The Role of Ion Association in the Substitution Reactions of

1 The Role of Ion Association in the Substitution Reactions of Octahedral Complexes in Nonaqueous Solution Downloaded by UNIV OF NORTH CAROLINA on October 23, 2015 | http://pubs.acs.org Publication Date: January 1, 1965 | doi: 10.1021/ba-1965-0049.ch001

M A R T I N L. TOBE* University College, London,

England

The k i n e t i c s o f r e p l a c i n g X in c o m p l e x e s o f the t y p e [Co en2 A X]+n h a v e b e e n r e v i e w e d . The rate dependence on the concentration of the entering reagent varies from first-order to z e r o - o r d e r a n d reflects t h e p r e a s s o c i a t i o n e q u i l i b r i a of the r e a g e n t s a n d n o t t h e m o l e c u larity of the a c t u a l substitution. The n a t u r e , stoic h i o m e t r y , a n d e q u i l i b r i u m constants o f t h e s e ion a g g r e g a t e s h a v e b e e n discussed i n t e r m s o f c o m p e t i t i o n b e t w e e n s o l v e n t a n d solute f o r a p o s i t i o n i n the i n n e r s o l v a t i o n s h e l l of the c o m p l e x . It is p r o p o s e d t h a t , a l t h o u g h t h e k i n e t i c b e h a v i o r d o e s n o t r e f l e c t t h e substitution m e c h a n i s m , t h e r e a r e circumstances in w h i c h it is p o s s i b l e to distinguish b e t w e e n a unimolecular mechanism a n d a borderline bimolecular m e c h a n i s m in w h i c h the e n t e r i n g g r o u p does not contribute to t h e enthalpy of the transition state.

i n m o l e c u l a r i t y s t u d i e s of s u b s t i t u t i o n a t a n o c t a h e d r a l t r a n s i t i o n m e t a l i o n , t h e '

r e a c t i o n s of c o b a l t (111) c o m p l e x e s w i t h e t h y l e n e d i a m i n e , of t h e t y p e , [ G o

A X]

+ n

, h a v e p r o v i d e d m u c h of t h e d a t a .

en

2

I t w a s r e a l i z e d e a r l y t h a t t h e c h o i c e of

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

conveniently

T h e o n l y reac

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

I n e a c h case a n i m p o r t a n t e n t i t y of t h e r e a c t i o n w a s

solvent molecule or its lyate i o n . equivocal or impossible.

Therefore, a n y kinetic interpretation was either

I n the solvolytic reaction, the zero-order w i t h respect t o

* Paper presented by Cooper H . Langford.

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

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

8

solvent is not related to the molecularity.

I n the anation reaction, extensive mass-

l a w r e t a r d a t i o n b y w a t e r w i l l l e a d t o a s e c o n d - o r d e r k i n e t i c f o r m for a u n i m o l e c u l a r r e a c t i o n (4).

I n t h e base h y d r o l y s i s r e a c t i o n , i t is n o t possible t o t e l l w h e t h e r t h e

h y d r o x i d e i n t h e p r o d u c t e n t e r e d as s u c h , o r c a m e i n as w a t e r i n a b a s e - c a t a l y z e d solvolysis. I t w a s o b v i o u s , therefore, t h a t a l t e r n a t i v e s o l v e n t s s h o u l d be u s e d . p r o b l e m w a s t o be c e r t a i n t h a t d i r e c t s u b s t i t u t i o n w a s o c c u r r i n g .

The

first

R e p l a c e m e n t of

one l i g a n d b y a n o t h e r has been s h o w n t o be a t w o stage process (6), cis-ICo e n N 0

Downloaded by UNIV OF NORTH CAROLINA on October 23, 2015 | http://pubs.acs.org Publication Date: January 1, 1965 | doi: 10.1021/ba-1965-0049.ch001

2

cis-[Co e n N 0 2

2

Cl]+ + H 0 — cis-[Co e n N 0 2

H 0]+

2

2

2

2

2

H 0]+ 2

+ S C N ~ -> a s - [ C o e n N 0 2

2

+ C I " (Aquation)

NCS]+ + H 0

2

2

(Anation)

T h e r e l a t i v e r e a c t i o n rates a n d t h e s t a b i l i t y of t h e a q u o c o m p l e x m a k e i t p o s s i b l e t o i d e n t i f y t h e a q u o c o m p l e x as a n i n t e r m e d i a t e a n d s t u d y t h e i n d i v i d u a l a c t s separately.

H o w e v e r , i f the s o l v e n t o c o m p l e x were less s t a b l e a n d t h e a n a t i o n r a t e

m u c h faster t h a n t h e s o l v o l y s i s , i t w o u l d n o t be p o s s i b l e t o o b s e r v e t h i s i n t e r m e d i a t e , a n d t h e process w o u l d be 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 a u n i m o l e c u l a r dissociative

process.

B o t h processes w o u l d e x h i b i t o v e r a l l

first-order

kinetics

a n d the usual mass-law retardation a n d other competitive phenomena characteristic of a n e x t r e m e l y r e a c t i v e i n t e r m e d i a t e . T h i s p r o b l e m r e q u i r e s a m o d i f i e d a p p r o a c h w h i c h G r a y (16) has s o l v e d i n t h e case of s u b s t i t u t i o n i n s q u a r e p l a n a r c o m p l e x e s .

H e uses t h e f a c t t h a t bases, l i k e

hydroxide, substitute very slowly but will immediately deprotonate,

a n d hence

stabilize, a protonic solvento

cannot

intermediate.

T h i s elegant a p p r o a c h

be

a p p l i e d t o the o c t a h e d r a l c o b a l t a m m i n e s w h o s e r e a c t i o n r a t e w i t h s u c h bases is very high. O u r a l t e r n a t i v e a p p r o a c h has been t o s y n t h e s i z e t h e s o l v e n t o i n t e r m e d i a t e a n d then study its reactions i n isolation.

W e thereby hope to show that its reactivity

a n d s t e r i c course is i n c o n s i s t e n t w i t h t h e p o s t u l a t e s t a t i n g t h a t i t is a n i n t e r m e d i a t e i n the substitution reactions.

C o m p l e x e s of t h e t y p e cis- a n d trans-[Co e n

C l ] + (7), a n d cis-[Co e n ( C H ) S O C l ] + (32) h a v e b e e n p r e p a r e d . 2

2

3

2

CH3OH

W e have shown

2

2

i n t h e first case t h a t t h e l a b i l i t y of t h e c o o r d i n a t e d m e t h a n o l does n o t s u f f i c i e n t l y explain the nonappearance trans-[Co e n C l ] 2

2

+

of t h e s o l v e n t o c o m p l e x

i n t h e r e a c t i o n s of cis- a n d

i n m e t h a n o l unless i t is n o t a n i n t e r m e d i a t e i n t h e r e a c t i o n .

d i m e t h y l s u l f o x i d e s o l u t i o n , cis- a n d trans-[Co e n

2

Cl ] 2

+

In

h a v e been s h o w n t o i s o m -

erize t o a n e q u i l i b r i u m m i x t u r e t h a t a l s o c o n t a i n s t h e s o l v e n t o i n t e r m e d i a t e

(32).

D e t a i l e d k i n e t i c s t u d i e s i n d i c a t e t h a t a b o u t 8 0 % of t h e t i m e i s o m e r i z a t i o n goes v i a the solvento intermediate.

B u t t h i s is n o t a r a t e - d e t e r m i n i n g s o l v o l y s i s , r a t h e r

a t e m p o r a r y d i v e r s i o n of t h e i n t e r m e d i a t e of a d i s s o c i a t i v e r e a c t i o n . has r e c e n t l y p r e p a r e d t h e d i m e t h y l f o r m a m i d e c o m p l e x , [ C o e n

2

Watts

D MF Cl]

+ 2

(33) and

has s h o w n t h a t i t c a n n o t be a n i n t e r m e d i a t e i n t h e i s o m e r i z a t i o n of cis- a n d trans[Co e n C l ] 2

2

+

in dimethylformamide.

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

2

A X]

+

n

i n nonaqueous solvents a n d w i l l show h o w the general conditions

t h a t a p p l y here c a n be e x t e n d e d t o a q u a t i o n a n d o t h e r s o l v o l y t i c r e a c t i o n s , a n d t o t h e base h y d r o l y s i s r e a c t i o n . B r o w n , I n g o l d , a n d N y h o l m (P, 10, 11) were t h e first s y s t e m a t i c a l l y t o s t u d y s u b stitution i n octahedral complexes i n methanol solution.

T h e y observed two types

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

1.

TOBE

Jtofe of

Ion-Association

9

of b e h a v i o r i n t h e r e a c t i o n b e t w e e n cis-[Co e n

Cl ]

2

2

+

a n d a v a r i e t y of a n i o n s : (a)

reagents of l o w n u c l e o p h i l i c i t i e s , e.g., NCS~~, C l ~ , B r " " a n d N 0 ~ , e n t e r e d a t a c o m 3

m o n r a t e t h a t w a s i n d e p e n d e n t of t h e i r n a t u r e a n d c o n c e n t r a t i o n a n d (b)

reagents

s u c h as C H 3 O - , N 3 - a n d N 0 ~ were m o r e r e a c t i v e a n d e n t e r e d a t a r a t e t h a t i n 2

creased w i t h i n c r e a s i n g c o n c e n t r a t i o n .

These observations were interpreted i n

t e r m s of a d u a l m e c h a n i s m b u t w e r e l a t e r c h a l l e n g e d successfully b y B a s o l o , H e n r y , a n d P e a r s o n (26, 27).

T h e y s h o w e d t h a t a l l t h e reagents, e x c e p t m e t h o x i d e , e n t e r e d

a t a c o m m o n r a t e , a n d t h a t t h e e n h a n c e d r e a c t i v i t y o b s e r v e d for a z i d e a n d n i t r i t e b y B r o w n et al. w a s caused b y t h e s o l v o l y t i c d i s t u r b a n c e c a u s e d b y these b a s i c anions. Cl ]


2

+

B a s o l o et al. a l s o s h o w e d t h a t t h e t r u e r e a c t i o n r a t e b e t w e e n cis-[Co e n

a n d N3"" i n m e t h a n o l d e p e n d e d s o m e w h a t u p o n [Nf]

2

at low concentrations.

B u t i t reached a m a x i m u m , concentration-independent rate a t higher concentra tions.

T h i s b e h a v i o r w a s a s c r i b e d t o i o n a s s o c i a t i o n b e t w e e n t h e reagents, t h e

free i o n b e i n g s o m e w h a t less r e a c t i v e t h a n t h e i o n p a i r . W e h a v e r e c e n t l y c o m p l e t e d a s t u d y of t h e i s o m e r i z a t i o n , r a c e m i z a t i o n , a n d c h l o r i d e exchange rates of cis-[Co e n C l ] i n m e t h a n o l i n t h e presence of o n l y s m a l l 2

q u a n t i t i e s of c h l o r i d e .

2

+

W e found that the rate depended upon chloride concentra

t i o n w h e n i t w a s l o w a n d b e c a m e i n d e p e n d e n t of c h l o r i d e c o n c e n t r a t i o n i n t h e h i g h e r r e g i o n s t u d i e d b y B r o w n et al.

T h e rate constants are p l o t t e d i n F i g u r e 1 as

f u n c t i o n of c h l o r i d e i o n c o n c e n t r a t i o n (8).

These observations are very similar to

those of B a s o l o et al. f o r t h e s u b s t i t u t i o n b y a z i d e a n d c a n be e x p l a i n e d i n t h e s a m e w a y , i.e., b y i n v o k i n g i o n a s s o c i a t i o n .

T h e k i n e t i c c u r v e s c a n be r e p r o d u c e d b y a n

i o n a s s o c i a t i o n c o n s t a n t , K, for t h e e q u i l i b r i u m , as-[Co en C l ] 2

2

+

κ. + Cl~~ +± cis-[Co e n C l ] 2

h a v i n g a v a l u e of 250/mole a t 35°C.

2

· · · Cl"~

+

E v e n t h o u g h i o n a s s o c i a t i o n causes s p e c t r u m

changes i n t h e r e g i o n a r o u n d 3000A., s p e c t r o p h o t o m e t r y e s t i m a t i o n s a r e u n r e l i a b l e since t h e change i n a b s o r p t i o n i s s m a l l .

T h e relationship between the various rate

c o n s t a n t s gives t h e s t e r i c c o u r s e of t h e c h l o r i d e s u b s t i t u t i o n .

T h i s is reported i n

Table I.

T a b l e I»

The Steric Course o f Substitution o f C h l o r i d e In c i s - [ C o e n a n d i n its C h l o r i d e Ion P a i r In M e t h a n o l a t 35°C. %

%

X 10 /min.

ke^t z

X 10 /min. z

X 10 /min. z

trans

cisinv.

3.2 7.1

4.6 8.2

4.6 8.2

70 86

15 8

free ion ion pair

N o t i c e t h e s i m i l a r i t y of

krae.

s t e r i c courses

kexch.

Ch]

+

2

%

cisnt. 15 8

i n t h e s u b s t i t u t i o n r e a c t i o n s of

free i o n a n d t h e i o n p a i r , a n d t h e c o m p l e t e

loss of o p t i c a l a c t i v i t y f o r

the

every

a c t of s u b s t i t u t i o n . I n t h e i s o m e r i z a t i o n r e a c t i o n s of cis- a n d trans-[Co e n C l ] i n d i m e t h y l f o r m a m i d e 2

2

+

and dimethylacetamide, the e q u i l i b r i u m isomer ratios depend upon the concentra t i o n of c h l o r i d e .

T h i s has been i n t e r p r e t e d i n t e r m s of i o n a s s o c i a t i o n , a n d t h e

e q u i l i b r i u m c o n s t a n t s h a v e been c o m p u t e d (31).

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

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

P r e l i m i n a r y chloride exchange experi-



ments indicated that the rate difference was due to a difference i n the rate of substitution and not just to a change i n the steric course. T h e rate constants for the reactions of the free ions and chloride ion pairs of the cis- and trans-[Co e n C l ] 2

2

+

complexes in methanol, dimethylformamide, a n d d i -

methylacetamide are in Table I I .

