a d i a b a t i c t r a n fer.)

I n this, the n o n a d i a b a t i c r e g i m e , the rate at a fixed d i s t a n c e

of

s e p a r a t i o n w i l l be g o v e r n e d b o t h b y the F r a n c k - C o n d o n b a r r i e r a n d a transition probability.

When

the i n t e r a c t i o n b e t w e e n the orbitals i n

creases, as e x p e c t e d w h e n the ions a p p r o a c h , crossover p o i n t increases.

the e n e r g y g a p at

the

W h e n this g a p b e c o m e s sufficiently

large—

a n d a c c o r d i n g to t h e o r y ( 13 ) a p p r o x i m a t e l y 0.5 k c a l suffices—the

system

w i l l r e m a i n o n the l o w e r c u r v e as i t traverses the e n e r g y b a r r i e r separat ing A and Β

(adiabatic

transfer).

I n this l i m i t , n o t c o n s i d e r i n g

e n e r g y i n v o l v e d i n b r i n g i n g the reactants

the

to a s u i t a b l e distance,

r e a c t i o n rate is g o v e r n e d solely b y t h e F r a n c k - C o n d o n b a r r i e r .

the

I n this

r e g i m e , i m p r o v i n g the i n t e r a c t i o n b e t w e e n the orbitals w i l l n o t l e a d to a r a t e increase, unless the d e r e a l i z a t i o n becomes great e n o u g h to l o w e r the e n e r g y m a x i m u m significantly. T h e o n l y t h i n g t h a t is l e a r n e d

from

r e a c t i o n rates a b o u t t u n n e l l i n g p r o b a b i l i t y i n the a d i a b a t i c r e g i m e

is

that i t is sufficiently h i g h . T h e n o n a d i a b a t i c reactions o n the other h a n d are p a r t i c u l a r l y i n t e r e s t i n g because b y

studying

them, the transition

p r o b a b i l i t y as a f u n c t i o n of distance a n d the properties of the m e d i u m c a n b e s t u d i e d . T h i s r e l a t i o n s h i p has not y e t b e e n e x a m i n e d for c h e m i c a l reactions. A n u m b e r of c r i t e r i a c a n be u s e d to gage w h e t h e r reactions are a d i a b a t i c or n o n a d i a b a t i c , a n d t h e y w i l l be c o n s i d e r e d i n t u r n . O n e c r i t e r i o n rests o n the a p p l i c a b i l i t y of the M a r c u s (11)

c o r r e l a t i o n of the rates of

cross reactions w i t h the self-exchange reactions of the couples i n v o l v e ^ . I t seems u n l i k e l y that the c o r r e l a t i o n w o u l d be successful i f t u n n e l l i n g p r o b a b i l i t y , w h i c h is e x e p c t e d to b e a sensitive f u n c t i o n of the d i m e n s i o n s of the b a r r i e r , w e r e a n i m p o r t a n t factor i n d e t e r m i n i n g the rates.

This

b a r r i e r is not that s h o w n i n F i g u r e 1; i t is the b a r r i e r w h i c h w o u l d mapped

out b y c o n s i d e r i n g

the p o t e n t i a l e n e r g y of t h e system

be

as a

f u n c t i o n of distance w h e n the e l e c t r o n is m o v e d f r o m the r e d u c i n g agent t h r o u g h the i n t e r v e n i n g m e d i u m to the o x i d i z i n g agent. t i o n of reactions

i n v o l v i n g the C o ( I I I ) - C o ( I I )

W i t h the e x c e p

couple, where

i t has

b e e n tested i n the s i m p l e i n o r g a n i c systems, the M a r c u s c o r r e l a t i o n w o r k s


7.

131


TAUBE

reasonably well.

R e a s o n a b l e a g r e e m e n t is a l l t h p t c a n b e e x p e c t e d since

t h e w o r k r e q u i r e d to assemble t h e reactants i n t h e p r e c u r s o r

complex

c a n n o t p r o p e r l y b e a l l o w e d f o r . E v e n w h e n t h e r e a c t a n t couples a r e o f t h e same c h a r g e t y p e , i d i o s y n c r a c i e s i n i n t e r a c t i o n , p e r h a p s b e c a u s e of changes i n s o l v a t i o n , m a y b e c o m e a factor.

T h i s is p a r t i c u l a r l y l i k e l y

to h a p p e n w h e n t h e r e a c t i o n p a r t n e r s a p p r o a c h c l o s e l y o n r e a c t i o n . F a c tors s u c h as t h e h y d r o p h o b i c / h y d r o p h i l i c n a t u r e of t h e reactants a n d , f o r l a r g e reactants, e v e n t h e i r shapes c a n l e a d to i d i o s y n c r a c i e s i n t h e w o r k r e q u i r e d to assemble t h e p r e c u r s o r complexes.

