Mechanisms of Inorganic Reactions - American Chemical Society

10 Spectrospecific Photolysis of Aqueous Cr (en) (OH) Ion +

2

2

ARTHUR W. ADAMSON University

of Southern

California,

Los

Downloaded by UNIV OF SYDNEY on April 7, 2013 | http://pubs.acs.org Publication Date: January 1, 1965 | doi: 10.1021/ba-1965-0049.ch010

The

Angeles,

Calif.

90007

quantum yields

for

p h o t o i s o m e r i z a t i o n of

photoaquation

cis-Cr(en) (OH) 2

and

vary

+

2

in

value and temperature dependence with w a v e l e n g t h of

irradiating

light

over

comprising the spin-allowed and bidden

transitions.

cis- a n d

The

trans-Cr(en) (OH) 2

isomerization, and tion.

The

more

thermal

+

2

the

the

region

spin-for-

lability

of

relates primarily to

only secondarily to

cis f o r m ,

the

aqua-

although more stable,

photosensitive,

and

photolysis

is

yields

a m a j o r p o r t i o n of a q u a t i o n p r o d u c t . F r o m such comparisons a n d t h e w a v e l e n g t h sensitivity of the

photolytic

behavior,

the

reaction path

f o l l o w i n g e x c i t a t i o n of a s p i n - a l l o w e d tion

transi-

differs f r o m t h a t f o l l o w i n g e x c i t a t i o n of a

spin-forbidden

transition,

and

both

differ

f r o m the t h e r m a l r e a c t i o n p a t h .

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

been

One continually interesting

q u e s t i o n has been t h e e x t e n t t o w h i c h t h e p h o t o c h e m i s t r y of a c o m p l e x d e p e n d s o n the absorption band irradiated.

I n t h e case of C o ( I I I ) a c i d o p e n t a m i n e s , s u c h as

C o ( N H ) 5 B r " , we f o u n d t h a t i r r a d i a t i o n of ( * A 3

f 2

—> T g) 1

l g

2

bands showing appreciable

c h a r g e t r a n s f e r l e d t o r e d o x a n d a q u a t i o n r e a c t i o n s w h i c h were c o m p e t i t i v e .

It

w a s r e a s o n a b l e t o s u p p o s e t h a t t h e c o m m o n p r e c u r s o r w a s t h e species f o r m e d b y a p r o m p t h e t e r o l y t i c b o n d fission (1).

The ( A 1

—> Ti ) l

l g

g

b a n d w a s f a r less p h o t o

a c t i v e , a n d i n m o d e l cases, i r r a d i a t i o n l e d o n l y t o a q u a t i o n .

E a c h excited state or

excited state manifold thus tended to show a distinct photochemistry, w h i c h meant t h a t c o n v e r s i o n f r o m one e x c i t e d s t a t e t o a n o t h e r w a s n o t i m p o r t a n t . T h e s i t u a t i o n h a s been less c l e a r for C r ( I I I ) c o m p l e x e s .

Only substitution

(2), (6), (13) ( a n d i s o m e r i z a t i o n (15)) p h o t o r e a c t i o n s are o b s e r v e d , i r r e s p e c t i v e of w h i c h l i g a n d field b a n d is i r r a d i a t e d .

I t has been p r o p o s e d t h a t t h i s u n i f o r m r e a c

t i o n m o d e ( a n d n e a r l y u n i f o r m q u a n t u m y i e l d ) c o u l d be a c c o u n t e d for i f s u b s t a n t i a l l y c o m p l e t e c o n v e r s i o n o c c u r r e d of t h e T 4

2 g

and T 4

l g

states to a lower l y i n g doublet,

237

In Mechanisms of Inorganic Reactions; Kleinberg, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1965.

238

MECHANISMS OF INORGANIC

presumably the

2

E

g

state.

REACTIONS

T h i s l a s t , i t w a s s u p p o s e d , w o u l d be s u f f i c i e n t l y l o n g -

l i v e d t o f u n c t i o n as a c h e m i c a l i n t e r m e d i a t e , e x c e p t i o n a l l y l a b i l e t o w a r d s s u b s t i t u t i o n (13).

T h e s e d e s i g n a t i o n s of e x c i t e d s t a t e s a r e r e a l l y for Ο A s y m m e t r y , b u t

serve t o i d e n t i f y l o o s e l y t h e a b s o r p t i o n b a n d s of c o m p l e x e s of d e s c e n d e n t

sym

metry. A n a l t e r n a t i v e i n c l i n a t i o n h a s been t o v i e w t h e i n i t i a l c h e m i c a l a c t f o l l o w i n g e x c i t a t i o n as a p r o m p t h e t e r o l y t i c b o n d fission of s o m e one c h r o m i u m - l i g a n d b o n d B o t h p i c t u r e s a d e q u a t e l y a c c o u n t for t h e so f a r n o t v e r y e l a b o r a t e d a t a ,

(2), (15).

t h e s e c o n d p e r h a p s m o r e i n k e e p i n g w i t h t h e p r i n c i p l e of s c i e n t i f i c p a r s i m o n y . P r o b a b l y t h e m o s t s e n s i t i v e d i a g n o s t i c test of w h e t h e r o r n o t p h o t o l y s i s of a complex proceeds t h r o u g h a n u b i q u i t o u s intermediate consists i n d e t e r m i n i n g the e x t e n t t o w h i c h c o p r e s e n t r e a c t i o n m o d e s s h o w different w a v e l e n g t h d e p e n d e n c i e s .


T h i s t y p e of t e s t w a s a v a i l a b l e for C o ( I I I ) s y s t e m s ; t h e p r o b l e m has been t o suitable C r ( I I I ) systems.

F o l l o w i n g a r e p o r t t h a t Co(en)2(H20)2

find

photoisomerized

+3

(10), i t o c c u r r e d t o us t h a t t h e a n a l o g o u s C r ( I I I ) species m i g h t p r o v i d e a n a n s w e r . If w e d e n o t e cis- a n d 2 r a f « - C r ( e n ) 2 ( H 0 ) 2 2

+3

b y H2C a n d H T , respectively, 2

a n d the four a c i d dissociation products b y H C , H T , C , a n d T , t h e n there are four a c i d i t y c o n s t a n t s (KH C, 2

KKC, ^ H T , a n d KUT) a n d t h r e e i s o m e r i z a t i o n c o n s t a n t s 2

(^Η Τ/Η Ο, KuT/HPf a n d Κτ/c). 2

2

A l l of these h a v e been e v a l u a t e d b y W o l d b y

(17),

a n d s o m e r a t e d a t a o n t h e Τ —• C c o n v e r s i o n h a s been r e p o r t e d b y O l s o n a n d G a r n e r (12).

P r e l i m i n a r y experiments showed that both photoaquation a n d photoisomeri-

z a t i o n d i d indeed occur, a n d the detailed investigations described below were there fore c a r r i e d o u t . Experimental Preparative Procedures. T h e s a l t s a s - [ C r ( e n ) 2 ( O H ) ( H 0 ) ] S 2 0 a n d trans[Cr(en)2(OH)(H 0)]Br were p r e p a r e d f r o m a s - [ C r ( e n ) C l 2 ] C 1 0 4 a c c o r d i n g t o W o l d b y e (17), a n d t h e l a s t c o m p l e x b y t h e p r o c e d u r e s i n " I n o r g a n i c S y n t h e s e s " (7). C r labelled complexes were s i m i l a r l y prepared. T h e ethylenediamine used was dried over sodium hydroxide a n d then distilled. H a r s h a w chromatographic a l u m i n a w a s u s e d for t h e c h r o m a t o g r a p h i c s e p a r a t i o n s . L o t N o . K 1 7 7 3 b e h a v e d e x c e p t i o n a l l y w e l l since c l e a r effluents were o b t a i n e d o n e l u t i o n w i t h t h e 1M p o t a s s i u m c h l o r i d e , 0.1 M p o t a s s i u m h y d r o x i d e s o l u t i o n . O t h e r c h e m i c a l s u s e d w e r e of reagent grade. T h e p r e p a r a t i o n s p r o v e d t o be 98 t o 9 9 % c h r o m a t o g r a p h i c a l l y p u r e , t h e c o n t a m i n a n t a l w a y s b e i n g t h e o t h e r i s o m e r . T h e v a r i o u s a b s o r p t i o n m a x i m a are r e p o r t e d i n T a b l e I . A l l s o l u t i o n s were 0.01 M i n c o m p l e x . T h o s e d e s i g n a t e d as H C o r H T w e r e m e a s u r e d e i t h e r i n w a t e r a t p H 5 - 6 , o r i n 0.1 A f s o d i u m a c e t a t e a c e t i c a c i d buffer a t p H 5.5 (no s p e c t r a l differences r e s u l t e d ) ; a n d t h o s e d e s i g n a t e d C o r Τ were a d j u s t e d t o p H 10.5 w i t h e t h y l e n e d i a m i n e . T h e v a l u e s agree w e l l i n p o s i t i o n w i t h those of W o l d b y e (17) ( w h i c h w e r e for s o l u t i o n s \M i n s o d i u m n i t r a t e ) , b u t o u r e x t i n c t i o n coefficients are c o n s i s t e n t l y a b o u t 1 0 % h i g h e r t h a n h i s . I n a d d i t i o n , w e n o t e a w e a k m a x i m u m for Τ a t 330 ϊημ a n d r e p o r t s o m e i n f o r m a t i o n o n t h e s p e c t r a i n t h e d o u b l e t r e g i o n . T h e s e l a s t m e a s u r e m e n t s w e r e m a d e o n filtered 0 . 0 4 M s o l u t i o n s , u s i n g 5 c m . cells. T h e g e n e r a l a b s o r p t i o n c u r v e s were o b t a i n e d b y a C a r y M o d e l 14 s p e c t r o p h o t o m e t e r , b u t for m o s t of t h e k i n e t i c s t u d i e s a n d a n a l y t i c a l m e a s u r e m e n t s , o p t i c a l d e n s i t i e s a t selected w a v e l e n g t h s were d e t e r m i n e d b y a D U B e c k m a n s p e c t r o p h o t o m e t e r . 2

2

2

6

2

5 1

K i n e t i c Studies. T h e r e is s o m e d i f f i c u l t y i n u n r a v e l l i n g t h e t h e r m a l r e a c t i o n s of t h e v a r i o u s f o r m s , since n o t o n l y does i s o m e r i z a t i o n o c c u r , b u t a q u a t i o n is n o t n e g l i g i b l e e v e n a t p H 10.5.

T h e s y s t e m , i n fact, resembles classic reaction triangle


10.

ADAMSON

Specfrospec/flc T a b l e I.

Complex H C 2

HC C H T 2

HT


Τ Aquo (HC) (HT) (C)

λ, πΐμ

c

368 (367) 392 (391) 377 (378) 365 (361) 398 (397) 330

48 (42.5) 47 (43) 69 (66) 43 (39. Ζ ) 49 (46) 19

415 418 378

Absorption Maxima λ, ΤΠμ

λ, τημ

€

70 (67.0) 72 (69.3) 65 (63.6) 32 (29.2) 38 (35.0) 33 (28.9) 31 30 37

485 (495) 515 (512) 528 (525) 445 (443) 495 (497) 398 (396) 570 570 548

33 34

2Z9

Photolysis

€

e 0.95 1.1 1.4

692sh

1.4

680

2.1

685sh

1.4

(22.5)

(508) 505 (505)

Note: Values in parentheses are those reported by Woldbye (17). spectra of solutions held at ca. 35°C. for six weeks.