T h e dots indicate that the information could

not be derived from the published data—not that these quantities are insignificant.

Table II. I o n A s s o c i a t i o n C o n s t a n t s a n d F i r s t - O r d e r R a t e C o n s t a n t s f o r Chloride Exchange o f Isomerlsation of cis- a n d trans-[Co e n C l ] 2

+

K/ mole

Kt/ mole

v min

k/ min

k ip/ min

kt.jp/ min

250 250 1800 1800 1700 1700

· · · 30 - · ·

0.0046 · · · 0.0017

0.00032* 0.003 0.0003

0.0082 0.008 0.0041

0.093

9

Methanol (35°C.) Dimethylformamide (60°C.) Dimethylacetamide (60°C.)

2

* Data from Pearson (26) measured at 25°C.

.. .

t

u

B y c o m b i n i n g t h e d a t a f o r t h e c h l o r i d e e x c h a n g e o f css-and trans-[Co e n C l ] i n 2

2

+

m e t h a n o l (26, 27) w i t h t h e k n o w n f a c t t h a t n o c i s i s o m e r c o u l d be d e t e c t e d a t e q u i l i b r i u m , i t w a s possible t o d e t e r m i n e t h a t t h e s t e r i c c o u r s e o f c h l o r i d e e x c h a n g e i n the trans isomer is almost entirely retentive.

T h i s is i n direct contrast t o the


1.

Jtofo o f Ion-Association

JOBS

s t e r i c courses

11

of t h e r e a c t i o n s i n d i m e t h y l f o r m a m i d e , d i m e t h y lace t a m i d e , a n d

d i m e t h y l s u l f o x i d e (32), w h e r e c h l o r i d e exchange o r s o l v o l y s i s of t h e t r a n s c o m p l e x gives m a i n l y t h e cis i s o m e r .

N o s a t i s f a c t o r y e x p l a n a t i o n h a s y e t been g i v e n for t h i s

observation. I t is easy t o a r g u e t h a t t h e b e h a v i o r of t h e [ C o e n C l ] i s o m e r s i s n o t s u r p r i s i n g . 2

+

2

T h e c o r r e l a t i o n of a q u a t i o n r a t e s of c o m p l e x e s of t h e t y p e , [ C o e n A C l ] 2

w i t h the

n +

e l e c t r o n d i s p l a c e m e n t p r o p e r t i e s of t h e n o n p a r t i c i p a t i n g l i g a n d , A , has l e d t o t h e belief t h a t l i g a n d s a b l e t o d o n a t e a s e c o n d p a i r of electrons t o t h e m e t a l c a n t h e r e b y s t a b i l i z e t h e 5-coordinate i n t e r m e d i a t e a n d hence p r o m o t e a u n i m o l e c u l a r r e a c t i o n C h l o r i n e is s u c h a l i g a n d , C l — C o - : - C l , a n d t h e e s s e n t i a l l y


(2, 18, 24).

first-order

k i n e t i c f o r m c o u l d be used as e v i d e n c e for a u n i m o l e c u l a r m e c h a n i s m , o n c e t h e i o n a s s o c i a t i o n p r e - e q u i l i b r i u m effects for t h e d i s p l a c e m e n t of c h l o r i d e u n d e r t h e elec t r o n - d i s p l a c i n g influence of t h e o t h e r c h l o r i n e a t o m h a v e been t a k e n i n t o a c c o u n t . C o n s e q u e n t l y , i t w a s i n t e r e s t i n g t o s t u d y t h e r e a c t i o n s of c o m p o u n d s of t h e t y p e , [Co en N 0 2

fluence

X]

2

of N 0

2

, w h e r e t h e d i s p l a c e m e n t of X u n d e r t h e e l e c t r o n - d i s p l a c e m e n t i n

+ n

w a s t h o u g h t t o t a k e place b i m o l e c u l a r l y .

T h e a m o u n t of i n t e r e s t

is reflected i n t h e a m o u n t of w o r k t h a t has been p u b l i s h e d .

A t first s i g h t , there

a p p e a r s t o be l i t t l e a g r e e m e n t b e t w e e n t h e o b s e r v a t i o n s of t h e different w o r k e r s i n the

field.

A s p e r g e r (3) f o u n d t h a t t h e r a t e of r e p l a c e m e n t of c h l o r i d e i n cis- a n d

trans-[Co e n N 0 2

2

C l ] b y t h i o c y a n a t e i n m e t h a n o l s o l u t i o n w a s i n d e p e n d e n t of t h e +

c o n c e n t r a t i o n of t h i o c y a n a t e for t h e cis i s o m e r , a n d h a d a m i x e d z e r o - a n d

first-

o r d e r d e p e n d e n c e for t h e t r a n s c o m p l e x .

first-

L a n g f o r d a n d T o b e (23) f o u n d a

o r d e r r a t e d e p e n d e n c e of t h i o c y a n a t e e n t r y i n t o trans-[Co e n N 0 2

2

B r ] i n sulfolane +

upon [ S C N ~ ] ; but they observed that chloride reacted m u c h more rapidly, a n d a l i m i t i n g , c h l o r i d e i n d e p e n d e n t r a t e w a s r e a c h e d as [Cl""] w a s i n c r e a s e d .

Langford

a n d L a n g f o r d (22) s h o w e d t h a t t h e r a t e of r e p l a c e m e n t of c h l o r i n e i n trans-[Co e n N0

2

Cl]

+

2

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

concentration. L a n g f o r d (21) has r e c e n t l y r e p o r t e d s o m e a n a t i o n r e a c t i o n s of trans-[Co e n Η 0] 2

2+

2

N0

2

i n sulfolane a n d we h a v e s t u d i e d s i m i l a r r e a c t i o n s o v e r a w i d e r r a n g e of s o l

v e n t (17).

T h e d a t a for t h i s r e a c t i o n a r e i n F i g u r e 2, w h e r e t w o a p p a r e n t t y p e s of

b e h a v i o r are c h a r a c t e r i z e d .

T h i o c y a n a t e enters a t a r a t e t h a t is i n d e p e n d e n t of

a n i o n c o n c e n t r a t i o n i n a l l three s o l v e n t s , a c e t o n e , d i m e t h y l f o r m a m i d e , a n d s u l folane.

The

rate is o n l y slightly dependent

b u t c o v e r s a t e n f o l d change i n r a t e c o n s t a n t .

u p o n t h e n a t u r e of t h e

solvent

H o w e v e r , for a b i m o l e c u l a r s u b s t i t u

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

N i t r a t e has a s i m i l a r k i n e t i c b e h a v i o r b u t t h e r a t e i s

1 5 % g r e a t e r t h a n t h a t of t h i o c y a n a t e . different k i n e t i c f o r m .

Chloride a n d bromide have a n entirely

A t r e l a t i v e l y l o w a n i o n c o n c e n t r a t i o n t h e r a t e has a m i x e d

z e r o - a n d first-order d e p e n d e n c e o n t h e a n i o n c o n c e n t r a t i o n . a l i m i t i n g r a t e is a t t a i n e d .

B u t as t h i s increases,

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

C o n d u c t i o m e t r i c s t u d i e s s h o w e d t h a t i o n aggregates e x i s t u n d e r these c o n d i t i o n s . I n d i m e t h y l f o r m a m i d e , t h e a s s o c i a t i o n between

one d i p o s i t i v e c a t i o n a n d

two

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

of a n i o n a n d c a t i o n were m i x e d , e v e n i n d i l u t e s o l u t i o n . for a t h i r d a n i o n t o be a d d e d t o t h e a s s e m b l y .

B u t there was no tendency

I n acetone, thiocyanate saturated

t h e i o n aggregate a t t h e 2:1 c o m p o s i t i o n a n d d i d n o t a d d f u r t h e r t o i t .

B u t chloride



12


Figure 2.

Pseudo first-order rate constants for the replace-

ment of the water in trans-[C0 eni NO2 H2O] (CIO'4)2 by Cl~, Br~, and SCN~ in nonaqueous solvents at 25°C. as a function of anion concentration. and bromide ions added on to the neutral 2:1 aggregate, trans-[Qo e n N 0 2

• · · 2Br~, to form the negatively charged 3:1 aggregate, trans-[Co e n N 0 2

2

H 0]

2

2

H 0] 2

+ 2

+ 2

• · · 3Br~. Consequently, all the data in Figure 2 can be explained in terms of the different reactivities of the various aggregates.

T h e insensitivity of the rate of thiocyanate

entry to the thiocyanate concentration simply shows that, over the whole concentration range studied, the substrate was always in the form of the ion triplet.

The

rate dependence on the chloride or bromide concentration represents the change in the distribution of the substrate between the 2:1 and the 3:1 aggregate as the anion concentration is increased. A general pattern emerges for all of the nonaqueous substitution reactions of the [Co e n A X ] 2

+ n

cations.

anion concentration.

In every case there is a limiting rate at a sufficiently high

Sometimes this limit may have been passed at the lowest

anion concentration studied, in which case the rate would appear to be quite i n -


1.

TOBB

Ro/a o f lofi-AssocioffOfi

d e p e n d e n t of t h e r e a g e n t c o n c e n t r a t i o n .

13 I n o t h e r cases i t m a y n o t be p o s s i b l e t o

a t t a i n a high enough anion concentration to detect the limiting rate.

I n the l i m i t

the rate w o u l d appear to depend linearly upon a n i o n concentration. W h a t is the significance of t h i s general b e h a v i o r ?

I t is easy

to assume

mis

t a k e n l y t h a t the s i m i l a r , o v e r a l l k i n e t i c f o r m of these n o n - a q u e o u s r e a c t i o n s of t h e cobaltammines indicates a similar mechanism.

Since the rates are not d i r e c t l y

p r o p o r t i o n a l t o t h e r e a g e n t c o n c e n t r a t i o n s , i t is e a s y t o a s s u m e t h a t t h i s i s a u n i molecular reaction. T h e l i m i t i n g r e a c t i o n r a t e c a n be e x p l a i n e d i n t e r m s of t h e p r e - e q u i l i b r i u m a s s o


c i a t i o n b e t w e e n t h e c a t i o n i c a n d a n i o n i c species. interact quite specifically w i t h its environment.

T h e complex, i n solution, can

I n t h e a b s e n c e of o t h e r i n t e r f e r i n g

s o l u t e p a r t i c l e s , t h i s e n v i r o n m e n t w i l l c o n s i s t of s o l v e n t m o l e c u l e s f o r m i n g t h e solvation shell.

W h e n w e c o n s i d e r t h e c a t i o n s of t h e t y p e , [ C o e n A X ] 2

+ n

, we

find

t h a t t h e r e i s c o n s i d e r a b l e a t t r a c t i o n for c e r t a i n a n i o n s , o f t e n e x c e e d i n g a n y s i m p l e electrostatic prediction.

T h e evidence

for t h i s t y p e of a s s o c i a t i o n ranges

from

kinetic a n d e q u i l i b r i u m studies to spectrometry a n d c o n d u c t i v i t y measurements. T h e s e i n t e r a c t i o n s l e a d t o f o r m i n g i o n aggregates w h i c h c a n be r e g a r d e d as species i n w h i c h a n i o n s o c c u p y p o s i t i o n s i n t h e i n n e r s o l v a t i o n s h e l l of t h e c o m p l e x i o n . T h e y c a n be c a l l e d " i n t i m a t e i o n a g g r e g a t e s " i n t h e W i n s t e i n sense of t h e w o r d (34) o r " o u t e r s p h e r e " c o m p l e x e s i n t h e T a u b e sense

(29).

Since the a t t r a c t i o n between the complex cation a n d the anions is balanced b y the repulsion between the anions a n d b y the solvent's a b i l i t y to solvate the anions, t h e r e m u s t be a m a x i m u m n u m b e r of a n i o n s t h a t c a n be a c c o m m o d a t e d i n t h e a g g r e gate.

T h i s n u m b e r m u s t d e p e n d u p o n t h e n a t u r e a n d c h a r g e of t h e c a t i o n , t h e

n a t u r e a n d c h a r g e of t h e a n i o n , a n d t h e n a t u r e of t h e s o l v e n t .

T h e r e is no reason

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

Neutral

r e a g e n t species c a n l i k e w i s e p r e a s s e m b l e i n t h e s o l v a t i o n s h e l l b u t t h e y l a c k t h e initial electrostatic advantage.

A l t h o u g h t h e y m a y be affected

b y differential

s o l v a t i o n effects, i n a p p r o p r i a t e c i r c u m s t a n c e s , t h e y c a n r e p l a c e c o m p l e t e l y

the

o r i g i n a l s o l v e n t so t h a t t h e i r effect m a y be m o r e p r o n o u n c e d i n t h e l o n g r u n . S i n c e m o v e m e n t w i t h i n t h e s o l v a t i o n s h e l l i n these c o m p l e x e s is r e l a t i v e l y s l u g gish, i t is postulated t h a t a complex remains a c t i v a t e d o n l y l o n g enough to react w i t h its immediate environment, the inner s o l v a t i o n shell.

In the reaction with

a n i o n i c species, a s i t u a t i o n c a n be r e a c h e d i n w h i c h n e a r l y a l l of t h e s u b s t r a t e i s i n t h e f o r m of t h e m a x i m u m i o n aggregate.

A n y increase i n t h e a n i o n c o n c e n t r a t i o n

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

In this w a y a limiting

r a t e c a n be i n d e p e n d e n t of t h e c o n c e n t r a t i o n of a d d e d a n i o n i c r e a g e n t , i r r e s p e c t i v e of t h e a c t u a l m e c h a n i s m of t h e a c t u a l a c t of s u b s t i t u t i o n . F u r t h e r m o r e , I suggest t h a t these r e a c t i o n s c a n be r e g a r d e d as r e a r r a n g e m e n t processes of t h e aggregate w h e r e b y a n i n n e r sphere l i g a n d changes p l a c e w i t h a n o u t e r sphere l i g a n d .

