Inorganic Compounds with Unusual Properties

12 The Thermally Equilibrated Excited (Thexi)

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State Chemistry of Some Co(III) Ammines ARTHUR W. ADAMSON University of Southern California, Los Angeles, Calif. 90007

The photochemistry of coordination compounds is discussed in terms of thermally equilibrated excited (thexi) states as the chemically reacting species. Such states are in thermo dynamic equilibrium with their surroundings and are essen tially high energy isomers of the ground state. By contrast, the species obtained by light absorption of wavelength around a ligand field band maximum are Franck-Condon states that have a nonthermodynamic distribution of vibra tional excitations; such states are treated as pseudo pure electronic states in ligandfieldtheory. New theory is needed to treat thexi states. Data on the ligandfieldsubstitutional photochemistry of trans- and cis-[Co(en) (NH )Cl] are reported. Data are discussed mechanistically as part of the thexi state chemistry of Co(III) ammines. 2+

2

3

' H p h e e m p h a s i s i n this p a p e r is o n a n aspect of t h e p h o t o c h e m i s t r y

of

c o o r d i n a t i o n c o m p o u n d s that is often r a t h e r u n d e r s t a t e d i n t h e l i t e r a ture. O t h e r aspects are i m p o r t a n t , b u t I b e l i e v e t h a t the present e m p h a s i s is essential to a n u n d e r s t a n d i n g of t h e e x c i t e d states r e a c h e d b y essentially d-d

field.

W e w i l l be dealing w i t h

transitions f r o m the g r o u n d state.

T h e t y p i c a l t r a n s i t i o n is t h a t w h i c h occurs u p o n a b s o r p t i o n of l i g h t i n the w a v e l e n g t h of the first ( L i ) or s e c o n d ( L ) l i g a n d field b a n d of the 2

c o m p l e x , p a r t i c u l a r l y w h e n s u c h b a n d s are of u s u a l i n t e n s i t y a n d are a p p a r e n t l y n o t c o m p l i c a t e d b y a d m i x t u r e w i t h c h a r g e transfer

(CT)

character. A c o m m o n m e t h o d of d i s p l a y i n g q u a l i t a t i v e l i g a n d field state energies is b y the w e l l k n o w n T a n a b e - S u g a n o d i a g r a m s ( J , 2, 3, 4,

5).

A s was n o t e d ( β ) , t h e r e is a tension b e t w e e n l i g a n d field concepts a n d the i m p l i c a t i o n s of p h o t o c h e m i c a l a n d p h o t o p h y s i c a l studies.

These

i m p l i c a t i o n s c o n c e r n the n a t u r e of e m i t t i n g a n d g e n e r a l l y c h e m i c a l l y r e a c t i v e e x c i t e d states; t h e y present some m a j o r t h e o r e t i c a l p r o b l e m s . 128 In Inorganic Compounds with Unusual Properties; King, R. Bruce; Advances in Chemistry; American Chemical Society: Washington, DC, 1976.

12.

Thexi State Chemistry

ADAMSON

of Co(IIl)

129

Ammines

L i g a n d field t h e o r y , l i k e its p a r e n t , c r y s t a l field t h e o r y , m a k e s t h e a p p r o x i m a t i o n i n t r e a t i n g g r o u n d a n d e x c i t e d states t h a t t h e s y m m e t r y a n d t h e i n t e n s i t y of t h e l i g a n d field are i n v a r i a n t . T h e s e states c o r r e s p o n d to v a r i o u s l y e n e r g e t i c a r r a n g e m e n t s of m e t a l d electrons ( r e l a t i v e to t h e free i o n ) i n t h e p a r t i a l l y degenerate ( u s u a l l y ) set of d o r b i t a l s , m o d i f i e d b y s u i t a b l e a c c o u n t a n c y for i n t e r e l e c t r o n i c r e p u l s i o n s . A s w a s m e n t i o n e d , the c o n c e p t u a l m o d e l is t h a t of a m e t a l i o n i m b e d d e d i n a c r y s t a l l a t t i c e so r i g i d t h a t the electrostatic e n v i r o n m e n t of t h e m e t a l i o n is

fixed.

The


c o r r e s p o n d i n g m o l e c u l a r o r b i t a l t r e a t m e n t a l l o w s for b o n d i n g c o n s i d e r a tions b u t s t i l l retains the a b o v e b a s i c a p p r o x i m a t i o n . A t this c o n v e n t i o n a l l e v e l of a p p r o x i m a t i o n , the e n e r g y

difference

b e t w e e n l i g a n d field states is g i v e n e x p e r i m e n t a l l y b y t h e w a v e l e n g t h of the a p p r o p r i a t e a b s o r p t i o n b a n d m a x i m u m . I t is this e n e r g y w h i c h is u s e d i n o b t a i n i n g t h e 10 Dq of l i g a n d field t h e o r y .