λ, mμ 667 640sh 681sh

34 (31.8)

Those labelled Aquo are terminal

(14), a n d e v e n i f i t i s a s s u m e d t h a t t h e reverse of a q u a t i o n i s u n i m p o r t a n t , s o l u t i o n s t o a r e a c t i o n t r i a n g l e , w h i l e a v a i l a b l e , a r e d i f f i c u l t t o use p e r c e p t i v e l y (9). F o r t u n a t e l y , a q u a t i o n o c c u r r e d s l o w l y e n o u g h , r e l a t i v e t o i s o m e r i z a t i o n , t o be t r e a t e d as a p e r t u r b a t i o n w h i c h g r a d u a l l y d i m i n i s h e d t h e effect of e q u i l i b r i u m c i s a n d trans concentrations w i t h o u t m a t e r i a l l y affecting their r a t i o .

A p p l y i n g this

a p p r o a c h r e q u i r e d p a r a l l e l d a t a for s o l u t i o n s i n i t i a l l y a l l t r a n s a n d i n i t i a l l y a l l c i s . Assuming

first-order

k i n e t i c s , the o p t i c a l d e n s i t y of the i n i t i a l l y a l l cis s o l u t i o n

for a p a t h l e n g t h of 1 c m . i s g i v e n b y :

De

a

-

^ (6o + Ke ) T

+

aK

(ec -

c ) e"*
Τ e q u i l i b r i u m c o n s t a n t a n d k = k\ + k

2

c o n c e n t r a t i o n is g i v e n b y a. D

T

-

-

(1)

T

+ K) ; t h e t o t a l

= k (l 2

S i m i l a r l y , for a s o l u t i o n i n i t i a l l y a l l t r a n s :

~

1 -f- A

(ec + Ker)

-

—7-= 1 -f-

Λ

(ec -

e ) e"* T

(2)

1

I t then follows t h a t D C T = De

+ KDT

= a(€c + i £ e r ) = a c o n s t a n t

P r e l i m i n a r y d a t a h a v e been o b t a i n e d f o r O.Olikf s o l u t i o n s a t p H 2.0

(3) (added

h y d r o c h l o r i c a c i d ) a n d s o l u t i o n s buffered t o p H 5.5 w i t h O.likf a c e t a t e buffer.

The

p r i n c i p a l i n t e r e s t , h o w e v e r , w a s i n t h e C - T s y s t e m ( s o l u t i o n s b r o u g h t t o p H 10.5 w i t h a d d e d e t h y l e n e d i a m i n e ) for w h i c h m o s t of t h e p h o t o c h e m i c a l r e s u l t s were obtained.

A n e x a m p l e of t h e d i r e c t d a t a for t h e C - T s y s t e m is s h o w n i n F i g u r e 1.

O v e r t h e p e r i o d of 120 h o u r s , D C T decreased b y a b o u t 3 % , c o r r e s p o n d i n g t o a b o u t 6 % aquation. effective D

00

T h e procedure was then to calculate from each measurement an

v a l u e : D°° = D r/(l C

time should then superimpose.

+ K).

P l o t s of (D°°-D ) T

a n d of (Dc-D°°)/K

vs.

T h i s w a s r e a s o n a b l y t h e case, as i l l u s t r a t e d i n

F i g u r e 2, for d a t a a t t w o w a v e l e n g t h s . T h e insight p r o v i d e d b y this approach led to the conclusion t h a t a n accurate Κ v a l u e c o u l d n o t be o b t a i n e d f r o m i s o m e r i z a t i o n d a t a u s i n g j u s t one f o r m , a n d t h a t



240

M E C H A N I S M S O F I N O R G A N I C REACTIONS

T i m e , hours Figure

1.

(OH)^

at 25°C;

Isomerization of 0.01 M

cis-and

trans-Cr(en)i-

solutions adjusted to pH 10.5 with ethyl

enediamine; optical density at 525 ?ημ. even using the c o m b i n e d d a t a for b o t h forms, the r e l a t i v e l y s m a l l a q u a t i o n t h a t occurred introduced a 5 0 % uncertainty.

T h u s , f o r t h e 3 5 ° C . d a t a of F i g u r e 2, Κ

v a l u e s of 0.2 a n d 0.3 r e p r e s e n t e d t h e l i m i t s s t i l l g i v i n g a c c e p t a b l e s t r a i g h t l i n e p l o t s . T h e results are s u m m a r i z e d i n T a b l e I I ; Κ values i n parentheses are from W o l d b y e (17) (for IMsodium

T a b l e II.

nitrate medium).

I s o m e r i z a t i o n a n d A q u a t i o n Kinetics % Aquation in One Isomerimtion Half Life

System

t°c.

Κ

k (hr)

H2C, H2T

25

0.25 db 0.05 (0.20) 0.25 ± 0.05

1 3 1.2

( p H 10.5)

42 25

0.25 =b 0.05 (0.41)

0.0425 (0.0245 ) 0.262 (0.104 ) 0.97 ca. 0.45

25

(0.078)

ca. 0.11

35

1

e

1

6

HC, H T ( p H 5.5) H C, H T ( p H 2.0) 2

2

β b

Reported by Olson and Garner (12) for p H 10.9. Similarly reported for p H 10.6.


10.

ADAMSON

Spectrospeciflc

241

Photolysis

P h o t o c h e m i c a l E x p e r i m e n t s . T h e general experimental procedure was t h a t p r e v i o u s l y d e s c r i b e d (25). T h e b o l o m e t e r w a s u s e d as a r e l a t i v e i n t e n s i t y i n d i c a t o r s i n c e c o l l i m a t i o n i n t h e c y l i n d r i c a l g e o m e t r y of a n A H 6 l a m p is p o o r , a n d a l l o w a n c e f o r s c a t t e r i n g i s d i f f i c u l t . C a l i b r a t i o n a t e a c h w a v e l e n g t h w a s b y m e a n s of K C r ( N H ) 2 ( N C S ) 4 s o l u t i o n s for w h i c h w e n o w h a v e a c c u r a t e a b s o l u t e q u a n t u m y i e l d s f o r w a v e l e n g t h s f r o m 450 ηΐμ t o 750 πΐμ (16). F o r t h e r u n s a t > 6 8 0 m/*, a 500 w a t t t u n g s t e n l a m p w a s u s e d s i n c e t h e o u t p u t of t h e A H 6 m e r c u r y l a m p w a s uncomfortably low at this wave length. W a v e l e n g t h s e l e c t i o n w a s b y glass filters. F o r r u n s l a b e l l e d 370 τημ, a B a i r d A t o m i c filter w i t h c u t offs a t 300 τημ a n d 400 πΐμ w a s u s e d , so t h a t a b s o i p t i o n o c c u r r e d m a i n l y i n t h e ( A2 -> T ) b a n d . R u n s l a b e l l e d 550 τημ a n d 500 πΐμ w e r e c a r r i e d o u t w i t h e i t h e r C o r n i n g C S - 2 - 6 4 o r C S - 3 - 7 1 filters w h o s e s h o r t w a v e l e n g t h c u t offs w e r e 530 τημ a n d 480 τημ, r e s p e c t i v e l y ; a b s o r p t i o n t h u s o c c u r r e d m a i n l y i n the ( A —> T 2 ) t y p e b a n d . F i n a l l y , r u n s l a b e l l e d > 6 8 0 τημ w e r e c a r r i e d o u t u s i n g a C o r n i n g C S - 3 - 6 9 filter w h o s e c u t off w a s a t 680 m/*, so t h a t a b s o r p t i o n was p r i m a r i l y i n the doublet region. W e u s e d a b s o r b e d l i g h t i n t e n s i t i e s of a b o u t ΙΟ"" E/minute a n d i r r a d i a t i o n t i m e sufficient t o p r o d u c e 10 t o 2 5 % r e a c t i o n (never more t h a n a n hour). 3

4


4

4

2 g

g

4

l g

g

6

I t w a s n o t c e r t a i n t h a t t h e a q u a t i o n p r o d u c t w o u l d be i d e n t i c a l i n s p e c t r u m to t h a t o b t a i n e d b y t h e r m a l reaction, a n d analysis was therefore carried out chromatographically, rather than spectrophotometrically. T h e procedure was t h a t described b y W o l d b y e , h i g h l y s t a n d a r d i z e d to ensure m a x i m u m r e p r o d u c i b i l i t y . P a r a l l e l separations were carried out o n u n i r r a d i a t e d solutions, a n d the c h r o m a t o g r a p h i c y i e l d s of t h e c i s a n d t r a n s w e r e i n s o m e cases d e t e r m i n e d b y u s i n g C r 6 1

0

2

4

6

Time, Figure

2.

Isomerization

Cr(en) ο are n e a r l y t h e same, a n d l i k e w i s e , Δ 5 Ϊ ο - » τ a n d Δ ό ' Ϊ τ - κ } since Κ i t s e l f is a l w a y s close t o u n i t y . T h i s i m p l i e s t h a t t h e t r a n s i t i o n


s t a t e for i s o m e r i z a t i o n is h i g h i n e n e r g y , h i g h i n e n t r o p y ( p o s s i b l y s u g g e s t i n g a l o o s e l y b o n d e d a r r a n g e m e n t ) , a n d c o n f o r m s s y m m e t r i c a l l y t o cis a n d t r a n s g e o m e t r i e s . T h e f a c t t h a t ΔΕ% is m o r e t h a n t w i c e t h a t for t h e r a c e m i z a t i o n of Cr(C20 ) ~" 4

3

3

(15) suggests t h a t , i n t h e p r e s e n t case, i t i s a C r - N r a t h e r t h a n a C r - 0 b o n d t h a t must loosen. A s e c o n d p o i n t of c o n t r a s t b e t w e e n t h e t h e r m a l a n d t h e p h o t o c h e m i c a l be h a v i o r i s t h a t , w h i l e t h e cis i s o m e r s a r e a l l t h e r m o d y n a m i c a l l y f a v o r e d , a n d c o n s e q u e n t l y a l l t h e t r a n s f o r m s a p p e a r m o r e l a b i l e , t h e s i t u a t i o n o n p h o t o l y s i s is varied.

T h e q u a n t u m y i e l d s f o r C a r e t e n t o t w e n t y t i m e s those for Τ ; H T i s m o r e

s e n s i t i v e t h a n H C ( b u t o n l y t o w a r d s p h o t o i s o m e r i z a t i o n ) , a n d H 2 C is m o r e p h o t o sensitive t h a n H 2 T . A n y a t t e m p t to explain the results u n a v o i d a b l y w i l l require postulating signi ficantly

different i n t e r m e d i a t e s t a t e s for t h e p h o t o c h e m i c a l a n d t h e r m a l r e a c t i o n

sequences.