C o n s e q u e n t l y , e a c h aggregate c a n be a s s i g n e d i t s o w n

order rate constant for its rearrangement.

first-

A n y d e p e n d e n c e of r a t e u p o n a n i o n

c o n c e n t r a t i o n arises f r o m changes c a u s e d i n d i s t r i b u t i n g t h e s u b s t r a t e b e t w e e n t h e v a r i o u s possible aggregates.

F o r e x a m p l e , i n t h e r e a c t i o n s d e s c r i b e d i n F i g u r e 2,

the r a t e d e p e n d e n c e o n b r o m i d e c o n c e n t r a t i o n i n a c e t o n e arises f r o m t h e t r a n s f o r m a t i o n of the 2:1 aggregate i n t o t h e 3:1 species.

T h e l i m i t i n g rate corresponds


14


t o a l l of t h e s u b s t r a t e b e i n g i n t h e f o r m of t h e h i g h e r aggregate a n d is e q u a l t o i t s rearrangement rate. T h i s c o n c e p t c a n n o t use k i n e t i c s t o e l u c i d a t e t h e m e c h a n i s m .

T h i s t h e n raises

t h e q u e s t i o n of a p p l y i n g t h e m o l e c u l a r i t y c o n c e p t t o processes t h a t i n v o l v e r e a r r a n g i n g a p r e v i o u s l y a s s e m b l e d aggregate of reagents.

W h a t is p r o b a b l y more i m p o r

t a n t is d i s t i n g u i s h i n g between the possibilities once we accept t h e i r existence.

The

c o n c e p t u a l d i s t i n c t i o n c a n be m a d e b y c o n s i d e r i n g t h e t i m i n g of t h e b o n d - m a k i n g a n d b o n d - b r e a k i n g processes.

If bond m a k i n g a n d breaking are synchronous, the

process i s c l e a r l y b i m o l e c u l a r .

If bond


mechanism is unimolecular with a

b r e a k i n g precedes b o n d

five-coordinate

intermediate.

making, the

If bond m a k i n g

o c c u r s first, t h e process w i l l be a s s o c i a t i v e w i t h a s e v e n - c o o r d i n a t e i n t e r m e d i a t e . A possible w a y t o d i s t i n g u i s h t h e m e c h a n i s m i n t h e a c t u a l a c t of s u b s t i t u t i o n i s o u t l i n e d i n F i g u r e s 3 a n d 4.

H e r e the s o l v a t i o n shell is represented b y a circle

a r o u n d t h e c o m p l e x , a n d a n y i o n aggregate i s r e p r e s e n t e d b y a n a n i o n i n t h e c i r cumference. to give the

T h e u n i m o l e c u l a r r e a c t i o n of t h e free i o n r e q u i r e s a s l o w d i s s o c i a t i o n five-coordinate

intermediate.

for a short w h i l e w i t h i n the s o l v a t i o n shell.

The

departing group

is t h e n

held

T h r e e p o s s i b l e fates a w a i t t h i s i n t e r

m e d i a t e : (1) a s o l v e n t m o l e c u l e e n t e r s t h e i n t e r m e d i a t e f r o m t h e s o l v a t i o n s h e l l a n d solvolysis occurs.

T h i s is the n o r m a l behavior i n water a n d happens quite often i n

d i m e t h y l s u l f o x i d e ; (2) t h e l e a v i n g g r o u p m a y r e - e n t e r t h e c o o r d i n a t i o n s h e l l a n d t h u s n o n e t s u b s t i t u t i o n o r e x c h a n g e has o c c u r r e d .

T h i s act is observable i n the

sense of b e i n g a t y p e of m a s s - l a w r e t a r d a t i o n ; (3) t h e i n t e r m e d i a t e w i l l l a s t l o n g e n o u g h f o r a n i o n of reagent Y ~ t o t a k e a p o s i t i o n i n t h e s o l v a t i o n s h e l l a n d t h e n enter the coordination shell to give s u b s t i t u t i o n .

+

χ

• X

UNI MOLEC U LA R I n t e r m s of o r i e n t a t i o n w i t h i n t h i s aggregate, t h e l e a v i n g g r o u p , X , w i l l h o l d a p o s i t i o n i n t h e s o l v a t i o n s h e l l w h i c h i s n o t v e r y different f r o m i t s p o s i t i o n i n t h e original octahedron.

T h e c o n s e q u e n c e s of t h i s h a v e a l r e a d y been d i s c u s s e d i n c o n

n e c t i o n w i t h t h e s t e r i c course of u n i m o l e c u l a r a q u a t i o n (13).

B u t i n the general

c o n t e n t of t h i s r e a c t i o n i t m e a n s t h a t t h e p o s i t i o n t a k e n u p b y t h e e n t e r i n g r e a g e n t Y " " w i l l be s i m i l a r i n t h e i o n aggregate w h i c h is o b t a i n e d i n t h i s w a y t o t h a t o b t a i n e d a f t e r t h e i n i t i a l d i s s o c i a t i o n of t h e i o n p a i r f o r m e d w i t h t h e o c t a h e d r a l

complex.

I n o t h e r w o r d s , i t i s of l i t t l e c o n s e q u e n c e w h e t h e r , i n t h e i o n aggregate i n v o l v i n g t h e five-coordinate

i n t e r m e d i a t e , X h a s l e f t the c o o r d i n a t i o n s h e l l before o r a f t e r Y h a s



1.

TOBE

15

Rolo of Ion -Association

ΒIMOLECULAR entered the solvation shell.

S i m i l a r s t e r i c courses of s u b s t i t u t i o n i n t h e free i o n

a n d t h e i o n p a i r w o u l d r e s u l t a n d a c t u a l l y h a v e been o b s e r v e d . T h e u n i m o l e c u l a r r e a c t i o n of t h e i o n a g g r e g a t e follows a s i m i l a r c o u r s e a n d t h e i n t e r m e d i a t e faces t h e same t h r e e p o s s i b i l i t i e s for r e a c t i o n .

T h e r a t e of b o n d

fis

s i o n w i l l n o t n e c e s s a r i l y be t h e s a m e as t h a t of t h e free i o n because t h e s o l v a t i o n e n v i r o n m e n t has c h a n g e d . r e a c t i o n s (1).

W e see t h i s effect i n t h e i o n p a i r - c a t a l y z e d s o l v o l y t i c

I n a d d i t i o n , since t h e reagent Y i s i n p o s i t i o n before t h e

five-coor

d i n a t e i n t e r m e d i a t e is f o r m e d , t h e p a t h b y w h i c h X re-enters t h e c o o r d i n a t i o n shell b e c o m e s less p r o b a b l e a s a r e s u l t of m o r e effective c o m p e t i t i o n b y Y , a n d t h e r a t e is i n c r e a s e d . T h e b i m o l e c u l a r process i s d e p i c t e d i n F i g u r e 4, w h e r e t h e free i o n has l i t t l e c h o i c e i n its reaction, being surrounded o n l y b y solvent molecules.

If i t can

undergo

s o l v o l y s i s , t h e n one of these m o l e c u l e s w i l l a t t a c k ; i f n o t , t h e r e w i l l be n o r e a c t i o n . T h e i o n aggregate c o n t a i n s s u i t a b l e reagents i n t h e i n n e r s o l v a t i o n s h e l l , a n d t h e bimolecular reaction can take place, either b y solvolysis or b y a n i o n attack. H o w t h e n c a n w e d i s t i n g u i s h k i n e t i c a l l y b e t w e e n these t w o p o s s i b l e

mechanisms?

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

If the s o l v o l y t i c reaction d i d not interfere, a

first-

o r d e r r a t e d e p e n d e n c e o n a n i o n c o n c e n t r a t i o n w o u l d be o b s e r v e d i f t h e r e a c t i o n w e r e b i m o l e c u l a r since o n l y t h e i o n p a i r c a n be i n v o l v e d i n t h e s u b s t i t u t i o n . [R [R -

X]+

X]+

n

+ Υ- £

[R -

· · · Υ " Λ [R -

n

Rate -

k[R

X]+ Y]+

-

X

.·· γ-

... X -

n

+

n

n

ion pair (bimolecular)

- · · Y"]

and [R -

X+

n

· · · Y-] -

K[R

-

X+ ] n

[Y1

If o n l y a s m a l l p a r t of t h e c o m p l e x a n d Y ~ a r e e n g a g e d i n i o n a s s o c i a t i o n , t h e n these t r u e c o n c e n t r a t i o n s w i l l be a p p r o x i m a t e l y e q u a l t o t h e c o n c e n t r a t i o n s of material and, [R — X

+ n

]

= [complex] a n d [ Y ~ ] = [ a n i o n i c reagent]

so t h a t , R a t e = ^ [ c o m p l e x ] [ a n i o n i c reagent]


added


16

I n o t h e r w o r d s , the r a t e decreases t o zero as t h e c o n c e n t r a t i o n of a n i o n i c reagent decreases t o zero. T h e r e a c t i o n s of the free i o n a n d the i o n p a i r c o n t r i b u t e t o t h e u n i m o l e c u l a r reaction. [R -

X]+

n

*

fR -


[R -

X]+

n

[R]< +!>+ · · · X - — ^ [ R fast

Y]+

n

X]

+

n

··· Y " ^

+ Y " £>[R [R -

Y]+

n

X]

+

·-· X -

n

··· Y -

n

··· X -

(unimolecular)

Then, Rate = ki[R -

X

+ n

] + k'[R -

X

+

n

· · · Y"],

w h i c h , u n d e r t h e c o n d i t i o n s d i s c u s s e d a b o v e gives, R a t e = {&i + &'i£[anionic reagent]}

[complex].

H e n c e , t h e r a t e w i l l r e m a i n finite as the c o n c e n t r a t i o n of t h e

anionic

reagent

a p p r o a c h e s zero. T h i s a p p r o a c h a p p l i e s o n l y w h e n w e a r e c e r t a i n t h a t t h e s u b s t r a t e is m a i n l y i n t h e f o r m of t h e free i o n a t t h e lowest a n i o n c o n c e n t r a t i o n s . c h l o r i d e exchange of cis~[Co e n C l ] 2

2

+

T h i s is t r u e i n t h e

i n methanol a n d we can safely conclude t h a t

t h e m e c h a n i s m is u n i m o l e c u l a r (8, 9, 10, 11, 26, 27).

T h i s condition d i d not exist

w h e n we s t u d i e d the d i s p l a c e m e n t of w a t e r i n tran$-[Co e n N 0 H 0 ] 2

2

2

+ 2

b y anions

w h e r e , because of t h e large i o n a s s o c i a t i o n c o n s t a n t s , n o n e of the s u b s t r a t e w a s i n t h e free i o n f o r m u n d e r r e a c t i o n c o n d i t i o n s . trans-[Co e n N 0 2

2

H o w e v e r , i n the reaction

between

B r ] " a n d thiocyanate i n sulfolane, the substrate was m a i n l y i n

t h e free i o n f o r m .

4

T h e observed second-order

kinetic form was fully

consistent

w i t h a s s i g n i n g a b i m o l e c u l a r m e c h a n i s m t o t h e r e a r r a n g e m e n t of t h e i o n p a i r . Is i t possible t o d i s t i n g u i s h the m e c h a n i s m b y the e x t e n t t o w h i c h t h e r e a r r a n g e m e n t r a t e of the i o n aggregate d e p e n d e d u p o n t h e n a t u r e of t h e e n t e r i n g a n i o n ? S m a l l differences w o u l d be e x p e c t e d for a n S j y l m e c h a n i s m since t h e presence a n i o n s i n t h e s o l v e n t shell w o u l d e x e r t a " s o l v e n t e f f e c t " u p o n t h e r a t e of fission.

I n a d d i t i o n , t h e n a t u r e a n d l o c a t i o n of t h e a n i o n s i n t h e aggregate

of

bond will

d e t e r m i n e the e x t e n t of the c o m p e t i t i o n w i t h t h e r e t u r n processes a n d hence, affect the observed rate.

If the r e a c t i v i t y difference of t h e e n t e r i n g n u c l e o p h i l e s were

large (i.e., s e v e r a l p o w e r s of 10), a s i n t h e case of b i m o l e c u l a r s u b s t i t u t i o n a t t e t r a h e d r a l c a r b o n (15) a n d o c t a h e d r a l s i l i c o n (25), one w o u l d n o t h e s i t a t e t o a s s i g n a bimolecular mechanism. H o w e v e r , if, as h a s been suggested (23), t h e a c t i v a t i o n e n e r g y is c o l l e c t e d m a i n l y b y the c o m p l e x a n d the f u n c t i o n of t h e e n t e r i n g g r o u p is t o be p r e s e n t w h e n t h e c o m p l e x is a c t i v a t e d , one w o u l d n o t e x p e c t t h e r a t e of t h i s t y p e of b i m o l e c u l a r process t o be g r e a t l y s e n s i t i v e t o the n a t u r e of t h e e n t e r i n g g r o u p .

This might

e x p l a i n L a n g f o r d ' s (20) o b s e r v a t i o n t h a t s o l v o l y s i s of C o ( I I I ) e t h y l e n e d i a m i n e c o m plexes is f a r less s e n s i t i v e t o the s o l v e n t i o n i z i n g p o w e r t h a n u n i m o l e c u l a r s o l v o l y s e s of t e t r a h e d r a l c a r b o n c o m p o u n d s ; b u t , he a l s o p o i n t s o u t t h a t t h e C o - X b o n d i n t h e g r o u n d s t a t e is m u c h m o r e i o n i c t h a n the C - X b o n d a n d hence there is a s m a l l e r difference i n t h e s o l v a t i o n of t h e g r o u n d s t a t e a n d t h e t r a n s i t i o n s t a t e .


1.

TOBE

Hole of

Ion-Association

17

A t present, a l t h o u g h t h e u n i m o l e c u l a r m e c h a n i s m has been a d e q u a t e l y

demon

s t r a t e d for c o m p l e x e s of the t y p e [ C o e n C l ] , t h e b i m o l e c u l a r m e c h a n i s m p o s t u 2

l a t e d for c o m p l e x e s of t h e t y p e , [ C o e n N 0 2

2

2

+

X]

+

n

(2, 18, 24) is b y n o m e a n s c e r t a i n .