I n addition, the measure


ments b e i n g c o n s i d e r e d a r e often m a d e u n d e r v a r y i n g c o n d i t i o n s , a n d a l l are subject to e x p e r i m e n t a l error. A s a c o n s e q u e n c e , i t is difficult to u s e the M a r c u s c o r r e l a t i o n to r e v e a l s m a l l b u t r e a l effects a r i s i n g f r o m n o n adiabaticity. Reactions involving the C o ( I I I ) - C o ( I I )

couple, for w h i c h

there is a l a r g e s p i n c h a n g e , m i g h t p r o v i d e examples of n o n a d i a b a t i c e l e c t r o n transfer ( n o n m i x i n g o f states of different m u l t i p l i c i t y ) .

How

ever, i n t h e u s u a l i n t e r p r e t a t i o n of reactions i n w h i c h C o ( I I I ) is r e d u c e d , the system c i r c u m v e n t s a n o n a d i a b a t i c t r a n s i t i o n i n t h e a c t i v a t e d c o m p l e x b y u n d e r g o i n g a s p i n c h a n g e p r i o r to o r f o l l o w i n g t h e a c t i v a t e d complex. Table I.

Reactions of R u ( I I ) Complexes with F e a q at 2 5 ° 3 +

Reactant

Medium

Ru(NH ) Ru(NH ) isn Ru(NH ) bipy 3

3

3

6

5

4

(M'

1

0ΛΜ H C 1 0 1M H 0 S C F 1M H C 1 0

2 +

3

2 + e

1

(M

a

6

4

(calc)

1

3.4 X 1 0 2.6 X 1 0 7.2 X 1 0

3 d

5

3

sec' )

1

4.3 X 1 0 4.3 X 1 0 ' 2.1 X 1 0 '

4

2 + e

sec' )

5

4 3

b,e

1.3 X 1 0 ' 0.6 X 1 0 ' 0.8 Χ 10"

2

2 2

Self-exchange rate for Ru(III)-Ru(II) couples. Calculated self-exchange rate for F e - aq. K determined for conditions specified by cyclic voltammetry (Ru couples) and potentiometric titration [Ru(NH )e ' and Fe « aq]. Ref. 15. isn = isonicotinamide ; bipy = 2,2'-bipyridine ; nic = nicotinic acid ; phen = 1,10-phenanthroline. 'From [Ru(NH ) isn] + [Ru(NH ) nic] \ 'From [Ru(NH ) phen] + [Ru(NH ) bipy] . a

b

c

3 +

2 +

eq

3

3+

2+

3+

2+

d

e

3

5

3

4

3+

3

3+

2

5

3

4

2+

I n some r e c e n t e x p e r i m e n t s ( 1 4 ) o n rates of o x i d a t i o n b y F e

3 +

of a

n u m b e r of r e l a t e d R u ( I I ) species, t h e rates of t h e self-exchange reactions for R u ( I I I ) - R u ( I I )

couples

a n d the equilibrium data were

as m u c h as p o s s i b l e u n d e r u n i f o r m c o n d i t i o n s .

measured

I n d e t e r m i n i n g self-ex

c h a n g e rates, a series of reactions of t h e t y p e R u ( N H ) L 3

5

2 +

+ Ru(NH ) L 3

5

/ 3 +

w e r e s t u d i e d i n w h i c h L a n d 1 / a r e p y r i d i n e s w h i c h differ i n o n e s u b stituent i n the 3 o r 4 p o s i t i o n . N o r a t e differences a s c r i b a b l e t o differences i n L a n d L ' w e r e o b s e r v e d , a p a r t f r o m t h e effect o n d r i v i n g force. T h e result of these studies a r e s u m m a r i z e d i n T a b l e I . T h e rate o f t h e F e

a q

3 +

'

2 +

s e l f - e x c h a n g e as c a l c u l a t e d f r o m t h e d a t a


132

BIOINORGANIC CHEMISTRY

is several h u n d r e d times s l o w e r t h a n that m e a s u r e d ( c a . 4 M'

sec"

1

II

for

1

t h e c o n d i t i o n s of the experiments ) ( 2 ). T h e d i s c r e p a n c y w a s n o t e d also i n the e a r l y w o r k ( 1 5 )

on the R u ( N H ) 3

6

3 +

'

2 +

self-exchange, b u t i n v i e w

of the d i f f i c u l t y a t t e n d i n g these m e a s u r e m e n t s , n o t m u c h significance w a s a t t a c h e d to i t . A s i m i l a r d i s c r e p a n c y has b e e n n o t e d for the F e a series of f e r r i c i n i u m - f e r r o c e n e cross reactions (16).

a q

3 +

«

and

2 +

T h e discrepancies

c a n h a r d l y b e c o n s i d e r e d p r o o f t h a t t h e r e is a n o n a d i a b a t i c c o n t r i b u t i o n s o m e w h e r e i n the process.