T h u s for a d

c o m p l e x , 10 Dq is just t h e e n e r g y c o r r e s p o n d i n g to the L mum;

octahedral

3

for a d c o m p l e x , t h e r e is a c o r r e c t i o n of 35 F , F 6

4

4

band maxi-

x

b e i n g one of t h e

C o n d o n - S h o r t e l y i n t e r e l e c t r o n i c r e p u l s i o n p a r a m e t e r s ( I , 2, 3, 4, 5 ) .

It

is thus the e n e r g y of b a n d m a x i m a w h i c h gives the w e l l k n o w n s p e c t r o c h e m i c a l series; the same is t r u e for t h e n e p h e l a u x e t i c series ( 3 ) .

The

10 Dq values o b t a i n e d f r o m b a n d m a x i m a a r e u s e d to c a l c u l a t e c r y s t a l field s t a b i l i z a t i o n s a n d to estimate a c t i v a t i o n energies for l i g a n d s u b s t i t u t i o n reactions

(7).

T h e r i g i d c r y s t a l l a t t i c e a p p r o x i m a t i o n is s i m i l a r l y u s e d f o r o c t a h e d r a l t y p e c o m p l e x e s t h a t h a v e m o r e t h a n one k i n d of l i g a n d . B a n d s p l i t t i n g s on going from O

h

fixed

to D h s y m m e t r y , for e x a m p l e , are t r e a t e d i n terms of 4

p s e u d o - o c t a h e d r a l axes of n o n - e q u a l l i g a n d field strengths ( I , 2, 3,

4, 5 ) a n d , a g a i n , positions of b a n d m a x i m a are u s e d to o b t a i n t h e e n e r g y separations of t h e s o - d e d u c e d excited-state t e r m system. T h e a p p r o x i m a t i o n is u s e d for other geometries, s u c h as s q u a r e p l a n a r a n d t e t r a h e d r a l . I n b r i e f , c o n v e n t i o n a l l i g a n d field t h e o r y assumes t h a t energies at b a n d m a x i m a represent specific p u r e e l e c t r o n i c e x c i t e d states i n a system of fixed

geometry. It m i g h t b e e x p e c t e d , i n terms of l i g a n d field t h e o r y , that dr-d t r a n s i -

tions w o u l d be v e r y s h a r p . T h e a c t u a l s i t u a t i o n is i l l u s t r a t e d i n F i g u r e 1. Such absorption bands m a y sharpen somewhat and develop indications of v i b r a t i o n a l s t r u c t u r e at v e r y l o w t e m p e r a t u r e , b u t the effect is t y p i c a l l y n o t v e r y d r a m a t i c [there are exceptions,

as w i t h C r ( C N )

( i

3

"

(8,

9)].

L i g a n d field t h e o r y does not s p e c i f i c a l l y treat t h e m a t t e r of b a n d w i d t h . I t has b e e n a s c r i b e d i n a g e n e r a l w a y to p e r t u r b a t i o n s of a n e l e c t r o n i c n a t u r e ( s u c h as J a h n - T e l l e r effects)

(2).

It is m y o p i n i o n t h a t the a b o v e g e n e r a l p i c t u r e w a s i r r e t r i e v a b l y compromised spin-allowed)

b y t h e observations emission from

(JO, J J )

Cr(III)

that

fluorescence

(that is,

c o m p l e x e s is d r a m a t i c a l l y r e d -

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

130

INORGANIC

COMPOUNDS

WITH

UNUSUAL PROPERTIES


60

λ,ηημ Chemical Reviews

Figure 1.

Absorption spectrum for aqueous [Cr(NH ) y+(15) 3

6

s h i f t e d r e l a t i v e to the a b s o r p t i o n b a n d . T h e classic case of C r ( u r e a )

6

3 +

is p l o t t e d i n F i g u r e 2. T h e i n e s c a p a b l e c o n c l u s i o n is that a s o - c a l l e d l i g a n d field

e x c i t e d state (of

energy given b y b a n d m a x i m a )

is i n r e a l i t y a

v i b r a t i o n a l l y excited state of a species w h o s e c h e m i c a l ( that is, electronic o n l y ) e n e r g y is l o w e r b y 1 0 - 2 0 k c a l / m o l e t h a n that g i v e n b y the b a n d m a x i m u m . T h i s c h e m i c a l - o n l y energy c a n b e e s t i m a t e d f r o m the crossing r e g i o n of the a b s o r p t i o n a n d e m i s s i o n b a n d s a n d b y other w a y s i n the absence of e m i s s i o n

(6).