P h o t o a c t i v a t i o n m u s t then c a r r y its o w n stereospecificity a n d cannot

be t h o u g h t of a s j u s t a n e x t e r n a l s o l i c i t a t i o n of e n e r g y t o ease a n e s s e n t i a l l y t h e r m a l r e a c t i o n sequence. P h o t o l y s i s of cîi-Cr(en)2(OH)2 . +

The

temperature

and

wave

length

p e n d e n c e s t u d i e s o n C p r o v i d e a b a s i s for s o m e m i n i m a l c o n c l u s i o n s .

de

Consider

first t h e c o n t r a s t b e t w e e n p h o t o i s o m e r i z a t i o n a n d a q u a t i o n a t t h e 370 πιμ a n d 550 τημ w a v e l e n g t h regions.

T h e former shows a considerable temperature dependence

i n q u a n t u m y i e l d , t h e a p p a r e n t a c t i v a t i o n e n e r g y a v e r a g i n g 9.6 k c a l . for t h e t w o w a v e l e n g t h r e g i o n s , w h i l e t h e l a t t e r process o b e y s a s m a l l n e g a t i v e a n d a s m a l l p o s i t i v e a p p a r e n t a c t i v a t i o n e n e r g y for these t w o regions, r e s p e c t i v e l y . I t m i g h t be t h o u g h t t h a t p h o t o i s o m e r i z a t i o n a n d p h o t o a q u a t i o n c o u l d

be

c o m p e t i t i v e towards a c o m m o n precursor excited state, b u t this e x p l a n a t i o n fails on quantitative testing.

B r i e f l y , t h e q u a n t u m y i e l d for p h o t o a q u a t i o n i s t o o h i g h

t o a c c o m m o d a t e its negative a p p a r e n t a c t i v a t i o n energy.

Photoaquation must

t h e n be c o m p e t i t i v e w i t h s o m e o t h e r process s u c h as d e a c t i v a t i o n , o r r e t u r n t o t h e o r i g i n a l cis species. N e x t i s t h e c o n t r a s t b e t w e e n t h e 370 πιμ t o 550 πιμ b e h a v i o r a n d t h a t for w a v e l e n g t h s g r e a t e r t h a n 680 πιμ.

W e a r e n o w c o m p a r i n g t h e consequence of e x c i t i n g

a quartet band and a doublet band.

T h e a c t i v a t i o n energy for p h o t o i s o m e r i z a t i o n

h a s i n c r e a s e d t o a b o u t 13 k c a l . , a n d t h e r e is a d r a m a t i c s h i f t i n t h e a q u a t i o n b e h a v i o r w h i c h n o w s h o w s 20 k c a l . a p p a r e n t a c t i v a t i o n e n e r g y a n d q u a n t u m y i e l d s a p p r o a c h ing u n i t y at low temperatures.

T h e m i n i m a l c o n c l u s i o n here i s t h a t i r r a d i a t i o n of


10.

ADAMSON

SpectrospeciHc

245

Photolysis

the doublet region produces a n intermediate differently disposed towards aquation f r o m t h a t r e s u l t i n g f r o m i r r a d i a t i o n of a q u a r t e t b a n d .

Specifically, the quartet to

d o u b l e t c o n v e r s i o n m e c h a n i s m , w h e r e b y a l l c h e m i c a l c o n s e q u e n c e s a r e filial t o a l o n g - l i v e d d o u b l e t i n t e r m e d i a t e , c a n n o t be i n effect here. Q u a n t u m y i e l d s f o r successive a q u a t i o n s t e p s of C r ( N H ) 6 3

b y E d e l s o n a n d P l a n e (6).

h a v e been r e p o r t e d

+ 3

T h e h i g h e r q u a n t u m y i e l d for w a v e l e n g t h s i n t h e d o u b l e t

a b s o r p t i o n r e g i o n t h a n f o r t h o s e i n t h e q u a r t e t r e g i o n w a s r e g a r d e d as s u g g e s t i n g that the conversion mechanism applied.

I t seems l i k e l y t h a t a d e t a i l e d s t u d y of

a q u a t i o n a n d i s o m e r i z a t i o n c o u l d y i e l d o b s e r v a t i o n s a n d a final c o n c l u s i o n s i m i l a r t o t h o s e f o u n d here.

O u r d a t a for p h o t o a q u a t i o n a t 2 5 ° C . s h o w t h e s a m e t r e n d of

increasing q u a n t u m y i e l d w i t h decreasing wave length, a n d o n l y the full picture


s h o w s t h a t t h i s p a r t i c u l a r c o r r e l a t i o n is t r i v i a l .

R e c e n t results i n this laboratory

s h o w n o i n c r e a s e i n q u a n t u m y i e l d i n t h e case of C r ( N H 3 ) e

+ 3

o n i r r a d i a t i o n of t h e

doublet band. Mechanisms.

S i n c e t h e a b o v e c o n c l u s i o n s a r e m i n i m u m , i t is i n t e r e s t i n g t o

see i f a m o r e d e t a i l e d p i c t u r e e x i s t s w h i c h c a n a c c o u n t for a l l of t h e o b s e r v a t i o n s . Four

different sequences

(at

least)

must

be

provided: thermal isomerization,

p h o t o l y s i s i n t h e d o u b l e t r e g i o n of w a v e l e n g t h , a n d p h o t o a q u a t i o n a n d p h o t o i s o merization i n the quartet region. I n a s s e m b l i n g t h i s c o l l e c t i o n , m a y w e first t u r n t o s o m e a q u a t i o n s t u d i e s of K C r ( N H ) ( N C S ) 4 (3). T h e s e l e d t o t h e c o n c l u s i o n t h a t s u b s t i t u t i o n o c c u r r e d 3

2

by

a c o n c e r t e d process, n a m e d SJV2FS, w h e r e b y a r r i v i n g a n d d e p a r t i n g g r o u p s i n t e r acted through hydrogen bonding.

F i g u r e 4 b s h o w s h o w t h i s m e c h a n i s m c a n be

e x t e n d e d t o the p r e s e n t i n s t a n c e . an N H

2

H y d r o g e n b o n d i n g interactions assist i n s h i f t i n g

g r o u p f r o m one p o s i t i o n t o a n o t h e r (needed t o a c c o m p l i s h i s o m e r i z a t i o n )

b u t k e e p i t s e q u e s t e r e d so t h a t t h e N H g r o u p n e v e r loses r a p p o r t w i t h t h e c o m p l e x . 2

S u c h loss of r a p p o r t w o u l d c o n s t i t u t e t h e first s t e p t o w a r d s c o m p l e t e

detachment

or a q u a t i o n . T h e proposed

i n t e r m e d i a t e h a s t h e s y m m e t r y suggested

b y the activation

p a r a m e t e r s , a n d m a n y r e l a t e d c o n f i g u r a t i o n s w o u l d be a v a i l a b l e t o c o n t r i b u t e t o a high activation entropy.

P o s s i b l y a s i m i l a r d e t a i l e d s e q u e n c e h o l d s for t h e r a c e m i -

z a t i o n of Cr(C 04)3~~ , w h i c h a g a i n i s m u c h f a s t e r t h a n a q u a t i o n , y e t i n v o l v e s t h e 3

2

—C 0

f u n c t i o n p a r t i c i p a t i n g w i t h s o l v e n t t o a degree l e a d i n g t o m a s s i v e O

2

c h a n g e (15).

T h e case of C o ( e n ) ( O H ) 2

zation was fairly concomittant w i t h O Co(en) (H 0) 2

2

2

+ 3

.

2

+

1 3

ex

m a y a l s o be q u i t e s i m i l a r since i s o m e r i z a -

e x c h a n g e (10), a n d e n t i r e l y so i n t h e case of

1 8

T h e r e i s a l s o a close r e s e m b l a n c e of t h e a c t i v a t i o n t h e r m o d y

namics for the C r a n d C o systems. Considering next the photochemistry

of

the doublet

wave

length

region,

i t i s h e l p f u l t o a c c e p t t h e p o s t u l a t e t h a t t h e r e a l l i f e t i m e of t h e E s t a t e ( a c t u a l l y a 2

v i b r o n i c s t a t e i n a l l l i k e l i h o o d (4)) i s l o n g e n o u g h for i t t o f u n c t i o n a s a r e a c t i o n intermediate.

W e n o w h a v e a species a b l e t o b o n d , p e r h a p s w i t h s o m e a c t i v a t i o n ,

t o a s e v e n t h g r o u p (-e.g., u s i n g d , 2

x

d -£, 2

z

and d

xy

orbitals if the s y m m e t r y were

CM) a n d t h e a r r a n g e m e n t s h o w n i n F i g u r e 4c i s m e r e l y a s u g g e s t i o n .

A s Gillespie

n o t e s (8), t h e e l e c t r o s t a t i c p r e d i c t i o n of h e p t a c o o r d i n a t i o n g e o m e t r y i s a m b i g u o u s , a n d t h e a r r a n g e m e n t s h o w n c o u l d be a p e r s p e c t i v e e i t h e r of a p e n t a g o n a l b i p y r a m i d , o r of G i l l e s p i e ' s 1,3,3 a r r a n g e m e n t .

I n e i t h e r case, s m a l l l u x a t i o n s c o u l d p r e p a r e

t h i s i n t e r m e d i a t e t o r e t u r n t o h e x a c o o r d i n a t i o n b y d i s c h a r g e of e i t h e r a n O H , o r




246

Figure 4. (OH)^;

Possible geometries; (a) (b) substitution by SN2FS

cis-Cr(en)2mechanism;

(c) a heptacoordinated intermediate; (d) a consequence of Cr-N bond fission; (e) a consequence of Cr-0 bond fission. N H 2 f r o m t h e c o o r d i n a t i o n sphere.

If the latter, a n intermediate having monoden-

date ethylenediamine w o u l d result, a n d this i n t u r n could proceed

t o t h e aquo

p r o d u c t b y s u b s t i t u t i v e d e t a c h m e n t of t h e o t h e r e n d . T h e r e i s n o p r e s e n t basis f o r d e c i d i n g w h i c h o r h o w m a n y of t h e s e v e r a l i s o m e r s of F i g u r e 4c m i g h t be i n v o l v e d ; b u t w e w o u l d c o n c l u d e t h a t

heptacoordinated

C r ( I I I ) is stable enough to require a c t i v a t i o n to revert i t t o hexacoordination. L a s t l y we w i l l consider w h a t happens w h e n photolysis is occasioned b y light of w a v e l e n g t h c o r r e s p o n d i n g t o one of t h e s p i n - a l l o w e d t r a n s i t i o n s , a n d l o o k a t t h e p o s s i b l e consequences of h o m o l y t i c b o n d

fission.

F o r t h e cis c o m p l e x , t w o p o s

s i b i l i t i e s a r e s h o w n i n F i g u r e 4 d a n d 4e, a s s u m i n g f o r s i m p l i c i t y t h a t a p r o m p t c o l l a p s e t o t r i g o n a l b i p y r a m i d a l f o r m occurs.

If i t is a C r - 0 bond that is broken, then

on regrouping, o n l y isomerization can result, b u t not a q u a t i o n .

If, h o w e v e r , i t i s a

C r - N b o n d t h a t f a i l s , w e a g a i n h a v e a s i t u a t i o n b u t one s t e p r e m o v e d f r o m a q u a t i o n , a n d , we w o u l d suppose, irreversibly c o m m i t t e d . I t i s a l s o possible t o a c c o u n t q u a l i t a t i v e l y f o r t h e greater p h o t o s e n s i t i v i t y of t h e of t h e c i s f o r m , e s p e c i a l l y t o w a r d s a q u a t i o n , t h a n t h e t r a n s .

