Mechanisms of Substitution Reactions of Cobalt (III ... - ACS Publications

2 Mechanisms of Substitution Reactions of Cobalt (III) Cyanide Complexes

Downloaded by UNIV OF MASSACHUSETTS AMHERST on October 8, 2015 | http://pubs.acs.org Publication Date: January 1, 1965 | doi: 10.1021/ba-1965-0049.ch002

ALBERT H A I M , ROBERT J. G R A S S I , a n d University

of Southern

California,

Los Angeles,

W A Y N E K. Calif.

WILMARTH

90007

A r e v i e w h a s b e e n m a d e o f t h e kinetic data f o r t h e substitution reactions of t h e Co(CN) X-3 ions, w h e r e X refers to a n y one of various l i g a n d s . The e v i d e n c e s u g g e s t s t h a t the p e n t a - c o o r d i n a t e Co(CN)5-2 is g e n e r a t e d a s a r e a c t i v e i n t e r m e d i a t e in t h e substitution of w a t e r in Co(CN)5OH2-2 b y v a r i o u s n u c l e o p h i l e s a n d i n t h e aquation of the various C o ( C N ) X ions. P r e l i m i n a r y studies indicate t h a t t h e interm e d i a t e detected by the scavenger action of Brand SCN- in the reaction of H N O and Co(CN)5N3-3 is a d i f f e r e n t s p e c i e s t h a n t h e o n e discussed a b o v e . 5

3

5

2

Jhe

r e s u l t s b e l o w c o n s i s t of k i n e t i c s t u d i e s of t h e f o l l o w i n g r e a c t i o n s : a) t h e s u b s t i t u t i o n of w a t e r i n C o ( C N ) O H ~ ~ b y v a r i o u s n u c l e o p h i l e s (3, 7) i n c l u d i n g 6

2

2

H 2 O , b) t h e a q u a t i o n (3, 7) of C o ( C N ) $ X " ~ , w h e r e X " ~ r e p r e s e n t s one of a v a r i e t y of 1 8

3

n u c l e o p h i l e s , c) t h e s c a v e n g e r (3, 7) a c t i o n of S C N ~ " for t h e r e a c t i v e i n t e r m e d i a t e g e n e r a t e d i n t h e a c i d - c a t a l y z e d a q u a t i o n of C O ( C N ) B N " " , a n d d ) t h e 3

scavenger

3

a c t i o n (5) of B r ~ a n d S C N ~ " for t h e r e a c t i v e i n t e r m e d i a t e f o r m e d i n t h e r e a c t i o n of C o ( C N ) N ~ w i t h H N 0 , a process w h i c h p r o d u c e s C o ( C N ) O H ~ i n t h e a b s e n c e of 6

3

3

2

5

2

2

scavengers.

Anation

of

Co(CNhOH 2

2

A l l e x p e r i m e n t s were c a r r i e d o u t a t c o n s t a n t i o n i c s t r e n g t h a n d

constant

c a t i o n c o n c e n t r a t i o n , a r e s t r i c t i o n d e s i g n e d t o m i n i m i z e m e d i u m effects (10,

11).

N e g a t i v e l y c h a r g e d n u c l e o p h i l e s were i n t r o d u c e d i n t o t h e s o l u t i o n as s o d i u m s a l t s , a n d t h e i o n i c s t r e n g t h w a s a d j u s t e d t o t h e d e s i r e d v a l u e b y a d d i t i o n of t h e a p p r o p r i a t e a m o u n t of N a C 1 0 4 .

I n t h e presence of a l a r g e excess of a

nucleophile,

X ~ , t h e r a t e of f o r m a t i o n of C o ( C N ) 5 X " ~ is c h a r a c t e r i z e d a t e a c h c o n c e n t r a t i o n of 3

X " ~ b y a p s e u d o - f i r s t - o r d e r r a t e c o n s t a n t , k, t h e n u m e r i c a l v a l u e of w h i c h m a y b e e v a l u a t e d f r o m t h e slope of t h e l i n e a r p l o t of l o g (D -D ) œ

t

vs. t i m e , w h e r e D% a n d

D

œ

a r e t h e m o l a r a b s o r b a n c i e s of t h e s o l u t i o n a f t e r a r e a c t i o n t i m e t, a n d a f t e r a t i m e long enough

for t h e s y s t e m t o a p p r o a c h e q u i l i b r i u m , r e s p e c t i v e l y .

Numerical

v a l u e s of k w e r e o b t a i n e d for e a c h n u c l e o p h i l e a t a n u m b e r of X " ~ c o n c e n t r a t i o n s . 31

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

32

M E C H A N I S M S O F I N O R G A N I C REACTIONS T h e f o r m a t i o n of C o ( C N ) 5 X ~ b y a r a t e - d e t e r m i n i n g b i m o l e c u l a r r e a c t i o n of 3

C O ( C N ) B O H 2 ~ " a n d X " " w o u l d i m p l y t h a t i n t h e a b s e n c e of m e d i u m effects the r a t i o 2

k/ÇKr) s h o u l d be a c o n s t a n t , i n d e p e n d e n t of t h e X ~ c o n c e n t r a t i o n , for a n y g i v e n nucleophile.

However, i n experiments at unit ionic strength, it was found that the

q u a n t i t y k/ÇKr)

decreased w i t h i n c r e a s i n g X ~ c o n c e n t r a t i o n , w i t h t h e d e v i a t i o n

f r o m c o n s t a n c y g e n e r a l l y i n c r e a s i n g w i t h i n c r e a s i n g r e a c t i v i t y of t h e n u c l e o p h i l e . T h i s b e h a v i o r is i l l u s t r a t e d i n F i g u r e 1, a p l o t of k vs. X ~ for t w o t y p i c a l n u c l e o p h i l e s , Ns~ and B r ~ .

F o r e a c h n u c l e o p h i l e t h e p o i n t a t (X"~) = Ο represents the a q u a t i o n

r a t e of C o ( C N ) 5 X * ~ , t h e a p p r o p r i a t e i n t e r c e p t for e i t h e r a n S # l o r Sjy2 m e c h a n i s m .


3

T h e p o i n t s for Br~", a r a t h e r u n r e a c t i v e n u c l e o p h i l e , define a s t r a i g h t l i n e t o w i t h i n t h e l i m i t of e r r o r of t h e m e a s u r e m e n t s .

B y c o n t r a s t , the p o i n t s for t h e m u c h m o r e

r e a c t i v e N " ~ f a l l o n a c u r v e w i t h t h e l i m i t i n g slopes, c o r r e s p o n d i n g t o t h e d o t t e d 3

lines i n t h e figure, h a v i n g t h e n u m e r i c a l v a l u e s of 80 χ 1 0 ~ a n d 38 χ 1 0 ~ a t zero a n d 5

1.0M

respectively.

δ

O t h e r n u c l e o p h i l e s , t o be d i s c u s s e d

below,

analogous behavior.

kx I0 ,sec"i 60 5

50 -

40 -

//

30

20

f

10

0

u

1

1

1

1

.2

.4

.6

.8

1

1.0 (X"),M

Figure

1.

Pseudo

first-order

rate constants vs. anion

tion at 40° C. and ionic strength

concentra

1.0AI


show

an

2.

HAIM ET A L .

Cobalt (III) Cyanide

Complexes

33

C u r v a t u r e i n a p l o t of k vs. (X~~) suggests t h a t t h e s u b s t i t u t i o n r e a c t i o n u n d e r c o n s i d e r a t i o n is o c c u r r i n g b y a l i m i t i n g t y p e of Sjvl m e c h a n i s m i n w h i c h X ~ is r e a c t i n g w i t h a r e a c t i v e i n t e r m e d i a t e , f o r m e d f r o m C o ( C N ) 5 0 H ~ a n d h a v i n g a life 2

2

time l o n g enough to distinguish between various nucleophiles i n the s o l u t i o n .

In

t h e r e a c t i o n sequence b e l o w , w h i c h w i l l be a d o p t e d i n t h e f o l l o w i n g d i s c u s s i o n of m e c h a n i s m s , the p r o p o s e d CO(CK)B~

2

r e a c t i v e i n t e r m e d i a t e has been a s s i g n e d t h e f o r m u l a

a n d is r e g a r d e d as a

five-coordinate

C o ( I I I ) complex

of

unspecified


geometry. Co(CN) OH 5

2

Co(CN) ~ 5

2

^

+

2

Co(CN) 5

X -

â

+

2

H 0

(1)

2

Co(CN) X-«

(2)

6

T h e f o r w a r d a n d reverse p a t h s of R e a c t i o n s 1 a n d 2 serve t o define t h e r a t e c o n s t a n t s fa, fa, fa, a n d fa, t h e s y m b o l s p l a c e d o v e r o r u n d e r t h e a p p r o p r i a t e a r r o w s , w i t h t h e e x c e p t i o n t h a t t h e c o n c e n t r a t i o n of w a t e r is i n c l u d e d i n fa i n t h e c u s t o m a r y f a s h i o n . If w e a s s u m e t h a t t h e a c t i v i t y coefficients of X ~ " a n d H 0 are i n d e p e n d e n t of 2

the X ~ concentration at a n y given ionic strength, then the usual steady

state

t r e a t m e n t l e a d s , w i t h o u t f u r t h e r a p p r o x i m a t i o n , t o E q u a t i o n 3, a r e l a t i o n s h i p b e t w e e n t h e p s e u d o first-order r a t e c o n s t a n t a n d t h e o t h e r k i n e t i c p a r a m e t e r s .

+ -

+

h/h

fa

fa/fa

. ν

(Χ")

w

W h e n t h e reverse of R e a c t i o n 2, t h e a q u a t i o n of C o ( C N ) 5 X ~ , is r e l a t i v e l y s l o w , as i t 3

i s for N3"* a n d S C N ~ , t h e n fa(X~) y> fa fa/fa a n d t h e o m i s s i o n of t h e s e c o n d t e r m i n t h e n u m e r a t o r of E q u a t i o n 3 i s a v a l i d a p p r o x i m a t i o n .

Equation 3 may

be

r e a r r a n g e d a n d w r i t t e n a s E q u a t i o n 4, a f o r m w h i c h i n d i c a t e s m o r e c l e a r l y t h a t £1 — fa a n d fa/fa m a y be e v a l u a t e d f r o m t h e i n t e r c e p t 1 k

—

fa fa —

W h fa

(fa —

fa)(X

a n d t h e r a t i o of s l o p e t o i n t e r c e p t i n a p l o t of l/(k fa a r e l i s t e d i n T a b l e I V .

( 4 )

)

— fa) vs. l/(X~).

V a l u e s of

W h e n fa(Xr~) ϊζ> fafa/fa, t h e n fa m a y be o m i t t e d f r o m t h e

d e n o m i n a t o r of e a c h of t h e t e r m s of E q u a t i o n 4, a n d fa a n d fa/fa m a y be e v a l u a t e d f r o m a p l o t of l/k vs. l / ( X ~ ) . T h e i n v e r s e p l o t s of l/(k

— fa) vs. l / ( X " ~ ) for t h e n u c l e o p h i l e s N 3 ~ , S C N ~ , I"",

a n d B r ~ a r e p r e s e n t e d i n F i g u r e 2, w i t h t h e p o i n t s r e p r e s e n t i n g t h e d a t a a t 40°C. and unit ionic strength.

O n l y t h e d a t a a t c o n c e n t r a t i o n s a b o v e 0.1 M h a v e been

i n c l u d e d i n o r d e r t o p r o v i d e a n a d e q u a t e e x p a n s i o n of scale, so t h a t t h e r e g i o n n e a r t h e i n t e r c e p t m a y be c l e a r l y v i s i b l e .

A n y reasonable straight line d r a w n t h r o u g h

t h e p o i n t s i n F i g u r e 2 for N$~, S C N ~ ~ , a n d p e r h a p s I , w h e r e t h e d a t a a r e s o m e w h a t -

less r e l i a b l e , w o u l d s e e m t o r e q u i r e a n o n z e r o i n t e r c e p t , t h e c h a r a c t e r i s t i c feature of a l i m i t i n g S^l m e c h a n i s m .

T h e t w o p o i n t s for B r ~ are i n c l u d e d o n l y t o i n d i c a t e

t h e r e l a t i v e p o s i t i o n of t h e p o i n t s , a l l of w h i c h lie o n a s t r a i g h t l i n e , b u t w i t h a slope so great t h a t t h e i n t e r c e p t is p o o r l y defined a n d t h e z e r o v a l u e c o u l d e q u a l l y w e l l h a v e been c h o s e n .



34

MECHANISMS OF INORGANIC

REACTIONS

l/(X");M"'

Figure

2.

A

plot of

Kf/(k-k )

vs. the

4

reciprocal of the anion concentration for data obtained at 40° C. and ionic strength 1.0 M T h e intercept o n the ordinate i n F i g u r e 2 corresponds t o a numerical value of &i of 1.60 χ 10~~ /sec.

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

3

c o n s t a n t f o r g e n e r a t i o n of C o ( C N ) ~ 5

from C o ( C N ) O H .

2

5

2

I t also represents t h e

r a t e c o n s t a n t f o r f o r m a t i o n of C o ( C N ) s X ~ b y a h y p o t h e t i c a l n u c l e o p h i l e s o efficient 3

i n i t s s c a v e n g e r a c t i o n t h a t i t w o u l d b e a b l e t o c a p t u r e a l l of t h e C O ( C N ) B "

2

gener

a t e d i n R e a c t i o n 1 before r e a c t i o n w i t h w a t e r c o u l d o c c u r . T h e r a t i o o f s l o p e t o i n t e r c e p t f o r t h e v a r i o u s l i n e s i n F i g u r e 2 leads t o n u m e r i c a l v a l u e s of k /k 2

z

of 1.90, 2.95, 5.15 a n d 10 f o r N ~ , S C N ~ , I ~ , a n d B r ~ , r e s p e c t i v e l y . 3

T h e s e n u m b e r s r e p r e s e n t t h e r e l a t i v e efficiencies w i t h w h i c h w a t e r c o m p e t e s w i t h the various nucleophiles for CO(CN)Ô"" , t h e competing reactions being R e a c t i o n 2 2

a n d t h e réverse o f R e a c t i o n 1.