I f i t does e x i s t , i t is c o n c e p t u a l l y different f r o m b i m o l e c u l a r s u b s t i t u t i o n a t p l a t i n u m (II) a n d tetrahedral carbon.

T h e r e is, as y e t , n o reason d e f i n i t e l y t o assign a u n i

m o l e c u l a r m e c h a n i s m t o t h e i r r e a c t i o n s a n d m u c h m o r e w o r k w i l l be before a n y final c o n c l u s i o n c a n be d r a w n .

necessary

I t is i n t e r e s t i n g t o n o t e t h a t , i n g o i n g

from solvolytic aquation to substitution reactions i n noninterfering solvents, we s t i l l h a v e n o t been a b l e t o a p p l y k i n e t i c s d i r e c t l y t o relate m o l e c u l a r i t y t o o r d e r .


A l t h o u g h i n w a t e r the k i n e t i c f o r m reflected t h e p e r m a n e n t u n c h a n g e a b l e s o l v e n t e n v i r o n m e n t , the n o n a q u e o u s w o r k shows no r e l a t i o n s h i p b e t w e e n t h e c o n t r o l l a b l e c o n c e n t r a t i o n of the reagent i n the b u l k s o l v e n t a n d the c o m p o s i t i o n of the k i n e t i c a l l y i m p o r t a n t e n v i r o n m e n t of t h e c o m p l e x .

T h e k i n e t i c a p p r o a c h is a g a i n use

less, a n d one m u s t use the m o r e e q u i v o c a l a p p r o a c h e s

w h i c h were a p p l i e d

to

aquation. T h e c o n c e p t of p r e a s s e m b l y as a r e q u i r e m e n t for s u b s t i t u t i o n m a y t h r o w l i g h t u p o n t h e v e x e d q u e s t i o n of t h e m e c h a n i s m of t h e base h y d r o l y s i s r e a c t i o n . l o n g been k n o w n t h a t c o m p l e x e s of the t y p e , [ C o e n A X ] 2

hydroxide i n aqueous solution.

+

n

I t has

can react r a p i d l y w i t h

T h e kinetic form is cleanly second-order

even at

h i g h h y d r o x i d e c o n c e n t r a t i o n s , p r o v i d e d t h a t t h e i o n i c s t r e n g t h is h e l d c o n s t a n t . H y d r o x i d e i s u n i q u e i n t h i s r e s p e c t for these c o m p l e x e s . been suggested.

T w o mechanisms have

T h e first is a b i m o l e c u l a r process; t h e second is a b a s e - c a t a l y z e d

d i s s o c i a t i v e s o l v o l y s i s i n w h i c h the base r e m o v e s a p r o t o n f r o m t h e n i t r o g e n i n p r e e q u i l i b r i u m t o f o r m a d i s s o c i a t i v e l y l a b i l e a m i d o species (5, 19,

30).

W i t h o u t d i s c u s s i n g the r e l a t i v e m e r i t s of t h e t w o m e c h a n i s m s i t is i n t e r e s t i n g t o p o i n t o u t t h e i n f o r m a t i o n t h a t does n o t r e a d i l y fit e i t h e r m e c h a n i s m : (1) t h e h i g h r e a c t i v i t y of h y d r o x i d e is p e c u l i a r t o c e r t a i n C o ( I I I ) a n d R u ( I I I )

complexes

a n d t h e a n a l o g o u s c o m p l e x e s of P t ( I V ) , R h ( I I I ) , a n d I r ( I I I ) a p p e a r t o h a v e l i t t l e o r n o excess l a b i l i t y i n t h e presence of h y d r o x i d e ; (2) i n m a n y cases, t h e g r e a t r e a c t i v i t y difference between w a t e r a n d h y d r o x i d e c o m e s m a i n l y f r o m t h e a c t i v a tion entropy a n d not the a c t i v a t i o n energy

(12).

A l l of t h i s suggests t h a t t h e i o n a s s o c i a t i o n e x p l a n a t i o n m a y be a p p l i e d here t o a n e s s e n t i a l l y b i m o l e c u l a r (or a s s o c i a t i v e ) p h e n o m e n o n .

C o n s i d e r i n g the

difference

b e t w e e n h y d r o x i d e a n d a n y o t h e r reagent i n w a t e r , a p a r t f r o m i t s b a s i c i t y , one c o n cludes t h a t its m o b i l i t y m u s t p l a y a n i m p o r t a n t p a r t .

Whereas a l l the other rea

gents m u s t be i n a s u i t a b l e p o s i t i o n w i t h i n the s o l v a t i o n s h e l l before t h e y c a n e n t e r the c o m p l e x , t h e h y d r o x i d e i o n , b y m e a n s of a G r o t t h u s c h a i n p r o t o n t r a n s f e r , c a n be t r a n s m i t t e d t o a n y p o s i t i o n w h e r e i t i s needed w h i l e the c o m p l e x b e c o m e s a c t i vated.

I t c a n therefore be l o o k e d u p o n as a n u n s a t u r a t a b l e i o n aggregate

h y d r o x i d e f u l l y " d e l o c a l i z e d " a b o u t the c o m p l e x . any departure from

the

first-order

with

C o n s e q u e n t l y , we d o n o t o b s e r v e

dependence upon

hydroxide

concentration.

T h i s c o n t r i b u t i o n to the r e a c t i v i t y w i l l appear i n the a c t i v a t i o n entropy rather t h a n in the enthalpy t e r m . I n c o n c l u s i o n , t h i s e x t r e m e i m p o r t a n c e of p r e a s s o c i a t i o n of reagents a p p e a r s t o be peculiar to the c o b a l t a m m i n e systems a n d m a y v e r y well arise from some p r o p e r t y of t h e N - H b o n d (27). Cl ] 2

+

R e c e n t w o r k o n the s u b s t i t u t i o n r e a c t i o n s of [ C o d i a r s

2

(diars = o-phenylenebis(dimethylarsine)) i n m e t h a n o l shows t h a t , even i n the

case of t h e cis c o m p l e x , there is a b s o l u t e l y no k i n e t i c effect i n i s o m e r i z a t i o n , i n



18

c h l o r i d e e x c h a n g e o r i n t h i o c y a n a t e s u b s t i t u t i o n w h i c h c a n be assigned t o i o n asso c i a t i o n (28).


Literature

C o n d u c t i v i t y s t u d i e s suggest t h a t no a s s o c i a t i o n o c c u r s

(14).

Cited

(1) A d a m s o n , A. W . , W i l k i n s , R . , J. Am. Chem. Soc., 7 6 , 3379 (1954). (2) Ašperger, S., Ingold, C . , J. Chem. Soc. 1956, 2862. (3) Ašperger, S., Pavlović, D., Orhanović, M., J. Chem. Soc. 1961, 2142. (4) Basolo, F., Pearson, R . , " M e c h a n i s m s of Inorganic R e a c t i o n s , " p. 139, J o h n W i l e y a n d Sons, Inc., N e w Y o r k , 1958. (5) Ibid., p. 124. (6) B a s o l o , F., Stone, B . , B e r g m a n n , J., Pearson, R . , J. Am. Chem. Soc., 7 6 , 3079 (1954). (7) B o s n i c h , Β., unpublished w o r k . (8) B o s n i c h , B . , Ingold, C . , T o b e , M., J. Chem. Soc., i n press. (9) B r o w n , D. D., Ingold, C . , N y h o l m , R . , J. Chem. Soc. 1953, 2674. (10) B r o w n , D. D., Ingold, C . , J. Chem. Soc. 1953, 2680. (11) B r o w n , D. D., N y h o l m , R . , J. Chem. Soc. 1953, 2696. (12) C h a n , S., T o b e , M., J. Chem. Soc. 1962, 4531. (13) C h a n , S., T o b e , M., J. Chem. Soc. 1963, 5700. (14) D o l c e t t i , C . Pelos, A. personal c o m m u n i c a t i o n , 1964. (15) E d w a r d s , J., Pearson, R . , J. Am. Chem. Soc., 8 4 , 16 (1962). (16) G r a y , H., O l c o t t , R . , Inorg. Chem., 1, 481 (1962). (17) Hughes, M., T o b e , M., J. Chem. Soc. 1965, 1204. (18) Ingold, C . , N y h o l m , R . , T o b e , M., Nature, 187, 477 (1960). (19) I n g o l d , C . , N y h o l m , R . , T o b e , M., Nature, 194, 344 (1962). (20) L a n g f o r d , C . , Inorg. Chem., 3, 228 (1964). (21) L a n g f o r d , C . , J o h n s o n , M., J. Am. Chem. Soc., 8 6 , 229 (1964). (22) L a n g f o r d , C . , L a n g f o r d , P., Inorg. Chem., 2, 300 (1963). (23) L a n g f o r d , C., T o b e , M., J. Chem. Soc. 1963, 506. (24) Pearson, R . , Basolo, F., J. Am. Chem. Soc., 7 8 , 4878 (1956). (25) Pearson, R . , E d g i n g t o n , D., B a s o l o , F., J. Am. Chem. Soc., 8 4 , 3233 (1962). (26) Pearson, R . , H e n r y , P., B a s o l o , F., J. Am. Chem. Soc., 79, 5379 (1957). (27) Pearson, R . , H e n r y , P . , Basolo, F., J. Am. Chem. Soc., 7 9 , 5382 (1957). (28) Pelos, A. T o b e , M., J. Chem. Soc. 1964, 5063. (29) T a u b e , H., Posey, F. Α., J. Am. Chem. Soc., 7 5 , 1463 (1953). (30) T o b e , M., Science Prog., 4 8 , 483 (1960). (31) T o b e , M., W a t t s , D., J. Chem. Soc. 1962, 4614. (32) T o b e , M., W a t t s , D., J. Chem. Soc. 1964, 2991. (33) W a t t s , D., personal c o m m u n i c a t i o n , 1963. (34) W i n s t e i n , S., R o b i n s o n , G., J. Am. Chem. Soc., 8 0 , 169 (1958). R E C E I V E D April 3, 1964.

Discussion Cooper H . Langford:

I

don't

pretend

to

be

a

chemically

satisfactory

s u b s t i t u t e for M a r t i n T o b e , b u t I t h o u g h t i t m i g h t be useful for m e q u i c k l y t o s u m m a r i z e s o m e of t h e m a i n c o n c l u s i o n s c o n t a i n e d i n h i s p a p e r .

Perhaps I may

a l s o a c c e p t K e n t M u r m a n n ' s i n v i t a t i o n t o a d d j u s t one o r t w o t h i n g s t h a t h a v e i n terested us r e c e n t l y w h i c h f o l l o w D r . T o b e ' s suggestions c o n c e r n i n g t h e i m p o r t a n c e of p r e a s s o c i a t i o n . T h e i n t e r e s t i n n o n a q u e o u s s o l u t i o n s for s t u d y i n g 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 amminecobalt(III)

s y s t e m s b e g a n v e r y o b v i o u s l y f r o m t h e difficulties c a u s e d

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

by

There was hope that


1.

TOBE

Discussion

19

p e r h a p s there w o u l d be some cases i n n o n a q u e o u s s o l v e n t s w h e r e t h e k i n e t i c s w o u l d be, i n a s t r a i g h t f o r w a r d w a y , d i a g n o s t i c of t h e m o l e c u l a r i t y of t h e r e a c t i o n .

The

n o t i o n t h a t the e n t e r i n g g r o u p m i g h t a p p e a r i n t h e r a t e l a w (some e n t e r i n g g r o u p o t h e r t h a n t h e s o l v e n t ) a n d l e a d t o a clear c u t d e c i s i o n o n t h e role of t h e e n t e r i n g g r o u p i n t h e r e a c t i o n , w a s one of t h e m o t i v a t i o n s for d o i n g n o n a q u e o u s w o r k . A s w o r k has progressed i n D r . T o b e ' s h a n d s , a n e w a m b i g u i t y has a p p e a r e d i n t h e k i n e t i c s for n o n a q u e o u s s y s t e m s .

O n e does find s u b s t i t u t i o n r e a c t i o n s w h o s e

k i n e t i c s d e p e n d o n t h e c o n c e n t r a t i o n s of e n t e r i n g a n i o n s ; b u t , i n a l m o s t a l l cases, d e t a i l e d a n a l y s i s reveals t h a t t h i s d e p e n d e n c e c a n be a c c o u n t e d

for b y


t r a c k of t h e n a t u r e of i o n aggregates, o r o u t e r sphere c o m p l e x e s f o r m e d .

keeping

The com

p l i c a t e d r a t e l a w s f o u n d are r a t h e r e a s i l y s o r t e d o u t i f one c a n o b t a i n s o m e d a t a a b o u t t h e e q u i l i b r i u m b e t w e e n different i o n aggregates a n d a s s i g n c o r r e c t s u b s t i t u t i o n rates for e a c h of t h e v a r i o u s o u t e r sphere c o m p l e x e s . i n t e r e s t i n g t h i n g s d o emerge f r o m c o n s i d e r i n g these s y s t e m s .

C e r t a i n l y some v e r y D r . T o b e has f o u n d

i n s e v e r a l s y s t e m s t h a t r e a c t i o n s t e r e o c h e m i s t r i e s i n the o u t e r sphere

complexes,

o r i o n aggregates, a r e s i m i l a r t o t h e s t e r e o c h e m i s t r i e s of t h e r e a c t i o n s of t h e free ions.

I a m n o t sure t h a t t h i s has been t h o r o u g h l y e x p l a i n e d , a n d p e r h a p s i t is a n

i n t e r e s t i n g p o i n t for t h i s d i s c u s s i o n . C a r e f u l c o n s i d e r a t i o n of t h e o u t e r sphere c o m p l e x e s does l e a d t o a n u n d e r s t a n d i n g of c i r c u m s t a n c e s u n d e r w h i c h s o m e t h i n g d e f i n i t e a b o u t t h e m o l e c u l a r i t y of s u b s t i t u t i o n r e a c t i o n s m i g h t be s a i d .