A s B a l l h a u s e n (4)

r e m a r k s , s i m p l e c r y s t a l field t h e o r y relates t h e e n e r g y p o s i t i o n s of t h e s p i n - a l l o w e d t r a n s i t i o n s t o a t e t r a g o n a l i t y p a r a m e t e r w h i c h d e p e n d s o n s u m s of a x i a l charges.


10.

ADAMSON

Spoctrospecifie

Photolysis

247

F o r t h e cis c o m p l e x , we h a v e t w o 0 - N a n d one N - N t y p e a x i s w h i l e for t h e t r a n s there a r e t w o N - N a n d one 0 - 0 t y p e a x i s .

W e can a r g u e f r o m t h e s p e c t r o c h e m i c a l

series t h a t t h e effective c r y s t a l field a l o n g a n a x i s increases i n t h e o r d e r : 0 - 0 , 0 - N , and N - N .

If, n o w , l i g h t a b s o r p t i o n d i r e c t l y o r e v e n t u a l l y leads t o b o n d w e a k e n i n g

a l o n g t h e w e a k e s t l i g a n d field a x i s , t h e n for t h e cis c o m p l e x , a n 0 - N p a i r w i l l be affected, l e a d i n g t o t h e d i c h o t o m o u s p o s s i b i l i t i e s of F i g u r e 4 d a n d 4e.

I n t h e case

of t h e t r a n s f o r m h o w e v e r , t h e 0 - 0 a x i s w o u l d be affected, a n d c o n s e q u e n t l y t h i s w o u l d lead only to isomerization. I t is t h u s p o s s i b l e t o a c c o u n t for t h e general f r a m e w o r k of o b s e r v a t i o n s .

It

does n o t seem possible, h o w e v e r , t o s p e c i f y a n y u n i q u e i n t e r p r e t a t i o n of t h e a c t i v a tion energy quantities.

F o l l o w i n g l i g h t a b s o r p t i o n , t h e r e c a n be e l e c t r o n i c r e t u r n

t o t h e i n i t i a l species b y 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 ; t h e r e c a n be s o m e c o n v e r s i o n


f r o m one s t a t e t o a n o t h e r .

T h e chemical intermediates m a y retain the original

configuration, m a y iscmerize, a n d m a y aquate.

Possibly solvent exchange studies

w o u l d h e l p r e d u c e the p l e t h o r a of p o s s i b i l i t i e s . W e do conclude, however, that C r ( I I I ) complexes can have a photochemistry w h i c h is spectrospecific as t o m e c h a n i s m , a n d we suspect t h a t w h e r e t h i s a p p a r e n t l y is n o t t h e case, i t is because o n l y one p r o d u c t is p o s s i b l e o r is s o u g h t for.

Acknowledgements T h e t h e r m a l i s o m e r i z a t i o n a n d a q u a t i o n r a t e s t u d i e s were c a r r i e d o u t b y M r . R . S. H a n n a n d M i s s L . B . M a r t i n e z . These investigations have been supported i n p a r t b y C o n t r a c t

AT(11-1)-113

b e t w e e n t h e U n i v e r s i t y of S o u t h e r n C a l i f o r n i a a n d t h e U . S . A t o m i c E n e r g y C o m mission.

Literature

Cited

(1) (2) (3) (4)

A d a m s o n , A. W., Discussion Faraday Soc. N o . 29, 163 (1960), a n d preceding papers. A d a m s o n , A. W., J. Inorg. and Nuclear Chem. 13, 275 (1960). A d a m s o n , A. W., J. Am. Chem. Soc. 80, 3183 (1958). A d a m s o n , A. W., A b s t r a c t s of the 144th M e e t i n g of the A m e r i c a n C h e m i c a l Society, M a r c h 1963. (5) B a l l h a u s e n , " I n t r o d u c t i o n to L i g a n d F i e l d T h e o r y , " p. 107, McGraw-Hill, New Y o r k , 1962. (6) E d e l s o n , R., P l a n e , R. Α., Inorg. Chem. 3, 234 (1964), a n d preceding papers. (7) F e r n e l i u s , W. C., ed., " I n o r g a n i c Syntheses," Vol. II, p. 196, McGraw-Hill, New Y o r k , 1946. (8) G i l l e s p i e , R. J., " A d v a n c e s i n the 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 , " p. 34, MacMillan, N e w Y o r k , 1961. (9) K o r v e z e e , A. E., Rec. trav. chim. 59, 913 (1940). (10) K r u s e , W., T a u b e , H., J. Am. Chem. Soc. 83, 1280 (1961). (11) N i k o l a i s k i , E., Thesis, J o h a n n W o l f g a n g Goethe U n i v e r s i t a t , Frankfurt/M, 1962. (12) O l s o n , D. C., G a r n e r , C. S., Inorg. Chem. 2, 415 (1963). (13) Schläfer, H. L., Kling, O., J. Inorg. Nucl. Chem. 8, 320 (1958). (14) S k r a b e l , Α., Ζ. phys. Chem. 6, 382 (1929). (15) Spees, S. T., A d a m s o n , A. W., Inorg. Chem. 1, 531 (1962). (16) Wegner, E., U n p u b l i s h e d results i n this L a b o r a t o r y . (17) W o l d b y e , F., Acta Chem. Scand. 12, 1079 (1958). RECEIVED

A p r i l 3, 1964.


248


Discussion Arthur

Adamson:

T h e r e has been s o m e d i s c u s s i o n a n d c o m m e n t o n w h a t I

c a l l e d a cage m e c h a n i s m , a n d I w a n t t o s a y i n a l i t t l e m o r e o r g a n i z e d w a y w h a t I t h i n k is i n v o l v e d . F i r s t , I a m not t r y i n g to a t t a c k the t r a n s i t i o n state theory.

What I am aiming

a t is a l i t t l e d i f f e r e n t . T r a n s i t i o n s t a t e t h e o r y i t s e l f is a b e a u t i f u l u n i o n of w a v e m e c h a n i c s i n one respect a n d t h e r m o d y n a m i c s i n t h e o t h e r .

H o w e v e r , i t c a n n o t be a p p l i e d f u l l y t o

t h e s y s t e m s t h a t w e h a v e been d i s c u s s i n g .

I n these c i r c u m s t a n c e s t h e t h e o r y is


used i n a n a p p r o x i m a t e w a y , i t m a y e v e n be u s e d as a language t o p r e s e n t r e s u l t s . V e r y o f t e n t h e e n t r o p y of a c t i v a t i o n t h a t i s r e p o r t e d for a r e a c t i o n i s n o t m u c h m o r e t h a n a f o r m a l t r a n s l a t i o n of d a t a i n a m e c h a n i c a l w a y , a n d i t is d a n g e r o u s t o read all t r a n s i t i o n state i m p l i c a t i o n s i n t o the result. If w e are g o i n g t o t a l k a b o u t a p p r o x i m a t i o n s a n d a p p r o x i m a t e s i t u a t i o n s , t h e n I suggest u s i n g one o r a n o t h e r e m p h a s i s o r p o i n t of v i e w a s a u s e f u l a p p r o x i m a t i o n o r language t h a t r e n d e r s less o b j e c t i o n a b l e

innuendoes.

O n e p r o b l e m , I t h i n k , i n a d e t a i l e d a c c e p t a n c e of s i m p l e t r a n s i t i o n s t a t e t h e o r y r e g a r d i n g s o l u t i o n s y s t e m s c o n c e r n s the c e n t r a l s u p p o s i t i o n t h a t the t r a n s i t i o n state is i n e q u i l i b r i u m w i t h t h e r e a c t a n t s .

If t h e t r a n s i t i o n s t a t e is a species w h i c h p r o

ceeds i r r e v e r s i b l y a n d o n one v i b r a t i o n p e r i o d t o p r o d u c t s , i t is a l i t t l e d i f f i c u l t t o demonstrate, I t h i n k , that this t h e r m o d y n a m i c e q u i l i b r i u m exists.

T h e c o n c e p t of

e q u i l i b r i u m a n d t h e c o n c e p t of a n i r r e v e r s i b l e process a t s o m e p o i n t m u s t be d i s t i n c t . O n e c a n r e t r e a t a s t e p a n d t a l k a b o u t , s a y , a species w h o s e e n e r g y

content

m i g h t be w r i t t e n as A H J m i n u s a s m a l l i n c r e m e n t , t h a t i s , a species b e l o w t r a n s i t i o n s t a t e i n e n e r g y a n d a r g u e t h a t t h i s is n o w i n e q u i l i b r i u m w i t h r e a c t a n t s .

T h e added

e n e r g y t h a t c a r r i e s t h i s t o p r o d u c t s is t h e n c o n s i d e r e d t o be a s m a l l p e r t u r b a t i o n . B u t I t h i n k t h i s is a d e b a t a b l e q u e s t i o n since i t argues t h a t e n e r g y a r r i v e s i n s m a l l increments.

M a y b e i t does, b u t t h i s i s n o l o n g e r a p u r e l y t h e r m o d y n a m i c q u e s t i o n .

I suggest t h a t for c e r t a i n t y p e s of r e a c t i o n s y s t e m s i n s o l u t i o n i t is u s e f u l t o suppose t h a t the energy, if i t does a r r i v e i n increments, m u s t a r r i v e i n a r e l a t i v e l y s h o r t p e r i o d of t i m e c o m p a r e d t o t h e l i f e t i m e of t h e cage. P e r h a p s a n o t h e r w a y of l o o k i n g a t t h i s i s t o c o n s i d e r t h e r e a c t i o n ' s g o i n g i n reverse ( s t a r t i n g w i t h a t r a n s i t i o n s t a t e w h i c h t h e n b r e a k s u p i n t o r e a c t a n t s ) a n d to say t h a t , i n solution a n d perhaps especially w i t h complex ions i n hydrogenb o n d e d s o l v e n t s , t h e r e is g o o d c o m m u n i c a t i o n b e t w e e n t h e m o l e c u l e a n d i t s e n v i r o n m e n t i n a v i b r a t i o n a l sense.

T h e r e f o r e , as a t r a n s i t i o n s t a t e d e c a y s b a c k w a r d s

t o r e a c t a n t s , i t s excess e n e r g y w i l l be lost i n r e a s o n a b l y large j u m p s a n d r e a s o n a b l y quickly.

C o n s e q u e n t l y , t h e r e a c t a n t species w i l l be b a c k *to t h e r m a l energies i n a

t i m e t h a t is c o m p a r a b l e , i f n o t s h o r t e r , t h a n t h e t i m e i t w i l l t a k e t h e m t o diffuse away. I a g a i n e m p h a s i z e t h a t I a m t h i n k i n g p r i m a r i l y of c o m p l e x i o n r e a c t i o n s i n v o l v i n g a f a i r l y large m o l e c u l e , a n d a l o t of i n t e r a c t i o n w i t h t h e s o l v e n t s , u s u a l l y through hydrogen bonding.

I a m n o t t h i n k i n g of s m a l l m o l e c u l e s w h i c h m a y n o t

be so i n t i m a t e l y s o l v a t e d .


10.