I n c o m p a r i n g t h e r e l a t i v e efficiencies o f w a t e r a s a

s c a v e n g e r f o r C o ( C N ) 5 ~ w i t h t h a t of a g i v e n n u c l e o p h i l e , i t m i g h t b e s o m e w h a t 2

m o r e r e a l i s t i c t o define t h e r a t e of t h e reverse of R e a c t i o n 1 a s e q u a l t o

fotCoCCN^""* ]

[ Η 0 ] , a change w h i c h w o u l d f o r m a l l y c o r r e c t f o r t h e w a t e r c o n c e n t r a t i o n . 2

p r o c e d u r e were a d o p t e d , a l l of t h e v a l u e s of k /k 2

a p p r o x i m a t e c o n c e n t r a t i o n of w a t e r i n t h e s y s t e m . m a y b e a p p l i e d t o t h e r a t i o k /k 2

z

z

2

If this

w o u l d be d i v i d e d b y 52, t h e However, a n y correction which

i n a n effort t o c o r r e c t f o r t h e w a t e r c o n c e n t r a t i o n

is, a t best, a r a t h e r a r b i t r a r y o n e .

T h e water molecule entering the coordination

sphere o f t h e C o ( C N ) ~ i o n m a y w e l l be p a r t of a h i g h l y o r d e r e d s o l v a t i o n s h e l l , a n d 6

2

t h e reverse of R e a c t i o n 1 a m u l t i m o l e c u l a r process i n v o l v i n g t h e s y n c h r o n i z e d a c t i o n of a n u m b e r of w a t e r m o l e c u l e s (8).


2.

HAIM §1 AL

35

Cobalt (III) Cyanido Complexes

Experimental. I n o r d e r t o s t u d y t h e n u c l e o p h i l i c p r o p e r t i e s of Is*", i t w a s necessary t o a d d excess I"~ t o t h e s o l u t i o n s t o p r e v e n t p r e c i p i t a t i o n of I . T h e r a t e of f o r m a t i o n of C o ( C N ) 5 i ~ w a s f o l l o w e d s p e c t r o p h o t o m e t r i c a l l y a f t e r t h e l a i n a l i q u o t s of t h e s o l u t i o n t a k e n a t s u i t a b l e t i m e i n t e r v a l s w a s r e d u c e d t o Γ" b y a r s e n i t e i o n . A t y p i c a l set of e x p e r i m e n t s w a s c a r r i e d o u t a t 4 0 ° C . a n d u n i t i o n i c s t r e n g t h , w i t h a l l s o l u t i o n s c o n t a i n i n g 0.5 Ai l~ a n d v a r i a b l e I3"" a t a m a x i m u m c o n c e n t r a t i o n of 0 . 2 8 M , t h e a p p r o x i m a t e u p p e r l i m i t i m p o s e d b y s o l u b i l i t y r e s t r i c t i o n s . T h e r e s u l t s a r e p r e s e n t e d i n F i g u r e 3 as a p l o t of k', t h e s y m b o l used for t h e p s e u d o first-order r a t e c o n s t a n t for t h i s s y s t e m , vs. l / ( I ) . I t i s a p p a r e n t t h a t I ~~ i s a r e m a r k a b l y efficient n u c l e o p h i l e , w i t h a r e a c t i o n r a t e c o n s i d e r a b l y g r e a t e r t h a n t h a t f o u n d for I ~ a t c o m p a r a b l e c o n c e n t r a t i o n s . T h e points i n F i g u r e 3 also show d e t e c t a b l e d e v i a t i o n f r o m l i n e a r i t y , d e s p i t e t h e l i m i t e d r a n g e of 13" c o n c e n t r a t i o n which was available. 2

3


3

kxlO

5

_

3

sec"' 35

30

25

20

(I - ) , M

Figure

3.

Pseudo first-order rate constant vs.

Iz~ concentration at 40°C.

and ionic strength

1.0M A s t h e d i s c u s s i o n b e l o w w i l l i n d i c a t e , t h e i n t e r p r e t a t i o n of t h e I 3 " d a t a i s s o m e w h a t m o r e a m b i g u o u s t h a n t h a t of t h e s y s t e m s c o n s i d e r e d a b o v e .

An

obvious

m e c h a n i s m , w h i c h a d e q u a t e l y r e p r e s e n t s t h e d a t a , m a y be b a s e d o n E q u a t i o n s 1, 2, 5, a n d 6. Co(CN) ~ 6

2

+

Ir

£

Ii + I-

CoCCNW-

^

3

+

If

h"


(5) (6)

36

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

T h e s t e a d y state t r e a t m e n t c o u p l e d w i t h the m i c r o s c o p i c r e v e r s i b i l i t y r e s t r i c t i o n y i e l d s E q u a t i o n 7. fafe(ir) ^

=

k

——

+

-

fa fa

j

+

r

(7)

(i-)

+(ir)

«5 L ^ 3


In E q u a t i o n 7 the symbols

a n d k are used t o represent t h e p s e u d o

r a t e c o n s t a n t for t h e I ~ s y s t e m , t h e p s e u d o

first-order

3

first-order

r a t e c o n s t a n t for t h e

0.5M

s o l u t i o n n o t c o n t a i n i n g I ~\

I n s p e c t i o n of E q u a t i o n 8, o b t a i n e d b y r e a r r a n g i n g

E q u a t i o n 7, shows t h a t

2

3

+ (I~)] m a y be e v a l u a t e d f r o m t h e r a t i o of slope

fa/fa[k /fa

t o i n t e r c e p t i n a p l o t of l/(k'

— k) vs. l / ( I * ~ ) . 3

ι

k

3

k' -

+ ^

k k/kz(l~)

k

2

2

falfa

' "

J

7

(8)

fak(h-)/fa(l~)

T h e p o s i t i o n of t h e s o l i d l i n e i n F i g u r e 3 has been d r a w n u s i n g t h e r e s u l t i n g v a l u e of

fa/fa

T h e n u m e r i c a l v a l u e of t h e r a t i ofa/fa= 0.61 i m p l i e s t h a t I "~ i s a

= 0.31.

3

s o m e w h a t m o r e r e a c t i v e n u c l e o p h i l e t h a n I ~ o r a n y of t h e o t h e r species so f a r c o n sidered.

T h e possible significance of t h i s r e s u l t w i l l be d i s c u s s e d b e l o w .

A t t h i s p o i n t i t is necessary t o c o n s i d e r s e v e r a l possible sources of a m b i g u i t y i n i n t e r p r e t a t i o n of t h e I "~ d a t a .

F i r s t , i t s h o u l d be n o t e d t h a t l m a y f o r m a n a d d i 2

3

t i o n c o m p l e x (2, 9) w i t h C o ( C N ) I ~ , j u s t as i t does w i t h a l k y l i o d i d e s i n n o n a q u e o u s 5

solution.

3

( E x p e r i m e n t s b e a r i n g o n t h i s q u e s t i o n a r e i n progress.)

Fortunately,

t h e presence of s u c h a c o m p l e x w o u l d n o t a l t e r t h e n u m e r i c a l v a l u e s of t h e k i n e t i c parameters.

S e c o n d l y , there i s t h e p o s s i b i l i t y t h a t R e a c t i o n 5 is a t r i m o l e c u l a r

process, w i t h t h e I a n d Γ~ r e a c t a n t s a d d i n g t o C o ( C N ) e " " a t s e p a r a t e stages of the 2

2

reactant. Co(CN) 5

2

+

I

2

+

I"

^

Co(CN) I5

3

+

I

(9)

2

T h i s f o r m u l a t i o n , w h i c h is k i n e t i c a l l y i n d i s t i n g u i s h a b l e f r o m t h a t g i v e n b y E q u a t i o n 5, w o u l d r e q u i r e t h a t the n u m e r i c a l v a l u e of 0.31 be a s s i g n e d t o t h e q u a n t i t y faK/fa , f

w h e r e Κ is t h e a s s o c i a t i o n c o n s t a n t for I

3

formation.

_

Finally, it should

b e e m p h a s i z e d t h a t n o t h i n g i s k n o w n a b o u t t h e g e o m e t r y of t h e a c t i v a t e d c o m p l e x g e n e r a t e d b y e i t h e r R e a c t i o n 5 o r 9.

T o be m o r e specific, t h e I2 m o l e c u l e m a y be

b o n d e d t o I"", t o a c y a n i d e l i g a n d , o r t o t h e t

e l e c t r o n s of t h e C o ( I I I ) i o n .

2g

In w e a k l y acidic solutions the various anation reactions are not p H dependent. H o w e v e r , w h e n f o r m a t i o n of C O ( C N ) Ô O H ~ t i o n , t h e r e i s a m a r k e d decrease i n r a t e . in some detail i n this p H region.

3

becomes a p p r e c i a b l e i n a l k a l i n e s o l u

T h e a n a t i o n r e a c t i o n of N ~ " w a s s t u d i e d 3

T y p i c a l results are given i n T a b l e I.

T w o a l t e r n a t e S # l r e a c t i o n s m u s t be c o n s i d e r e d for t h e r e a c t i o n i n b a s i c s o l u tion.

T h e first a l t e r n a t i v e assumes t h a t C o ( C N ) O H 5

- 3

is completely

unreactive

a n d t h a t a t a n y g i v e n p H , N * ~ reacts o n l y w i t h t h a t f r a c t i o n of the c o m p l e x 3

as C o ( C N ) 5 0 H ~ . 2

2

present

I n t h i s f o r m u l a t i o n the p H a n d a z i d e i o n d e p e n d e n c e of

p s e u d o first-order r a t e c o n s t a n t is g i v e n b y E q u a t i o n 10.


the

2.

HAIM ET AL.


T a b l e I.

37

Rate of Formation of Co(CN) N 5

3

at 4 0 C .

3

Ionic strength 1.0M, p H 6.4 k χ 10* 10*xk/(Nr), Wr), M M~ seer seer

\Co(CN) OHc*], Μ χ 10*

k χ 10* sec. (calcd.)

t

b


Complexes

l

1

1

0.86

0.0355

9.30

3.30

1.72

.071

8.80

6.25

6.89

.142

7.89

2.95 5.78 11.2

11.2

6.89

.212

7.48

16.1

16.0

5.32

.45

6.78

30.5

30.6

2.83

.45

6.82

30.7°

30.6

1.00

.45

6.85

30.8°

30.6

5.67

.725

6.14

44.5

44.4

7.50

.725

6.03

43.7

5.68

.90

5.61

50.5

5.48

54.8 '

5.70

1.0

E

44.4 51.5

b

6

55.2

C

" I n these experiments the C o ( C N ) * O H 2 was p r e p a r e d b y h y d r o l y s i s of C o C C N ^ B r . U n b u f f e r e d perchloric a c i d solutions_at p H 6.7. In this experiment the Co(CN)60H2 was prepared b y a c i d h y d r o l y s i s of C o f C N H N s " " . 2

- 5

6

2

e

8

,

M H ) (ΝΓ) +

" In equation

10

K

a

Kh/h

+

(ΝΓ)][Κ.

+

refers t o t h e a c i d i t y c o n s t a n t of

(H+)]

U

Co(CN)50H2~" .

10

;

I t has t h e

2

n u m e r i c a l v a l u e of 2.0 χ 10~~ a t 4 0 ° C . a n d u n i t i o n i c s t r e n g t h .

u

I n the s e c o n d of t h e

two alternative mechanisms, the further assumption is made that CO(CN)ô"" m a y 2

a l s o b e g e n e r a t e d b y R e a c t i o n 11. ki ^

f

Co(CN) OH-* 6

(In

fa'

Co(CN) 5

2

+

O H -

(11)

d e r i v i n g E q u a t i o n 1 0 a n d 11 t h e a q u a t i o n r a t e of t h e r e a c t i o n

Co(CN)5N3~~ has been neglected. 3

product

T h i s is a v a l i d a p p r o x i m a t i o n w h i c h somewhat

s i m p l i f i e s t h e f o r m o f t h e e q u a t i o n s a n d h e l p s t o c l a r i f y t h e p h y s i c a l significance o f the various kinetic parameters.) W h e n R e a c t i o n 11 i s i n c l u d e d i n t h e m e c h a n i s m , t h e e x p r e s s i o n f o r t h e r a t e constant is given b y E q u a t i o n 12.

fe(H+) + tfq](N 1 3

[(fa/fa)

+

(fa*/fa) ( O H " )

+

(ΝΓ)] [K

a

+

(H+)]

{

E q u a t i o n 1 0 does n o t p r o v i d e a n a d e q u a t e r e p r e s e n t a t i o n o f t h e d a t a .

l

}

First,

the rate constants calculated using this equation for experiments carried o u t a t a p H greater t h a n 10 are 1 5 - 2 0 % smaller t h a n t h e e x p e r i m e n t a l l y determined values. A s e c o n d , a n d p e r h a p s m o r e c o m p e l l i n g p o i n t i s t h a t E q u a t i o n 10 does n o t c o r r e c t l y p r e d i c t t h e d e p e n d e n c e of r a t e u p o n Ν3"" c o n c e n t r a t i o n i n t h e m o r e a l k a l i n e s o l u tions.

I n q u a l i t a t i v e t e r m s w h a t i s o b s e r v e d i s t h a t t h e p o i n t s i n a p l o t of k vs.

(N3-") a p p r o a c h l i n e a r i t y a s t h e a l k a l i n i t y of t h e s o l u t i o n i s i n c r e a s e d .

A p l o t of t h e

d a t a i n T a b l e I I o b t a i n e d a t p H 10.1 e x h i b i t s m u c h less c u r v a t u r e t h a n t h a t p r e s e n t e d i n F i g u r e 1.