P e r h a p s t h e y b e t t e r i l l u s t r a t e the p o i n t t h a t

at the present l e v e l of refinement i n c a l c u l a t i n g the w o r k of b r i n g i n g the Downloaded by UNIV OF OKLAHOMA on August 17, 2013 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/ba-1977-0162.ch007

reactants together a n d w i t h e x i s t i n g d a t a , the a p p l i c a t i o n of t h e M a r c u s r e l a t i o n is n o t v e r y u s e f u l i n r e v e a l i n g s m a l l effects r e s u l t i n g f r o m n o n adiabatic behavior. A second c r i t e r i o n w h i c h c a n b e a p p l i e d i n s e a r c h i n g for the effects of n o n a d i a b a t i c i t y is b a s e d o n charge t r a p p i n g b y t h e o r i e n t a t i o n of solvent molecules.

I n i l l u s t r a t i n g the significance of F i g u r e 1, i t w a s

a s s u m e d that the t r a p p i n g of the c h a r g e is effected b y the m o l e c u l e s i n t h e first c o o r d i n a t i o n sphere. T h e t h e o r e t i c a l treatments of the e l e c t r o n transfer process i n s o l u t i o n (17, 18)

s h o w t h a t f o r species of o r d i n a r y

size, charge t r a p p i n g b y the solvent m a k e s a n i m p o r t a n t c o n t r i b u t i o n to t h e e n e r g y b a r r i e r . O b s e r v a t i o n s o n the c h a n g e i n energy of t h e i n t e r v a l e n c e t r a n s i t i o n i n / A - 4 , 4 - b i p y r i d i n e b i s ( p e n t a a m m i n e r u t h e n i u m ) i n the /

m i x e d ( [2, 3] ) v a l e n c e state b e a r d i r e c t l y o n this p o i n t ( 19, 20) a n d l e n d s u p p o r t to the t h e o r e t i c a l treatments

(21).

T h e results o b t a i n e d

for

these systems suggest that i n the ions of the t y p e specified, a b o u t h a l f of t h e o v e r a l l energy b a r r i e r is a s c r i b a b l e to c h a r g e t r a p p i n g b y the solvent. T h e r e f o r e i f a series of complexes is s t u d i e d i n w h i c h the p o l a r g r o u p s a t t a c h e d to t h e m e t a l ions are h e l d constant b u t the l i g a n d s are m a d e m o r e b u l k y b y a d d i n g s a t u r a t e d h y d r o c a r b o n m a t t e r , the r e a c t i o n rate s h o u l d increase w i t h the b u l k i n e s s of t h e groups i f t h e reactions are a d i a b a t i c for the series. I m p l i c i t i n this c o n c l u s i o n is the p r e m i s e t h a t t h e h y d r o c a r b o n m a t t e r is less effective at t r a p p i n g the charge t h a n is the solvent, a n d this seems reasonable f o r reactions i n w a t e r . I f the rate decreases o n i n c r e a s i n g reactant b u l k i n the m a n n e r specified, i t is r e a s o n a b l e to assume that the rate decrease is c a u s e d b y a r e d u c t i o n i n e l e c t r o n transfer p r o b a b i l i t y . No

s y s t e m a t i c studies i n v o l v i n g n e t c h e m i c a l c h a n g e

have

been

r e p o r t e d w h i c h d e m o n s t r a t e n o n a d i a b a t i c b e h a v i o r , b u t t w o studies of self-exchange s h o w effects w h i c h are e x p e c t e d i f the p r o b a b i l i t y of b a r r i e r p e n e t r a t i o n is b e c o m i n g ( Χ 10"

1

ening i n

5 5

3

3 +

T h e specific rates

1

M n for M n ( C N R )

6

2 +

«

are 64 f o r R =

+

at 7 ° i n a c e t o n i t r i l e (22).

terf-butyl phen)

a r a t e - d e t e r m i n i n g factor.

M " sec" ) of self-exchanges as m e a s u r e d b y N M R l i n e b r o a d

4

> , Fe(4,7-phenylphen) 2 +

3

3 +

E t a n d 4.0 for R

For Fe(phen)

' , 2 +

and

3

3 +

' , 2 +

=

Fe(3,4,5,8-me-

Fe(4,7-cyclohexylphen)


3

3 +

' , 2 +

7.

Nonadiabatic

TAUBE

Electron

133

Transfer

specific rates ( Χ 10" M " sec" ) i n a c e t o n i t r i l e at 2 5 ° are 6 ± 0.6, 16.9 ± 6

1

1

1.2, 8.0 =b 0.8, a n d 0.41 ± and

Wahl.