ADAMSON

249

Discussion

P e r h a p s a n o t h e r w a y of e x p l a i n i n g w h a t I a m t r y i n g t o e m p h a s i z e is t h a t w h e n r e a e t a n t s c o m e t o g e t h e r I t h i n k p r o b a b l y t h e y w i l l h a v e diffused t o g e t h e r before a c q u i r i n g their a c t i v a t i o n energy. I t i s reasonable t o a s k j u s t w h a t t h e o p e r a t i o n a l m e a n i n g of t h e cage p i c t u r e i s . W h a t differences does i t r e a l l y m a k e ?

If there a r e n ' t a n y , w h y discuss i t ?

T h e first e m p h a s i s here is o n t h e n o t i o n of a p r e a s s e m b l y of r e a e t a n t s . b i m o l e c u l a r r e a c t i o n A a n d Β first diffuse t o w a r d s e a c h o t h e r .

i o n , t h e n B , i f a s o l v e n t species, m u s t first diffuse i n t o t h e s o l v a t i o n s h e l l of A . is a n i o n , t h e s i t u a t i o n w o u l d be d e s c r i b e d as i o n p a i r i n g .

In a

If A i s a c o m p l e x If Β

I n a n y event, A a n d Β

first diffuse i n t o a c o m m o n " c a g e " ; t h e n d u r i n g t h i s p e r i o d of a s s o c i a t i o n , r e a c t i o n w i l l o c c u r i f a sufficient a c c i d e n t a l confluence of e n e r g y occurs.


T h e s e c o n d e m p h a s i s i s t h a t i t i s h e l p f u l a n d suggestive t o s u p p o s e t h a t t h e p r o b a b i l i t y of f o r m i n g a t r a n s i t i o n s t a t e f a v o r a b l e t o t h e r e a c t i o n i s h i g h l y d e t e r m i n e d b y t h e p r o b a b i l i t y t h a t a s i m i l a r b u t n o n a c t i v a t e d c o n f i g u r a t i o n is a v a i l a b l e i n t h e p r e a s s e m b l e d o r " c a g e d " species.

T h u s , i n t h e case of a n a t i o n , t h e stereo

c h e m i s t r y of t h e r e a c t i o n w o u l d be d e t e r m i n e d p a r t l y b y w h e t h e r t h e i n i t i a l i o n p a i r had a favorable configuration.

I n t h e case of s o l v a t i o n i n a m i x e d s o l v e n t , t h e

s o l v e n t d i s t r i b u t i o n i n t h e s o l v a t i o n s h e l l r a t h e r t h a n t h e average

composition

s h o u l d be t h e i m p o r t a n t v a r i a b l e d e t e r m i n i n g r e a c t i o n r a t e a n d o r d e r . S o m e possible consequences of t h i s e m p h a s i s a r e t h e f o l l o w i n g : T h e c o n c e p t of a p r e a s s e m b l y focuses a t t e n t i o n o n t h e a r r a n g e m e n t of p o t e n t i a l r e a e t a n t s i n t h e s o l v e n t cage a n d suggests t h a t there i s n o t m u c h v a l i d i t y i n u s i n g s u c h f o r m a l labels as S j y l , l i m i t i n g o r n o n l i m i t i n g , o r Sjy2 o r t h e c o m b i n a t i o n I h a v e been g u i l t y of—Si\r2FS.

I t i m p l i e s t h a t these labels are t o o r i g i d , t o o c o n s t r i c t i v e ,

a n d t o o o r i e n t e d t o w a r d s gas phase r e a c t i o n k i n e t i c s . A n o t h e r possible effect o r difference is t h a t o r d i n a r i l y t h e r e i s n o r e a l w a y independently to s t u d y the transition state—independently verify AfiTj or A 5 j . B u t i f one supposes, as a useful a p p r o x i m a t i o n , t h a t c o n f i g u r a t i o n s i n t h e cage w i l l i n c l u d e those of i n t e r e s t i n t h e t r a n s i t i o n s t a t e , one c a n n o w s t u d y t h e s o l v a t i o n shell o r t h e i o n p a i r s t r u c t u r e , o r i n o t h e r w o r d s , s t u d y t h e s i t u a t i o n i n t h e cage w i t h o u t r e q u i r i n g i t t o be a c t i v a t e d .

T h i s m e a n s t h a t one c a n s t u d y i o n p a i r

f o r m a t i o n a n d c o n s i d e r t h i s as a useful guide t o s u b s t i t u t i o n r e a c t i o n s a n d h o w t h e y m i g h t proceed.

In a mixed solvent system i n v o l v i n g a substitutive solvation such

as a q u a t i o n , t h e r e a c t i o n cage c o n c e p t suggests t h a t i t w o u l d be h e l p f u l t o s t u d y t h e s o l v e n t c o m p o s i t i o n of the s o l v a t i o n cage. w o u l d n o w be a p p l i c a b l e .

Ordinary physical chemical

methods

W e h a v e been t a k i n g a p a r t i c u l a r c o m p l e x w h o s e a b s o r p

t i o n s p e c t r u m is s o m e w h a t s e n s i t i v e t o s o l v e n t e n v i r o n m e n t a n d l o o k i n g a t t h i s s p e c t r u m i n m i x e d s o l v e n t s t o get a t w h a t seems t o be t h e i m m e d i a t e s o l v e n t e n vironment. A n o t h e r w a y of p i c t u r i n g t h e cage is as a large, loose m o l e c u l e .

I n a sense,

w e a r e s a y i n g a b i m o l e c u l a r r e a c t i o n forms a large, loose m o l e c u l e i n t h e case of c o m p l e x i o n , w h i c h t h e n i s o m e r i z e s ; t h a t i s , t h e i n n e r - o u t e r sphere exchange

is

t h o u g h t of as a n i s o m e r i z a t i o n o r a n i n t r a m o l e c u l a r change. I n t h e case of m i x e d s o l v e n t s , i f t h e t w o s o l v e n t s are a b o u t e q u a l l y g o o d , one m a y s u p p o s e t h a t i n t h e s o l v a t i o n s h e l l there is v e r y l i t t l e f r a c t i o n a t i o n a r o u n d t h e complex.

W i t h i s o m e r i z a t i o n o r i n t r a m o l e c u l a r changes as a v i e w of w h a t i s

o c c u r r i n g , one c a n a r g u e t h a t p e r h a p s j u s t t h e s p a t i a l o c c u p a n c y , o r r o u g h l y t h e



250

v o l u m e f r a c t i o n of t h e s o l v e n t c o m p o n e n t variable.

t h a t w i l l s u b s t i t u t e , m a y be a g o o d

T h e r e a c t i o n r a t e s h o u l d t h e n be p r o p o r t i o n a l t o t h e v o l u m e f r a c t i o n .

I n some cases t h i s p r e d i c t i o n seems t o w o r k o u t . T h i s is one of t h e specific suggestions t h a t arises f r o m t h e r e a c t i o n cage c o n c e p t , w h e r e a s t r a n s i t i o n s t a t e t h e o r y i n i t s s i m p l e a p p l i c a t i o n w o u l d suggest t h a t t h e a c t i v i t y of t h e s o l v e n t c o m p o n e n t , r a t h e r t h a n i t s v o l u m e f r a c t i o n , s h o u l d be t h e proper variable. S t i l l a n o t h e r c o n s i d e r a t i o n is t h a t t h i s p o i n t of v i e w , e s p e c i a l l y w h e r e a t least one of t h e m o l e c u l e s i s large a n d c o m p l e x , l e n d s i t s e l f r e a d i l y t o t h i n k i n g i n t e r m s of m o r e t h a n one r e a c t i o n p a t h .

T h a t i s , i t i s m o r e a p p a r e n t t h a t t h e r e c a n be

v a r i o u s s i m i l a r c o n f i g u r a t i o n s w h o s e o v e r a l l c o n t r i b u t i o n t o t h e r a t e m a y be s i m i l a r ,


a n d hence c o m p e t i t i v e .

Some m a y have a high a c t i v a t i o n energy but represent a

v e r y p r o b a b l e s t a t e of a f f a i r s ; o t h e r s m a y h a v e a l o w a c t i v a t i o n e n e r g y b u t a n i m p r o b a b l e s t a t e of affairs.

I n o t h e r w o r d s , m a n y r e l a t e d r e a c t i o n p a t h s c a n be

present. A s a n i l l u s t r a t i o n , a n d n o t t o d r a w specific c o n c l u s i o n s a b o u t t h e p a r t i c u l a r set of r e a c t i o n s , t h e t a b l e t h a t P r o f . T a u b e s h o w e d ( T a b l e I) i n h i s p a p e r o n r a t e s of c h r o m o u s r e a c t i o n w i t h v a r i o u s p e n t a m m i n e c o b a l t ( I I I ) c o m p l e x e s , p r o v i d e a set of r a t e c o n s t a n t s t h a t a r e v e r y s i m i l a r , w i t h i n a f a c t o r of t w o o r t h r e e . energies suggest t h a t t h e r a t e s s h o u l d v a r y a m i l l i o n - f o l d . e n t r o p i e s of a c t i v a t i o n . each other.

Yet, activation

T h e same i s t r u e for t h e

S o m e h o w these t w o f a c t o r s h a v e m a n a g e d j u s t t o b a l a n c e

T h i s is t o o m o n u m e n t a l a c o i n c i d e n c e t o be o n l y t h a t .

T h e r e m u s t be

a n explanation a n d perhaps we can draw a n analogy from the following situation i n catalysis.

F o r a particular reaction and a particular catalyst, but activated to

d i f f e r e n t degrees, one o f t e n o b s e r v e s t h a t t h e r e a c t i o n proceeds a t a b o u t t h e same r a t e for t h e v a r i o u s l y a c t i v a t e d c a t a l y s t s , b u t w i t h q u i t e different a c t i v a t i o n energies.

T h i s w a s c o n s i d e r e d i n d e t a i l b y surface c h e m i s t s s o m e t i m e a g o a n d t h e y

p r o p o s e d a d i s t r i b u t i o n of sites o n t h e c a t a l y s t ; those t h a t a r e m o r e a c t i v e a n d reduce t h e a c t i v a t i o n e n e r g y are fewer i n n u m b e r so t h e r e i s a c o m p e n s a t i o n of f r e q u e n c y a n d a c t i v a t i o n factors. I a m s u g g e s t i n g t h a t o f t e n t h e a p p l i c a b i l i t y of B a r k l e y - B u t l e r t y p e p l o t s , t h a t is, t h e l i n e a r r e l a t i o n s h i p b e t w e e n t h e e n t r o p y a n d e n t h a l p y of a c t i v a t i o n i n a series m a y c o m e a b o u t because of t h e r e b e i n g a d i s t r i b u t i o n of r e a c t i o n p a t h s .

Small

v a r i a t i o n s i n t h e i m p o r t a n c e of l o w a c t i v a t i o n e n e r g y , l o w p r o b a b i l i t y p a t h s c o u l d t h e n a c c o u n t for t h e d a t a i n D r . T a u b e ' s t a b l e .

B y contrast, transition state

t h e o r y i n i t s a p p r o x i m a t e a p p l i c a t i o n , i n v a r i a b l y l e a d s t o d i a g r a m s of e n e r g y vs. r e a c t i o n p a t h w h i c h , i n s p i t e of a l l p r o t e s t , one r e a c t i o n p a t h , w h a t e v e r i t is, one t r a n s i t i o n s t a t e , a n d one e n e r g y . A g a i n let me say I a m proposing a shift i n emphasis a n d a n a p p r o x i m a t i o n to d e a l w i t h w h a t is i n e v i t a b l y a s t a t e of a p p r o x i m a t i o n .