I n a s i m i l a r p l o t of t h e d a t a i n T a b l e I I I o b t a i n e d a t a h y d r o x i d e

i o n c o n c e n t r a t i o n of 9 χ 10""~ Af t h e r e i s n o d e t e c t a b l e d e v i a t i o n f r o m l i n e a r i t y . 3

T h e d a t a u n d e r c o n s i d e r a t i o n a l l agree w e l l w i t h t h e p r e d i c t i o n s of E q u a t i o n 1 1 . T o test t h e v a l i d i t y of t h i s e q u a t i o n , i t is necessary t o e v a l u a t e t h e n e w k i n e t i c p a r a m e t e r s fa' a n d fa'/ fa. F r o m i n s p e c t i o n of t h e e q u a t i o n v a l u e s of [K

a


+

(H )]/ +


38 T a b l e II.

REACTIONS

Rata of Formation of Co(CN) N ~ at 4 0 C . 5

3

3

k χ 10* seer (calcd.)

1

M


0.18 .27 .45 .72 .90

2.67 2.49 2.36 2.29 2.19

T a b l e III.

4.64 6.80 10.8 16.2 19.5

4.81 6.72 10.6 16.5 19.7

Rate of Formation of Co(CN) N 5

3

a t 40°C.

3

-ι

WD, M 0.19 .45 .90

2.37 2.29 2.28

0.43 1.03 2.05

0.42 1.04 2.05

[ & i ( H ) -f- fa'K ] a n d of (fa/fa) + fa*/fa) (OH"") m a y be o b t a i n e d f r o m t h e i n t e r c e p t +

a

a n d t h e r a t i o of t h e s l o p e t o t h e i n t e r c e p t i n t h e l i n e a r p l o t s of l / k vs. l / ( N " ~ ) . 3

Use

of t h e d a t a of T a b l e s I I a n d I I I a n d t h e p r e v i o u s l y e v a l u a t e d q u a n t i t i e s fa, fa/fa, and K , a

w e o b t a i n Κχ

= 6.5 χ 1 0 - / s e c . 4

a

n

d k /fa

= 3.0 χ 10 . s

2

T h e excellent

a g r e e m e n t b e t w e e n t h e o r y a n d e x p e r i m e n t m a y be seen b y r e f e r r i n g t o T a b l e s I, I I , a n d I I I w h e r e e x p e r i m e n t a l v a l u e s of k m a y b e c o m p a r e d w i t h t h o s e c a l c u l a t e d f r o m E q u a t i o n 11. I n s p e c t i o n of t h e n u m e r i c a l v a l u e s of t h e k i n e t i c p a r a m e t e r s i n d i c a t e s t h a t R e a c t i o n 11 i s s o m e w h a t less efficient t h a n R e a c t i o n 1 as a g e n e r a t i n g s o u r c e for Co(CN)b"" . 2

A

fa'/fa,a n

m o r e s u r p r i s i n g r e s u l t i s t h e large v a l u e of

i n d i c a t i o n of

t h e efficiency w i t h w h i c h O H ^ c o m p e t e s w i t h w a t e r for c a p t u r e of C o ( C N ) 5 .

fa'/faw i t h

p a r i s o n of t h e v a l u e of

fa/fav a l u e s c i t e d suggests t h a t t h e

the

of r e a c t i o n of O H ~ w i t h C o ( C N ) ~ differs f r o m t h a t of o t h e r n u c l e o p h i l e s . 3

5

Com

mechanism Quite

p o s s i b l y t h e c a p t u r e of O H ~ is f a c i l i t a t e d b y a G r o t t h u s c h a i n m e c h a n i s m i n v o l v i n g p r o t o n t r a n s f e r t h r o u g h t h e s o l v a t i o n s p h e r e of t h e C o ( C N ) 5 ~ i o n . 2

T h e observa

t i o n t h a t O H ~ c o m p e t e s so f a v o r a b l y w i t h w a t e r i m p l i e s t h a t t h e w a t e r r e a c t i o n has e i t h e r a r a t h e r a p p r e c i a b l e a c t i v a t i o n e n e r g y , o r a n u n f a v o r a b l e a c t i v a t i o n e n t r o p y (or b o t h ) .

I t i s p r e s u m a b l y these f a c t o r s w h i c h e n a b l e o t h e r n u c l e o p h i l e s

t o c o m p e t e w i t h w a t e r , e v e n w h e n t h e l a t t e r is p r e s e n t a t a m u c h h i g h e r c o n c e n t r a tion. O u r d i s c u s s i o n of t h e r e a c t i o n m e c h a n i s m i n a l k a l i n e s o l u t i o n has a s s u m e d t h a t t h e b i m o l e c u l a r r e a c t i o n of C o ( C N ) O H ~ a n d N ~ does n o t p r o v i d e a n i m p o r t a n t 3

6

3

p a t h for f o r m a t i o n of C o ( C N ) N ~ . B

Co(CN) OH~ 6

3

3

+

3

N

3

-

Co(CN) N 6

3

3

+

O H -

(13)

F o r t u n a t e l y , t h e r e is a n e x p e r i m e n t a l test of t h e v a l i d i t y of t h i s a s s u m p t i o n .

I n the

d i s c u s s i o n of t h e a q u a t i o n r e a c t i o n s , t o be p r e s e n t e d b e l o w , i t w i l l be s h o w n t h a t t h e a q u a t i o n r a t e C o ( C N ) N 3 ~ i n a l k a l i n e s o l u t i o n is p H i n d e p e n d e n t , c l e a r l y i n d i 6

c a t i n g t h a t t h e reverse of R e a c t i o n 13 is u n i m p o r t a n t .

Consequently, the m i c r o

s c o p i c r e v e r s i b i l i t y r e s t r i c t i o n r e q u i r e s n o a p p r e c i a b l e f o r m a t i o n of

Οο(ΟΝ)^ζ~^

b y R e a c t i o n 13.


2.

HAIM ET AL.


Complexes

39

A f t e r c o m p l e t i o n of t h e e x p e r i m e n t s a t u n i t i o n i c s t r e n g t h , e x p l o r a t o r y s t u d i e s were c a r r i e d o u t a t m u c h h i g h e r n u c l e o p h i l e c o n c e n t r a t i o n s , d e s p i t e t h e possible u n c e r t a i n t y i n v o l v e d i n i n t e r p r e t a t i o n of t h e p h y s i c a l significance of s u c h d a t a . T h e r e s u l t s of a n a t i o n s t u d i e s u s i n g Ν 3""" a n d S C N " * a t 2 0 ° C . a n d a n i o n i c s t r e n g t h of 5.0 are p r e s e n t e d i n F i g u r e 4 as a p l o t of k vs. t h e X " c o n c e n t r a t i o n .

T h e points

show a v e r y pronounced d e v i a t i o n from linearity, as E q u a t i o n 3 w o u l d predict, a n d a n d i n t h e N e " s y s t e m t h e r a t e b e c o m e s a l m o s t z e r o - o r d e r i n N3"" a b o v e a c o n c e n t r a t i o n a p p r o x i m a t e l y 3.0ilf. of h = 51 χ 10*" s e c .


5

- 1

T h e i n v e r s e p l o t s , b a s e d o n E q u a t i o n 4, y i e l d e d a v a l u e

a n d k /kz v a l u e s of 3.0 a n d 5.0 for N e " a n d S C N ~ 2

respectively.

T h e p o s i t i o n of t h e c u r v e s i n F i g u r e 4 were c a l c u l a t e d u s i n g these n u m e r i c a l v a l u e s of t h e k i n e t i c p a r a m e t e r s .

T h e d a t a for t h e S C N " " s y s t e m c o n f o r m c l o s e l y t o t h e

p r e d i c t i o n s of E q u a t i o n 4.

H o w e v e r , the observed deviations from linearity i n the

N 3 - system are somewhat more pronounced t h a n the theory would predict. kxl0 ,sec-' 5

Figure 4.

Pseudo first-order rate constants vs. anion concen tration at 20°C. and ionic strength 5.0M

M o r e r e c e n t l y , u n p u b l i s h e d s t u d i e s (6) h a v e been c o m p l e t e d i n t h e B r " * s y s t e m a t 2 0 ° C . a n d a t a n i o n i c s t r e n g t h of 5.0.

T h e r e s u l t s o b t a i n e d a t 0.5, 1.0, 3.0, a n d

5.0ikf Br~* m a y be a d e q u a t e l y r e p r e s e n t e d b y t h e E q u a t i o n , k = [1.2 4.9 Χ 10~" ( B r ~ ) ] / s e c . δ

X

10~"

6

+

T h e i n t e r p r e t a t i o n of t h e c o n s t a n t 1.2 X 1 0 ~ V s e c . p r e s e n t s

n o p r o b l e m since i t w o u l d r e p r e s e n t a n o t u n r e a s o n a b l e v a l u e for k\ t h e r a t e c o n y

s t a n t for s o l v o l y s i s of C o ( C N ) ô B r ~ . 3

H o w e v e r , t h e l i n e a r d e p e n d e n c e of k u p o n t h e

B r " " c o n c e n t r a t i o n i s n o t i n a g r e e m e n t w i t h c a l c u l a t i o n s b a s e d o n t h e v a l u e of k\ cited above.

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

p a s t , t h a t t h e r e i s c o n s i d e r a b l e u n c e r t a i n t y i n t h e i n t e r p r e t a t i o n of k i n e t i c d a t a o b t a i n e d a t so h i g h a n i o n i c s t r e n g t h .

A p a r t f r o m t h i s n o t e of c a u t i o n , n o v e r y

u s e f u l g e n e r a l i z a t i o n m a y be d r a w n because of t h e v e r y l i m i t e d a m o u n t of a n a l o g o u s data i n the chemical literature. B e f o r e c o n c l u d i n g t h e d i s c u s s i o n of t h e a n a t i o n r e a c t i o n , s o m e c o n s i d e r a t i o n s h o u l d be g i v e n t o a l t e r n a t i v e f o r m u l a t i o n s o f t h e r e a c t i o n m e c h a n i s m .


In parti-

40


cular, there is the question whether the reaction might not be proceeding by an Sj\r2 mechanism, with the decrease in k/ (X~)

with increasing (X~~) merely representing a

medium effect arising from the replacement of CIO4"" by X ~ .

M e d i u m effects could

be attributed either to the usual long range interaction of ions, or to ion pairs, or triplets having the formulas C o ( C N ) O H - - X ~ , or C o ( C N ) O H - N a + - X ~ . 5

2

2

5

2

Driv

ing force for the formation of ion pairs presumably would arise from hydrogen bond ing of the sort C o - r O r V ·Χ~~ or from electrostatic interaction of X ~ with an i n completely shielded Co(III) ion.


It does not seem to us that long range interaction provides a very plausible explanation of the data.

In a reaction of two negative ions both theory and ex

periment seem to indicate that the value of the rate constant is not sensitive to the nature of the negative ions in the solution as long as the positive ion environment is held constant (11).

If the rate law for the Nf

system were formulated in terms of

the Brônsted-Bjerrum equation, the data of Figure 1 would require that the activity coefficient ratio change by a factor of two when the medium is changed from l . O M NaNe to l.OM

NaC104.

Further, the data would require the rather unlikely

coincidence that large changes in the activity coefficient ratios occur only with reactive nucleophiles. A n explanation based upon ion pairing would require that a substantial fraction of the C o ( C N ) O H 2 ~ be associated with X~~ ions. 2

5

In view of the unfavorable

electrostatic interaction of two anions, it hardly seems likely that extensive ion pairing would occur. It is interesting to consider whether an S ^ l mechanism might have been anticipated in the present system.

In organic systems the presence of a highly electro-

negative cyano substituent would tend to favor an Sjv2 rather than an S.yl mechanism.

T h e presence of a single cyano ligand in a positively charged complex ion

appears to have similar mechanistic consequences.

However, when the number

of cyano ligands is increased to five, the accumulation of negative charge probably produces a relatively high electron density at the cobalt atom, despite the tendency for x-bonding to spread the charge throughout the ligand sphere.

Such an accumu-

lation of negative charge at the cobalt atom should lead to a relatively weak C 0 - O H 2 bond, a weakly acidic complex, and a relatively favorable activation energy for an SjvI reaction path.

T h e observed p K of the complex and the comparatively rapid

water exchange are consistant with this view. For a limiting type of SatI mechanism, it is also necessary that the energy of the reactive intermediate be appreciably lower than that of the preceding activated complex.

In the present system, it can perhaps be argued that the decrease in

coordination number from six to five is accompanied by an increase in bond angles and a decrease in electrostatic repulsion of the negative ligands.

However, it is

difficult to assess the importance of this electrostatic argument since the changes in bond angles and coordination number also imply important changes in bond energies.

Finally, as we have suggested above, it is quite probable that the nature

of the solvation sphere plays an important role in determining the life time of the reactive intermediate.

E n t r y of a water molecule into the solvation sphere of the

Co(CN)5"~ intermediate by a complicated synchronized motion of a number of 2

water molecules obviously represents a process which might have a quite unfavorable enthalpy and entropy of activation.


2.

HAIM ET AL.


Rate of Exchange of Co(CN) OHr

with O

2

h

Complexes Labelled

1 8

41 Water

A q u i t e c o n c l u s i v e test of t h e v a l i d i t y of t h e m e c h a n i s m based o n E q u a t i o n s 1 a n d 2 w o u l d be p r o v i d e d b y a n a c c u r a t e m e a s u r e m e n t of t h e exchange r a t e of t h e water i n C o ( C N ) O H " " with H 0 5

2

2

2

has n o t y e t been c o m p l e t e d .

1 8

labelled solvent, a project w h i c h unfortunately

If t h e p r o p o s e d m e c h a n i s m is v a l i d , t h e r a t e c o n s t a n t

for w a t e r exchange s h o u l d e q u a l k\, t h e r a t e c o n s t a n t for f o r m a t i o n of C o ( C N ) ~ 5

from

Co(CN)OH ~ .