T o b e b l u n t , i t is

fictional

to treat b a n d m a x i m a energies as p u r e

electronic energies as does l i g a n d field theory. T h e broadness of t y p i c a l l i g a n d field b a n d s is n o w r e c o g n i z e d as reflecting p r i m a r i l y the F r a n c k C o n d o n o v e r l a p factor

( 8 , 9);

the most p r o b a b l e t r a n s i t i o n is to that

v i b r a t i o n a l l y e x c i t e d l e v e l of the e l e c t r o n i c a l l y e x c i t e d state for w h i c h t h e probable

n u c l e a r positions are close to those of the m o s t

populated

g r o u n d state v i b r a t i o n a l l e v e l ( a m o r e d e t a i l e d w a y of s t a t i n g this is


12.


ADAMSON

suggested b e l o w ) .

of Co(III)

T h e ( 0 , 0 ) ( g r o u n d state υ =

131

Ammines

ο to e x c i t e d state ν =

ο)

t r a n s i t i o n is i m p r o b a b l e b e c a u s e of t h e s m a l l o v e r l a p of the n u c l e a r w a v e f u n c t i o n s , a n d transitions i n v o l v i n g energies m u c h greater t h a n t h a t of the b a n d m a x i m u m are likewise improbable.


Emission

Zeitschrift fuer Physikalische Chemie

Figure 2.

Absorption

and (low temperature) emission for (10)

\_Cr(urea) '\ 6

3+

T h e f u r t h e r a n d c e n t r a l l y i m p o r t a n t i m p l i c a t i o n is t h a t the p r o b a b l e n u c l e a r positions for the p u r e e l e c t r o n i c e x c i t e d state a r e s i g n i f i c a n t l y different f r o m those i n the g r o u n d state. I n other w o r d s , t h e t w o states are d i s t o r t e d r e l a t i v e to e a c h other. T h e s i t u a t i o n is i l l u s t r a t e d s c h e m a t i c a l l y i n F i g u r e 3, w h i c h also d e p i c t s the o r i g i n of t h e r e d shift o n rescence.

fluo

( T h e c o m m o n l y seen p a r a b o l i c p o t e n t i a l e n e r g y d i a g r a m s are

m i s l e a d i n g i n t h a t t h e y do n o t a l l o w t h e s h o w i n g of m o r e vibrational ladder, corresponding

to a l t e r n a t i v e m o d e s of

than

one

distortion.)

A b s o r p t i o n of a l i g h t q u a n t u m p r o d u c e s a v i b r a t i o n a l l y e x c i t e d

species

w h i c h t h e r m a l l y e q u i l i b r a t e s to a m b i e n t t e m p e r a t u r e v i b r a t i o n a l - r o t a t i o n energy.

B e c a u s e of the c o n s e q u e n t shifts i n n u c l e a r p o s i t i o n s , the m o s t

probable

fluorescent

t r a n s i t i o n is t h e one that terminates at a v i b r a t i o n a l l y

e x c i t e d g r o u n d state. N o n e of t h e a b o v e inferences are n e w b y n o w ; t h e y are i n f a c t g e n erally accepted

(S, 9 ) .

Y e t c e r t a i n i m p o r t a n t consequences are r a r e l y


132

INORGANIC

COMPOUNDS WITH

?ν^Λΐ =£4~


CHEMICAL REACTION

UNUSUAL

S

PROPERTIES

CHEMICAL *~ REACTION

DISTORTION "Inorganic Reaction Mechanisms"

Figure 3. Energy level diagram for a Cr(HI) complex (6) The distortion coordinate is schematic; various ladders represent radiationless processes which may be either vibrational only or vibronic in nature. A , absorption; P, phosphorescent emission; F, fluorescent emission; Q, radiationless deactivation; and X, intersystem crossing.

stressed; t h e y are u n c o m f o r t a b l e

to c o n v e n t i o n a l l i g a n d

F i r s t , the e l e c t r o n i c - o n l y e x c i t e d state energies pounds

of

m a y b e a r l i t t l e r e l a t i o n s h i p to the u s u a l l i g a n d

schemes.

field

theory.

coordination field

com energy

T h i s is true w i t h respect to b o t h absolute energies a n d e n e r g y

r a t i o s ; e v e n t h e o r d e r i n g of states o b t a i n e d b y a p p l i c a t i o n of l i g a n d field t h e o r y c a n b e w r o n g . A clear i l l u s t r a t i o n is that p r o v i d e d b y the C r ( I I I ) f a m i l y of complexes.

T h e r e are s e v e r a l c o m p l e x e s for w h i c h the t h e r m a l l y

e q u i l i b r a t e d ( o r e l e c t r o n i c - o n l y ) e x c i t e d first q u a r t e t state is b e l o w t h e first d o u b l e t state i n e n e r g y (see R e f . 11).

L i g a n d field t h e o r y , o r d e r i n g

these states a c c o r d i n g to the a p p r o p r i a t e a b s o r p t i o n b a n d m a x i m a , p l a c e s t h e first q u a r t e t w e l l a b o v e the d o u b l e t state.