I a m not attempting to

k n o c k d o w n t h e p i l l a r s of t r a n s i t i o n s t a t e t h e o r y ( F i g u r e A ) . O v e r t h e p a s t s e v e r a l y e a r s w e h a v e been i n t e r e s t e d i n d e t e r m i n i n g t o w h a t e x t e n t t h e p h o t o c h e m i s t r y of c o m p l e x i o n s of v a r i o u s t r a n s i t i o n m e t a l i o n s r e s e m b l e t h e r m a l r e a c t i o n c h e m i s t r y as t o p r o d u c t s , a n d t o w h a t e x t e n t t h e b e h a v i o r v a r i e s w i t h t h e w a v e l e n g t h o r t y p e of e x c i t e d s t a t e p r o d u c e d . I n t h e case of t h e c o b a l t c o m p l e x e s , a n d i n p a r t i c u l a r a series of a c e t o p e n t a m m i n e s , g e n e r a l l y i r r i d a t i o n i n t h e w a v e l e n g t h v i c i n i t y of a c h a r g e t r a n s f e r b a n d l e d


10.

ADAMSON

251

Discussion

F i r s t Stage CoA5X+

2

-+

[Co(II)A*—X]+

4-

2

Δ

M — X b o n d weakened, v i b r a t i o n a l energy, Δ , dissipated Second State [Co(II)Ar-X]

^

+ 2

k

t

CoAfiX+

+

2

Δ'

atom returns with further Δ ' dissipated CoAfi+ ~-H 0—X 2

2

i n t e r p o s i t i o n of s o l v e n t


Figure A.

Suggested mechanism

for photolysis

of Co (111)

complexes

t o a g o o d d e a l of r e d o x d e c o m p o s i t i o n — i . e . , o x i d a t i o n of l i g a n d a n d r e d u c t i o n of t h e cobalt.

A c c o m p a n y i n g t h i s w a s o f t e n a good d e a l of s u b s t i t u t i o n , b u t c o m p e t i t i v e

t o t h e r e d o x m o d e of r e a c t i o n . homolytic bond

fission,

W e p r o p o s e d i n 1957 t h a t t h e p r i m a r y a c t w a s a

a n d t h a t either a redox or a n a q u a t i o n reaction ensued.

T h i s d e p e n d e d o n w h e t h e r t h e f r a g m e n t s escaped c o m p l e t e l y o r w h e t h e r , before c o m p l e t e escape, a n e l e c t r o n r e t u r n e d t o reverse t h e r e d o x p a r t b u t s t i l l a l l o w t h e a c i d o g r o u p t o escape.

O n t h e o t h e r h a n d , i r r a d i a t i o n , i n t h e case of these c o b a l t

c o m p l e x e s , of a p u r e r l i g a n d field b a n d , l e d t o v e r y l o w q u a n t u m y i e l d s a n d a q u a tion only.

T h u s , t h e choice of w a v e l e n g t h o r e s s e n t i a l l y t h e choice of t y p e of b a n d

t o i r r a d i a t e m a d e a c o n s i d e r a b l e difference as t o t h e p h o t o c h e m i s t r y . T h e g e n e r a l o b s e r v a t i o n i n t h e p u b l i s h e d w o r k has been t h a t for species s u c h as h e x a m m i n e c h r o m i u m , thiocyanatopentammine, or the hexa-aquo i o n where

O

1 8

exchange w a s l o o k e d a t , i r r a d i a t i o n p r o d u c e d a s u b s t i t u t i o n r e a c t i o n a n d n o t h i n g else.

M o r e o v e r t h e r e a c t i o n m o d e w a s i n d e p e n d e n t of w a v e l e n g t h , a n d t h e q u a n

t u m yields d i d not change m u c h .

F r o m a m o r p h o l o g i c a l p o i n t of v i e w , t h e r e are

e s s e n t i a l l y three t y p e s of e x p l a n a t i o n s .

F i r s t a l l e x c i t e d states i n d e p e n d e n t l y l e a d

t o t h e s a m e c h e m i c a l sequence, a n d w e suppose t h a t t h e p r i m a r y a c t is s i m p l y a heterolytic bond

fission.

Second, the excited quartet states convert to the doublet state w h i c h t h e n acts as a c o m m o n c h e m i c a l i n t e r m e d i a t e .

T h i s a t t r a c t i v e suggestion, o w i n g p a r t l y t o

t o P l a n e a n d p a r t l y t o S c h l a f e r , is p l a u s i b l e because t h i s c o n v e r s i o n i s k n o w n t o occur i n rigid media. H o w e v e r , a t h i r d p o s s i b i l i t y i s t h a t v a r i o u s e x c i t e d states a r e p o t e n t i a l l y different i n t h e i r p h o t o c h e m i s t r y , b u t t h a t t h e s y s t e m s c h o s e n for s t u d y d i d n ' t h a v e a n y m e a n s of r e f l e c t i n g t h i s difference. T h e p r e s e n t p a p e r i s c o n c e r n e d w i t h c h o o s i n g s y s t e m s w h i c h reflect s u c h differences i f t h e y e x i s t .

F i g u r e Β r e p r o d u c e s t h i s s u m m a r y of r e s u l t s .

T h e c o m p l e x c h o s e n for s t u d y w a s a * 5 - d i h y d r o x y b i s ( e t h y l e n e d i a m i n e ) c h r o m i u m (III).

T w o t y p e s of s u b s t i t u t i o n r e a c t i o n s c a n o c c u r h e r e : i s o m e r i z a t i o n a n d a q u a

tion.

W e n o w a r e a b l e t o o b s e r v e r e a c t i o n r a t i o s as w e l l as o v e r a l l q u a n t u m y i e l d s .

W e d o see s o m e differences w i t h w a v e l e n g t h .

T h e m o s t s t r i k i n g is t h a t w h i c h

o c c u r s i n t h e q u a n t u m y i e l d for t h e a q u a t i o n r e a c t i o n . i n d e p e n d e n t for s h o r t w a v e - l e n g t h i r r a d i a t i o n s .

T h i s is nearly temperature

A t the longer-wave lengths where

a t l e a s t a f a i r p r o p o r t i o n of l i g h t i s b e i n g a b s o r b e d i n t o t h e d o u b l e t t r a n s i t i o n , i t i s quite temperature dependent.

T h e isomerization quantum yields given b y


the

MECHANISMS OF INORGANIC

252

REACTIONS

1.01 Aquation


ο,ο,ι

triangles show rather similar behavior at a l l the wave lengths b u t w i t h a gradually i n c r e a s i n g t e m p e r a t u r e d e p e n d e n c e as t h e w a v e l e n g t h is d e c r e a s e d . I t is i m p o s s i b l e t o s u p p o s e t h a t , i n t h e case of p h o t o l y s i s a t t h e s h o r t e r w a v e lengths, aquation a n d isomerization are competitive towards a c o m m o n precursor. T h e a r g u m e n t is a q u a n t i t a t i v e one c o n c e r n i n g t h e h i g h q u a n t u m y i e l d s for b o t h processes.

T h e a l g e b r a of a c o m p e t i t i v e r e a c t i o n scheme c a n n o t e x p l a i n t w o p r o c

esses, one w i t h a h i g h t e m p e r a t u r e coefficient a n d one w i t h a n a p p a r e n t z e r o - t e m p e r a t u r e coefficient, i f b o t h o c c u r i n c o m p a r a b l e y i e l d s .


10.

ADAMSON

Discussion

253

I p r o c e e d i n t h e p a p e r t o s p e c u l a t e a b o u t i n t e r m e d i a t e s t h a t π i g h t be p r o d u c e d . T h e s e are e s s e n t i a l l y s p e c u l a t i o n s , a f r a m e for d i s c u s s i o n .

I w i l l leave t h a t aspect

o p e n for q u e s t i o n s . Harry Gray: chemistry.

Photochemistry

is

a

new

and

exciting

area i n inorganic

I a m sure t h e e x p e r t s i n t h e field, P r o f . A d a m s o n , P r o f . P l a n e , a n d o t h e r s

here, w i l l agree t h a t t h e progress i n i n o r g a n i c p h o t o c h e m i s t r y t h u s f a r has been a d o g ' s d i n n e r c o m p a r e d t o o r g a n i c p h o t o c h e m i s t r y s i m p l y because we h a v e a t t r a c t e d fewer p e o p l e i n t o t h i s n e w

field.

I t h i n k i t is g o i n g t o be m o r e c o m p l i c t e d t h a n t h e

o r g a n i c p h o t o c h e m i s t r y since o u r s y s t e m s are c o n s i s t e n t l y m o r e i n t e r e s t i n g a n d intricate. I h a v e s o m e of o u r r e s u l t s t o p r e s e n t . problems i n this area.

A l s o , I w o u l d like to p o i n t out some

T h e r e are three p r o b l e m s t h a t I t h i n k m u s t be s o l v e d before


we c a n d o a n y s i g n i f i c a n t p h o t o c h e m i s t r y . T h e first is t h e q u e s t i o n of m o n o c h r o m a t i c l i g h t .

Monochromatic light will

h a v e t o be a v a i l a b l e , I t h i n k , a t l e a s t =b 1 0 0 A . , o r e v e n less. T h e s e c o n d p r o b l e m is t h e i d e n t i f i c a t i o n of a b s o r p t i o n b a n d s , a p r o b l e m t h a t P r o f . A d a m s o n m a k e s c l e a r i n t h e first page of h i s p a p e r .

I n discussing transitions

t o t h e v a r i o u s d-d e x c i t e d states i n o c t a h e d r a l c h r o m i c c o m p l e x e s , he uses t h e u s u a l octahedral designations.

T h e s e d e s i g n a t i o n s serve t o i d e n t i f y these b a n d s l o o s e l y .

B u t we m u s t do considerably better t h a n this.

T h u s , the second

p r o b l e m is

certainly to identify the absorption bands i n considerably more detail. T h e t h i r d p r o b l e m i s t o s t u d y s y s t e m a t i c a l l y t h e v a r i o u s t y p e s of t r a n s i t i o n s i n v o l v e d i n m e t a l complexes.

F i r s t , i f one w a n t s t o s t u d y t h e p h o t o c h e m i s t r y of

d-d b a n d s he s h o u l d n o t s t u d y t h e p h o t o c h e m i s t r y of d-d b a n d s o v e r l a p p e d w i t h charge transfer bands.

T h a t i s , h e s h o u l d select s y s t e m s w h i c h h a v e d-d b a n d s

clearly separated from the other transitions. transfer.

S e c o n d , t h e r e a r e t w o t y p e s of c h a r g e

L i g a n d - t o - m e t a l c h a r g e t r a n s f e r i n t h e h a l i d e s is one.

Systems should

be selected i n w h i c h t h e L —> M b a n d s a r e i s o l a t e d f r o m t h e o t h e r s , a n d one s h o u l d s t u d y t h e q u a n t u m y i e l d s as a f u n c t i o n of w a v e l e n g t h . t r a n s f e r s y s t e m s are t h i r d .

M e t a l - t o - l i g a n d charge

T h i s h a s n ' t been stressed u p t o t h i s p o i n t .