0.04, r e s p e c t i v e l y , w e r e m e a s u r e d b y C h a n

I n t h e a b o v e , p h e n represents

o-phenanthroline, a n d the

n u m e r a l s specify t h e positions of s u b s t i t u t i o n b y C H , C H , a n d C H n . 3

6

5

c

I n b o t h systems, t h e rate declines w i t h b u l k y substituents. T h e r e s u l t r e c o r d e d f o r the s e c o n d set of s y s t e m s — a s m a l l increase i n b u l k a c t u a l l y c a u s i n g a rate i n c r e a s e — i s p a r t i c u l a r l y i n t e r e s t i n g . T a k e n at face v a l u e , t h e o b s e r v a t i o n i m p l i e s that self-exchange f o r F e ( p h e n )

3

3 +

«

2 +

is w e l l w i t h i n

t h e a d i a b a t i c r e g i m e , a n d i t bolsters the c o n c l u s i o n t h a t w i t h the b u l k i e s t Downloaded by UNIV OF OKLAHOMA on August 17, 2013 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/ba-1977-0162.ch007

s a t u r a t e d g r o u p , n o n a d i a b a t i c effects are b e i n g felt. A l o w transition probability i n the activated complex w i l l be re flected

i n a decrease i n e n t r o p y of a c t i v a t i o n , so t h a t i n p r i n c i p l e , t h e

e n t r o p y of a c t i v a t i o n c a n r e v e a l n o n a d i a b a t i c e l e c t r o n transfer. U n f o r t u n a t e l y , w h e n reactions are s t u d i e d i n t h e b i m o l e c u l a r m o d e , a n d p a r t i c u l a r l y w h e n b o t h t h e r e a c t i n g species are c h a r g e d , the e n t r o p y changes associated w i t h f o r m i n g the p r e c u r s o r c o m p l e x are l a r g e , a n d i t is i m p o s s i b l e to separate this c o n t r i b u t i o n f r o m t h e o v e r a l l e n t r o p y change.

Some

values of A S 4 for systems of s i m i l a r c h a r g e t y p e b u t f e a t u r i n g d i f f e r i n g e l e c t r o n i c structures a r e s h o w n i n T a b l e I I . T h i s selection w a s m a d e Table II. Entropies of Activation as Functions of Electronic Structure of Reactants

(NH Cr (NH Cr (NH Cr (NH Eu (NH

ΛΗ+ (kcal mol' )

+

0.35

8.2

-33

1.0

2

8

2 t

+

2.7 Χ 1 0

1.0

-34

0.1

25

*

+

8.8 Χ 10"

14.7

-30

0.4

26

3 +

+

0.074

9.3

-33

0.4

27

*

+

0.53

8.2

-32

1.0

28

10.2

11.6

-15

1.0

29

1

) CoOAc * ) RuOAc

3

2 t

5

(cal mol deg- )

kat25° (M' sec )

Reaction

5

1

4

1

1

1

Γ

Ref.

2 +

3

) CoNH 5

3

3

5

2 +

3

) CoOH 5

2

2 +

3

) CoOH 5

2

3

[(NH ) Co0 C— " C—CH ] ~+ V 3

5

2

3

2 +

2 +

II

ο

α

Ionic strength.

b e c a u s e i t seems r e a s o n a b l e t h a t t h e t r a n s i t i o n p r o b a b i l i t y w i l l b e sensi t i v e to s o m e t h i n g as b a s i c as o r b i t a l s y m m e t r y , a n d i f i t w e r e

indeed

p a r t l y r a t e d e t e r m i n i n g , t h i s w o u l d b e r e v e a l e d i n t h e entropies

of

activation. T h e systems i n c l u d e a n o x i d i z i n g agent, C o ( I I I ) , w h i c h accepts a n e l e c t r o n i n a n o r b i t a l of σ s y m m e t r y ; R u ( I I I ) , w h e r e t h e a c c e p t o r o r b i t a l


134


II

has Tr s y m m e t r y ; a n d r e d u c i n g agents w h i c h lose electrons f r o m σά o r b i tals ( C r * ) , 7rd o r b i t a l s ( V ) , a n d / o r b i t a l s ( E u ) . N o significant d i f ferences i n e n t r o p y of a c t i v a t i o n for the same c h a r g e t y p e are o b s e r v e d , e x c e p t for the last entry. T h e r e a c t i o n of V w i t h the p y r u v a t o p e n t a a m m i n e c o b a l t ( I I I ) is one of a l a r g e class i n w h i c h s u b s t i t u t i o n o n V(H 0) appears to b e rate d e t e r m i n i n g . I n a l l the others, t h e p r e cursor c o m p l e x , w h e t h e r i n n e r - s p h e r e or outer-sphere, is i n e q u i l i b r i u m w i t h the reactants, a n d o v e r a l l reflects a c o n t r i b u t i o n f r o m the for m a t i o n of the p r e c u r s o r c o m p l e x as w e l l as f r o m the electron transfer act itself. 2