Co(CN)5X~


2

2

F u r t h e r , the

r a t e of

w a t e r exchange

and

the

f o r m a t i o n s h o u l d be c o m p e t i t i v e processes, w i t h t h e w a t e r

3

rate

2

of

exchange

b e i n g i n h i b i t e d i n t h e presence of a r e a c t i v e n u c l e o p h i l e . E x c h a n g e d a t a w o u l d a l s o test t h e v a l i d i t y of a n a l t e r n a t i v e m e c h a n i s m in

which

the

reactive

Co(CN) (OH ) 6

2

2

2

intermediate

is

formulated

as

the

seven

coordinate

complex.

Co(CN) OHr 5

Co(CN)

5

(OH ) ~ 2

2

+

2

H 0 2

+

2

X-

%

â±

C o ( C N ) (OH )r 5

2

Co(CN) X-

+

5

(14)

2

2 H 0

(15)

2

T h e a n a t i o n d a t a p r e s e n t e d a b o v e c o u l d e q u a l l y w e l l be i n t e r p r e t e d i n t e r m s of s u c h a mechanism.

H o w e v e r , i f t h e w a t e r molecules i n the c o m p l e x f o r m e d i n E q u a t i o n

14 b e c o m e e q u i v a l e n t , a p l a u s i b l e , a l t h o u g h n o t i n e v i t a b l e , consequence of

the

m e c h a n i s m , t h e n the p r e d i c t e d r a t e c o n s t a n t for w a t e r exchange w o u l d e q u a l k\/2. E a r l i e r a t t e m p t s t o measure t h e w a t e r exchange were b a s e d o n a n a n a l y t i c a l p r o c e d u r e i n v o l v i n g p r e c i p i t a t i o n of t h e p a r t i a l l y l a b e l l e d A g C o ( C N ) O H d e h y d r a 2

t i o n , a n d a n a l y s i s of t h e d e h y d r a t e d w a t e r for O

1 8

5

content.

2

A t t e m p t s to obtain

q u a n t i t a t i v e d a t a were f r u s t r a t e d b y a s i l v e r i o n i n d u c e d exchange process w h i c h occurred w i t h extreme r a p i d i t y , even when the precipitation was carried out i n a q u e o u s m e t h a n o l a t — 50°C.

H o w e v e r , b y v a r y i n g the O

c o n t e n t of t h e m e d i u m

1 8

used i n p r e c i p i t a t i n g A g C o ( C N ) O H , i t w a s possible t o d r a w t h e t e n t a t i v e c o n 2

5

2

c l u s i o n t h a t t h e exchange r a t e c o n s t a n t h a d a n u m e r i c a l v a l u e a p p r o x i m a t e l y e q u a l t o t h a t of k\.

M o r e r e c e n t l y , a l t e r n a t e s e p a r a t i o n p r o c e d u r e s h a v e been d e v e l o p e d ,

a n d i t is h o p e d t h a t exchange d a t a w i l l s o o n b e c o m e a v a i l a b l e .

A s t u d y of t h e

c a t a l y z e d exchange i n d u c e d b y v a r i o u s L e w i s a c i d s w i l l also be u n d e r t a k e n . T h e a q u a t i o n of t h e v a r i o u s C o ( C N ) 5 X " ~ c o m p l e x e s m u s t o c c u r b y a r e a c t i o n 3

p a t h w h i c h is m e r e l y t h e reverse of t h a t f o l l o w e d i n t h e a n a t i o n .

If t h e p r o p o s e d

m e c h a n i s m for t h e a n a t i o n r e a c t i o n is v a l i d , t h e reverse of R e a c t i o n s 1 a n d 2 m a y be u s e d t o describe t h e e q u a t i o n .

I n a n y g i v e n e x p e r i m e n t t h e r a t e of a p p r o a c h t o

e q u i l i b r i u m m a y be c h a r a c t e r i z e d b y a

first-order

t o t h e o t h e r k i n e t i c p a r a m e t e r s b y E q u a t i o n 3.

r a t e c o n s t a n t k w h i c h is r e l a t e d W h e n k%k /kz ^> & i ( X " ~ ) , as i t i s i n 4

t h e a q u a t i o n of C o ( C N ) B r " ~ i n t h e absence of a d d e d B r ~ , t h e n t h e a q u a t i o n p r o 3

5

ceeds t o c o m p l e t i o n , a n d k e q u a l s k . 4

T h e a q u a t i o n r e a c t i o n s of t h e o t h e r c o m p l e x e s d o n o t p r o c e e d t o c o m p l e t i o n , e v e n i n t h e absence of a d d e d X ~ . m a y be u s e d t o e v a l u a t e k . 4

U n d e r these c o n d i t i o n s t w o a l t e r n a t i v e m e t h o d s

I n t h e first m e t h o d k is o b t a i n e d f r o m a s t u d y of t h e 4

i n i t i a l r a t e of a q u a t i o n i n t h e t i m e v i t e r v a l w h e n t h e a n a t i o n r e a c t i o n m a y neglected.

be

I n the second m e t h o d the a q u a t i o n is s t u d i e d i n a l k a l i n e s o l u t i o n w h e r e

the f o r m a t i o n of C o ( C N ) s O H ~ t e n d s t o d r i v e t h e r e a c t i o n t o c o m p l e t i o n . 3


42


REACTIONS

I n a l l of t h e s y s t e m s so f a r i n v e s t i g a t e d t h e a q u a t i o n r a t e has been f o u n d t o be p H i n d e p e n d e n t i n a l k a l i n e s o l u t i o n , a t least u p t o 0.1 I f O H , t h e largest c o n c e n t r a -

tion investigated.

I t m a y be n o t e d t h a t t h i s b e h a v i o r i s e n t i r e l y a n a l o g o u s t o

t h a t of t r i t y l c h l o r i d e a n d o t h e r o r g a n i c h a l i d e s w h i c h u n d e r g o s o l v o l y s i s b y w e l l e s t a b l i s h e d SNI m e c h s n i s m s . (S) N u m e r i c a l v a l u e s of k a n d K, t h e e q u i l i b r i u m c o n s t a n t f o r C o ( C N ) 5 X " ~ f o r m a 3

4

t i o n , h a v e been a s s e m b l e d i n T a b l e I V .

I n t h e three cases w h e r e

temperature

coefficient d a t a i s a v a i l a b l e , i t c a n be seen t h a t t h e r e l a t i v e v a l u e s of £4 a r e d e t e r


m i n e d b y t h e differences i n b o t h AH a n d AS.

A c o m p a r i s o n of k a n d Κ i n d i c a t e s 4

t h a t a n increase i n Κ is a c c o m p a n i e d b y a decrease i n £4.

F o r a n y given nucleophile

k a n d Κ m a y b e r e l a t e d b y t h e e x p r e s s i o n Κ — hikz/kifa. 4

T h i s latter expression

h a s been u s e d t o c a l c u l a t e t h e n u m e r i c a l v a l u e s o f Κ f o r t h e S C N ~ ù n d N3""" s y s t e m s w h e r e e q u i l i b r i u m d a t a is n o t a v a i l a b l e .

Table IV. x-

J0 k seer 5.5 80 3.7 78 74 4950 1680

Γ, °C. 40 60 40 60 40 69.9 40

N 3

SCN IBr-

7

1

4

Κ

AH

AS

27.3

-0.6

1530

31.1

11.5

1460

29.7

12.5

39 0.88

42 0.95

T h e r a p i d a n a t i o n r e a c t i o n o b s e r v e d i n t h e presence of I 3 i m p l i e s t h a t I2 -

s h o u l d b e a v e r y efficient c a t a l y s t f o r t h e a q u a t i o n of Co(CN)5Ï~~ . Q u a l i t a t i v e 3

observations confirm this prediction, b u t a careful s t u d y of t h e catalyzed a q u a t i o n has n o t y e t been completed.

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

a n d L e w i s a c i d s w i l l a l s o be u n d e r t a k e n i n f u t u r e w o r k . Acid-Catalyzed

Aquation

of

Co(CN)&Nr*

I n a c i d i c s o l u t i o n t h e a q u a t i o n o f C o ( C N ) N ~ ~ w a s f o u n d t o be a c i d - c a t a l y z e d · 6

In

3

3

t h e a b s e n c e of a n i o n s o t h e r t h a n CIO4"", t h e o n l y r e a c t i o n p r o d u c t s

C o ( C N ) O H ~ and H N 3 . 6

2

2

were

T y p i c a l results obtained a t u n i t ionic strength a n d 40°C.

a r e p r e s e n t e d i n c o l u m n 2 o f T a b l e V as p s e u d o first-order r a t e c o n s t a n t s .

Table V .

Acid-Catalyzed Aquation of Co(CN) N - .

(m) 0.0042 .0084 .0168 .0336 .0364 .0505 .091 .166 β

6

t = 40°C, μ 10% sec." 5.90 12.1 23.5 43.2 47.2 64 94 132 1

1.0

3

3

10*k seer calcd. 6.15 12.1 23.5 43.6 47.0 61 95 140

1

a

Calculated using Equation 17.

T h e m e c h a n i s m of t h e a c i d - c a t a l y z e d a q u a t i o n m a y be f o r m u l a t e d i n t e r m s o f E q u a t i o n 16 a n d 17.


2.

HAIM ET A l .

Cobalt (III) Cyanide Co(CN) N 5

Co(CN) N H6

3

2

3

+

- 3

+

H

2

Complexes κ ^±

H+ 0

-»

43

Co(CN) N H 5

3

Co(CN) OH 5

2

(16)

- 2

+

2

H N

(17)

3

A s w e s h a l l s h o w b e l o w , R e a c t i o n 17 does n o t o c c u r i n a single s t e p , b u t t h i s c o m p l i c a t i o n m a y be i g n o r e d f o r t h e m o m e n t .

I f w e a s s u m e t h a t R e a c t i o n 16 i s

r a p i d l y r e v e r s i b l e a n d t h e R e a c t i o n 17 i s r a t e - d e t e r m i n i n g , t h e n i t m a y r e a d i l y b e shown that the expected

d e p e n d e n c e of k u p o n h y d r o g e n i o n c o n c e n t r a t i o n i s


g i v e n b y E q u a t i o n 18.

k

ι + K

=

m

(

I n E q u a t i o n 18, Κ a n d k a r e t h e e q u i l i b r i u m c o n s t a n t a n d

first-order

a

for R e a c t i o n s 16 a n d 17, r e s p e c t i v e l y .

1

8

)

rate constant

T h e c o n s t a n t s k a n d Κ m a y be e v a l u a t e d a

b y u s i n g E q u a t i o n 18 a n d t h e d a t a g i v e n i n c o l u r h n s 1 a n d 2 of T a b l e V . T h e p r o c e d u r e i n v o l v e s o b t a i n i n g l/k

a

a n d Κ f r o m t h e i n t e r c e p t a n d t h e r a t i o of i n t e r

c e p t t o slope i n t h e l i n e a r p l o t of l/k vs. l / ( H ) a n d leads t o k = 3.2 χ 1 0 +

and Κ

= 4.7.

sec.

- 3

a

- 1

T h e e x c e l l e n t a g r e e m e n t b e t w e e n t h e o r y a n d e x p e r i m e n t m a y be

seen b y c o m p a r i n g t h e e x p e r i m e n t a l a n d c a l c u l a t e d v a l u e s of k g i v e n i n T a b l e V . M a j o r r e s u l t s were o b t a i n e d w h e n t h e a c i d - c a t a l y z e d a q u a t i o n was c a r r i e d o u t in

solutions containing S C N ions.

B y carrying out the

-

spectrophotometric

analyses a t a p p r o p r i a t e wave lengths, i t was possible t o follow simultaneously t h e r a t e of d i s a p p e a r a n c e of C o ( C N ) 5 N

3

a n d t h e r a t e of f o r m a t i o n of C o ( C N ) § O H

- 3

I t w a s f o u n d t h a t t h e presence of S C N of C o ( C N ) N r . 5

-

However, both C o ( C N ) O H

3

6

2

and C o ( C N ) N C S -

- 2

5

3

- 2

.

a p p e a r e d as

reaction products, even i n time intervals so short t h a t the C o ( C N ) 6 N C S h a v e been f o r m e d b y r e a c t i o n of C o ( C N ) O H 5

2

and S C N

- 2

these e x p e r i m e n t a l r e s u l t s a r e p r e s e n t e d i n T a b l e V I . constants

2

d i d n o t influence t h e r a t e of d i s a p p e a r a n c e

f o r t h e d i s a p p e a r a n c e of C o ( C N ) N " " 6

3

3

- 3

- 3

could not

D a t a illustrating

.

T h e pseudo

first-order

are listed i n column

rate

3 under

t h e h e a d i n g 1 0 km s e c . , t h e s u b s c r i p t 3 8 0 i n d i c a t i n g t h a t t h e s p e c t r o p h o t o 6

- 1

m e t r i c m e a s u r e m e n t s were c a r r i e d o u t a t λ = 380 πιμ w h e r e C o ( C N ) O H 6

CO(CN)ÔNCS~

have identical molar absorbancy indices.

3

these r a t e c o n s t a n t s agree w i t h those of T a b l e V .

2

- 2

and

W i t h i n the l i m i t of error

B y contrast, the pseudo

first-order

r a t e c o n s t a n t s i n c o l u m n 4 of T a b l e V I , w h i c h m e a s u r e t h e i n i t i a l r a t e of a p p e a r a n c e of C o ( C N ) 5 0 H SCN

-

2

2

- 2

, s h o w a d i s t i n c t decrease i n m a g n i t u d e i n t h e presence of a d d e d

because of t h e p a r a l l e l f o r m a t i o n of C O ( C N ) B N C S .

T O indicate

- 3

more

c l e a r l y t h e n a t u r e of t h e s t u d i e s , t h e r e s u l t s o f a single e x p e r i m e n t are p r e s e n t e d i n m o r e d e t a i l i n F i g u r e 5.

Table V I .

0.019 .092 .091 .091 .166 .166 .166

A c i d - C a t a l y z e d A q u a t i o n o f C o ( C N ) N - a t 40°C.