I t becomes clear, t h a t i n order to understand

t h e m o l e c u l a r i t y of a s u b s t i t u t i o n r e a c t i o n , one m u s t find d a t a for t h e c o n c e n t r a t i o n r e g i o n of a n e n t e r i n g o r a n i o n i c g r o u p , w h e r e i t is p o s s i b l e t o m e a s u r e r e a c t i o n of t h e free i o n .

I f one c a n p r o c e e d t o s u f f i c i e n t l y l o w c o n c e n t r a t i o n of t h e e n t e r i n g g r o u p

so t h a t t h e p r e d o m i n a n t species i n s o l u t i o n a r e t h e free c o m p l e x i o n a n d t h e

1:1

o u t e r sphere c o m p l e x , a n a p p r o p r i a t e e x t r a p o l a t i o n c a n l e a d t o m o l e c u l a r i t y d e termination.

A s I u n d e r s t a n d D r . T o b e ' s p o i n t of v i e w , i f t h e r e i s n o p a t h w a y i n

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

N o w , of c o u r s e , one m u s t s a y , " m a y be u n i

m o l e c u l a r , " because t h e same d i f f i c u l t y t h a t arises i n a q u e o u s s o l u t i o n arises i n these solvents.

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

s o l v o c o m p l e x , a n d t h i s p o s s i b i l i t y has t o be r e s o l v e d before one c a n d e c i d e a b o u t t h e m o l e c u l a r i t y of t h e s u b s t i t u t i o n p r o c e s s . T h i s i s a n o t h e r of t h e v e r y i n t e r e s t i n g c o n t r i b u t i o n s i n T o b e ' s p a p e r . h a s s t u d i e d 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

Tobe

dichloro-bis(ethylenediamine)cobalt(III)

i o n i n m e t h a n o l , r e p o r t e d t h e p r e p a r a t i o n of t h e s u p p o s e d s o l v o i n t e r m e d i a t e t h a t w o u l d be r e q u i r e d , a n d s t u d i e d t h e r a t e of t h e c h l o r i d e a n i o n e n t r y i n t o t h i s s u p posed solvo intermediate.

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

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

Y e t i t is possible t o o b t a i n , i n t h e case of t h e d i c h l o r o -

c h l o r i d e exchange, a t e r m i n t h e r a t e l a w for t h e free i o n .

T h i s leads t o t h e c o n

c l u s i o n t h a t , i n f a c t , one has a g e n u i n e l y u n i m o l e c u l a r s u b s t i t u t i o n process. If t h e r e i s n o p a t h w a y i n d e p e n d e n t of t h e c h l o r i d e , t h a t i s , i f e x t r a p o l a t i o n t o v e r y l o w c h l o r i d e c o n c e n t r a t i o n of c h l o r i d e e x c h a n g e r e a c t i o n l e d t o a z e r o i n t e r c e p t a n d n o free i o n p a t h w a y , t h e n we w o u l d h a v e a b i m o l e c u l a r r e a c t i o n .

T h i s defini

t i o n of b i m o l e c u l a r i t y o n l y r e q u i r e s c h l o r i d e t o be a c o m p o n e n t of t h e s e c o n d c o o r d i n a t i o n sphere for c h l o r i d e e x c h a n g e t o o c c u r .



20

A quibble i n designating the molecularity appears.

T h e process m i g h t be c o n

s i d e r e d u n i m o l e c u l a r i f one considers t h e s e c o n d c o o r d i n a t i o n s p h e r e as p a r t of t h e c o m p l e x i n t h i s single s p e c i e s ; i t m i g h t be c o n s i d e r e d b i m o l e c u l a r i f one prefers t o c o n s i d e r t h e c h l o r i d e as e x t e r i o r t o t h e c o m p l e x — a n d t h e aggregate t w o p a r t i c l e s . B u t i t is q u i t e p o s s i b l e t h a t t h e r e a c t i o n m e c h a n i s m i s n o t different i n a n y i m p o r t a n t w a y f r o m t h e r e a c t i o n m e c h a n i s m t h a t leads t o a c l e a r c u t u n i m o l e c u l a r p a t h w a y . L e t ' s i m a g i n e a n i n t e r m e d i a t e , i n t h e sense of t r a n s i t i o n s t a t e t h e o r y , t h a t has a m i n i m u m p o t e n t i a l energy surface w i t h " w e a k " bonds to b o t h chlorides i n the outer coordination sphere.

W e c o u l d h a v e a n i n t e r m e d i a t e i n t h e sense of t r a n s i t i o n


s t a t e t h e o r y b u t i t w o u l d n o t be k i n e t i c a l l y d e t e c t a b l e , because i t m i g h t r e a c t so r a p i d l y t h a t i t s s e c o n d c o o r d i n a t i o n sphere c o u l d n o t r e a r r a n g e i n t h e t i m e for i t s reaction.

T h e n , of c o u r s e , i t w o u l d n e v e r a p p e a r i n t h e r a t e l a w a t a l l .

O n e w o n d e r s i f w e a r e l o o k i n g , i n s o m e sense, i n t h e r i g h t d i r e c t i o n b y a t t e m p t i n g t o a n a l y z e these r e a c t i o n s w i t h a n o t a t i o n t h a t designates m o l e c u l a r i t y r a t h e r t h a n f o c u s i n g a t t e n t i o n o n t h e e n e r g e t i c role o r l a c k of e n e r g e t i c r o l e of t h e e n t e r i n g group.

D r . T o b e p o i n t s o u t t h a t a large b o d y of t h e s t u d i e s o n t h e a m m i n e c o b a l t ( I I I )

s y s t e m s c a n be i n t e r p r e t e d s u c c e s s f u l l y f r o m a p o i n t of view s u g g e s t i n g t h a t t h e r

r e a c t i o n s r e q u i r e p r e a g g r e g a t i o n of t h e r e a c t a n t s (the o r i g i n a l c o m p l e x a n d t h e e n t e r i n g g r o u p ) , b u t t h a t t h e a c t i v a t i o n e n e r g y for t h e s u b s t i t u t i o n d e r i v e s l a r g e l y w i t h i n t h e o r i g i n a l c o m p l e x a n d is n o t s i g n i f i c a n t l y r e d u c e d b y p a r t i c i p a t i o n of t h e entering group. I t h i n k D r . T o b e ' s p a p e r c l e a r l y suggests t h a t c o n c e p t s d e v e l o p e d i n c o n n e c t i o n w i t h r e a c t i o n s i n n o n a q u e o u s s o l v e n t s , p r e a s s o c i a t i o n , a n d i t s i m p o r t a n c e for r e a c t i o n , m a y have i m p o r t a n t applications i n s t u d y i n g reactions i n aqueous solution. I n t h a t c o n t e x t I w o u l d l i k e t o offer a few t h o u g h t s o n t h e m o r e o r less c l a s s i c r e a c t i o n of w a t e r w i t h t h e c h l o r o p e n t a m m i n e c o b a l t i c i o n . T h e f o r w a r d r e a c t i o n , t h e r e p l a c e m e n t of c h l o r i d e b y w a t e r , w a s s t u d i e d q u i t e s o m e y e a r s ago.

A l t h o u g h t h e r e has been s o m e i n t e r e s t i n t h e reverse r e a c t i o n , t h e

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

I n p a r t i c u l a r , of c o u r s e , t h e r e a r e a few

cases w h e r e one m a y i d e n t i f y r a t e s of t h e same s o r t t h a t a r e d i s c u s s e d i n t h e p a p e r o n n o n a q u e o u s s y s t e m s - i . e . , r a t e s of i n t e r c h a n g e b e t w e e n o u t e r a n d i n n e r sphere ligands. I n t h e case of sulfate a n d d i h y d r o g e n p h o s p h a t e w h e r e t h e i o n p a i r a s s o c i a t i o n c o n s t a n t s h a v e been c l e a r l y i d e n t i f i e d , t h e a n a t i o n rates a r e k n o w n for t h e 1:1 o u t e r sphere c o m p l e x .

These rate values v a r y somewhat, a n d this perhaps indicates

p a r t i c i p a t i o n of t h e e n t e r i n g g r o u p . w h a t is g o i n g o n .

B u t there m a y be a n o t h e r w a y t o i n t e r p r e t

T h e s e t w o r a t e s of a n i o n e n t r y a r e s m a l l e r t h a n t h e r a t e of w a t e r

e x c h a n g e of t h e a q u o p e n t a m m i n e c o m p l e x .

In fact, the u n i v a l e n t a n i o n enters t h e

c o m p l e x f r o m t h e o u t e r sphere a t a p p r o x i m a t e l y o n e - e i g h t h of t h e r a t e of w a t e r e x c h a n g e , a n d t h e d i v a l e n t a n i o n e n t e r s t h e c o m p l e x f r o m t h e o u t e r sphere a b o u t t w i c e as fast. P e r h a p s these r e s u l t s c o u l d be i n t e r p r e t e d b y a d o p t i n g a m o d e l w h e r e t h e e n t e r i n g g r o u p does n o t p a r t i c i p a t e i n t h e a c t i v a t i o n process a t a l l b u t t h a t w h e n t h e b o n d f r o m t h e c o b a l t t o t h e w a t e r is s u f f i c i e n t l y w e l l b r o k e n , w h a t e v e r g r o u p i s i n place ( s t a t i s t i c a l l y ) , falls i n .

F r o m t h i s p o i n t of v i e w t h e n u m b e r e i g h t (as a m a g i c


1.

TOBE

21

ùiêcvssion

number) for the number of molecules in the second coordination sphere is not entirely unreasonable. We have some preliminary experiments on the rate of chloride entry in the aquopentammine complex over a range of chloride concentrations, which perhaps also can be incorporated into a picture of this kind. If we plot the observed rate vs. the concentration of sodium chloride of less than O.lOAf to 2M, the rate in the low concentration region depends on the chloride concentration.

T h e reaction is approximately second-order.

controlling the ionic strength.)

(This is without

Slightly above 0.101/ there is a definite change of

slope, and the reaction rate is less sensitive to chloride concentration.

Perhaps


here again the change of slope represents the formation of the 1:1 outer sphere complex, and in this region the additional rate increase is caused by additional chloride association with the ion.

If we extrapolate the high chloride portion of

the curve to zero, the intercept is again approximately one-eighth of the water exchange rate, or in pretty close agreement to the rate of entry of dihydrogen phosphate. A t this point I am not sure that I am prepared to defend this interpretation. But I suggest that these results indicate that perhaps D r . Tobe's concept of preassociation should be seriously considered in accounting for reaction process in aqueous solution which have been regarded more in terms of the dissociative process. R a l p h G . Pearson :

I would like to say a few things in connection w ith the r

ideas brought out by D r . Tobe's paper.

First, the thing that impressed me very

much was the fact that just as in an aqueous solution, for these octahedral complexes (at least the ammineeobalt(III) systems) we find that the rate of the reaction is remarkably independent of the nature and concentration of the incoming group.

T h i s has, of course, led many people to discuss a reaction path in which

bond making was relatively unimportant, and bond breaking led bond making to a substantial degree. I would be interested in any comment from the audience about the chronological development of this concept.

A s far as I can remember, in 1958 a number of

people simultaneously brought this idea forward.

Roughly, it was that if we are

going to replace a coordinated chlorine by something, then just as Tobe's diagram represents, we would have a mechanism in which lengthening of the cobalt—chlorine bond would be the critical step.

Figure A.

We have to supply activation energy from within

Solvent assisted dissociation, or SAD

mechanism; X-solvent molecule or

other ligand in second coordination shell. the complex, an extreme vibration, so that eventually this cobalt—chlorine bond becomes greatly lengthened and is on the verge of breaking.

B u t I feel that for

many cases of the normal substituents on cobalt it would be quite impossible for this chlorine just to leave and for a five-coordinated intermediate to remain behind.



22

A l s o , r e a r r a n g e m e n t i n m a n y cases w o u l d be d i f f i c u l t because of t h e c r y s t a l field s t a b i l i z a t i o n effects.

B u t i t c o u l d be possible t h a t s o m e w h e r e , w h e n t h i s b o n d h a d

l e n g t h e n e d s u f f i c i e n t l y , t h i s g r o u p X c o u l d s l i p i n ; a n d as D r . L a n g f o r d i n d i c a t e d , we w o u l d then have an intermediate, or transition s t a t e / o r something w h i c h would


h a v e these t w o g r o u p s p a r t i a l l y b o n d e d .

Figure B.

Partial bonding of X and Cl groups

T h e n i n w a t e r s o l u t i o n i t seems t h a t t h i s X g r o u p is w a t e r , a l m o s t u n i v e r s a l l y . B u t t h i s w o u l d a c c o u n t for t h e g e n e r a l i n d e p e n d e n c e of t h e r e a c t i o n r a t e o n t h e n a t u r e of t h i s X

group.

T h i s is the m e c h a n i s m w h i c h has been c a l l e d t h e s o l v e n t a s s i s t e d d i s s o c i a t i v e mechanism, or the S A D mechanism.

I t h i n k the name "interchange m e c h a n i s m "

m i g h t be a g o o d one, p e r h a p s b e t t e r t h a n t h e S A D m e c h a n i s m . I n 1958, a s I r e m e m b e r i t , F r e d B a s o l o a n d I , A r t h u r A d a m s o n a n d H e n r y T a u b e , a n d H . R . H u n t came out w i t h this mechanism basically, stressing slightly different features.

T h e n D r . T o b e joined the bandwagon w i t h this mechanism i n

1959, a n d t h e t e r m " s o l v e n t assisted d i s s o c i a t i o n " w a s c o i n e d b y W a l l a c e a n d h i s g r o u p i n C a n a d a i n 1961.

T h a t is t h e s t o r y a s I see i t a t t h e p r e s e n t t i m e

I w o u l d l i k e t o s h o w s o m e figures t o set u p s o m e b a c k g r o u n d a n d l a n g u a g e . Base OH0 H" NH NH HPOr NH2OH FNOr H 0 I-

A

1 x 10-» 1 X 10" 4 X 10" 7 Χ ΙΟ" 2 Χ ΙΟ"* 2 X 10-* 4 Χ ΙΟ" 55

12

2

2

k

K

e

2

8

2

4

2

Figure C.