Metal

c a r b o n y l s , a c e t y l a c e t o n a t e s , a n d m e t a l n i t r o s y l s a r e i n t e r e s t i n g cases i n w h i c h one can isolate the metal-to-ligand charge

transfer bands

from the other

bands.

F i n a l l y , t h e l i g a n d - l i g a n d t r a n s i t i o n s are t y p e s t h a t t h e o r g a n i c c h e m i s t s h a v e investigated thoroughly. O n t h e second p r o b l e m t h e p e o p l e i n m y a r e a a r e r e s p o n s i b l e for t h e p o o r s t a t e of a f f a i r s .

I t a k e m o s t r e s p o n s i b i l i t y because o u t s i d e of o c t a h e d r a l a n d t e t r a h e d r a l

complexes no complete assignments have been made.

B u t , we do have new results,

a n d I t h i n k i t is of s o m e i n t e r e s t t o p r e s e n t t h e m .

Cooper Langford and I at

C o l u m b i a n o w h a v e c o n c l u s i v e r e s u l t s o n t h e e n e r g y levels i n t h e d i s t o r t e d o c t a h e d ral cobalt compounds.

T h e s e are t h e 0 ) ( Ν Η 3 ) δ Χ

+ 2

complexes.

T h e s p e c t r a of these c o m p l e x e s were c a r e f u l l y w o r k e d o u t b y L e n h a r t a n d W e i g e l some t i m e ago. D r . L a n g f o r d a n d I d e c i d e d t o l o o k a t these s p e c t r a a g a i n .

W e remeasured

t h e s p e c t r u m of C o C N H a ^ F " * " because t h i s is t h e one i n w h i c h t o r e s o l v e t h e f o u r t h 2

d-d b a n d .

T h e b a n d c o m e s o u t n i c e l y a n d we d i d n ' t e v e n need a G a u s s i a n a n a l y s i s .

A G a u s s i a n a n a l y s i s c h e c k e d o u t L e n h a r t a n d W e i g e l ' s first three b a n d s v e r y n i c e l y a n d t h u s we h a v e r e s o l v e d i n one case, o u t s i d e of o c t a h e d r a l a n d t e t r a h e d r a l s y m m e t r y , a l l of t h e s p i n - a l l o w e d b a n d s .

T h e b a n d f r o m xy t o x?-y? s h o u l d h a v e t h e


254


ΔΠ

Ε

>


Αι

I yz

I xx

I

*

»»lj t

I x*-y*

Figure C. Energy levels in the distorted octahedral cobalt complexes same energy as i n C o ( N H ) 3

6

+ 3

because t h e h a l i d e s u b s t i t u t i o n is i n a p o l e p o s i t i o n .

T h e r e a r e t w o a l l o w e d b a n d s , t h e t w o E ' s , a n d t w o f o r b i d d e n ones, A2 a n d B2. T h e p o l a r i z a t i o n s h a d been d o n e a n d i n t e r p r e t e d b y C a r l B a l l h a u s e n a n d M o f f i t t , a n d a l l t h a t r e m a i n e d w a s t o get t h e f o u r t h b a n d .

W e have done the complete

t h e o r e t i c a l p r o b l e m as w e l l as p o s s i b l e i n s t r o n g field w i t h c o m p l e t e c o n f i g u r a t i o n i n t e r a c t i o n , a n d we n o w h a v e , I t h i n k , t h e c o r r e c t o n e - e l e c t r o n o r d e r i n g for t h e halopentammines. I t t u r n s o u t t h a t z — a l i t t l e s u r p r i s i n g — i s s u b s t a n t i a l l y a b o v e x —y . 2

2

xz—yz l e v e l i s f u r t h e r a b o v e xy, h o w e v e r , a n d t h e n e t effect places t h a n NH3 i n the spectrochemical

series.

2

fluoride

The lower

F l u o r i d e gives s t r o n g e r σ - a n t i b o n d i n g

b u t even stronger ir-antibonding a n d thus net reduction. F o r C o ( N H ) F + , Δττ 3

6

2

=

3450/cm., Δ σ

T h a t i s , z i s 2400 w a v e n u m b e r s a b o v e x —y . 2

2

2

«

2400/cm.

T h e chloropentammine,

aw i s

4 0 0 0 / c m . t h e b r o m o p e n t a m m i n e , aw is 4 5 5 0 / c m . , a n d t h e i o d o p e n t a m m i n e aw is 5550/cm. Dr. Adamson:

O n e c a n get IOOA. r e s o l u t i o n w i t h i n t e r f e r e n c e

filters.

We

use t h e m m a i n l y because of t h e i r h i g h l i g h t i n t e n s i t y , b u t we d o h a v e d e t a i l e d d a t a b y w a v e l e n g t h r e g i o n o n one c h r o m i u m c o m p o u n d , a n d we d o see s m a l l d i p s a n d changes i n the q u a n t u m yields.

I a m r e f e r r i n g t o some w o r k b y D r . B e g n e r i n o u r

department. T h e d e v e l o p m e n t of t h e o r y is b e a u t i f u l , a n d I w o u l d s a y t h a t t h e o r e t i c i a n s h a v e t o be p r o m p t e d b y e x p e r i m e n t a l r e s u l t s . I s h o w e d , a t a F a r a d a y d i s c u s s i o n i n 1960, a slide d e p i c t i n g a s p e c t r u m of t h e fluoropentammine

w h i c h showed the s m a l l peak.

W e weren't alert enough theoreti

c a l l y t o see i t s significance. I h a v e one final c o m m e n t o n t h e c h r o m i u m s y s t e m . state, a doublet Γ

2 α

T h e r e is another doublet

t h a t is supposed to be present a n d u s u a l l y h i d d e n b y the q u a r t e t -

quartet transitions.


10.

ADAMSON

Discussion

255

P o s s i b l y s o m e of o u r r e s u l t s i n v o l v e c o n v e r s i o n t o t h a t s t a t e .

D o you have a n y

comment? Robert Plane:

I w o u l d l i k e t o c o m m e n t o n t h e p h o t o c h e m i s t r y of these c o m

p l e x e s , p a r t i c u l a r l y t h e c h r o m i u m w h i c h I t h i n k a r e w e l l c h o s e n for a t l e a s t t w o of D r . G r a y ' s reasons.

F i r s t , i n t h e case of c h r o m i u m , u n l i k e c o b a l t , t h e

charge

t r a n s f e r b a n d i s w e l l s e p a r a t e d , so t h a t one c a n s t u d y t h e d-d t r a n s i t i o n s , a t l e a s t i n c e r t a i n s y s t e m s w h e r e one has s i x l i g a n d s a l l a l i k e , a n d t h e r e i s n o J a h n - T e l l e r s p l i t t i n g , a n d one h a s a f a i r l y g o o d i d e a as t o t h e a s s i g n m e n t of b a n d s . T h e first t h e o r y t o a c c o u n t for p h o t o c h e m i s t r y i n these series w a s t h a t b y D r . A d a m s o n w h i c h p o s t u l a t e s t h e b o n d fission as t h e p r i m a r y a c t .

Simultaneously,

H . L . S c h l a f e r (9) a n d J o h n H u n t a n d I (8) p r o p o s e d t h e i n v o l v e m e n t of t h e d o u b l e t Ε state.

A l t h o u g h these s e e m e d s i m i l a r , t h e y were r a d i c a l l y d i f f e r e n t suggestions.


Schlafer's p o i n t was t h a t on i r r a d i a t i o n the molecule found itself u l t i m a t e l y i n the spin-forbidden state where the energy was stored.

O n releasing energy d u r i n g

r e t u r n from the doublet Ε state to the ground state, reaction occurred.

Our

postulate also i n v o l v e d the s p i n d o u b l e t state w h i c h is a spin-forbidden state.

But

our p o i n t was that the reaction occurred while the molecule was i n the doublet state. I n other words, we s a i d t h a t b y i r r a d i a t i n g the s y s t e m we are s t u d y i n g the c h e m i s t r y of a n e x c i t e d s t a t e of c h r o m i u m .

I think Prof. Adamson's d a t a have shown very

w e l l t h a t t h e t h e r m a l r e a c t i o n a n d t h e p h o t o c h e m i c a l r e a c t i o n are d i f f e r e n t t h i n g s . T h e y l e a d t o d i f f e r e n t p r o d u c t s , a n d t h e t e m p e r a t u r e coefficients a r e v e r y d i f f e r e n t . S u c h r e s u l t s are c o m p l e t e l y c o n s i s t e n t w i t h t h e p o s t u l a t e of r e a c t i o n i n t h e e x c i t e d state.

H o w e v e r , t h i s p o s t u l a t e d o e s n o t e x c l u d e t h e b o n d fission p o s t u l a t e .

If y o u

l i k e , w e a r e t r y i n g t o d e c i d e w h a t species i s r e a c t i n g : i s i t t h e d o u b l e t m o l e c u l e o r t h e ground state molecule? finally

A l s o w h a t i s t h e m e c h a n i s m b y w h i c h t h e p r o p e r species

reacts? I w o u l d like t o c o m m e n t on the statement i n D r . A d a m s o n ' s paper w h i c h says

t h a t o u r w o r k as a f u n c t i o n of w a v e l e n g t h i s t r i v i a l i f we o n l y k n e w a l l t h e f a c t s . Perhaps this is true.

B u t I a m n o t w i l l i n g t o a d m i t t h a t i t i s t r i v i a l o n t h e b a s i s of

a n y fact that I k n o w yet.

F o r the H 2 O

1 8

e x c h a n g e w i t h C r ( H 2 0 ) e , one c a n i r r a d i + 3

ate a l l three a b s o r p t i o n bands separately, a n d the q u a n t u m yields are the same (8).

T h i s is r e m i n i s c e n t of w h a t i n v a r i a b l y h a p p e n s i n o r g a n i c

w h e n one excites t o a n y of s e v e r a l s p i n - a l l o w e d s t a t e s .

photochemistry

T h e molecule immediately

converts i n t e r n a l l y t o the lowest state, a n d a n y t h i n g t h a t is interesting happens f r o m the lowest.

H e n c e , i t d o e s n ' t m a t t e r w h i c h s t a t e i t goes t o

first.

W e b e l i e v e i t goes

n e x t i n t o t h e d o u b l e t s t a t e a n d so w e d i d t h e s e c o n d p a r t of t h e e x p e r i m e n t .

This

c o u l d n o t be d o n e w i t h t h e h e x a q u o case because t h e s p i n d o u b l e t o v e r l a p s w i t h one of t h e s p i n - a l l o w e d s t a t e s .

Instead we used C r ( N H ) 6 3

+ 3

where we can i r r a d i a t e

the doublet directly a n d p u t the molecule i n t o this state.

W l i e n we d o so, o u r

q u a n t u m y i e l d c l i m b s to u n i t y , w h i c h I t h i n k is significant ( i ) .

I n t h i s case t h e

evidence is good t h a t molecules are r e a c t i n g i n their excited states. Dr. Adamson:

T h i s is a c o r r e c t i o n .

I d i d n ' t s a y y o u r r e s u l t s were t r i v i a l .

I s a i d t h a t b y h a v i n g t h e m a t o n l y one t e m p e r a t u r e p o s s i b l y t h e o r d e r i n g of t h e q u a n t u m yields w i t h wave length led to a conclusion t h a t was not general.

At

a n o t h e r temperature y o u m i g h t have found a different order. Dr. Plane:

W e found them temperature independent.