(SCN-) 0 0.50 .70 .90 0 0.40 .80

s

μ = 1.0, i n the presence of S C N " 10* k Q, 10* k is, sec.~ sec. 94 83 ± 3 * 96 94 80 ± 3 91 66 ± 4 132 133 132 121 ± 5 133 104 ± 4 U

2

1

l

3

3

10* k seer , calcd.


i7S

' 85 76 69 116 104

1



44

7

8

9

10 II

time,

Figure

5.

An experiment

etry in the presence measurements

12

REACTIONS

13 14 15 16 17 18 19 2 0 21 2 2

min.

illustrating

of thiocyanate

the change in

at 380 and 278 mμ, respectively;

represents our evaluation

stoichiom-

and Ο

ion; ·

represent

the dotted

of the initial

line

slope.

T h e d a t a of T a b l e I V s t r o n g l y suggest t h a t w a t e r a n d S C N " a r e r e a c t i n g i n a c o m p e t i t i v e fashion, w i t h a reactive intermediate generated i n or after the rated e t e r m i n i n g s t e p of t h e a c i d - c a t a l y z e d a q u a t i o n .

E q u a t i o n s 19, 2 0 , a n d 21 r e p r e

s e n t a p l a u s i b l e m e c h a n i s m f o r g e n e r a t i o n of t h e r e a c t i v e i n t e r m e d i a t e , a s s u m e d t o be C o ( C N ) 5 ~ , a n d i t s c o m p e t i t i v e r e a c t i o n s w i t h w a t e r a n d S C N ~ ~ . 2

Co(CK) N H5

3

Co(CN) -

2

5

Co(CN) 5

2

+

H

+

2

0

2

5

+

2

determining

Co(CN) OH 5

rs Co(CN) -

rate

H N ,

2

(20)

2

CoCCN^NCS-

SCN'

(19)

(21)

3

If w e a s s u m e t h a t t h e c h e m i c a l p r o p e r t i e s o f t h e C o ( C N ) ~ g e n e r a t e d i n R e a c t i o n 19 5

2

a r e i d e n t i c a l w i t h t h o s e of R e a c t i o n 1, t h e n t h e m e a s u r e d v a l u e of k a n d t h e p r e a

v i o u s l y t a b u l a t e d v a l u e of k /k% = 2.95 m a y be u s e d t o c a l c u l a t e km, t h e r a t e 2

constant for f o r m a t i o n of C o ( C N ) O H ~ a t v a r i o u s S C N ~ " concentrations. 6

2

2

The

c a l c u l a t i o n w a s c a r r i e d o u t u s i n g E q u a t i o n 22, a n e q u a t i o n w h i c h w a s d e r i v e d i n t h e a p p e n d i x of o u r o r i g i n a l p u b l i c a t i o n ( 7 ) . kk /fa 2

(SCN") T h e excellent agreement between

+

(22)

k /k 2

z

t h e c a l c u l a t e d a n d e x p e r i m e n t a l v a l u e s o f km

l i s t e d i n T a b l e V I w o u l d s e e m t o p r o v i d e s t r o n g s u p p o r t f o r t h e p r o p o s e d S^rl mechanisms. T h e k i n e t i c p a r a m e t e r s o b t a i n e d i n t h e a b s e n c e of S C N ~ a r e of s o m e i n h e r e n t interest.

T h e a c i d - c a t a l y z e d a q u a t i o n of l i g a n d s w h i c h a r e c o n j u g a t e bases of w e a k

a c i d s h a s been o b s e r v e d i n a n u m b e r of o t h e r s y s t e m s ( i ) , b u t p r e v i o u s s t u d i e s


2.

HAIM FT AL.


Complexes

45

f r e q u e n t l y p r o v i d e a v a l u e o n l y f o r t h e p r o d u c t k K.

T h e n u m e r i c a l values of k

and

Both

a

a r e therefore

Κ

Co(CN) N3~"

some

inherent

appear to undergo

3

5

of

a

interest.

Co(CN) N H"~ and 6

2

3

solvolysis b y a n S ^ l mechanism.

T h e large

i n c r e a s e i n l a b i l i t y c a u s e d b y a d d i t i o n of t h e p r o t o n t o t h e a z i d e l i g a n d i s g i v e n b y t h e r a t i o k /k a

= 5800.

4

I t i s i n t e r e s t i n g t o c o m p a r e t h e a c i d i t y c o n s t a n t of C o ( C N ) c N 3 H HN . 3

T h e a d d i t i o n of C o ( C N )

5

- 2

b y a f a c t o r of a p p r o x i m a t e l y 5 χ 10 .

w i t h t h a t of

T h e a c i d i t y c o n s t a n t of C o ( C N ) 5 0 H

3

l a r g e r t h a n t h a t of H 0 b y a f a c t o r of 2 χ 10 . 5

2


- 2

t o t h e l a t t e r species increases t h e a c i d i t y c o n s t a n t 2

- 2

is

T h e s o m e w h a t different b e h a v i o r

of t h e t w o p a i r s of a c i d s i s n o t p a r t i c u l a r l y s u r p r i s i n g , since t h e s t r u c t u r e s of Co(CN) OH 6

Reaction

2

- 2

and Co(CN) N H 5

of Co(CN) Nr*

and

&

An

attempt

3

was made

- 2

are q u i t e different.

HN0

2

t o generate

Co(CN)

5

- 2

b y the rapid reaction

of

C o ( C N ) 5 N 3 ~ a n d H N 0 , a p r o c e s s (4) w h i c h y i e l d s o n l y t h e p r o d u c t s l i s t e d b e l o w 3

2

i n t h e a b s e n c e of a n i o n s o t h e r t h a n CIO4"". Co(CN) N + 5

3

+

3

HN0

-f

2

H+

-»

Co(CN) OH 6

2

+

-

N

+

2

N

0 (23)

2

It w a s h o p e d t h a t a n i n t e r m e d i a t e i n R e a c t i o n 23, p e r h a p s C o ( C N ) 5 N 4 0 , w o u l d T

- 2

contain the nitrogen atoms i n a very weakly bonded ligand, a situation which would f a v o r l i g a n d e x p u l s i o n a n d g e n e r a t i o n of C o ( C N ) 5 ~ .

T h e procedure designed t o

2

d e t e c t t h e presence of C o ( C N ) & ~ i n v o l v e d s t u d y i n g t h e r a t e a n d s t o i c h i o m e t r y of 2

the r e a c t i o n i n t h e presence of a d d e d B r a n d S C N ; t h i s m e t h o d of a p p r o a c h is -

-

e n t i r e l y a n a l o g o u s t o t h a t a d o p t e d i n t h e s t u d y of t h e a c i d - c a t a l y z e d a q u a t i o n of Co(CN)5N

- 3

3

.

However,

t h e r e s u l t s were s o m e w h a t

obtained i n the latter system.

more complex

than

those

A t c o n c e n t r a t i o n s of B r o r S C N i n t h e range of -

-

0.01 ilf, t h e s t o i c h i o m e t r y of t h e r e a c t i o n r e m a i n e d t h a t g i v e n b y E q u a t i o n 23, t h e expected

result a t this l o w scavenger

either B r

-

concentration.

H o w e v e r , t h e presence o f

o r S C N w a s f o u n d t o increase t h e r e a c t i o n r a t e , q u i t e p o s s i b l y because -

of t h e a p p e a r a n c e of a n e w r e a c t i o n p a t h i n v o l v i n g N O X , a r a t h e r c o m m o n of H N 0

feature

reaction mechansims.

2

W h e n t h e c o n c e n t r a t i o n of B r o r S C N w a s i n c r e a s e d f r o m 2.0M t o 5.0il/, a n -

increase i n rate a n d the expected Co(CN)6Br

or C o ( C N ) N C S

- 3

5

- 3

-

scavenger

a c t i o n were

both observed,

a p p e a r i n g as r e a c t i o n p r o d u c t s .

with

I t c a n be c o n

c l u d e d t h a t these l a t t e r p r o d u c t s w e r e f o r m e d b y r e a c t i o n o f B r o r S C N w i t h a -

-

r e a c t i v e i n t e r m e d i a t e , since there w a s n o c o r r e l a t i o n b e t w e e n t h e c h a n g e i n r a t e a n d t h e c h a n g e i n s t o i c h i o m e t r y c a u s e d b y t h e presence of t h e s c a v e n g e r s . e v e r , a n a n a l y s i s o f t h e s t o i c h i o m e t r y d a t a leads t o v a l u e s of kmo/kiC

How

w h i c h are t w o

t o t h r e e t i m e s s m a l l e r t h a n t h e c o r r e s p o n d i n g v a l u e s of k /kz o b t a i n e d f r o m t h e 2

a n a t i o n studies.

P r e l i m i n a r y results are s u m m a r i z e d i n T a b l e V I I .

S e v e r a l a l t e r n a t i v e i n t e r p r e t a t i o n s m a y be suggested results.

First,

CO(CN)Ô , - 2

but with

Co(CN) N40 6

i t is possible

- 2

some

t h a t t h e scavenger other

mentioned above.

reactive

to explain the above

ions are reacting, not w i t h

intermediate

such

a s t h e species

Secondly, there is the p o s s i b i l i t y t h a t C O ( C N ) B

is g e n e r a t e d i n b o t h t h e a c i d - c a t a l y z e d a q u a t i o n a n d t h e H N 0

2

- 2

reaction, but that

t h e t w o r e a c t i o n s m a y p r o d u c e different i s o m e r i c f o r m s of C o ( C N )

6

- 2

, one f o r m


46


Table VII·

Competition of X ~ a n d H 0 f o r t h e Intermediate G e n e r a t e d In t h e C o ( C N ) N - - H N 0 S y s t e m 2

s

3

3

2

kmo/k

x

t 20 40

μ 5.0 1.0

Br~ 15 - 7 . 7 33

SCN3.7 6.7

e

° Values at 1.0, 3.0, and 5.0 M (Br~) are 15, 12 and 7.7, respectively.


perhaps being trigonal b i p y r a m i d a l a n d the other tetragonal p y r a m i d a l .

As a

t h i r d a n d s o m e w h a t less p l a u s i b l e e x p l a n a t i o n , i t m i g h t be a s s u m e d t h a t a n a t i o n R e a c t i o n 1 a n d 2 a c t u a l l y p r o c e e d e d b y t w o p a r a l l e l r e a c t i o n p a t h s , one c o r r e s p o n d i n g t o t h e m e c h a n i s m d i s c u s s e d a b o v e , a n d t h e o t h e r t o a b i m o l e c u l a r S#2 process. T h i s s i t u a t i o n w o u l d l e a d t o a n i n c o r r e c t a s s i g n m e n t of k /kz v a l u e s .

However,

2

t h i s e x p l a n a t i o n w o u l d seem t o be r u l e d o u t b y t h e fact t h a t i d e n t i c a l v a l u e s of k /kz were o b t a i n e d i n t w o different s y s t e m s , t h e a n a t i o n r e a c t i o n a n d t h e a c i d 2

c a t a l y z e d a q u a t i o n of C o ( C N ) N 3 ~ . 5

Literature

3

Cited

(1) Basolo, F., Pearson, R . G . , " M e c h a n i s m s of Inorganic R e a c t i o n s , " p. 152, W i l e y , N e w Y o r k , 1958. (2) B u j a k e , J. E., Jr., N o y e s , R . M., J. Am. Chem. Soc. 83, 1555 (1961). (3) G r a s s i , R . J., W i l m a r t h , W. K., "Proceedings of the Seventh I n t e r n a t i o n a l C o n f e r ence o n C o o r d i n a t i o n C h e m i s t r y , " p. 242, B u t t e r w o r t h , L o n d o n , 1963. (4) H a i m , A l b e r t . P r e l i m i n a r y results. F u r t h e r w o r k is i n progress. (5) H a i m , A l b e r t , unpublished experiment. (6) H a i m , A l b e r t , unpublished experiments. (7) H a i m , Α., W i l m a r t h , W. K., Inorg. Chem. 1, 573, 583 (1962). (8) Ingold, C . K., " S t r u c t u r e a n d M e c h a n i s m s i n Organic C h e m i s t r y , " section 27d, p. 376, C o r n e l l U n i v e r s i t y Press, I t h a c a , 1953. (9) K a t z i n , L. E., McBeth, E. L., J. Phys. Chem. 62, 253 (1958). (10) K i n g , E . L., chapter i n " H o m o g e n e o u s C a t a l y s i s II," Ed., P . H. E m m e t t , R e i n h o l d , N e w Y o r k , 1955. (11) O l s o n , A. R., Simonson, T . R . , J. Chem. Phys. 17, 1167 (1949). RECEIVED

April

3,

1964.

Discussion I t is a pleasure t o r e p o r t t h e w o r k c a r r i e d o u t b y A l b e r t

Wayne K . W i l m a r t h :

H a i m a n d R o b e r t G r a s s i d e a l i n g w i t h s u b s t i t u t i o n r e a c t i o n s of t h e

pentacyano-

c o b a l t ( I I I ) complexes. I n the p a p e r , we e x p l o r e t h e p o s s i b i l i t y of i n t e r p r e t i n g these r e s u l t s i n t e r m s of a l i m i t i n g t y p e of S i v l m e c h a n i s m w i t h a n i n t e r m e d i a t e w h i c h has a p r o p o s e d l i f e t i m e sufficient t o d i s c r i m i n a t e between v a r i o u s n u c l e o p h i l e s p r e s e n t i n t h e s y s t e m . I h a d h o p e d t h a t we m i g h t h a v e m o r e w o r k t o r e p o r t a t t h i s t i m e , a n d w h i l e we h a v e c a r r i e d o u t c e r t a i n o t h e r s t u d i e s , i n t h e m a i n , t h e y d o n o t change g r e a t l y t h e c o n c l u s i o n s of t h e p a p e r .

I t h i n k , therefore, i n h a r m o n y w i t h t h e p h i l o s o p h y of t h e

F a r a d a y Society, that I w i l l merely conclude the talk at this point a n d a t t e m p t to answer pertinent questions.


2.

HAIM ET AL.