—

1.7 X 10» M~ 8.0 X 10* 33 3.2 X 10-* 1.2 1.7 Χ ΙΟ" 8.8 X 10-» 4.9 X 10-* 3 Χ ΙΟ" ?

l

sec."

1

8

4

Rate constants for base-catalyzed hydrolysis of

Si(acac)z

+

F i g u r e C s h o w s a n e x t r e m e case of t h e d e p e n d e n c e of a s u b s t i t u t i o n r e a c t i o n r a t e o n t h e n a t u r e of t h e i n c o m i n g g r o u p .

T h i s h a p p e n s t o be t h e h y d r o l y s i s of t h e

t r i s a c e t y l a c e t o n a t e c o m p l e x of s i l i c o n (I V ) , c a t i o n i c species, w h i c h K i r c h n e r s t u d i e d first—the

r a t e of r a c e m i z a t i o n o r r a t e of d i s s o c i a t i o n .

We

studied the

base-

c a t a l y z e d r a t e of d i s s o c i a t i o n a n d s h o w e d t h a t a large n u m b e r of a n i o n s a n d n u c l e o p h i l i c g r o u p s , i n g e n e r a l , w o u l d c a t a l y z e i n t h e d i s s o c i a t i o n process.

W e found that

t h e r e a c t i o n r a t e s were a c t u a l l y for a s e c o n d - o r d e r process, so these u n i t s a r e l i t e r s per mole per second.

B u t the reaction rate d i d v a r y over a n enormous r a n g e — i n

t h i s case, a b o u t a f a c t o r of 1 0 — a n d t h i s i s t y p i c a l of the s o r t of v a r i a t i o n i n r a t e s 9

of r e a c t i o n ( t h a t y o u c a n get) for processes t h a t seem t o be Sj\r2 b i m o l e c u l a r d i s p l a c e m e n t processes.


1·

TOBE

Discussion

23

T h e m e c h a n i s m , i n c i d e n t a l l y , for t h i s a c e t y l a c e t o n a t e case is n o t u n a m b i g u o u s a n d i t is possible t h a t n u c l e o p h i l i c a t t a c k o c c u r s a t t h e l i g a n d r i n g i n one of t h e c a r b o n y l groups. Y o u w i l l n o t i c e i n t h i s case t h a t the n u c l e o p h i l i c r e a c t i v i t y c o n s t a n t s — t h e s e rate c o n s t a n t s — v a r y roughly w i t h the basicity.

H y d r o x i d e i o n i s a l m o s t the m o s t

p o w e r f u l n u c l e o p h i l i c reagent because i t is the strongest base t h a t c a n e x i s t i n a q u e ous s o l u t i o n .

W a t e r is t h e w e a k e s t n u c l e o p h i l i c reagent, s i m p l y because a n y t h i n g

w e a k e r t h a n w a t e r is n o t d e t e c t a b l e i n a n a q u e o u s s o l u t i o n .

S o t h i s n u m b e r is t h e

l o w e r l i m i t for n u c l e o p h i l i c r e a c t i v i t y .


F o r o t h e r n u c l e o p h i l e s s u c h as i o d i d e — r e a l l y one c a n ' t decide w h e t h e r there i s a n u c l e o p h i l i c r a t e c o n s t a n t for i o d i d e o r n o t , because y o u h a v e t o h a v e e n o r m o u s c o n c e n t r a t i o n s of i o d i d e t o d e t e c t a r a t e c o n s t a n t of t h i s v a l u e .

S o t h i s n u m b e r here

is r e a l l y j u s t a n u p p e r l i m i t t o w h a t the r e a c t i v i t y of i o d i d e is. N o t i c e — a n d I w a n t t o m a k e a p o i n t of t h i s — t h e h y d r o g e n p e r o x i d e a n i o n is a b e t t e r n u c l e o p h i l i c reagent t h a n the h y d r o x i d e i o n b y a f a c t o r of a b o u t 50, e v e n t h o u g h h y d r o g e n p e r o x i d e is a s t r o n g e r a c i d t h a n w a t e r b y a f a c t o r of a b o u t W e c o n v e r t w a t e r t o the same u n i t s as h y d r o g e n

10 . 4

peroxide.

T h i s is a n e x a m p l e of w h a t J o h n E d w a r d s a n d I c a l l the a l p h a effect.

I think

i t i s v a l u a b l e because i t does e n a b l e us t o generate q u i t e e a s i l y a v e r y p o w e r f u l n u c l e o p h i l i c reagent i n w a t e r , t h i s h y d r o g e n p e r o x i d e a n i o n . T h i s p a r t i c u l a r n u c l e o p h i l i c r e a c t i v i t y series w h i c h w e find for s i l i c o n ( I V ) is n o t n e c e s s a i i l y c h a r a c t e r i s t i c of w h a t one w o u l d e x p e c t t o find for a l l different m e t a l ions.

A n d i n f a c t , we k n o w i n t h e case of p l a t i n u m ( I I ) , w h e r e SAT2 r e a c t i o n s seem

t o o c c u r q u i t e c o m m o n l y , t h a t we get q u i t e a different n u c l e o p h i l i c r e a c t i v i t y series. T h e v a r i a t i o n i n r a t e s for p l a t i n u m ( I I ) is a l m o s t as large as t h i s . t h a t a n a c t u a l range of 10

9

I don't

know

has been c o v e r e d y e t , b u t the o r d e r of t h e different

n u c l e o p h i l e s is q u i t e d i f f e r e n t . T h e n e x t t w o figures i n t r o d u c e some t e r m i n o l o g y a b o u t h a r d a n d soft t h a t I a m p u s h i n g a t t h e m o m e n t , because I t h i n k i t is u s e f u l .

D a r y l e B u s c h suggested

t h i s t o m e one t i m e a n d I p i c k e d i t u p a n d f o u n d i t u s e f u l . Hard Bases OH-, F S O r , PO4-3 CH3COO-, R O Cl", N H 2

3

Figitre D.

Soft Bases I", R S, R P CO, C N - , R C N C H , C H H-, R2

6

6

3

2

4

Examples of some hard and soft bases

W e define a h a r d base s i m p l y as one t h a t is n o t v e r y p o l a r i z a b l e .

It usually

m e a n s a l s o t h a t t h e b a s i c a t o m is one of h i g h e l e c t r o n e g a t i v i t y a n d n o t v e r y e a s i l y oxidized.

T h e t y p i c a l h a r d bases w o u l d be o x y g e n a t o m l i g a n d s a n d fluoride i o n ,

w i t h c h l o r i d e i o n a n d a m m o n i a c e r t a i n l y n o t as h a r d as those, because c h l o r i d e is m o r e p o l a r i z a b l e t h a n fluoride, a n d a m m o n i a is m o r e p o l a r i z a b l e t h a n w a t e r . B y soft bases I m e a n h i g h l y p o l a r i z a b l e bases.

T h i s usually means not o n l y

large a t o m s s u c h as i o d i d e a n d s u l f u r , b u t a l s o u n s a t u r a t e d s y s t e m s s u c h as t h e n i t r i l e s , t h e olefins, the a r o m a t i c s , a n d t h e a l k y l a n d h y d r i d e i o n s w h i c h are k n o w n e x p e r i m e n t a l l y t o be h i g h l y p o l a r i z a b l e .

S o f t bases a r e of l o w e l e c t r o n e g a t i v i t y

and easily oxidized. F i g u r e Ε shows t h e c o r r e s p o n d i n g e n t r i e s i n t h e case of t h e a c i d s w h i c h c o o r d i n a t e to these v a r i o u s bases, f o r m i n g o u r f a m i l i a r c o o r d i n a t i o n c o m p o u n d s i n some


24

MECHANISMS

O F I N O R G A N I C REACTIONS

Hard Acids

Soft Acids

H+, Mg+ , A l Cr+ , Si+ , As+ BF , PR0 , R S 0 R C+, RCO+ H X ( h y d r o g e n bonders) 2

3

3

+ 3

4

Cu+, Pt+ , Hg+ R S + , I+, H 0 + I9, n i t r o b e n z e n e , q u i n o n e s O, C l , R C M e t a l atoms 2

b

3

2

+

2

+

3

Figure

E.

2

3

Examples

of some hard and soft acids

cases, o r p e r h a p s o t h e r t y p e s of complexes, s u c h as charge t r a n s f e r complexes, i n o t h e r cases.


W e define t h e h a r d a c i d s s i m p l y as those of low p o l a r i z a b i l i t y . of s m a l l size a n d h i g h p o s i t i v e c h a r g e .

T h e y w o u l d be

T h e soft a c i d s w o u l d be those of h i g h

p o l a r i z a b i l i t y , large size, a n d l o w p o s i t i v e c h a r g e .

A s y o u c a n see, there are m a n y

o t h e r a c i d s besides m e t a l ions w h i c h c a n be classified i n t h i s w a y . N o w as f a r as m e t a l ions are c o n c e r n e d , t h i s classific at ion is i d e n t i c a l w i t h — i n fact I use t h e same o p e r a t i o n a l d e f i n i t i o n as t h a t w h i c h C h a t t , A h r l a n d , a n d D a v i e s use.

H a r d a c i d s w o u l d be C l a s s A m e t a l ions a n d soft a c i d s w o u l d be C l a s s

Β m e t a l i o n s ; b u t I d o n ' t t h i n k C l a s s A a n d C l a s s Β m e a n q u i t e t h e same t h i n g as h a r d a n d soft, because h a r d a n d soft h a v e w i d e r a p p l i c a b i l i t y i n t h a t y o u c a n d isc u ss o t h e r s y s t e m s as w e l l .

A n d f u r t h e r m o r e , I t h i n k bases c a n be classified as h a r d a n d

soft, a n d t h e n one has t h i s v e r y useful r u l e t h a t h a r d a c i d s l i k e t o c o m b i n e w i t h h a r d bases, a n d soft a c i d s l i k e t o c o m b i n e w i t h soft bases.

T h i s is j u s t a n e m p i r i c a l

o b s e r v a t i o n — v a r i o u s theories c a n be p u t f o r w a r d t o a c c o u n t for i t — b u t i t is a f a c t , a n d t h e reason we s h o u l d be a w a r e of i t is because if we a r e l o o k i n g a t k i n e t i c s , i f we are l o o k i n g a t r e a c t i o n r a t e s , t h e n we s h o u l d r e m e m b e r t h a t o u r s u b s t r a t e , t h a t is, t h e c o m p l e x w h i c h is g o i n g t o lose a g r o u p , w i l l p r o v i d e t h e a c i d site. a t o m w i l l be t h e e l e c t r o p h i l i c c e n t e r o r t h e a c i d site.

T h e metal

Depending upon whether that

is a h a r d a c i d site o r a soft a c i d site, we w i l l e x p e c t t h a t n u c l e o p h i l e s , e i t h e r h i g h l y basic t o w a r d s the p r o t o n (the p r o t o n is t h e k e y h a r d a c i d , t h e p r o t o t y p e h a r d a c i d ) o r else the p o l a r i z a b i l i t y p h e n o m e n o n w i l l be i m p o r t a n t — o n e o r t h e o t h e r .

For

s i l i c o n ( I V ) i t seems t h a t b a s i c i t y is i m p o r t a n t , for p l a t i n u m ( I I ) i t seems t h a t p o l a r i z a b i l i t y o r l o w e l e c t r o n e g a t i v i t y is i m p o r t a n t .

S o we e x p e c t t o get different

n u c l e o p h i l i c orders, c e r t a i n l y . I w a n t t o s a y s o m e t h i n g a b o u t the e n d of D r . T o b e ' s p a p e r o n base h y d r o l y s i s . T h e U n i v e r s i t y C o l l e g e S c h o o l has been f e u d i n g w i t h s o m e of us o n t h i s side of t h e ocean r e g a r d i n g t h e m e c h a n i s m of base h y d r o l y s i s of c o b a l t p e n t a m m i n e s for s o m e years, a n d t h e y are extremely ingenious a t coming up w i t h alternative explanations for a l l of the c o n c l u s i v e e x p e r i m e n t s t h a t we seem t o d o .

However, I a m strongly

of t h e o p i n i o n t h a t m u c h of t h i s resembles the c l o u d s of b l a c k i n k t h a t t h e c u t t l e f i s h is s a i d t o e m i t w h e n i t is e s c a p i n g f r o m s o m e p u r s u e r . Co(NH ) Cl+ + O H " Co(NH ) NH Cl+ + H 0 Co(NH ) NH Cl+ Co(NH ) NH + + Cl~ C o ( N H ) N H + + H 0 -» Co(NH ) OH+ 3

2

5

3

3

4

3

2

4

Figure

3

2

2

F.

2

SNICB

4

4

2

2

2

2

3

5

2

Mechanism

F i g u r e F shows t h e c o n j u g a t e base m e c h a n i s m for base h y d r o l y s i s .

D r . Tobe

suggests e s s e n t i a l l y t h a t i n base h y d r o l y s i s , the h y d r o x i d e i o n o c c u p i e s a u n i q u e p o s i t i o n for one of s e v e r a l reasons. P e r h a p s t h e h y d r o x i d e i o n is h y d r o g e n b o n d e d


1·

TOBE

25

Discussion

to an N H group, and that is the reason why it is special. wouldn't be bonded to an N H group escapes me.

Why

other anions

T h e other thing is that perhaps

the hydroxide ion can penetrate to a reaction site very quickly by a " G r o t t h u s " chain mechanism.

In other words, we might always have water close to chloride

ion, which is going to be a leaving group; then as the cobalt—chlorine bond stretches, that water molecule might instantaneously be converted to a hydroxide ion by a proton transfer mechanism.

I think this sort of thing can indeed happen, and it is

certainly worthwhile considering the possibilities of rapid proton removal converting a weakly nucleophilic water to a strongly nucleophilic hydroxide ion.


Incidentally, I should say that the thermodynamic data on cobalt(III) indicates that it is a hard acid, but just barely so, and one might say borderline. little harder than tetrahedral carbon and alkyl halides.

It is a

So hydroxide ion in that

sense would be expected perhaps to be a good nucleophilic reagent, as hard acids would like hydroxide ion. T h e alternate mechanism which opposes the hydroxide ion as actually being the group that slips in either an S.v2 mechanism or some variation of an S#2 mechanism is, of course, the conjugate base mechanism proposed by Garrich in 1937.