2 +

2 +

2 +


2

6

2 +

E v e n t h o u g h a c a l c u l a t i o n of rates of e l e c t r o n transfer u s i n g o n l y m o l e c u l a r parameters for the reactants a n d the d i e l e c t r i c p r o p e r t i e s of the solvent is b e y o n d the c o m p e t e n c e of c u r r e n t theory, t h e o r y is n o n e theless u s e f u l i n rate c o m p a r i s o n s a n d c a n r e v e a l u n u s u a l b e h a v i o r . F o r reasons w h i c h h a v e a l r e a d y b e e n stated, self-exchange rates are m o r e a m e n a b l e to t h e o r e t i c a l analysis t h a n are the rates of cross reactions. I n the f o l l o w i n g , the F e - self-exchange rate is t a k e n as a reference v a l u e for other couples of the 3-f-, 2-f- charge t y p e . A c c o r d i n g to S u t i n ( 1 3 ) , the increase i n rate for the F e ( p h e n ) self-exchange c o m p a r e d w i t h Fe c a n be u n d e r s t o o d o n the basis of the l a r g e r size of the f o r m e r ions, w h i c h decreases t h e c o n t r i b u t i o n b y the solvent to the F r a n c k C o n d o n b a r r i e r a n d of the r e d u c e d i n n e r - s p h e r e r e o r g a n i z a t i o n energy w h i c h arises f r o m b a c k - b o n d i n g i n t e r a c t i o n b e t w e e n F e a n d the π a c i d l i g a n d . T h e c a l c u l a t e d rate increase of 1 0 is consistent w i t h the m e a s u r e m e n t of the self-exchange rate i n a c e t o n i t r i l e ( the self-exchange rates are a p p a r e n t l y l a r g e r i n w a t e r . ) T h e self-exchange rate for R u ( N H ) ' , ca. 10 , seems reasonable i n r e l a t i o n to that of F e ( H 0 ) b e c a u s e the c h a n g e i n b o n d distance w i t h c h a n g e i n o x i d a t i o n state is less for the r u t h e n i u m t h a n it is for i r o n . A s l o w e r rate for C r ( H 0 ) selfexchange c o m p a r e d w i t h F e ( H 0 ) is expected, because C r ( H 0 ) absorbs a n a n t i - b o n d i n g e l e c t r o n , a n d a l a r g e d i s t o r t i o n attends t h e r e d u c t i o n , b u t i t is not c e r t a i n i f this c a n a c c o u n t for a rate r e d u c t i o n b y a factor (30, 31) i n excess of 10 . T h e E u self-exchange rate (k < 3 X 10" M " sec" ) seems m o r e c l e a r c u t . O w i n g to the l a r g e r size of the ions, b o t h the i n n e r - s p h e r e a n d solvent r e o r g a n i z a t i o n energies m u s t b e less t h a n for F e ( H 0 ) , yet t h e rate of self-exchange is so s l o w t h a t i t has not b e e n m e a s u r e d for the a q u o ions a l t h o u g h the rate w a s m e a s u r e d for the p a t h i n v o l v i n g C I " ( 3 2 ) . It is possible that the E u > c o u p l e p r o v i d e s a n e x a m p l e of n o n a d i a b a t i c transfer. S i n c e the / o r b i t a l s are b u r i e d i n the k e r n e l of the i o n , it is reasonable that the E u - E u c o u p l e , of a l l those considered, w o u l d show nonadiabatic behavior. T h i s argument was pre sented also i n a r e v i e w a r t i c l e (33). T h e c h l o r i d e i o n m a y affect the rate b y s t a b i l i z i n g a n i n n e r - s p h e r e a c t i v a t e d c o m p l e x , thus d e c r e a s i n g the b a r r i e r p e n e t r a t i o n distance. 3 +

2 +

3 + , 2 +

3 + , 2 +

2 +

7

3

3

2

6

2

2

5

5

1

6

6

G

2

3 + 2 +

3 + , 2 +

3+

2 +

3 +

2 +

3 + , 2 +

3 + , 2 +

1

2

6

3 + , 2 +

2+

3 +


6

3 +

7.

135


TAUBE

I n this c o n n e c t i o n , several groups of w o r k e r s h a v e h a d d i f f i c u l t y i n g e t t i n g reasonable a n d / o r r e p r o d u c i b l e measurements t r o n couples observed

i n reactions

with

each

other.

w i t h the

Sullivan

and

/-elec

Thompson

i n the E u ( I I ) - N p ( I V ) system w i t h the reactants i n a p

(34)

p r o x i m a t e l y e q u i v a l e n t concentrations that the rate of r e a c t i o n is first order i n E u ( I I ) U

3 +

-Eu

b u t zero o r d e r i n N p ( I V ) .

reaction (35)

3 +

I n separate studies of the

b y T e m p l e t o n a n d N i c o l i n i , the rates w e r e i r r e -

p r o d u c i b l e a n d d i d not c o n f o r m to second-order Yb

and E u

2 +

L a v a l l e e a n d N e w t o n (36) Downloaded by UNIV OF OKLAHOMA on August 17, 2013 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/ba-1977-0162.ch007

kinetics. T h e reaction

w a s f o u n d b y C h r i s t e n s e n also to b e i n t r a c t a b l e

3 +

f o u n d that the U ( I I I ) - N p ( I V )

not r e p r o d u c i b l e a n d that the d a t a d i d not f o l l o w s i m p l e kinetics.