Diêcuuion

47

J o h n B a i l a r p o i n t e d o u t t h a t i n the e a r l y d a y s there

Richard G . Y a l m a n :

were s o m e 30-odd p r o p o s a l s for a W a l d e n r e a r r a n g e m e n t .

A s I l o o k a t the p a p e r s

t o d a y a n d t h e i r different m e c h a n i s m s i t seems t h a t w e a r e t r y i n g t o e s t a b l i s h , for t h e m o s t p a r t , single m e c h a n i s m s t o d e s c r i b e — a n d t h a t ' s a l l we are d o i n g , w e a r e d e s c r i b i n g t h i n g s h e r e — a w i d e v a r i e t y of s y s t e m s .

W h a t I a m s u g g e s t i n g is t h a t

w h e n we l o o k a t the p e n t a m m i n e s y s t e m , we are l o o k i n g a t the p e n t a m m i n e s y s t e m . W h e n w e are l o o k i n g a t the p e n t a c y a n o s y s t e m , we a r e l o o k i n g a t t h e p e n t a c y a n o system.

W h e n we l o o k a t the b i s e t h y l e n e d i a m i n e s y s t e m , w e a r e l o o k i n g a t t h e


bisethylenediamine system.

I t does n o t follow t h a t a n y of these s y s t e m s m a y h a v e

related mechanisms. I w o u l d even say that to t r y to look at a system and then analyze our data i n t e r m s of t h e p r o p o s a l of t h e a u t h o r i t y figures is p r o b a b l y d o i n g ourselves a d i s service.

T h i s is n o t t o s a y t h a t these d o n o t serve as w o n d e r f u l guide lines, a n d

t h a t p e r h a p s we w i l l e n d u p u s i n g t h o s e m e c h a n i s m s , b u t I d o h a v e a w o r d of c a u tion. A n o t h e r p o i n t w h i c h I w o u l d l i k e t o b r i n g u p is t h e i m p o r t a n c e of w h a t I w i l l call " o f f - s i t e " reactions.

W e have already touched on this t o d a y i n our discussions

of o t h e r sphere a s s o c i a t i o n . I t h i n k i n the c u r r e n t p a p e r , D r . W i l m a r t h ' s p a p e r w o r k e d on b y D r . H a i m , t h e a c i d c a t a l y s i s of t h e a q u a t i o n of t h e a z i d e s y s t e m i s a n e x a m p l e of w h a t I c a l l an "off-site" reaction.

T h e a t t a c h m e n t of h y d r o g e n t o n i t r o g e n , w h i c h is three

a t o m s a w a y f r o m t h e c o b a l t a t o m b r i n g i n g a b o u t a w e a k e n i n g of the c o b a l t n i t r o g e n b o n d a n d — i f I r e m e m b e r the figures c o r r e c t l y — a 3500-or 5800-fold increase i n the r a t e of a q u a t i o n . F o r those w h o are t u r n i n g t o w a r d b i o l o g i c a l a p p l i c a t i o n s , i t is t h e " o f f - s i t e " r e a c t i o n s w h i c h I t h i n k are g o i n g t o be m o r e i m p o r t a n t t h a n t h e o n - s i t e r e a c t i o n s w h i c h h a v e been t h e s u b j e c t of t h e p r e v i o u s d i s c u s s i o n . A n o t h e r e x a m p l e is t h e increase i n the a q u a t i o n of t h e i o d i d o p e n t a c y a n o s y s t e m i n t h e presence of t h e t r i - i o d i d e i o n .

A g a i n , t h i s is w h a t I w o u l d c a l l a n " o f f - s i t e "

or O S R reaction. A t h i r d e x a m p l e is the r e a c t i o n of n i t r i t e w i t h the a z i d e c o m p l e x . to discuss this more.

I don't want

I t h i n k that C a r l B r u b a k e r would like to talk about this.

I w i s h t o p o i n t o u t t h a t these r e a c t i o n s were s t u d i e d i n e i t h e r n e u t r a l o r a c i d i c solutions where the cyanide cobalt system is really unstable t h e r m o d y n a m i c a l l y . I raise the question about oxidation-reduction i n the iodo complex. mentioned i n the paper.

T h i s wasn't

I t seems t o m e i t w o u l d p r o v i d e a n a l t e r n a t e p a t h w h i c h

m i g h t increase t h e r e a c t i o n rates i n t h e case of the i o d i d e c o m p l e x . I h a v e t o u c h e d o n s o m e of t h e p e r i p h e r a l m a t t e r s . Carl H . Brubaker, J r . :

I agree w i t h D r . Y a l m a n t h a t t h i s r e p r e s e n t s a v e r y

c o m p l e t e piece of w o r k , a n d I t h i n k , t h e m a j o r i t y of t h e c o n c l u s i o n s a r e f a i r l y c l e a r cut.

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

I would hope

t h a t a l i t t l e l a t e r P r o f . W i l m a r t h o r o t h e r s w i l l s p e c u l a t e a b o u t t h e s t r u c t u r e s of t h i s t r a n s i t i o n s t a t e species, o r s e v e r a l species of t h e p e n t a c y a n o c o b a l t a t e ( I I )

t h a t are

s u p p o s e d t o be the t r a n s i t i o n s t a t e c o m p l e x , o r a n i n t e r m e d i a t e . I a l s o h o p e t h a t someone w o u l d c o m m e n t r e g a r d i n g D r . Y a l m a n ' s c o m m e n t s a b o u t t h e r e a c t i o n between the n i t r i t e i o n a n d t h e a z i d o c o m p l e x .

O n e finds t h a t

t h e s c a v e n g i n g a c t i v i t y o f — I believe i t w a s b r o m i d e a n d t h i o c y a n a t e — i s m u c h m o r e effective i n t h e presence t h a n i n t h e absence of n i t r i t e i n the a z i d e s y s t e m .

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

The

48


REACTIONS

s t a t e m e n t i s m a d e t o w a r d the e n d of the p a p e r t h a t t h e e n h a n c e d s c a v e n g i n g a c t i v i t y of t h i o c y a n a t e o r b r o m i d e m i g h t be the r e s u l t of m o r e r e a d y a t t a c k b y t h i o c y a n a t e o r b r o m i d e o n t h e p r o d u c t of t h e r e a c t i o n b e t w e e n t h e n i t r i t e a n d t h e a z i d o c o m p l e x , i n o t h e r w o r d s , a t h i n g d e s i g n a t e d as p e r h a p s b e i n g a n N4 o r a n N4O group; or the pentacyanocobaltate a n o t h e r e n t i r e l y different species.

species i n t h i s case m i g h t be i s o m e r i c w i t h I w o n d e r i f i t w o u l d e n ' t be w o r t h w h i l e t o s p e c u

late as t o w h y one m i g h t e x p e c t t h a t a different species w o u l d r e s u l t f o l l o w i n g t h e e l i m i n a t i o n of a n N4O, o r w h a t e v e r t h e f r a g m e n t s a r e , a n d w h y one m i g h t e x p e c t


t h a t t h i s N4O u n i t m i g h t be so m u c h m o r e e f f i c i e n t l y d i s p l a c e d b y t h i o c y a n a t e o r bromide.

I n f a c t , I t h i n k i t m i g h t be w o r t h w h i l e t o d i s c u s s t h e possible s t r u c t u r e

of t h i s p e n t a c y a n o c o b a l t a t e . W i t h respect t o t h e r e a c t i o n of t h e a z i d o c o m p l e x a n d n i t r o u s

Dr. W i l m a r t h :

a c i d , since t h i s w o r k w a s d o n e b y D r . H a i m , I t h i n k he s h o u l d c o m m e n t i f he cares t o d o so. In a d d i t i o n , I t h i n k workers i n D r . T a u b e ' s lab have s t u d i e d i n some detail the r e a c t i o n of a n u m b e r of a z i d o c o m p l e x e s w i t h n i t r o u s a c i d , a n d i t m i g h t be i n t e r e s t i n g t o h a v e a few c o m m e n t s o n t h i s w o r k . I t h i n k t h a t D r . W i l m a r t h a n d his c o - w o r k e r s h a v e p r e s e n t e d

Henry Taube:

e x c e l l e n t e v i d e n c e for t h e e x i s t e n c e of a genuine p e n t a c o o r d i n a t e d i n t e r m e d i a t e i n the c y a n o system.

T h e result reported b y t h e m t h a t the intermediate generated

b y t h e s p o n t a n e o u s r e a c t i o n i s different f r o m t h a t w h i c h m a y be g e n e r a t e d b y t h e r e a c t i o n of H O N O w i t h C o ( C N ) 5 N

3

+ 3

casts d o u b t o n s o m e of t h e c o n c l u s i o n s w h i c h

D r . H a i m a n d I r e a c h e d o n t h e r e a c t i o n of n i t r o u s a c i d w i t h t h e a z i d o p e n t a a m m i n o cobaltic ion.

I n a d d i t i o n , y o u w i l l r e m e m b e r t h a t s o m e of t h e r e s u l t s w h i c h R a l p h

P e a r s o n m e n t i o n e d a l s o cast d o u b t o n o u r c o n c l u s i o n s . I w a n t t o enlarge a l i t t l e o n t h e t h e m e of l o o k i n g for g e n u i n e i n t e r m e d i a t e s . I d o n o t k n o w h o w to u n d e r s t a n d t h e r e s u l t s of D r . P e a r s o n a n d c o - w o r k e r s , b u t a t t h e s a m e t i m e I b e l i e v e e v i d e n c e for the existence of i n t e r m e d i a t e s i n t h e c a t i o n i c species is a c c u m u l a t i n g . which D.

I w o u l d l i k e t o offer a s u g g e s t i o n r e l a t i n g t o t h e w o r k

D r . Pearson has cited a n d

then m e n t i o n some a d d i t i o n a l evidence

by

L o e l i g e r w h i c h fits i n w e l l w i t h t h e r e s u l t s of A . S a r g e s o n , a n d w i t h those of

others.

The

r e s u l t s in

support

toto

the

assumption

that

pentacoordinated

i n t e r m e d i a t e s are f o r m e d i n some s y s t e m s . D r . Pearson presented d a t a on the o p t i c a l density observed w h e n the n i t r a t o c o b a l t i c c o m p l e x r e a c t s w i t h t h i o c y a n a t e i o n , a n d there is n o t h i n g t o o b j e c t t o i n these r e s u l t s .

B u t I t h i n k one m i g h t be c o n c e r n e d a b o u t t h e t h e o r e t i c a l c u r v e

c a l c u l a t e d u s i n g t h e c o m p e t i t i o n r a t i o i n a t a b l e w hich H a i m a n d T a u b e p r e s e n t e d r

in the j o u r n a l . A n i m p o r t a n t p o i n t i n t h i s w o r k w h i c h m u s t be k e p t i n m i n d i s t h a t t h e r e a c t i o n of H O N O w i t h ( N H ) C o N 3

5

3

+ 2

often implicates the ligand, X .

T h o u g h i n some

i n s t a n c e s i t w a s s h o w n t h a t t h e f o r m a t i o n of t h e p r o d u c t ( N H 3 ) C o X 5

+ 2

was not

related to the c o n t r i b u t i o n to the t o t a l reaction b y the p a t h ( Ν Η ) Ο ο Ν 3

HONO

4- X~~, t h i s w a s n o t d o n e i n a l l cases.

δ

3

+

+ 2

I n a n y i n d i v i d u a l case, t h e p o s s i

b i l i t y exists t h a t a species N O X i n r e m o v i n g N ~ places X " ~ o n C o ( I I I ) .

T h e mere

fact t h a t ( N H ) C o X

3

3

3

6

+ 2

is f o r m e d is n o p r o o f t h a t a n i n t e r m e d i a t e ( N H ) C o " 6

, _ 3

f o r m e d ; a n y p r o o f rests r a t h e r o n d e m o n s t r a t i n g t h a t t h e r a t i o , ( ( N H ) Ô C O X 3

( ( N H ) 5 C o O H 2 ) is linear i n ( X ~ ) a n d on 3

+ 3

finding

4 2

is )/

t h e same c o m p e t i t i o n r a t i o i n


2.

HAIM BT AL.

Discussion

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

49

I n o u r w o r k , t h e v a r i a t i o n of c o m p e t i t i o n r a t i o as a f u n c

t i o n of (X~~) w a s t e s t e d o n l y i n a few cases, a n d o u r r e s u l t s i n a n y e v e n t were n o t v e r y precise.

F o r the system w i t h X ~ = NCS~~ the c o m p e t i t i o n ratio was found

n o t t o be i n d e p e n d e n t of (X~~), a n d t h i s itself is e v i d e n c e t h a t t h e r e is a c o m p l i c a t i o n i n t h i s r e a c t i o n ; t h e c o m p l i c a t i o n m a y be t h a t there is a s t r o n g c o m p o n e n t (ΧΗ ) Ο)Ν 3

δ

3

+ N O X where X

+ 2

is i n s e r t e d w h e n a z i d e i o n leaves (as Ν 2 +

-

from N2O).

R . B . J o r d a n , h a v i n g l e a r n e d a b o u t the w o r k of P e a r s o n a n d M o o r e , has b e c o m e i n t e r e s t e d i n the issues a n d has s e a r c h e d for t h e f o r m a t i o n of ( N H ) 5 C o B r 3

when ( N H ) C o N 0


3

5

3

+ 2

reacts i n the presence of B r ~ .

H i s observations

+ 2

suggest

t h a t t h e b r o m o c o m p l e x does a p p e a r i n i t i a l l y , b u t o n c a l c u l a t i n g the c o m p e t i t i o n r a t i o finds i t t o be less b y a f a c t o r of a b o u t 2 t h a n t h a t r e p o r t e d b y H a i m a n d m y self.

J o r d a n has d o n e o n l y a single e x p e r i m e n t o n t h i s .

T h e s u b j e c t is o b v i o u s l y

w o r t h g o i n g i n t o i n some d e t a i l , b u t a t t h i s p o i n t I a m n e i t h e r p r e p a r e d t o s a y t h a t Jordan's result supports the conclusions which D r . H a i m a n d I reached, nor that it does n o t . D r . L o e l i g e r has been s t u d y i n g t h e changes i n c o n f i g u r a t i o n w h i c h a c c o m p a n y substitution in acid solution.