I n g e n e r a l , a n y p a i r of

states for w h i c h the m e t a l - t o - l i g a n d b o n d i n g characteristics are apt to b e different are also a p t to h a v e energies

t h a t a r e q u i t e different,

both

a b s o l u t e l y a n d r e l a t i v e l y , f r o m those o b t a i n e d f r o m l i g a n d field t h e o r y a n d b a n d m a x i m a positions. A f u r t h e r c o n s e q u e n c e is t h a t t h e concepts of t h e s p e c t r o c h e m i c a l a n d n e p h e l a u x e t i c series b e c o m e c o n f u s e d a n d a r e n o t g e n e r a l l y u s a b l e . S i n c e the t h e r m a l l y e q u i l i b r a t e d e x c i t e d state m a y b e of different g e o m e t r y f r o m the g r o u n d state a n d h a v e different b o n d lengths, t h e r i g i d l a t t i c e a p p r o x i m a t i o n is n o t correct, a n d n o u n i q u e l i g a n d field exists. T h e t e r m l i g a n d field has thus lost its u s u a l 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 meaning.

M o r e o v e r , t h e t e r m designations of c o n v e n t i o n a l l i g a n d


field

12.


ADAMSON

of Co(III)

133

Ammines

t h e o r y m a y n o t b e correct for t h e t h e r m a l l y e q u i l i b r a t e d e x c i t e d state since the p o i n t g r o u p s y m m e t r y m a y n o t b e t h e s a m e as for the g r o u n d state. Thus a hexacoordinated O , D , h

or C

4h

4v

g r o u n d state c o m p l e x m i g h t b e C

5 v

i n a t h e r m a l l y e q u i l i b r a t e d e x c i t e d state; a s q u a r e p l a n a r c o m p l e x m i g h t b e t e t r a h e d r a l . Y e t n o t o n l y does e m i s s i o n a p p e a r to c o m e f r o m t h e r m a l l y e q u i l i b r a t e d e x c i t e d states, b u t , as is d e v e l o p e d t h e s u b s t i t u t i o n a l p h o t o c h e m i s t r y of c o o r d i n a t i o n

f u r t h e r b e l o w , so does compounds.

B e c a u s e of the s e v e r a l q u a l i t a t i v e differences b e t w e e n t h e c o n c e p t s of


ligand

field

a n d t h e r m a l l y e q u i l i b r a t e d e x c i t e d states, i t is u s e f u l

develop a distinguishing vocabulary.

to

C o n v e n t i o n a l l i g a n d field e x c i t e d

states w i l l be c a l l e d F r a n c k - C o n d o n states since the energies a r e those of b a n d m a x i m a a n d the transitions b e t w e e n t h e m a r e essentially those v e r t i c a l ones w i t h m a x i m u m F r a n c k - C o n d o n o v e r l a p . t h e x i state has b e e n p r o p o s e d state.

(12)

T h e abbreviation

for a thermally equilibrated excited

T h e gist of t h e f o r e g o i n g is that c o n v e n t i o n a l l i g a n d field t h e o r y

treats h y p o t h e t i c a l e l e c t r o n i c e x c i t e d states w h i c h are i n r e a l i t y F r a n c k C o n d o n states, w h e r e a s i t is t h e x i states t h a t are i m p o r t a n t i n p h o t o p h y s i c a l a n d p h o t o c h e m i c a l processes.

N e w t h e o r y o r n e w extensions of present

t h e o r y a r e c l e a r l y n e e d e d to treat t h e latter t y p e of state. F u l l a p p r e c i a t i o n of t h e differences b e t w e e n a F r a n c k - C o n d o n state and

a t h e x i state has l i k e l y b e e n h i n d e r e d b y the f a c t t h a t t h e w o r d

" s t a t e " has q u i t e different m e a n i n g s i n the t w o expressions. t i o n is that b e t w e e n a s p e c t r o s c o p i c

The distinc-

state a n d a t h e r m o d y n a m i c state.

T h e f o r m e r has to d o w i t h the d e t a i l e d q u a n t u m m e c h a n i c a l d e s c r i p t i o n of a n i n d i v i d u a l m o l e c u l e (as, for e x a m p l e , b y g i v i n g e l e c t r o n i c , v i b r a t i o n a l , a n d r o t a t i o n a l q u a n t u m n u m b e r s ) ; the latter has to do w i t h t h e p h e n o m e n o l o g i c a l specification of a n e n s e m b l e of m o l e c u l e s i n t h e r m a l a n d m e c h a n i c a l e q u i l i b r i u m w i t h t h e i r s u r r o u n d i n g s . A b s o r p t i o n of l i g h t of w a v e l e n g t h a r o u n d a b a n d m a x i m u m p r o d u c e s a c o l l e c t i o n of v i b r a tionally and electronically excited

species

each

with

a

spectroscopic

s p e c i f i c a t i o n ; the c o l l e c t i o n is not, h o w e v e r , a t h e r m o d y n a m i c

ensemble.