The

loss of a proton, with the amido group acting as a powerful activator owing to 7rbonding, would be the first step. five-coordinated

Unimolecular dissociation then occurs forming a

intermediate which has some moderate kinetic stability, at least

under suitable circumstances.

T h e n water is picked up by this

five-coordinated

intermediate, followed by rearrangement of protons and the attainment of the final product. Fred Basolo and I have come up with at least three critical tests of this particular mechanism to distinguish it from an S#2 mechanism.

One critical test is based

on the fact that this mechanism requires acidic protons, whereas other straightforward displacement mechanisms certainly would not require such acidic protons. Admittedly, the concept of hydroxide ion binding to an N H group also requires acidic protons. B y now a great many studies oi such complexes have been made, and D r . Tobe reports one himself, the diarsine complex.

F o r all those cases involving an uni-

dentate leaving group, the rate of the hydrolysis under basic conditions is no different from the rate of hydrolysis under acid conditions.

A s far as I can tell, for

unidentate ligands, at least where the results perhaps are a little less unambiguous compared to chelate leaving groups, the requirements of a conjugate base mechanism have stood up to tests even on the side of the opposition. T h e second critical test of this conjugate base mechanism is based on the fact that this five-coordinated intermediate, if indeed it exists, would not always have to react with the solvent, though the solvent would be what it would react with under most circumstances.

We have run this type of base hydrolysis in the presence

of many anions of high concentration, and the only thing that we can find is the hydroxo complex; so at least in water solution, water seems to be what this fivecoordinated intermediate picks up.

But in dimethylsulfoxide it certainly is pos-

sible to throw in various anions, and since dimethylsulfoxide is not as good as water in coordination, other nucleophiles may react.

We do find in dimethylsulfoxide

that a base, such as hydroxide ion, speeds up the rate of base hydrolysis; but the product, instead of being a hydroxo compound, is the complex corresponding to whatever anion we have added, such as nitrite ion, azide ion, and thiocyanate ion.


26

MECHANISMS OF INORGANIC

REACTIONS

I t is i n t e r e s t i n g t h a t T a u b e a n d G r e e n h a v e d o n e t h i s s a m e e x p e r i m e n t essen tially i n water.

I s a i d t h a t we c o u l d n ' t d o t h i s e x p e r i m e n t i n w a t e r because w a t e r

a l w a y s w a s the r e a c t a n t a n d n o t o t h e r a d d e d a n i o n s .

B u t Taube and Green very

c l e v e r l y t o o k a d v a n t a g e of t h e fact t h a t a n i s o t o p e d i s c r i m i n a t i o n f a c t o r for o x y g e n 18 s u c h as t h e r a t i o of o x y g e n - 1 8 t o o x y g e n - 1 6 i n w a t e r is n o t t h e same a s t h e r a t i o in hydroxide ion.

T h i s i n effect l a b e l s t h e h y d r o x i d e i o n a n d d i s t i n g u i s h e s i t f r o m

w a t e r , i n s p i t e of t h e f a c t t h a t r a p i d p r o t o n t r a n s f e r is o c c u r r i n g .

Green and Taube

s h o w e d t h a t i n base h y d r o l y s i s i n a q u e o u s s o l u t i o n , t h e h y d r o x o c o m p l e x t h a t t h e y o b t a i n e d h a d a n o x y g e n r a t i o s u c h t h a t t h e h y d r o x o g r o u p m u s t h a v e been d e r i v e d


from water a n d not from hydroxide ion.

T h i s e x p e r i m e n t seems t o m e t o

be

p r e t t y u n e q u i v o c a l a n d , i n f a c t , t h e e x a c t a n a l o g of t h e e x p e r i m e n t t h a t w e d i d i n dimethylsulfoxide. T h e t h i r d test of t h e c o n j u g a t e base m e c h a n i s m t h a t w e p u t f o r w a r d w as b a s e d r

o n t h e i d e a t h a t t h e first s t e p s h o u l d be w r i t t e n as a n e q u i l i b r i u m , a n d t h e r e a c t i o n r a t e s h o u l d s h o w specific h y d r o x i d e i o n c a t a l y s i s .

If t h i s i s i n d e e d i n e q u i l i b r i u m ,

a n d d e u t e r i u m exchange s t u d i e s s a y t h a t i t m u s t be, t h e n t h e r a t e of t h e r e a c t i o n m u s t d e p e n d o n t h e 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 , a n d o n n o t h i n g else. T h i s is e x p e r i m e n t a l l y w h a t i s f o u n d , of course ; b u t t h e n i t m a k e s one w o n d e r i f S#2 m e c h a n i s m s e x i s t w h e n one n e v e r gets a n y effect of a n y o t h e r a d d e d n u c l e o phile.

W e d i d w h a t I thought was the extreme t h i n g to distinguish between

S j s r l C B m e c h a n i s m s a n d S#2 m e c h a n i s m s .

W e added hydrogen peroxide to the

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

N o w , i f i t were a n Sjy2

r e a c t i o n , t h e a r g u m e n t w o u l d be t h a t t h e a n i o n h y d r o g e n p e r o x i d e i n e v e r y case t h a t h a s been t e s t e d — a n d t h e r e a r e a b o u t a d o z e n of t h e m — i s a n y w h e r e f r o m 35 t o 10,000 t i m e s m o r e r e a c t i v e t h a n t h e h y d r o x i d e i o n a s a n u c l e o p h i l i c r e a g e n t .

So,

if t h e r e is a n y Si\r2 c h a r a c t e r t o t h i s r e a c t i o n , i t seems t o us t h a t w e s h o u l d h a v e g o t t e n s o m e increase i n r a t e .

O n t h e o t h e r h a n d , because of t h e f a c t t h a t h y d r o g e n

p e r o x i d e is a s t r o n g e r a c i d t h a n w a t e r , a d d i n g h y d r o g e n p e r o x i d e , of c o u r s e , c u t s d o w n the hydroxide ion concentration drastically.

T h i s draws the equilibrium

w a y b a c k t o t h e left, a n d h y d r o g e n p e r o x i d e s h o u l d decrease t h e r a t e of t h e r e a c tion.

A t IM h y d r o g e n p e r o x i d e , i n fact, one s h o u l d get a decrease i n t h e r a t e of

t h e r e a c t i o n b y a f a c t o r of 150.

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

r e a c t i o n r a t e , o r a decreased r e a c t i o n r a t e b o t h b y a f a c t o r of a 100 o r so.

I hardly

need t o s a y , t h a t t h e r e a c t i o n r a t e decreased b y a f a c t o r of 100. I t seems t o m e t h a t we h a v e g i v e n t h i s p a r t i c u l a r r e a c t i o n m e c h a n i s m as m a n y tests as p o s s i b l e . N o w , let m e dispose of, I h o p e , T o b e ' s a r g u m e n t t h a t p e r h a p s a G r o t t h u s c h a i n transfer mechanism is i n v o l v e d .

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

c a n a p p e a r a n y w h e r e b y b e i n g generated f r o m w a t e r b y a p r o t o n t r a n s f e r .

I admit

t h i s , b u t t h e v e r y s i m p l e t h i n g i s , i f h y d r o x i d e i o n c a n generate h y d r o x i d e i o n b y a p r o t o n t r a n s f e r , w h y c a n ' t a l l o t h e r bases generate h y d r o x i d e i o n b y a p r o t o n t r a n s fer?

If a l l one needs t o cause a r a p i d r e a c t i o n of c o b a l t c o m p l e x is t o be a b l e t o

create h y d r o x i d e i o n o n d e m a n d b y a p r o t o n t r a n s f e r , a n y base w i l l d o t h i s ; a n d the hydrogen peroxide a n i o n , i n fact, m a y do i t better t h a n the hydroxide i o n , from t h e v i e w p o i n t of r a t e , n o t e q u i l i b r i u m . B u t we k n o w from m a n y experiments done b y m a n y workers t h a t no other base i n t h e case of t h e t y p i c a l p e n t a m m i n e c o b a l t o r c h l o r a m m i n e c o b a l t s y s t e m s — n o o t h e r base s h o w s a n y k i n e t i c effect a t a l l t h a t I a m a w a r e of.

I t seems t o m e


1.

TOBE

Discussion

27

a b s o l u t e l y i n c r e d i b l e t h a t o n l y t h e h y d r o x i d e i o n w o u l d be p r i v i l e g e d t o i n d u l g e i n p r o t o n t r a n s f e r , a n d t h a t a l l o t h e r bases w o u l d be e x c l u d e d . I n t h e a c i d h y d r o l y s i s of t h e p e n t a m m i n e c o b a l t c o m p l e x e s w h e r e y o u h a v e a l e a v i n g g r o u p , s u c h as n i t r a t o o r b r o m o , H . T a u b e a n d A . H a i m c a m e o u t w i t h s o m e v e r y i n t e r e s t i n g w o r k , w i t h w h i c h I a m sure y o u are a l l f a m i l i a r . that the

five-coordinated

T h e y suggested

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

c r i m i n a t e d b e t w e e n v a r i o u s n u c l e o p h i l i c reagents, s o m e t i m e s r e a c t i n g w i t h w a t e r , sometimes with, thiocyanate ion.

I n fact, t h e y were able t o measure these n u c l e o

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


different t y p e s of r e a c t i o n s i n w h i c h t h e p e n t a m m i n e c o b a l t w o u l d be g e n e r a t e d i n different w a y s . I w a s s h o c k e d w h e n I s a w t h i s p a p e r because w e h a v e been of t h e o p i n i o n for a l o n g t i m e t h a t t h i s p e n t a m m i n e c o b a l t species is m u c h t o o h i g h i n e n e r g y t o e x i s t f o r a l o n g e n o u g h t i m e t o d i s c r i m i n a t e b e t w e e n different n u c l e o p h i l i c reagents.

In

f a c t , e v e n i n t h e base h y d r o l y s i s w h e r e w e h a v e a s t r o n g l y a c t i v a t i n g a m i d o g r o u p i t d o e s n ' t seem possible t o get d i s c r i m i n a t i o n i n w a t e r s o l u t i o n . T h e r e are some s i m p l e tests of t h i s h y p o t h e s i s of H a i m a n d T a u b e .

A l l we

h a v e t o d o is t o c a r r y o u t a c i d h y d r o l y s i s of one of these t y p i c a l c o m p l e x e s i n t h e presence of a large a m o u n t of a n a n i o n .

T h e n w e s h o u l d go d i r e c t l y t o t h e a n i o n i c

complex according to H a i m a n d Taube.

I felt t h a t w e w o u l d n o t f o r m a n y large

a m o u n t of t h e a n i o n i c c o m p l e x , because the p e n t a m m i n e c o b a l t i n t e r m e d i a t e is n o t f o r m e d , a n d i n s t e a d w e w o u l d go t o t h e a q u o c o m p l e x a n d t h e n f r o m t h e a q u o c o m p l e x t o the t h i o c y a n a t e c o m p l e x , o r w h a t e v e r i t m i g h t be.

TIME (min.) Figure

G.

Plot

of

[Co(NII ) NO ](NO ) z

H

+

B

z

= 0,02M.

z

2

absorbancy vs.

time for

aquation

(0.01M) in the presence

of

ofOJOMNaSCN;

Upper curve calculated for mechanism involving

a five-coordinate intermediate.

Lower

curve calculated

mechanism involving conversion to Co(NHz)^H20

¥Z

sequently reacts to form Co(NHz)^NCS . +2

mental.

for

which sub-

Points are experi-

(Inorg. Chem. 3, 1334 (1964).



28

F i g u r e G shows t h e r e s u l t s of the f o l l o w i n g r e a c t i o n . [Co(NH ) N0 ]+ 3

5

3

2

!H^>

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

5

2

+

3

N0 ~ 3

T h i s i s t h e o p t i c a l d e n s i t y p l o t t e d a g a i n s t t h e t i m e for a h e x p e r i m e n t i n w h i c h we t o o k t h e n i t r a t o p e n t a m m i n e c o b a l t c o m p l e x i n t h e presence of 0.50ilf t h i o c y a n a t e i o n since t h e c o m p e t i t i o n r a t i o for t h i o c y a n a t e i o n a n d for w a t e r h a d been d e t e r mined by H a i m and Taube.

K n o w i n g the o p t i c a l d e n s i t i e s of a l l the possible r e a c -

t a n t s a n d p r o d u c t s , we c o u l d c a l c u l a t e w h a t t h e o p t i c a l d e n s i t y s h o u l d be as a f u n c t i o n of t i m e a c c o r d i n g t o the m e c h a n i s m of H a i m a n d T a u b e .

T h i s calculation de

p e n d s u p o n a r a t e c o n s t a n t for t h e a q u a t i o n of the n i t r a t o c o m p l e x w h i c h we t o o k


from the literature.

B u t one s h o u l d get a c o n t i n u o u s l y i n c r e a s i n g o p t i c a l d e n s i t y

because t h e t h i o c y a n a t e c o m p l e x has a h i g h e r o p t i c a l d e n s i t y t h a n a n y t h i n g else. H o w e v e r , i f r e a c t i o n w e n t b y w a y of a q u a t i o n t o a n a q u o c o m p l e x a n d t h e n a n a t i o n of i h e a q u o c o m p l e x t o the t h i o c y a n a t o , a n d we k n e w t h i s r a t e c o n s t a n t , t h e n t h e o p t i c a l d e n s i t y w o u l d d o t h e f o l l o w i n g : since the a q u o c o m p l e x has a l o w e r e x t i n c t i o n coefficient t h a n t h e n i t r a t o , t h e d e n s i t y w o u l d d r o p first a n d t h e n e v e n t u a l l y r i s e , a n d of course t h e t w o c u r v e s w o u l d c o m e t o g e t h e r e v e n t u a l l y . T h e e x p e r i m e n t a l p o i n t s here f o l l o w t h e p r e d i c t i o n s of t h e second

mechanism

v e r y c l o s e l y , a n d I w o u l d s a y t h a t n o t m o r e t h a n 2 % of t h i o c y a n a t e c o m p l e x formed directly.