T h e r e l e v a n c e of these e x p e r i m e n t s t o o u r present

c o n c e r n is t h i s : i f i n t e r m e d i a t e s are f o r m e d w h i c h h a v e p r o p e r t i e s i n d e p e n d e n t of h o w t h e y are f o r m e d , t h e n changes i n g e o m e t r y s h o u l d be i n d e p e n d e n t of h o w t h e net s u b s t i t u t i o n is b r o u g h t a b o u t .

S o m e of t h e d a t a w h i c h L o e l i g e r has o b t a i n e d

together w i t h those of o t h e r s are s h o w n i n t h e f o l l o w i n g t a b l e .

Configuration Changes Accompanying Spontaneous a n d Assisted Aquation Assumed Intermediate trans-Co trans-Co trans-Co trans-Co trans-Co trans-Co a h e

Method of

en N en N en N en H20 en H 0 en H 0 2

3

2

3

2

3

trans-Co trans-Co trans-Co trans-Co trans-Co trans-Co

+ 2

+ 2 + 2

2

2

2

2

2

en (N ) + + en (N ) -f en N Cl+ + en N H 0+ en ClH 0+ en (H 0) 2

3

2

2

2

2

2

3

2

3

2

2

2

2

2

2

%

trans Product

Formation

2

2

+ 3

HN0 Hg+ Hg+ + HN0 + Hg+ + H 0

100 100" 100* 60 ± 5° 60 65 e

2

2

2

2

6

2

c

2

Experiments by D . Loeliger. Sargeson, A . M . , Australian J. Chem., 17(3), 385 (1964). Kruse, W . , Taube, H . , J". Am. Chem. Soc, 83, 1280 (1961). T h e o b s e r v a t i o n s i n w h i c h c o n f i g u r a t i o n is r e t a i n e d (100% t r a n s p r o d u c t ) a r e

o n l y of l i m i t e d usefulness.

T h e e x p e r i m e n t a l r e s u l t s are n o t refined e n o u g h

to

d i s t i n g u i s h b e t w e e n 99.0, 99.9 a n d 99.99% f o r m a t i o n of t h e t r a n s p r o d u c t , y e t these n u m b e r s c o r r e s p o n d t o a v a r i a t i o n i n t h e p r o d u c t r a t i o s b y a f a c t o r of 100.

How

e v e r i n s e v e r a l i n s t a n c e s , p r o d u c t r a t i o s i n t h e range of 0.1 t o 10 h a v e been o b s e r v e d , a n d i f a p a r t i c u l a r p r o d u c t r a t i o i n t h i s range is m a i n t a i n e d w h i l e t h e m e a n s of f o r m i n g t h e a q u o p r o d u c t is a l t e r e d , t h i s o b s e r v a t i o n c a n be r e g a r d e d as s i g n i f i c a n t . O n e of t h e m o s t i n t e r e s t i n g o b s e r v a t i o n s is t h a t i n v o l v i n g / r a w s - C o e m ^ O " as t h e 4

presumed intermediate. the

same

whether the diaquo

Coen N H 0+ 2

3

2

2

3

T h e e x t e n t of i s o m e r i z a t i o n , w i t h i n e x p e r i m e n t a l e r r o r is

with H N 0

2

product

is formed

by

the

r e a c t i o n of

o r of / r a w s - C o e n C l H 0 + w i t h H g + . 2

isotopic studies w i t h 2 r a w 5 - C o e n ( H 0 ) 2

2

2

+ 3

2

2

2

trans-

I n the oxygen

t w o k i n d s of m e a s u r e m e n t s w h e r e m a d e :

t h e exchange of o x y g e n i n t o / m w j - C o e n 2 ( H ? 0 )

2

+3

a n d t h e r a t e of c h a n g e of t h e t r a n s

f o r m t o t h e cis, w h i c h i n v o l v e s i n c o r p o r a t i n g one s o l v e n t o x y g e n for e a c h cis i o n formed.

If i t is a s s u m e d t h a t b o t h processes i n v o l v e a c o m m o n i n t e r m e d i a t e ,


50


/raws-Coen2H20 , w h i c h is p i c k i n g u p a m o l e c u l e of s o l v e n t e i t h e r r e t a i n s c o n f i g u r a +3

t i o n o r changes t o t h e cis f o r m , a n u m b e r c o r r e s p o n d i n g t o t h e r e t e n t i o n of t h e t r a n s c o n f i g u r a t i o n c a n be c a l c u l a t e d w h i c h , w i t h i n e x p e r i m e n t a l e r r o r , is t h e s a m e as for the o t h e r t w o s y s t e m s . fluence

T h e r e s u l t s suggest t h a t t h e l e a v i n g g r o u p does n o t i n

t h e g e o m e t r i c course of t h e r e a c t i o n , a n d t h i s i n t u r n , suggests t h a t a c o m

m o n i n t e r m e d i a t e is f o r m e d w h i c h has lost m e m o r y of h o w i t w a s f o r m e d . Michael Anbar: paper.

I w o u l d l i k e t o a s k t w o o r three q u e s t i o n s p e r t a i n i n g t o t h i s

F i r s t , a c c o r d i n g t o t h i s p a p e r , t h e h y d r o x y l c o m p l e x is f o r m e d a l s o a c c o r d


i n g t o t h e same SATI m e c h a n i s m , as f a r as I c o u l d u n d e r s t a n d , w h i c h m e a n s t h a t t h e p r o t o n t r a n s f e r , g o i n g f r o m t h e a q u o t o a h y d r o x i d e , is s l o w e r t h a n a d i s s o c i a t i o n of the a q u o c o m p l e x t o f o r m t h e p e n t a c y a n o c o m p l e x .

C o u l d t h i s be c o r r o b o r a t e d b y

some i n d e p e n d e n t m e t h o d of l o o k i n g a t t h e r a t e of p r o t o n e x c h a n g e ?

In othet

w o r d s , t h e r e a c t i o n exchange a n d t h e o x y g e n exchange s h o u l d be a t t h e s a m e r a t e , according to this mechanism, if I understood it right. T h e o t h e r q u e s t i o n c o n c e r n s the c a t a l y t i c effect of i o d i n e o n t h e i o d i n a t i o n of the p e n t a c y a n o c o m p l e x .

D i d y o u ever o b s e r v e t h e same effect of I2 s a y o n t h e

s u b s t i t u t i o n of b r o m i d e , or t h i o c y a n a t e as w e l l ? A t h i r d q u e s t i o n is w h e t h e r y o u h a v e o b s e r v e d a n y specific c a t i o n i c effect i n changing, at y o u r high concentrations, from s o d i u m to potassium or to another a l k a l i m e t a l — i . e . , w h e t h e r t h i s has a n y effect o n t h e r a t e . ment.

T h e r e is one m o r e c o m

Y o u mentioned a n Sjyl mechanism referring to organic chemistry.

y e a r s ago we p u b l i s h e d r e s u l t s o n t h e h y d r o l y s i s of

fluoroborate

A few

ions where exactly

t h e s a m e m e c h a n i s m w a s p o s t u l a t e d ( / . Phys. Chem. 6 4 , 1896 ( I 9 6 0 ) ) .

Fluorobo

r a t e i o n s u n d e r g o h y d r o l y s i s i n t h e a l k a l i n e r e g i o n i n d e p e n d e n t of O H " c o n c e n t r a -

t i o n ; a n d a g a i n t h i s v e r y h i g h increase i n t h e r a t e of t h e Sjsrl m e c h a n i s m b y p r o t o n a tion occurs.

H B F 4 undergoes a g a i n a n S ^ l c l e a v a g e , b u t t h e r a t e of h y d r o l y s i s i s

a c c e l e r a t e d b y s e v e r a l orders of m a g n i t u d e . Dr. Wilmarth:

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

correctly, y o u are not q u o t i n g the m e c h a n i s m we proposed. ROH 110HR OH 2

£ k

'

R Ι «

+ νγ

H 0 2

>RN

3

I n a c i d s o l u t i o n i t i s a s s u m e d t h a t t h e a q u a p e n t a - c y a n o i o n reacts t o f o r m a n i n t e r m e d i a t e , a n d t h a t t h e i n t e r m e d i a t e m a y r e a c t w i t h w a t e r , o r m a y be p i c k e d u p b y a scavenger s u c h as a z i d e i o n t o f o r m p r o d u c t s .

I t is a l s o a s s u m e d t h a t t h e

c o m p l e x , w h i c h has a p K of 9.8 is i n e q u i l i b r i u m w i t h R O H , t h e p r e d o m i n a n t species in akaline solution.

T h e d e c i s i o n t h a t we h a d t o m a k e w a s w h e t h e r t h e decrease i n

r a t e o c c u r r e d because R O H w a s a c o m p l e t e l y i n e r t species, o r w h e t h e r R O H a l s o underwent a n S#l

m e c h a n i s m as w e l l as t h i s .

It was o u r conclusion t h a t the

e v i d e n c e f a v o r e d t w o p a r a l l e l Sjsrl m e c h a n i s m s . W i t h respect t o t h e g e n e r a l i o d i n e c a t a l y s i s o r h y d r o g e n c a t a l y s i s , t h i s s u b j e c t is s t i l l u n d e r i n v e s t i g a t i o n , a n d p o s s i b l y t h e m e c h a n i s m is m o r e c o m p l i c a t e d t h a n we h a v e i n d i c a t e d i n t h e p a p e r .

W e do k n o w , a m o n g other scattered observations,

t h a t t h e i o d o p e n t a c y a n o c o m p l e x r e a c t s i n t i m e of m i x i n g w i t h a q u e o u s b r o m i n e t o give the bromopentacyano complex.

T h e r e is some s u s p i c i o n t h a t the h y p o i o d o u s

a c i d w o u l d a l s o c a t a l y z e t h e s o l o v c l y s i s of i o d o p e n t a c y a n o c o m p l e x , so t h a t a r a t h e r


2.

MAIM IT A l .

51

Dhtutslon

c a r e f u l p H d e p e n d e n c e w i l l be r e q u i r e d t o d e t e r m i n e m o r e a c c u r a t e l y t h e p o s s i b l e mechanisms. W e k n o w r e l a t i v e l y l i t t l e a b o u t t h e effect of c h a n g e i n p o s i t i v e i o n i n t h e s y s t e m , b u t w e t h i n k t h a t t h e r a t e w o u l d p r o b a b l y be f a i r l y s e n s i t i v e t o t h i s .

The only

d e f i n i t e i n f o r m a t i o n w e h a v e i n v o l v e s r e c e n t s t u d i e s of t h e s u b s t i t u t i o n of w a t e r b y p y r i d i n e , a n e u t r a l n u c l e o p h i l e , a n d a r a t h e r efficient one.

H e r e there is perhaps a

20 o r 3 0 % difference i n r a t e d e p e n d i n g o n w h e t h e r w e a r e w o r k i n g i n IM ion or 1M p y r i d i n i u m i o n .

sodium

T h e r a t e is faster i n t h e p y r i d i n i u m i o n s o l u t i o n .


I n t e r e s t i n g l y , t h e p l o t of k vs. p y r i d i n e c o n c e n t r a t i o n does n o t c u r v e t o t h e extent t h a t we w o u l d expect.

P o s s i b l y this represents a general difficulty i n a t

t e m p t i n g to study neutral nucleophiles. I n t h e s t u d y of a n u c l e o p h i l e s u c h a s a z i d e , i t i s p o s s i b l e t o r e p l a c e p e r c h l o r a t e w i t h azide even t o l i l f concentration w i t h o u t a n appreciable change i n the a m o u n t of w a t e r o r t h e a c t i v i t y of w a t e r i n t h e s y s t e m ; b u t b y t h e t i m e one h a s r e a c h e d

1M

p y r i d i n e , 78 g r a m s of p y r i d i n e , t h e m e d i u m h a s c h a n g e d r a t h e r s u b s t a n t i a l l y , a n d t h e reverse p a t h i n v o l v i n g w a t e r m a y be s u b s t a n t i a l l y effected. Arthur Adamson:

I think D r . Wilmarth

and

co-workers

have

probably

s u p p l i e d t h e b e t t e r a v a i l a b l e e v i d e n c e for t h e p e n t a c o o r d i n a t e d i n t e r m e d i a t e i n a cobalt substitution reaction. H o w e v e r , I t h i n k t h a t a g a i n i t is p o s s i b l e t o t r e a t these d a t a , as I t h i n k a l s o t h a t D r . T a u b e n o t e d , i n t e r m s of t h e s o l v e n t cage p i c t u r e a n d i n t e r m s of t h e i d e a t h a t t h e r e a c t a n t s h a v e p r e a s s e m b l e d before t h e a c t i v a t i o n e n e r g y a r r i v e s . S p e c i f i c a l l y , I a m n o t s u r e t h a t i t ' s a l w a y s safe t o a s s u m e t h a t b e c a u s e o n e species i s n e u t r a l , o r because t h e t w o a r e l i k e i n c h a r g e , t h a t o n e w i l l therefore n o t h a v e i o n a s s o c i a t i o n o r a n y a p p r e c i a b l e preference t o w a r d s a s s o c i a t i o n . I t h a s been o b s e r v e d t h a t i o n - p a i r i n g c o n s t a n t s a r e d i f f e r e n t for C r ( I I I ) a n d C o ( I I I ) complexes.

analogous

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

n o t just the o v e r a l l charge is i m p o r t a n t .

T h e r e is a question as to just how far out,

for e x a m p l e , a p p r e c i a b l e d - e l e c t r o n d e n s i t y m a y be p r e s e n t ; w h e t h e r t h e r e m a y b e , i n f a c t , a g o o d d e a l of c o o r d i n a t i o n p o s s i b l e i n t h e s e c o n d c o o r d i n a t i o n sphere so t h a t o n e h a s forces f a v o r i n g a s s o c i a t i o n w h i c h a r e n o t j u s t e l e c t r o s t a t i c .