O n l y after t h e r m a l e q u i l i b r a t i o n does t h e c o l l e c t i o n of m o l e c u l e s c o n s t i tute a t h e r m o d y n a m i c state. I t w i l l n o w h a v e a c o n v e n t i o n a l m o l a r free energy, e n t h a l p y , a n d e n t r o p y ; i t w i l l h a v e a s t a n d a r d r e d o x p o t e n t i a l . N o n e of

these

q u a n t i t i e s h a v e m e a n i n g for

a F r a n c k - C o n d o n state.

A l t e r n a t i v e l y , a t h e x i state corresponds c o n c e p t u a l l y to a c h e m i c a l i s o m e r of the g r o u n d state.

I t possesses e q u i l i b r i u m s t r u c t u r e a n d d i s t i n c t i v e ,

a c t i v a t e d r e a c t i o n k i n e t i c s . T h e t h e x i state is a c h e m i c a l species. S o m e of t h e e v i d e n c e for the i m p o r t a n c e of t h e x i states i n t h e p h o t o c h e m i s t r y of c o o r d i n a t i o n c o m p o u n d s

is g i v e n i n the n e x t section, f o l -

l o w e d b y a d i s c u s s i o n of the r e a c t i o n c h e m i s t r y of the t h e x i state i m p o r t a n t for C o ( I I I )

ammines.

T h e c o n c l u d i n g s e c t i o n deals w i t h some f u r t h e r

aspects of F r a n c k - C o n d o n a n d t h e x i states.


134

INORGANIC

COMPOUNDS

WITH

UNUSUAL

PROPERTIES

Some Evidences for the Realtiy and Importance of Thexi States It is possible to s u r m i s e some of the attributes of the t y p i c a l energetic species w h i c h undergoes l i g a n d s u b s t i t u t i o n r e a c t i o n f o l l o w i n g i r r a d i a t i o n i n the w a v e l e n g t h r e g i o n of a n

or L

2

b a n d . T h e species is n o t g e n e r a l l y

a v i b r a t i o n a l l y h o t g r o u n d state m o l e c u l e , p r e v a l e n c e of photoreactions

first

of a l l , because of

the

different f r o m t h e r m a l ones. W e c a l l s u c h

b e h a v i o r a n t i t h e r m a l . T h e energetic species is therefore almost c e r t a i n l y a n e l e c t r o n i c a l l y excited one.

W i t h the C r ( I I I )

f a m i l y of

complexes,


there are some v e r y d i r e c t i n d i c a t i o n s that the p h o t o s u b s t i t u t i o n occurs f r o m a l o w l y i n g q u a r t e d e x c i t e d state ( 1 3 , 1 4 , 1 5 ) . N e x t , there are several i n d i c a t i o n s that the p h o t o r e a c t i v e

species is

n o t i n a F r a n c k - C o n d o n state, b u t r a t h e r i t is i n a t h e r m a l l y e q u i l i b r a t e d e x c i t e d state. O n e i n d i c a t i o n is that q u a n t u m y i e l d s as w e l l as t h e n a t u r e of the p h o t o r e a c t i o n

do not v a r y a p p r e c i a b l y as the i r r a d i a t i n g w a v e -

l e n g t h traverses the w i d t h of a l i g a n d field b a n d ( a l t h o u g h v a r i a t i o n s m a y o c c u r o n g o i n g f r o m one b a n d to a n o t h e r )

I t appears t h a t

(13, 14, 15).

a c o m m o n reactive state is r e a c h e d , regardless of the degree of v i b r a t i o n a l e x c i t a t i o n of the i n i t i a l l y p r o d u c e d

F r a n c k - C o n d o n state.

The

simplest

e x p l a n a t i o n is that this c o m m o n state is a t h e x i state. A s another l i n e of a p p r o a c h , there is at least i n d i r e c t e v i d e n c e that the photoreactive

species is m u c h longer l i v e d t h a n w o u l d b e e x p e c t e d

for a F r a n c k - C o n d o n state. T h e l i f e t i m e of this last, that is, the r e l a x a t i o n t i m e for t h e r m a l e q u i l i b r a t i o n to a m b i e n t t e m p e r a t u r e ,

should not

be

m u c h greater t h a n a f e w h u n d r e d v i b r a t i o n a l periods a n d i t is p r o b a b l y m u c h less.

T y p i c a l complexes are s t u d i e d i n p o l a r ,

hydrogen-bonding

solvents, a n d the ligands must be i n excellent v i b r a t i o n a l c o m m u n i c a t i o n w i t h the solvent m e d i u m .