T a u b e a n d H a i m ' s p r e d i c t i o n w o u l d be

W e h a v e d o n e t h e same e x p e r i m e n t , i n c i d e n t a l l y , for t h e and the results are precisely the same.

is

14%. bromopentammine

Y o u will recall that Langford mentioned

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

there was no mass law re

t a r d a t i o n s u c h as w o u l d be p r e d i c t e d f r o m t h e m e c h a n i s m of H a i m a n d T a u b e . Arthur W. Adamson:

I w o u l d l i k e t o s a y t h a t t w o y e a r s ago I h a d t h e p r i v

ilege of a y e a r ' s v i s i t a t U n i v e r s i t y C o l l e g e L o n d o n a n d t h a t D r . T o b e a n d C . K . I n g o l d r e p r e s e n t e d a n i s l a n d of E n g l a n d i n t h e A u s t r a l i a n S e a . I w a n t t o fill i n some of t h e d i s c u s s i o n s we h a d a t U C L o n w h a t I w a s c a l l i n g t h e cage m e c h a n i s m , t o a d d s t i l l a n o t h e r t o t h e l i s t of n a m e s . t h a n the S A D m e c h a n i s m .

I t h i n k i t is b e t t e r

B u t t h e p o i n t t h a t I t h i n k is e s s e n t i a l t o t h e g e n e r a l

i d e a is t h a t w h i c h c o m e s o u t of t h e f o l l o w i n g set of n u m b e r s .

If one considers a

b i m o l e c u l a r gas phase r e a c t i o n , a n d l e t ' s s a y 0.01 M reagents, one c a n e x p e c t a m o l e cule t o e x p e r i e n c e s o m e t h i n g l i k e 10 c o l l i s i o n s p e r s e c o n d . 9

T h e s i t u a t i o n is different w i t h s o l u t i o n s , w h e r e the r e a c t a n t molecules m u s t diffuse t o g e t h e r .

F o r the same 0.01 M c o n c e n t r a t i o n of A a n d Β r e a c t a n t s , t h e

f r e q u e n c y w i t h w h i c h e i t h e r m a k e s a d i f f u s i o n a l e n c o u n t e r w i t h t h e o t h e r w i l l be a b o u t 10 p e r s e c o n d . 7

T h e y w i l l t h e n u n d e r g o a b o u t 100 c o l l i s i o n s o r v i b r a t o r y

i m p a c t s before d i f f u s i n g a w a y , so t h a t i t is t h e p a t t e r n r a t h e r t h a n the t o t a l n u m b e r of c o l l i s i o n s t h a t is different f r o m t h e gas phase case. If, n o w , 20 o r 30 k c a l . of e n e r g y of a c t i v a t i o n are needed for r e a c t i o n , one s u p poses i n t h e gas phase s i t u a t i o n t h a t the c o l l i d i n g molecules m u s t h a v e t h i s a m o u n t of k i n e t i c e n e r g y o r of a v a i l a b l e i n t e r n a l e n e r g y b e t w e e n t h e m .

The approximate

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

components

I n t h e case of a l i q u i d p h a s e s i t u a t i o n , h o w e v e r , i t i s u n

l i k e l y t h a t a m o l e c u l e h a v i n g a c c i d e n t a l l y a c q u i r e d t h i s s o r t of e n e r g y w i l l be a b l e t o r e t a i n i t for m o r e t h a n a few v i b r a t i o n a l p e r i o d s . r e a c t a n t spends a b o u t 1 0

- 7

In the above illustration, a

seconds o r a b o u t 10 v i b r a t i o n a l p e r i o d s between e n 5

counters a n d about a hundred such periods d u r i n g an encounter.

I t seems c l e a r


1·

TOBE

Discussion

29

t h a t o n l y i f t h e needed a c t i v a t i o n e n e r g y h a p p e n s t o a r r i v e w h i l e r e a c t a n t s are u n d e r g o i n g a n e n c o u n t e r w i l l there be m u c h c h a n c e of r e a c t i o n . T h e a p p r o x i m a t e p i c t u r e p r o v i d e d b y t h e cage m e c h a n i s m i s t h e n one of a p r e assembly

of

the reactants b y diffusional encounters

(whose d u r a t i o n m a y

be

l e n g t h e n e d i f a t t r a c t i v e forces o r c h e m i c a l b o n d i n g a b i l i t y i s present) w i t h subse q u e n t a r r i v a l of t h e e n e r g y of a c t i v a t i o n . T h e c e n t r a l a r g u m e n t , n a m e l y t h a t excess e n e r g y c a n n o t be h e l d for l o n g t i m e s c o m p a r e d t o v i b r a t i o n a l p e r i o d s , is s u p p o r t e d b y o u r o w n w o r k o n p h o t o c h e m i s t r y . H e r e , large a m o u n t s of e n e r g y a r e d e l i v e r e d t o a c o m p l e x i o n a n d m o r e often t h a n


n o t , escape as v i b r a t i o n a l o r t h e r m a l e n e r g y w i t h o u t a n y r e a c t i o n ' s o c c u r r i n g .

Were

i t possible for s u c h e n e r g y t o be r e t a i n e d between e n c o u n t e r s , p h o t o c h e m i c a l q u a n t u m y i e l d s s h o u l d a l w a y s be v e r y h i g h . O n e i m p o r t a n t i m p l i c a t i o n of the cage m e c h a n i s m is t h a t those r e a c t i o n s s h o u l d be f a v o r e d for w h i c h there is a h i g h p r o b a b i l i t y for the r e a c t a n t s t o be i n each o t h e r ' s v i c i n i t y o r cage.

H e n c e , the p r e v a l e n c e of a q u a t i o n r e a c t i o n s a n d the i m p o r t a n c e

of i o n p a i r s as i n t e r m e d i a t e s i n a n a t i o n r e a c t i o n s .

I t h i n k i t is v e r y i n t e r e s t i n g t h a t

J o h n B a i l a r ' s e x a m p l e s of stereospecificity i n v o l v e r a t h e r d r a s t i c changes i n t h e i m m e d i a t e e n v i r o n m e n t a r o u n d the c o m p l e x since c o n c e n t r a t e d s y s t e m s were used i n the one set of cases, a n d d i l u t e ones, i n the o t h e r . D r . P e a r s o n has been s o m e w h a t b l u n t .

I t h i n k the Sjv d e s i g n a t i o n s of reac

t i o n s h a v e c o n s t i t u t e d a k i n d of w a s t e l a n d , i n t h a t there has been t o o m u c h t e n dency

to

make

d i s t i n c t i o n s w i t h o u t r e a l differences

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

much

pigeon

T h e effect has been t o reduce r a t h e r t h a n

and

too

enhance

understanding. M a r t i n Tobe:*

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

t h a n k s t o D r . C o o p e r L a n g f o r d for the excellent w a y i n w h i c h he rose t o t h e o c c a s i o n a n d presented m y paper. I w a n t t o t a k e issue w i t h P r o f . P e a r s o n o n t w o p o i n t s f r o m h i s s u p p l e m e n t a r y lecture.

I see t h a t he does n o t l i k e a d i s s o c i a t i v e m e c h a n i s m i n w h i c h t h e

c o o r d i n a t e i n t e r m e d i a t e has a finite existence.

five-

I a m n o t q u i t e sure w h e t h e r he i s

a t t a c k i n g t h i s c o n c e p t o n energetic g r o u n d s b u t , if he i s , we a r e h a r k i n g b a c k t o t h e a r g u m e n t s used a g a i n s t D r . I n g o l d i n t h e t h i r t i e s w h e n he p r o p o s e d

the

now

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

W e m u s t r e m e m b e r n o w , as t h e n , t h a t w h e n t h e r e i s sufficient s e p a r a t i o n

b e t w e e n r e a c t i o n c e n t e r a n d t h e l e a v i n g g r o u p , t h e g a i n i n s o l v a t i o n e n e r g y of t h e forming components overcompensates the energy required to stretch the bond fur ther a n d

finally

break it.

T h u s , i f D r . P e a r s o n is h a p p y t o s t r e t c h t h e m e t a l —

l i g a n d b o n d t o i t s c r i t i c a l d i s t a n c e , he s h o u l d n o t be s u r p r i s e d i f i t " c o m e s a p a r t i n his h a n d s . "

A s h a s been p o i n t e d o u t m a n y t i m e s , t h e t r a n s i t i o n s t a t e for a u n i

m o l e c u l a r process s t i l l has a p a r t i a l b o n d between t h e r e a c t i o n c e n t e r a n d t h e l e a v i n g g r o u p a n d s h o u l d n o t be c o n f u s e d w i t h t h e i n t e r m e d i a t e of l o w e r c o o r d i n a t i o n n u m b e r , w h i c h is s o m e w h a t m o r e s t a b l e . mediate a n d not the transition state.

T h e i n c o m i n g reagent enters t h i s i n t e r

W e r e a l l y need a

five-coordinate

intermediate

t o e x p l a i n w h y we h a v e s t e r e o c h e m i c a l change i n c e r t a i n r e a c t i o n s . T h e so c a l l e d " s o l v e n t assisted d i s s o c i a t i o n " m e c h a n i s m i n w h i c h reagent

Y

( w h i c h m a y or m a y n o t be s o l v e n t ) s l i p s i n w h e n t h e b o n d b e t w e e n t h e m e t a l a n d * These comments added after the conference b y i n v i t a t i o n .



30

t h e l e a v i n g g r o u p is s t r e t c h e d b e y o n d a c r i t i c a l l e n g t h , is i d e n t i c a l i n c o n c e p t t o t h e process t h a t I h a v e d e s c r i b e d for t h e r e a c t i o n s of the ( C o e m A X ) * c o m p l e x e s , w h e r e n

A = N O 2 , N H 3 , a n d C N , the q u a s i b i m o l e c u l a r r e a c t i o n .

I t is s t e r i l e t o a r g u e a b o u t

t h e w o r d s used t o describe i t as l o n g as there is general a g r e e m e n t a b o u t t h e p i c t u r e itself.

T h i s t y p e of process m o s t c e r t a i n l y e x p l a i n s t h e o b s e r v a t i o n t h a t these

complexes

undergo substitution w i t h complete

r e t e n t i o n of c o n f i g u r a t i o n .

m e c h a n i s m is p r o b a b l y general for t h e s u b s t i t u t i o n r e a c t i o n s of a n a l o g o u s and

The

Rh(III)

p o s s i b l y C r ( I I I ) c o m p l e x e s w h e r e r e t e n t i o n of c o n f i g u r a t i o n o n s u b s t i t u t i o n

a p p e a r s t o be the general r u l e .


F o r m y second p o i n t , I w a n t t o c o m m e n t o n P r o f . P e a r s o n ' s r e a c t i o n t o t h e m i n o r c o m m e n t t h a t t h e c o n j u g a t e base m e c h a n i s m does n o t e x p l a i n a n u m b e r of i m p o r t a n t o b s e r v a t i o n s o n t h e base h y d r o l y s i s r e a c t i o n , a n d t h e i m a g i n a r y feud t o w h i c h he refers.

F o r a n u m b e r of y e a r s , we h a v e a c c e p t e d t h a t a c l a s s i c a l b i

m o l e c u l a r m e c h a n i s m is u n l i k e l y i n t h e s u b s t i t u t i o n r e a c t i o n s of the a m m i n e c o b a l t s , b u t we feel t h a t the u n i q u e n e s s of t h e h y d r o x i d e r e a c t i o n c a n n o t be a t t r i b u t e d t o a n a i v e S&\CB m e c h a n i s m e i t h e r .

Neither mechanism can explain adequately

the

rapidly accumulating d a t a (coming m a i n l y from the group at Northwestern U n i v e r s i t y ) w h i c h s h o w t h a t t h e s p e c i a l r e a c t i v i t y of t h e l y a t e i o n is p e c u l i a r o n l y t o certain C o ( I I I ) a n d R u ( I I I ) complexes.

W h i l e one m a y s t i l l be i n t h e d a r k as t o

p r e c i s e l y w h a t i t is t h a t h y d r o x i d e does w h e n i t p r e s e n t s itself t o t h e c o m p l e x , I s t i l l t h i n k t h a t i t is reasonable t o a t t r i b u t e , as I h a v e d o n e i n t h e p a p e r , a great d e a l of its s p e c i f i c i t y t o the fact t h a t i t is m o b i l e t h r o u g h t h e s o l v e n t s h e l l ( w h i c h i s i n a c c o r d w i t h the o b s e r v a t i o n of large p o s i t i v e e n t r o p i e s of a c t i v a t i o n ) .

D r . Pear

son's " d e m o l i t i o n c h a r g e " for t h e G r o t t h u s c h a i n h y p o t h e s i s c l e a r l y f a i l e d t o e x p l o d e , p o s s i b l y because i t w a s a l i t t l e w e t .

I n the s o l v e n t s y s t e m t h a t he uses for

the h y d r o p e r o x i d e e x p e r i m e n t i t is v e r y u n l i k e l y t h a t the s o l v a t i o n shell of t h e c o m plex c o n t a i n s a n y h y d r o g e n p e r o x i d e m o l e c u l e s . h y d r o p e r o x i d e i o n ' s p r e s e n t i n g itself for r e a c t i o n .

T h e r e is therefore no chance of a (If the r e a c t i o n were c a r r i e d o u t

i n 1 0 0 % h y d r o g e n p e r o x i d e , I have no d o u b t t h a t the r e s u l t s w o u l d be s t a r t l i n g ! ) H y d r o g e n p e r o x i d e , b e i n g a s t r o n g e r a c i d t h a n w a t e r w o u l d m o n o p o l i z e m o s t of t h e G r o t t h u s c h a i n s ( w h i c h is s a y i n g m u c h the same as " h y d r o g e n p e r o x i d e reduces the c o n c e n t r a t i o n of h y d r o x i d e " ) a n d so leads t o a r e d u c t i o n of r a t e ( w h i c h is p r e c i s e l y w h a t was observed).


The Role of Ion Association in the Substitution Reactions of

Recommend Documents