Cooper L a n g f o r d :

M y comment

is not on

photochemistry,

but on

A d a m s o n ' s r e m a r k s o n cage aggregates a n d t h e t r a n s i t i o n s t a t e t h e o r y .


Prof.

256

M E C H A N I S M S O F I N O R G A N I C REACTIONS I s y m p a t h i z e w i t h t h e p o i n t of v i e w he i s a d v o c a t i n g , a n d I h o p e t h a t h i s s u g

gestions w o u l d n o t be c o n f u s e d

b y m i s c o n s t r u i n g t r a n s i t i o n state t h e o r y .

He

singles o u t large m o l e c u l e s y s t e m s i n c o n d e n s e d phases as places w h e r e t h e t r a n s i t i o n s t a t e t h e o r y w o u l d be i n t r o u b l e . M y u n d e r s t a n d i n g of t r a n s i t i o n state t h e o r y is t h a t t h i s is w h e r e t h e a p p r o x i m a t i o n s a r e m o s t l i k e l y t o be a c c u r a t e , e s p e c i a l l y i f t h e a c t i v a t i o n e n e r g y is l a r g e .

The

transition state theory, from the detailed statistical viewpoint, requires t h a t a l l degrees of f r e e d o m b u t one be i n e q u i l i b r i u m .

T h i s is most readily obtained i n

a c o n d e n s e d phase w h e r e e n e r g y t r a n s f e r is r a p i d . Dr. Adamson:

Y o u have a point there.

I t h i n k t h a t i n t h e large m o l e c u l e

s y s t e m , for e x a m p l e p e n t a m m i n e h a l i d e a n d w a t e r , t h e r e w i l l be m o r e t h a n one r e a c


t i o n p a t h a n d n o t a single t r a n s i t i o n s t a t e .

I t i s h e l p f u l t o c o n s i d e r t h i s as a k i n d of

intramolecular rearrangement. Robert Connick:

S i n c e t h e s u b j e c t has been r a i s e d , I w o u l d l i k e t o c o m m e n t

o n this general p r o b l e m . I agree w i t h l o o k i n g a t these m a t t e r s i n different w a y s .

A s I understand it,

y o u do not object to the t r a n s i t i o n state t h e o r y f o r m u l a t i o n .

T h e one w o r d of

c a u t i o n I w o u l d l i k e t o i n t e r j e c t , t h o u g h , i s t h i s : y o u s p o k e of t h e caged

species

before i t h a d r e a l l y r e a c h e d t h e a c t i v a t e d c o m p l e x c o n f i g u r a t i o n , a n d y o u s p o k e of this concept's being particularly valuable. I n a f o r m a l i s t i c sense t h e r a t e is c o n t r o l l e d , a c c o r d i n g t o t h e t r a n s i t i o n state t h e o r y , b y t h e s t a b i l i t y , c o n c e n t r a t i o n , a n d so o n of t h e a c t i v a t e d c o m p l e x . the k e y configuration.

T h i s is

T h i s t h e o r y says t h a t a l l o t h e r c o n f i g u r a t i o n s m a y be i n

teresting b u t t h e y don't really count w h e n i t comes to determing the rate.

There

fore, one is a l w a y s i n d a n g e r of i n c o r r e c t p r e d i c t i o n s a b o u t a r e a c t i o n r a t e i f one deliberately strays from the activated complex configuration.

I do not mean to say

t h a t one w i l l a l w a y s e r r , because i n m a n y r e a c t i o n s t h e c o n f i g u r a t i o n of t h e s y s t e m as i t a p p r o a c h e s t h e a c t i v a t e d c o m p l e x i s v e r y s i m i l a r t o w h a t i t is i n t h e a c t i v a t e d complex.

I n f a c t , one believes t h a t i n m o s t of these cases i t is t h e a c c u m u l a t i o n of

e n e r g y i n p a r t i c u l a r b o n d s r a t h e r t h a n changes i n g e o m e t r y w h i c h a r e o c c u r r i n g i n t h e final a c t i v a t i o n .

N e v e r t h e l e s s , s u c h a s i t u a t i o n does n o t a l w a y s e x i s t , a n d

p a r t i c u l a r l y i f one t a l k s a b o u t w h a t caged species w o u l d f o r m p r e f e r e n t i a l l y i n a solution.

O f t e n the c o n f i g u r a t i o n w h i c h is t h e r m o d y n a m i c a l l y preferable w i l l n o t

be t h e one w h i c h c o r r e s p o n d s t o t h e a c t i v a t e d c o m p l e x . r e a s o n w h y t h e y s h o u l d be i d e n t i c a l .

I n m a n y cases t h e r e i s n o

I t m u s t be r e m e m b e r e d t h a t t h e a c t i v a t e d

c o m p l e x is a n u n s t a b l e species, n o t a s t a b l e one.

I t is f o r m e d w i t h a great e x p e n d i

t u r e of e n e r g y a n d therefore, one m u s t n o t p o s t u l a t e p r o p e r t i e s a b o u t i t b a s e d o n g e n e r a l ideas of o v e r a l l s t a b i l i t y . Dr. Adamson: i n t h i s respect. state.'

I t is possible t h a t D r . C o n n i c k a n d I h a v e different v i e w p o i n t s

H e referred r e p e a t e d l y t o ' the a c t i v a t e d c o m p l e x ' a n d ' the t r a n s i t i o n

I h a v e s a i d , t h a t I feel i n these t y p e s of r e a c t i o n s one m a y h a v e a d i s t r i b u

t i o n of p a t h s t h a t a r e c o n t r i b u t i n g c o m p a r a b l y .

T h e y are n o t t e r r i b l y different,

b u t e n o u g h so t h a t i t i s d a n g e r o u s t o c o m p r e s s ones t h o u g h t s t o " t h e " t r a n s i t i o n state a n d " t h e " a c t i v a t e d complex.

I suspect

t h a t these

B a r k l e y - B u t l e r se

q u e n c e s m a y i n d i c a t e a d i s t r i b u t i o n of r e a c t i o n p a t h s . Edward K i n g :

O f course, i f these different t r a n s i t i o n states h a v e

composition, i t appears i n the rate law.


different

10.

ADAMSON

Discussion

Dr. Adamson:

257

L e t ' s go b a c k t o t h e i n t r a m o l e c u l a r r e a r r a n g e m e n t p i c t u r e .

T h e c o m p o s i t i o n of the t r a n s i t i o n s t a t e is a l w a y s t h e s a m e . I w a n t t o pose a q u e s t i o n .

Daniel Leussing: p e r i m e n t a l access?

Is t h i s p i c t u r e o p e n t o e x

I n o t h e r w o r d s , i f t h e r e i s a s p e c t r u m of t r a n s i t i o n states w i t h

different h e a t s , w i l l w e n o t get t h e a p p e a r a n c e of a h e a t c a p a c i t y of t h e t r a n s i t i o n s t a t e ; w h e r e a s i f t h e m o d e l s h o w e d a single t r a n s i t i o n state p r e d o m i n a t i n g w o u l d n o t t h e h e a t of r e a c t i o n be c o n s t a n t ?

W o u l d n ' t t h i s a l l o w us t o a n s w e r t h e q u e s

tion. D r . Adamson:

I f t h e r e is a d i s t r i b u t i o n of r e a c t i o n p a t h s , t h e n t h e a p p a r e n t

a c t i v a t i o n e n e r g y s h o u l d i n d e e d change w i t h t e m p e r a t u r e , a n d t h e effect w o u l d a p p e a r as a h e a t c a p a c i t y of a c t i v a t i o n .

H o w e v e r , i t does n o t seem possible

to

d i s t i n g u i s h t h i s s i t u a t i o n f r o m t h a t of a single r e a c t i o n p a t h w h e r e t h e t r a n s i t i o n


s t a t e h e a t c a p a c i t y i s different f r o m t h a t of t h e r e a c t a n t s .

T h a t is to say, the formal

t h e r m o d y n a m i c s w o u l d be i d e n t i c a l for t h e t w o cases. Gordon Atkinson:

I t h i n k t h e r e i s , a t least i n p r i n c i p l e , a w a y t o e x a m i n e a

s y s t e m for d i s t r i b u t i o n of v e r y s i m i l a r p a t h s .

T h i s is b y relaxation technique, p a r

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

But

there is a s u b t l e e x p e r i m e n t a l p r o b l e m i n v o l v e d i n d i s t i n g u i s h i n g b e t w e e n a single r e l a x a t i o n t i m e a n d a r a t h e r c l o s e l y s p a c e d d i s t r i b u t i o n of r e l a x a t i o n t i m e s . Ronald Archer:

W i t h r e g a r d t o base h y d r o l y s i s , I t h i n k t h e r e is some stereo

c h e m i c a l e v i d e n c e t h a t suggests b o t h sides are p r o b a b l y s o m e w h a t c o r r e c t .

There

is e v i d e n c e of the t y p e w h i c h J o h n B a i l a r has been g i v i n g us for a b o u t 30 y e a r s r e g a r d i n g t h e so c a l l e d D —• L i n v e r s i o n s of c o n f i g u r a t i o n (2, J , 4).

F r e d B a s o l o has

suggested s e v e r a l t i m e s t h a t one does n o t see t h i s i n v e r s i o n b y a s i m p l e S j v l t y p e process (6, 7).

O n t h e o t h e r h a n d , s o m e of t h e w o r k I h a v e been d o i n g i n l i q u i d

a m m o n i a suggests t h a t n o t o n l y does one see t h e i n v e r s i o n of c o n f i g u r a t i o n , b u t a l s o t h e c o n j u g a t e base p a t h (2).

T h e r e f o r e , I t h i n k one m u s t h a v e a p r e o r i e n t a t i o n

p l u s a n i n t e r m e d i a t e t h a t is p r o b a b l y b a s i c a l l y a n S ^ l t y p e ; n o , I w o n ' t s a y S j y l — l e t ' s s a y there is p o s s i b l y a

Literature

five-coordinate

i n t e r m e d i a t e of s o m e s o r t i n t h e m i d d l e .

Cited

(1) A r c h e r , R. D., "Proceedings E i g h t h I n t e r n a t i o n a l Conference o n C o o r d i n a t i o n C h e m i s t r y , V . G u t m a n n , ed., p p . 111-114, S p r i n g e r - V e r l a g , V i e n n a , 1964. (2) B a i l a r , Jr., J. C., A u t e n , W., J. Am. Chem. Soc. 56, 774 (1934). (3) B a i l a r , Jr., J. C., H a s l a m , J. H., Jones, Ε. M., J. Am. Chem. Soc. 58, 2226 (1936). (4) B a i l a r , Jr., J. C., J. Am. Chem. Soc. 86, 3656 (1964). (6) Basolo, F r e d , " 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 , J. C. B a i l a r , ed., C h a p t e r 8, R e i n h o l d , N e w Y o r k , 1956. (6) B a s o l o , F r e d , Chem. Rev. 52, 459 (1953). (7) E d e l s o n , M. R., P l a n e , R. Α., Inorg. Chem. 3, 231 (1964). (8) P l a n e , R. Α., H u n t , J. P., J. Am. Chem. Soc. 79, 3343 (1957). (9) Schlafer, H. L., Z. physik. Chem., Neue F o l g e , 11, 1/2, 5 (1957).





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