F r o m the fact that q u a n t u m y i e l d s t y p i c a l l y

are w e l l b e l o w u n i t y , i t m a y also be i n f e r r e d that excess v i b r a t i o n a l e n e r g y is d i s s i p a t e d q u i c k l y to the s u r r o u n d i n g m e d i u m . T h i s means that m o s t of the a b s o r b e d e n e r g y is d i s s i p a t e d n o n r a d i a t i v e l y , a n d it almost c e r t a i n l y passes t h r o u g h h i g h l y v i b r a t i o n a l l y e x c i t e d g r o u n d states (see a n d Ref. 9 ) .

Figure 3

T h e e n e r g y b e i n g d i s s i p a t e d c a n easily be m u c h

greater

t h a n the a c t i v a t i o n e n e r g y for a k n o w n t h e r m a l s u b s t i t u t i o n r e a c t i o n of t h e c o m p l e x . T h e latter is t y p i c a l l y 2 0 - 3 0 k c a l / m o l e , a n d t y p i c a l energies of a b s o r b e d l i g h t q u a n t a a n d h e n c e energies b e i n g d i s s i p a t e d are 5 0 - 7 0 kcal/mole.

C l e a r l y , v i b r a t i o n a l l y e x c i t e d g r o u n d states m u s t lose excess

e n e r g y to the m e d i u m i n less t h a n the t i m e r e q u i r e d for that e n e r g y to find

its w a y i n t o those p a r t i c u l a r n u c l e a r motions that l e a d to t h e r m a l

chemical reaction.

T h e same s i t u a t i o n is l i k e l y to be true for F r a n c k -

C o n d o n e x c i t e d states.

( A caveat is d i s c u s s e d b e l o w . )

B y contrast, lifetimes of a n a n o s e c o n d or longer c a n b e i n f e r r e d f o r the a c t u a l p h o t o r e a c t i v e

state.

F i r s t , since q u a n t u m y i e l d s are r a r e l y

greater t h a n a f e w tenths, t h e d o m i n a n t d i s s i p a t i v e process is n o t


one

12.


ADAMSON

of Co(III)

135

Ammines

of c h e m i c a l r e a c t i o n ; i t m u s t u s u a l l y b e t h a t of r a d i a t i o n l e s s d e a c t i v a t i o n (emission normally being

of

trivial

importance

under typical

photo

c h e m i c a l c o n d i t i o n s ). N e g l e c t i n g some p o s s i b l e c o m p l i c a t i o n s , the q u a n tum

y i e l d for

c h e m i c a l r e a c t i o n , φ, is t h e n g i v e n

approximately

by

^ c r A n r , the r a t i o of the rate constants f o r c h e m i c a l r e a c t i o n a n d for n o n r a d i a t i v e r e t u r n to the g r o u n d state, r e s p e c t i v e l y .

T h e temperature

de

p e n d e n c e of φ t h e n gives a n a p p a r e n t a c t i v a t i o n energy, Εφ, t h a t is e q u a l to (E

— E

cr


and

φ = θ

n r

) . R a d i a t i o n l e s s d e a c t i v a t i o n also c o m p l e t e s w i t h e m i s s i o n ,

1/τ & 0

where

ηΓ

emission lifetime.

τ

Thus

is the n a t u r a l

0

E

n r

may

be

(temperature-independent)

inferred from

the

temperature

d e p e n d e n c e of the e m i s s i o n y i e l d ; t y p i c a l v a l u e s are 3 - 5 k c a l / m o l e 14, 15)

a n d they m a y be larger. A p p r o x i m a t e l y , however, E

c r

=

(9,

Εφ +

3

in kcal/mole. T h e t e m p e r a t u r e d e p e n d e n c e of φ w a s r e p o r t e d f o r a n u m b e r

of

systems. W i t h C r ( I I I ) complexes, v a l u e s of Εφ r a n g e f r o m near z e r o u p to 10 or m o r e k c a l / m o l e (14, thus seem n o t u n c o m m o n . T h e rate constant f o r a (— E*/RT),

15).

E

cv

v a l u e s of 12 or m o r e

first-order

reaction should be about 10

neglecting activation entropy.

m o l e , the rate constant k

cr

E v e n for a n E

c r

kcal/mole

O n e m a y t h e n m a k e the f o l l o w i n g analysis. For E r = C

w o u l d b e a b o u t 1 0 sec" 4

of 4 k c a l / m o l e , k

cr

is a b o u t 1 0

1 0

1

exp-

1 3

E * —12

kcal/

at r o o m t e m p e r a t u r e .

sec"

1

w h i c h still corre

sponds to a n excited-state l i f e t i m e of t h o u s a n d s of v i b r a t i o n a l p e r i o d s . I n s u m m a r y , t h e p r e v a l e n c e of a c t i v a t e d p h o t o c h e m i s t r y s t r o n g l y suggests t h a t the p h o t o r e a c t i v e

state is m u c h l o n g e r l a s t i n g t h a n w o u l d b e

ex

p e c t e d w e r e i t a F r a n c k - C o n d o n state. T h e s i t u a t i o n is t h a t e x p e c t e d f o r a t h e x i state. A n o t h e r i n d i c a t i o n t h a t the t y p i c a l r e a c t i n g e x c i t i n g state is r e l a t i v e l y l o n g - l i v e d is i n the s e l e c t i v i t y , e s p e c i a l l y the stereoselectivity of c h e m i c a l s u b s t i t u t i o n reactions.