(35).

rates w e r e second-order

T h e researchers i n a l l these cases are e x p e r i e n c e d , a n d great

care was t a k e n w i t h p u r i t y of the m a t e r i a l s . A s a final e x a m p l e of u n e x p l a i n e d i r r e p r o d u c i b i l i t y i n a s e e m i n g l y s i m p l e system is the i n t e r a c t i o n of C r

2 +

with R u ( N H ) 3

strong (37),

2

5

3 +

i n the presence of C I " . N e i t h e r A r m o r n o r A r m

w h o w o r k e d w i t h the system i n d e p e n d e n t l y , was a b l e to get

reproducible (H 0) C1

6

or reasonable

behavior

i n the ratio of

Cr( H 0 ) 2

6

3 +

/Cr-

p r o d u c e d as a f u n c t i o n of C I " . N o n a d i a b a t i c processes m a y

2 +

b e u n u s u a l l y sensitive to changes i n the e n v i r o n m e n t , a n d the anomalies m e n t i o n e d m a y b e traceable to a n o n a d i a b a t i c c o n t r i b u t i o n to the rate, b u t of course cannot w i t h c e r t a i n t y b e t a k e n as d i a g n o s t i c of a d i a b a t i c effects.

C l e a r l y , the systems are w o r t h f u r t h e r s t u d y , b u t it is not at a l l

o b v i o u s w h a t measures w i l l b r i n g the systems u n d e r c o n t r o l . R e m a i n i n g s t i l l to b e c o n s i d e r e d are the o x i d a t i o n - r e d u c t i o n reac tions i n w h i c h e l e c t r o n transfer occurs over m a n y b o n d lengths. are best d e a l t w i t h as a f u n c t i o n of e l e c t r o n i c s t r u c t u r e t y p e . t h o r o u g h l y s t u d i e d a m o n g t h e m are reactions of C r cobalt(III)

complexes

2 +

These

T h e most

with pentaammine-

of l i g a n d s h a v i n g c o n j u g a t e d

b o n d systems

(σ

d o n o r , σ acceptor ). Efforts h a v e b e e n m a d e to relate the rates to p r o p e r ties s u c h as the m o b i l e b o n d o r d e r of the c o n j u g a t e d l i g a n d s . If s u c h a r e l a t i o n existed, it w o u l d at o n c e i m p l y n o n a d i a b a t i c transfer. B u t it n o w appears

(38,

39, 40)

that most of the reactions of C r

2 +

with Co (III)

complexes i n w h i c h there is remote attack p r o c e e d b y a stepwise m e c h a nism in which C r

2 +

transfers a n e l e c t r o n to the l i g a n d , a n d the r e s u l t i n g

r a d i c a l t h e n reacts w i t h C o ( I I I )

or w i t h C r ( I I I ) :

C o [ L ' . . . L] + Cr m

Co

n i

[L' . . . L] Cr

( I I I )

2 +

^±Co [L' m

->Co(II) +

. . . L] Cr [1/ .

( I I I )

. L]Cr

( I I I )

T h i s c o n c l u s i o n w a s s t r o n g l y s u p p o r t e d b y N o r d m e y e r ' s ( 7 ) results a n d has b e e n a m p l y b o r n e out b y f u r t h e r studies d o n e chiefly b y E . S. G o u l d a n d c o - w o r k e r s (41) (see, for e x a m p l e , R e f . 41 w h i c h refers to


136

BIOINORGANIC

earlier w o r k ) .

CHEMISTRY

I n these systems, a stepwise m e c h a n i s m m a y b e

II

forced

o n t h e s y s t e m b e c a u s e of the s y m m e t r y m i s m a t c h b e t w e e n t h e

redox

a c t i v e m e t a l i o n o r b i t a l s a n d the c a r r i e r o r b i t a l o n the l i g a n d , w h i c h i n a l l l i k e l i h o o d is a l o w - l y i n g ?r* o r b i t a l . T w o F r a n c k - C o n d o n b a r r i e r s — Cr

2 +

to l i g a n d a n d l i g a n d r a d i c a l to C o ( I I I ) — a f f e c t t h e rates. T h e r e is

n o reason to b e l i e v e t h a t t u n n e l l i n g p r o b a b i l i t y at either of these j u n c t i o n s is rate d e t e r m i n i n g a n d c e r t a i n l y not t h a t e l e c t r o n m o b i l i t y i n the l i g a n d is rate d e t e r m i n i n g . A n i m p o r t a n t p r o p e r t y of t h e l i g a n d w h i c h m a k e s r e m o t e e l e c t r o n transfer p o s s i b l e i n these systems is the a c c e s s i b i l i t y of Downloaded by UNIV OF OKLAHOMA on August 17, 2013 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/ba-1977-0162.ch007

t h e π* o r b i t a l . O t h e r t h i n g s b e i n g e q u a l , r e a d y r e d u c i b i l i t y of t h e l i g a n d is

important i n determining whether remote

attack w i l l

occur.

e x a m p l e , i t is p r o b a b l e t h a t t e r e p h t h a l a t o p e n t a a m m i n e c o b a l t ( I I I ) r e d u c e d b y remote attack (39) b a l t ( I I I ) c e r t a i n l y is (42).

while

For is not

p-formylbenzoatopentaammineco-

N u m e r o u s other comparisons can be

found

i n the extensive a n d i n s t r u c t i v e c o n t r i b u t i o n s w h i c h h a v e b e e n m a d e to this subject b y G o u l d (39,

41).