Thus

a s s o c i a t i o n s c o u l d p r o v i d e k i n e t i c i n t e r m e d i a t e s e v e n i n these s y s t e m s . J a c k Halpern:

I would like to comment further on the question that D r .

A n b a r r a i s e d a b o u t t h e p o s s i b i l i t y of g e n e r a t i n g t h e p e n t a c y a n o c o b a l t ( I I I ) electron transfer from

by

pentacyanocobalt(II).

S h u z o N a k a m u r a a t t h e U n i v e r s i t y of C h i c a g o h a s b e e n l o o k i n g i n t o t h i s possibility.

T h e reaction i n question is

Co(CN)

Co(CN) Br6

3

T h e r e a c t i o n p r o c e e d s w i t h a r a t e c o n s t a n t i n excess of

+ 10

9

~ sec."" a p

M~

l

1

p r o a c h i n g t h e d i f f u s i o n c o n t r o l l e d l i m i t a n d i m p l y i n g t h a t s u b s t i t u t i o n of a s i x t h l i g a n d i n t o t h e c o o r d i n a t i o n s h e l l of C o ( C N ) 5 ~ " is a n e x t r e m e l y r a p i d process. 3

N o w , i f one o x i d i z e s C o ( C N ) 5 ~ ~ w i t h a n o x i d i z i n g a g e n t s u c h as C o ( N H 3 ) e 3

+ 3


w h i c h c a n n o t s u p p l y a b r i d g i n g l i g a n d , so t h a t i n n e r sphere e l e c t r o n t r a n s f e r i s p r e c l u d e d , t h e n b y f a r t h e strongest preference of t h e C O ( C N ) Ô ~ u n d e r these c o n d i t i o n s 3

is, i f a t a l l p o s s i b l e , t o p i c k u p a s i x t h c y a n i d e f r o m t h e s o l u t i o n a n d t h u s t o f o r m C o ( C N ) " as t h e p r o d u c t . 3

6

T h e k i n e t i c s of t h i s r e a c t i o n a r e and Ο ο ( Ν Η ) β .

first-order

i n free C N ~ " a s w e l l as i n C o C C N J g "

3

T h i s behavior persists d o w n to extemely low C N ~ concentration.

4 3

3

C N ~ , even i n v e r y small concentration, is the preferred ligand to complete the co ordination shell. I f one r e a l l y forces t h e issue b y g o i n g t o e x t r e m e l y l o w c y a n i d e c o n c e n t r a t i o n s , the reaction becomes exceedingly slow, to the p o i n t where the results are almost unreliable.

U n d e r these c o n d i t i o n s t h e f o r m a t i o n of s o m e C o C C N ^ O F ^ " "

3

as

a

r e a c t i o n prodjuct is d e t e c t a b l e , a n d w e h a v e a t t e m p t e d t o see w h e t h e r t h i s m i g h t arise through the pentacoordinated cobalt(III) t h i s i n t h e p r e s e n c e of i o n s s u c h a s N ~ . of N

3

intermediate CO(CN)B"~ b y doing 2

H o w e v e r , we d o n o t o b s e r v e a n y p i c k u p

3

~ u n d e r these c o n d i t i o n s c o n s i s t e n t w i t h t h e d i s c r i m i n a t i o n p a t t e r n r e p o r t e d b y

H a i m and Wilmarth.

W e thus concluded t h a t the CO(CN)Ô"" reported b y t h e m is 2

n o t f o r m e d u n d e r a n y c o n d i t i o n s i n t h e o x i d a t i o n of C o ( C N ) 5 ~ . 3

T h e r e a r e t w o inferences t h a t n e e d t o be d r a w n h e r e .

O n e concerns the ex

t r e m e l y h i g h r a t e f o r these i n n e r s p h e r e o x i d a t i o n s , c o u p l e d w i t h o u r f a i l u r e t o f o r m Co(CN)50H2~" under a n y but most extreme conditions. 2

I t h i n k this argues v e r y

s t r o n g l y a g a i n s t t h e s u g g e s t i o n t h a t C o ( C N ) g ~ i s a c t u a l l y a n a q u o c o m p l e x (-i.e., 3

Co(CN) OH2~ ).

If i t were, i t is h a r d to u n d e r s t a n d w h y i t fails t o undergo direct

3

6

o x i d a t i o n t o C O ( C N ) B O H 2 ~ , w h i c h i s a p e r f e c t l y s t a b l e species w h e n g e n e r a t e d i n 2

other ways. T h e o t h e r i n f e r e n c e relates t o t h e f a c t t h a t i t a l s o a p p e a r s t o be v e r y d i f f i c u l t e v e n u n d e r c o n d i t i o n s w h e r e one h a s s l o w e d d o w n a l l t h e a l t e r n a t e p a t h s , t o c o n v e r t Co(CN) ~ 5

3

b y d i r e c t o x i d a t i o n t o a species w h i c h i s i d e n t i c a l t o t h e C O ( C N ) B ~

species of D r . H a i m a n d D r . W i l m a r t h .

2

T h i s suggests t h a t t h e t w o species m u s t be

s t r u c t u r a l l y v e r y different, so t h a t t h e t r a n s f o r m a t i o n of o n e t o t h e o t h e r d o e s n o t o c c u r r e a d i l y , b u t r a t h e r t h a t C o ( C N ) " ~ prefers t o e x p a n d i t s c o o r d i n a t i o n s h e l l b y 6

2

c a p t u r i n g a s i x t h l i g a n d before t r a n s f e r r i n g a n e l e c t r o n . I t seems l i k e l y t h e s t r u c t u r e of t h e C o ( C N ) ~ i s t r i g o n a l b i p y r a m i d a l , w h e r e a s 6

3

t h e s t r u c t u r e of t h e C O ( C N ) B ~ ~ i s s q u a r e p y r a m i d a l . 2

Dr. Bru b a k e r :

I have been interested personally i n the w o r k t h a t has come

from A u s t r a l i a i n the last year b y B e t t s a n d W i n f i e l d and others o n the oxidation a l s o of t h e p e n t a c y a n o w i t h o x y g e n .

H e r e t h e y find t h e o x y g e n b i n d s a

i n t e r m e d i a t e species w h i c h m a y be a m o n o p e r o x o m o n o m e r , scavenges

very

well

for

the

decacyano-M-peroxyldicobalt(III)

pentacyanocobalt(II) complex.

and

proposed

which this

forms

the

group

familiar

S o i n t h i s case t o o , t h e o x i d a n t s t i c k s ,

not water.


2.

HAIM ET AL.

Discussion

53

I w o u l d like to comment on the relative nucleophilic character

Anthony Poe:

t o w a r d s a c t i v e i n t e r m e d i a t e s a n d the fact t h a t i t h a s been s a i d t h a t t h e differences b e t w e e n t h e n u c l e o p h i l i c c h a r a c t e r s of a n i o n s a r e r a t h e r s m a l l . W e w o u l d h a v e t h o u g h t t h e s a m e t h i n g a f t e r s o m e s t u d i e s of t h e c o m p e t i t i o n between c h l o r i d e , b r o m i d e , a n d i o d i d e for s o m e r h o d i u m ( I I I ) c o m p l e x e s , because we f o u n d t h a t t h e r a t i o s of the r a t e c o n s t a n t s a t 50°C. were v e r y n e a r l y 1:1:1.

How

ever, w h e n a c t i v a t i o n energies were m e a s u r e d , we f o u n d t h a t these p r o d u c e a m u c h bigger d i s c r i m i n a t i o n .

I n f a c t , t h e a c t i v a t i o n e n e r g y for t h e a d d i t i o n of i o d i d e t o


o u r r e a c t i v e i n t e r m e d i a t e s is n o less t h a n 6 k c a l . g r e a t e r t h a n t h e a c t i v a t i o n e n e r g y for a d d i t i o n of c h l o r i d e a n d b r o m i d e . N o n e of these e x p e r i m e n t s o n r e a c t i v e i n t e r m e d i a t e s has been d o n e o v e r a r a n g e of t e m p e r a t u r e s .

A t least, I t h i n k t h a t is t r u e .

I t w o u l d be i n t e r e s t i n g t o see

w h e t h e r t h i s w o u l d p r o d u c e a bigger d i s c r i m i n a t i o n b e t w e e n t h e v a r i o u s n u c l e o p h i l e s t h a n is g i v e n b y t h e r a t e c o n s t a n t s . W e w o u l d be i n t e r e s t e d i n h a v i n g b e t t e r t e m p e r a t u r e coeffi-

Dr. Wilmarth:

c i e n t d a t a f o r — , b u t i t w o u l d r e q u i r e e x p e r i m e n t s of e x t r e m e a c c u r a c y to o b t a i n a n y i n f o r m a t i o n of i n t e r e s t here.

O n e w o u l d be d e a l i n g w i t h a r a t i o of slopes t o i n t e r

cepts i n plots at two temperatures to obtain this information. I n connection w i t h that, do y o u w a n t to comment on the fact

Dr. Y a l m a n :

t h a t t h e e n t r o p y change t o t h e a z i d e a q u a t i o n i s m a r k e d l y different f r o m t h a t f o r the thiocyanate a n d the iodide?

I t i s q u i t e p o s s i b l e t h a t t h e difference i s m o r e

apparent than real. Dr. Wilmarth: t h e difference i s r e a l .

I d o n ' t h a v e a n y r e a l e x p l a n a t i o n for t h i s , b u t I believe t h a t S i n c e t h e s t r u c t u r e of t h e t w o i o n s is different, i t seems t h a t

t h e AS* v a l u e s m i g h t differ. R e g a r d i n g these

Albert H a i m :

questions

of

the

reactions

of

the

azido

c o m p l e x e s w i t h n i t r o u s a c i d , t h e r e are t w o s y s t e m s t h a t I h a v e t r i e d t o e x p l o r e : the azidopentammine-cobalt(III)

complex

a n d the

azidopentacyanocobalt(III)

complex. W e h a v e t r e a t e d these c o m p l e x e s w i t h n i t r o u s a c i d i n t h e presence of v a r i o u s a n i o n s , a n d we t r i e d t o o b t a i n i n f o r m a t i o n o n t h e k i n e t i c s a n d s t o i c h i o m e t r i e s of t h e reactions.

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

a z i d o p e n t a m m i n e c o b a l t ( I I I ) o r a z i d o p e n t a c y a n o - c o b a l t ( I I I ) are e x t r e m e l y s e n s i t i v e t o the presence of a n i o n s o t h e r t h a n p e r c h l o r a t e .

I n regard to the kinetic sensi

t i v i t y , w e c a n i n d i c a t e b y a p l u s s i g n t h a t b o t h of these r e a c t i o n s a r e e x t r e m e l y s e n s i t i v e t o the presence of a n i o n s o t h e r t h a n p e r c h l o r a t e . Sensitivity of Kinetics Co(NH ) N + 3

6

3

Co(CN) Nr 5

3

+

2

+

HN0 HN0

2

2

Sensitivity of Stoichiometry

+ +

T h e o t h e r q u e s t i o n i s t h e s e n s i t i v i t y of t h e s t o i c h i o m e t r y of t h e r e a c t i o n .

I f there

is n o t h i n g b u t n i t r o u s a n d p e r c h l o r i c a c i d s i n these s y s t e m s , t h e p r o d u c t s of these reactions are the corresponding aquo complexes.

I f one has, i n a d d i t i o n , t h e a n i o n

X " * a t sufficiently h i g h c o n c e n t r a t i o n of X~~, one d e t e c t s s o m e X - p e n t a m m i n e c o b a l t -


54


(III) or some X - p e n t a c y a n o c o b a l t ( I I I ) .

B u t t h e s e n s i t i v i t y of t h e s t o i c h i o m e t r i c s

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

I think

t h a t t h e o n l y c o n c l u s i o n t h a t one c a n r e a c h f r o m t h i s t y p e of d a t a i s t h a t t h e r e i s a n intermediate i n the system. N o w the question is, w h a t is this intermediate ? been a b o u t .

T h i s is w h a t the a r g u m e n t has

A s w e suggest i n t h e p a p e r , t h e r e a r e v a r i o u s possible i n t e r m e d i a t e s .

T h e first one t h a t c o m e s t o m i n d i s , b y a n a l o g y t o t h e r e a c t i o n of free a z i d e i o n w i t h n i t r o u s a c i d w h i c h h a s been s t u d i e d b y k i n e t i c a n d t r a c e r s t u d i e s , t h e i n t e r m e d i a t e R N N N N O f o r m e d b y a d d i t i o n of N O " t o t h e a z i d e .


4

fate of t h i s i n t e r m e d i a t e i n t h e p r e s e n t s y s t e m s ?

N o w t h e q u e s t i o n is, w h a t is t h e

I t can react w i t h X ~ , it can react

x RNNNNOH 0 2

dissociation

w i t h H 0 ; or i t can dissociate to give the intermediate, R . 2

T h e p o i n t t h a t needs t o

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

T h a t is, the possible d i s c r i m i n a t i o n

of t h e R N N N N O i n t e r m e d i a t e for X ~ a n d H 0 m a y be different f r o m t h e possible 2

d i s c r i m i n a t i o n of the p e n t a c o o r d i n a t e d i n t e r m e d i a t e for X ~ a n d H 0 . 2

In addition

t o t h e q u e s t i o n of i n t e r m e d i a t e s of different s t o i c h i o m e t r y , ( R N N N N O a n d

R)

there is the q u e s t i o n of the g e o m e t r y of t h e p e n t a c o o r d i n a t e d i n t e r m e d i a t e .

At

t h i s p o i n t i t w o u l d be o n l y s p e c u l a t i o n t o s a y w h a t the g e o m e t r y of t h i s p e n t a c o ordinated intermediate is.

H o w e v e r , i t is p o s s i b l e , t h a t i n different

different geometries m i g h t be o b t a i n e d .


reactions,

Mechanisms of Substitution Reactions of Cobalt (III ... - ACS Publications

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