A s one

2

p h o t o a q u a t e s c h l o r i d e , b u t i t gives c t s - [ C r ( e n ) ( H 0 ) C l ] 2

t h e trans t h e r m a l r e a c t i o n p r o d u c t .

photo

example, £ r a n s - [ C r ( e n ) C l ] 2

2 +

2

+

rather than

F u r t h e r , i f the e t h y l e n e d i a m i n e s are

c o n n e c t e d b y the b e l t l i g a n d c y c l a m so t h a t trans-to-cis i s o m e r i z a t i o n b e c o m e s i m p o s s i b l e , the p h o t o a q u a t i o n y i e l d d r o p s a t h o u s a n d f o l d

(15).

S o m e i l l u s t r a t i o n s i n v o l v i n g C o ( I I I ) a m m i n e s are p r e s e n t e d b e l o w .

Such

specificity presents n o p r o b l e m for a t h e x i state. B y contrast, a F r a n c k C o n d o n e x c i t e d state s h o u l d b e too s h o r t - l i v e d to engage i n m o r e t h a n s i m p l e b o n d fission processes t h a t h a v e l i t t l e stereoselectivity. F i n a l e v i d e n c e t h a t p h o t o r e a c t i v e species are too l o n g - l i v e d to Franck-Condon

states

comes f r o m

the occasional

systems

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

for

be

which

T h e r e are

s e v e r a l C r ( I I I ) c o m p l e x e s w h i c h s h o w s p i n - f o r b i d d e n e m i s s i o n at r o o m temperature (17).

(16),

a n d l i f e t i m e s of m i c r o s e c o n d s

have been

measured

( E m i s s i o n l i f e t i m e s of 1.3, 1.7, a n d 46 /xsec w e r e f o u n d (18)


for

136

INORGANIC

[Cr(en) ] 3

3 +

,

[Cr(NH ) ] 3

3 +

e

COMPOUNDS

, and

temperature, aqueous solution. )

WITH

[Cr(bipyr) ] 3

UNUSUAL

PROPERTIES

respectively i n

3 +

room

T o t h e extent t h a t the e m i t t i n g d o u b l e t

state is ( a ) itself c h e m i c a l l y r e a c t i v e or ( b )

i n thermal equilibrium with

a r e a c t i v e q u a r t e t e x c i t e d state, one is l e d t o c o n c l u d e t h a t the p h o t o c h e m i s t r y i n v o l v e s t h e x i states. I n case ( a ) the r e a c t i n g a n d e m i t t i n g state are t h e same a n d i t is therefore k n o w n t h a t the l i f e t i m e of the f o r m e r is too l o n g f o r i t to b e a F r a n c k - C o n d o n state. I n case ( b ) t h e v e r y p o s t u l a t i o n of o r d i n a r y k i n e t i c i n t e r c o n v e r s i o n b e t w e e n t w o e l e c t r o n i c states i m p l i e s


t h a t b o t h are t h e x i states. E m i s s i o n l i f e t i m e s for s p i n - a l l o w e d d-d

transitions of c o o r d i n a t i o n

c o m p o u n d s a p p a r e n t l y h a v e n o t yet b e e n m e a s u r e d u n d e r p h o t o c h e m i c a l conditions.

T h e r a d i a t i v e l i f e t i m e , T , c a n b e e s t i m a t e d ( w i t h serious c

p o t e n t i a l e r r o r ) f r o m the area u n d e r t h e a b s o r p t i o n b a n d ( 9 ) ; t h e v a l u e s are i n the o r d e r of

microseconds.

l i f e t i m e of C r ( u r e a ) that E

n r

6

T h e low-temperature

w a s r e p o r t e d as 50 psec ( 9 , 1 9 ) .

3 +

fluorescence

I f one

assumes

averages a b o u t 3 k c a l / m o l e , a n o r d e r of m a g n i t u d e c a l c u l a t i o n

suggests a r o o m t e m p e r a t u r e l i f e t i m e of a b o u t 0.02 nsec, w h i c h is s t i l l long compared w i t h vibrational periods.