W h e n the o r b i t a l o n the o x i d i z i n g

agent has ττ s y m m e t r y as i n a p e n t a a m m i n e r u t h e n i u m ( I I I ) c o m p l e x , a n d the c o m p l e x is r e d u c e d b y C r ( I I ) , there is a p p a r e n t l y o n l y one F r a n c k C o n d o n b a r r i e r to s u r m o u n t — C r ( I I ) to l i g a n d — t h e e l e c t r o n b e i n g t r a n s f e r r e d to a n o r b i t a l t h a t is d e l o c a l i z e d o v e r m e t a l i o n a n d l i g a n d .

In

these systems, transfer f r o m C r ( I I ) to ( l i g a n d + m e t a l ) is rate deter m i n i n g , a n d a g a i n there is n o reason to i n v o k e t u n n e l l i n g p r o b a b i l i t y as a r e a c t i o n b a r r i e r . Systems i n w h i c h the r e d u c i n g agent loses a ττ e l e c t r o n and

t h e o x i d i z i n g agent gains a ττ e l e c t r o n w i l l b e d e a l t w i t h

below.

F o r completeness, i t m u s t b e m e n t i o n e d that i n these systems w h e n t h e b r i d g i n g l i g a n d s are c o n j u g a t e d , it is l i k e l y that t h e e l e c t r o n transfer reactions are a d i a b a t i c . T o s u m u p the s u r v e y of the past w o r k o n o x i d a t i o n - r e d u c t i o n r e a c t i o n s : the o n l y e x p e r i m e n t a l results o b t a i n e d thus f a r w h i c h s t r o n g l y i n d i c a t e n o n a d i a b a t i c effects are some o b t a i n e d b y M a t t e s o n a n d B a i l e y (22)

a n d b y C h a n a n d W a h l (23)

for self-exchange reactions. I n a d d i

t i o n , i t is v e r y l i k e l y that s u c h effects are significant also for reactions of /

electron

redox

agents,

particularly on

reaction w i t h

one

another.

M a r c u s has a d v o c a t e d consistently t h e p o s i t i o n that n o n a d i a b a t i c effects are r e l a t i v e l y u n i m p o r t a n t f o r t h e o r d i n a r y o x i d a t i o n - r e d u c t i o n reactions w h i c h have been studied. B u t m a n y experimentalists, i n c l u d i n g myself, h a v e b e e n m u c h s l o w e r to a r r i v e at i t . F u r t h e r w o r k m a y s h o w t h a t a n o n a d i a b a t i c f a c t o r is significant i n m a n y other processes, b u t at the present l e v e l of d e v e l o p m e n t

of the subject, there are not m a n y cases

w h e r e i t needs to b e i n v o k e d . B e f o r e d e s c r i b i n g e x p e r i m e n t s d e s i g n e d to s t u d y the n o n a d i a b a t i c r e g i m e f o r e l e c t r o n transfer i n o x i d a t i o n - r e d u c t i o n reactions s y s t e m a t i c a l l y , some systems i n w h i c h e l e c t r o n t u n n e l l i n g , i n the sense t h a t i t


7.

137


TAUBE

determines rate of e l e c t r o n transfer, is a f a c t o r s h o u l d b e Zamaraev a n d co-workers

(43, 44)

a n d others (45)

mentioned.

have developed

s t r o n g case for e l e c t r o n transfer b y t u n n e l l i n g i n c o n d e n s e d w h i c h have undergone radiation damage.

M i l l e r (46)

a

systems

also a d d e d e v i -

d e n c e for t u n n e l l i n g i n s t u d y i n g t h e r e a c t i o n : [biphenyl]" - f triphenylethylene = biphenyl + i n r i g i d e t h a n o l at — 7 7 ° .

[triphenylethylene]"

T h e o v e r a l l b e h a v i o r i n these systems is c o m -


p l i c a t e d b e c a u s e t h e reactants are at v a r y i n g distances. H o w e v e r i n the systems d e s c r i b e d b e l o w the o x i d a n t a n d r e d u c t a n t are at fixed r e l a t i v e positions.

T h e y represent a great s i m p l i f i c a t i o n p a r t i c u l a r l y w h e n

p a r e d w i t h the second-order

com-

r e d o x processes w h i c h h a v e b e e n s t u d i e d .

B y h a v i n g oxidant a n d reductant assembled i n a single molecule, intram o l e c u l a r e l e c t r o n transfer rates c a n b e m e a s u r e d a n d m a n y a m b i g u i t i e s a t t e n d i n g the i n t e r p r e t a t i o n of the s e c o n d - o r d e r

processes are a v o i d e d .