T h i s m a y be delayed Table I.

fluores-

Photochemistry

Photochemistry* Compound

Product

[Co(NH ) ] [Co(NH ) (H 0)] [Co(NH ) F] 3

6

3

5

3

3 +

3 +

2

2 +

5

[Co(NH ) (H 0)] [Co(NH ) (H 0) ] [Co(NH )4(H 0)F] [Co(NH ) (H 0)] [Co(NH )4(H 0)Cl] [Co(NH ) (H 0)] trans- [ C o (en) ( H 0 ) C 1 ] m-[Co(en) (H 0)Cl] trans-[Co(cyclam) (H 0)C1] ira/w-[Co(en) (H 0)Cl] m-[Co(en) (H 0)Cl] trans- [ C o (en) ( H 0 ) C 1 ] cis-a- [ C o (trien) ( H 0 ) C 1 ] cis-a- [ C o (trien) ( H 0 ) ] cis-β-[Co(trien) (H 0)C1] trans-[Co(trien) (H 0)C1] cis-β-[Co(trien) (H 0) ] trans-[Co(trien) (H 0)C1] cis-[Co(trien) (H 0)C1] [Co(tren)(H 0)Cl] [Co(tren)(H 0) ] 3

5

2

3

4

2

[Co(NH ) Cl] 3

5

2 +

5

2

5

2

trans- [ C o (en) C 1 ] 2

+

2

trans- [ C o ( c y c l a m ) C l ] cis- [ C o (en) C 1 ] 2

2 +

2

2 +

2

2

+

2

2 +

2

2 +

2

2 +

2

2 +

2

2

3 +

2 +

2

2 +

2

cis-β- [ C o (trien) ( H 0 ) C 1 ] 2

2 +

2

2

3 +

2 +

2

trans- [ C o (trien) ( H 0 ) C 1 ] 2

[Co(tren)Cl ] 2

2

2 +

2 +

2

2 +

2

2 +

MCo(tren)(H 0)Cl]

2 +

2

2 +

2 +

2

2

+

2

2 +

2

2

cis-[Co(en) (H 0)Cl] [ C o (trien) C l ] cis-a-[Co(trien) (H 0)C1] CÎS-/?-[CO (trien) C l ]

2 +

2

2

2

+

2

2 +

2 +

2

2

2

2 +

3 +

2

3

3

3 +

2

2

3

3

3 +

2

3 +

•From Refs. 20, 21, and 22; irradiations around L i band and at either 0°C or

25°Q


12.

ADAMSON

cence, however.

137


of Co(III)

Prompt

f r o m f r a n s - C r ( N H ) ( N C S ) 4 ~ is

fluorescence

Ammines

3

2

r e p o r t e d to b e < 3 psec ( M . W i n d s o r , G . P o r t e r a n d A . D . K i r k , p r i v a t e c o m m u n i c a t i o n ). Ligand Field Photochemistry of Some Co (III)

Ammines

U n t i l r e c e n t l y o n l y s c a t t e r e d d a t a w e r e r e p o r t e d for t h e i r r a d i a t i o n of C o ( I I I ) a m m i n e s i n t h e w a v e l e n g t h r e g i o n of t h e L i b a n d (14,

15).

W i t h a c i d o a m m i n e c o m p l e x e s , t h e a c i d o g r o u p m a y b e r e p o r t e d to p h o t o -


a q u a t e , b u t no a m m o n i a a q u a t i o n w a s l o o k e d for a n d the p h o t o c h e m i s t r y a p p e a r e d to b e m e r e l y a catalysis of the t h e r m a l r e a c t i o n .

Reported

q u a n t u m y i e l d s w e r e l o w ( e x c e p t w h e r e C T c h a r a c t e r w a s present, i n w h i c h case r e d o x d e c o m p o s i t i o n w a s also o b s e r v e d ) , a n d the l i g a n d field p h o t o c h e m i s t r y of this class of complexes s e e m e d r a t h e r u n i n t e r e s t i n g . R e c e n t w o r k i n this l a b o r a t o r y has d e m o n s t r a t e d t h a t s u c h p h o t o c h e m i s t r y is i n f a c t v e r y r i c h a n d v a r i e d (20, 21, 22).

T h e ligand aquated

is n o t a l w a y s the same as t h a t i n the t h e r m a l r e a c t i o n , a n d , m o r e o v e r , t h e p h o t o r e a c t i o n is stereospecific,

often d i f f e r e n t l y f r o m t h e g r o u n d state

r e a c t i o n (see T a b l e I ) . T h e findings c a n b e a c c o u n t e d f o r i n d e t a i l i f o n e of C o ( I I I ) Photo

Ammines

chemistry

b

Φ (X

W) 4

2.1 1.3 19.6 5.5 50.7 17.1 7.9 3.1 4.0 17.5 6.5 42

Inorganic Compounds with Unusual Properties

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