T h e systems c u r r e n t l y b e i n g s t u d i e d i n v o l v i n g m e t a l ions d o h a v e p r e c e d e n t i n p u r e l y o r g a n i c ones, f o r e x a m p l e , i n t h e p a r a c y c l o p h a n e r a d i c a l anions s t u d i e d b y W e i s s m a n n (47)

a n d r a d i c a l anions d e r i v e d f r o m the

4,4'-nitrobiphenyl studied by H a r r i m a n and M a k i

(48).

Although in

p r i n c i p l e the results i n these systems are i n the d i r e c t l i n e of o u r interest, i n p r a c t i c e t h e y go o n l y a s m a l l w a y t o w a r d y i e l d i n g the k i n d of i n f o r m a t i o n w h i c h is d e s i r e d , n a m e l y rates as a f u n c t i o n of a w i d e r a n g e i n the s t r u c t u r e of the b r i d g i n g groups s p a n n i n g t h e r e d o x centers a n d as a f u n c t i o n of t e m p e r a t u r e

(as m e n t i o n e d e a r l i e r , values of A S ^ c a n

p a r t i c u l a r l y s i g n i f i c a n t ) . F o r the o r g a n i c systems, rates w e r e b y the E S R t e c h n i q u e , a n d this is c o m p e t e n t

be

measured

only i n a rather narrow

r a n g e of specific rates. E v e n i f o t h e r a p p r o a c h e s for the o r g a n i c systems w e r e a v a i l a b l e , there w o u l d s t i l l b e i m p o r t a n t reasons for s t u d y i n g m e t a l ions as t h e r e d o x reactants, because the rates w i l l b e sensitive to o r b i t a l s y m m e t r y , to differences overlap.

i n e n e r g y of o r b i t a l s , a n d p e r h a p s to o r b i t a l

O n l y b y s t u d y i n g m e t a l - i o n c o n t a i n i n g systems are w e

likely

to u n d e r s t a n d the m a n y r e d o x reactions of m e t a l complexes w h i c h h a v e b e e n s t u d i e d i n the b i m o l e c u l a r m o d e . T h e a d v a n t a g e of s t u d y i n g i n t r a m o l e c u l a r r a t h e r t h a n i n t e r m o l e c u l a r e l e c t r o n transfer w a s a p p r e c i a t e d q u i t e e a r l y (49),

but only rather re-

c e n t l y h a v e reports of the results of s u c h measurements b e e n a p p e a r i n g f o r m e t a l - t o - m e t a l e l e c t r o n transfer. G a s w i c k a n d H a i m ( 5 0 )

reported

first-order

complex

rates

Co(NH ) H 0 3

5

2

3 +

for

electron

· Fe(CN)

6

4

transfer

the

outer-sphere

- , C a n n o n a n d G a r d i n e r (51)

sphere complex formed between a n d F e , a n d H u r s t a n d L a n e (52) 2 +

in

f o r the i n n e r -

[(NHs^CoOaCC^NiCHaŒ^H^] * 2

for [ ( N H ) R u f u m a r a t e ] 3

5

2 +

with C u

+

b o u n d to the d o u b l e b o n d of the l i g a n d . B u t since i n e a c h case m e n t i o n e d t h e o x i d i z i n g - r e d u c i n g agent b o n d is l a b i l e , the positions w h i c h


138


II

the m e t a l ions h a v e r e l a t i v e to e a c h other i n the a c t i v a t e d c o m p l e x are not defined.

T h e a m b i g u i t y r e f e r r e d to is m u c h r e d u c e d i n systems

under study.

Isied and T a u b e

(53)

now

d e v i s e d a strategy for m e a s u r i n g

i n t r a m o l e c u l a r e l e c t r o n transfer i n complexes

of t h e class C o

L' . . .

T I I

L R u , a strategy w h i c h s h o u l d b e a p p l i c a b l e to m o r e c o m p l e x systems, 1 1

i n c l u d i n g b i o l o g i c a l ones. B y the p r o p e r c h o i c e of p o l a r groups for the bifunctional bridging ligand 1/ . . . L , a substitution inert combination Co

L' . . . L Ru

l n

Ru(III)

M a n y r e d u c i n g agents react w i t h

( a ττ e l e c t r o n a c c e p t o r ) m o r e r a p i d l y t h a n w i t h C o ( I I I )

electron acceptor), Downloaded by UNIV OF OKLAHOMA on August 17, 2013 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/ba-1977-0162.ch007

is first f o r m e d .

I H

(a σ

a n d the system c a n b e " c o c k e d " f o r i n t r a m o l e c u l a r

e l e c t r o n transfer b y a d d i n g a r e d u c i n g agent s u c h as R u ( N H ) 3

2 +

6

. For

the complexes thus f a r s t u d i e d , R u ( I I ) is b o u n d to the b r i d g i n g l i g a n d b y a h e t e r o c y c l i c n i t r o g e n a n d a q u a t i o n of this l i n k a g e has a h a l f - l i f e i n excess of a m o n t h (54).

Moreover, R u ( I I )

in combination w i t h a pyri-

d i n e - l i k e l i g a n d shows a v e r y s t r o n g ?r*

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