4 Substitution Reactions of Square Planar Complexes FRED
BASOLO
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Northwestern University, Evanston,
Ill.
K i n e t i c studies s h o w t h a t substitution r e a c t i o n s of s q u a r e p l a n a r c o m p l e x e s g e n e r a l l y t a k e place b y a displacement mechanism. A t w o -term rate l a w is o b t a i n e d w h e r e o n e term (the s o l v e n t p a t h ) is z e r o - o r d e r i n r e a g e n t a n d t h e o t h e r t e r m (the r e a g e n t p a t h ) is f i r s t - o r d e r in r e a g e n t c o n c e n t r a t i o n . The r a t e of r e p l a c e m e n t of a g r o u p is very s e n s i t i v e t o t h e n a t u r e of the l i g a n d o p p o s i t e t o it i n t h e c o o r d i n a t i o n s p h e r e (trans effect). I n c r e a s e d steric h i n d r a n c e i n t h e c o m p l e x is a c c o m p a n i e d b y a d e c r e a s e in its r a t e . R e a g e n t s v a r y m a r k e d l y in their reactivities towards these substrates, a n d it is f o u n d t h a t t h e p o l a r i z a b i l i t y of t h e r e a g e n t is m o r e i m p o r t a n t t h a n b a s i c i t y i n d e t e r m i n i n g its r e a c t i v i t y . These effects a n d o t h e r s a r e discussed o n t h e basis o f t h e p r o p o s e d m e c h a n i s m of r e a c t i o n .
C q u a r e p l a n a r c o m p l e x e s a r e g e n e r a l l y of the l o w - s p i n J t y p e . 8
T h i s includes the
f o u r - c o o r d i n a t e d c o m p l e x e s of N i ( I I ) , P d ( I I ) , P t ( I I ) , A u ( I I I ) , R h ( I ) a n d I r ( I ) . T h e best k n o w n a n d m o s t e x t e n s i v e l y s t u d i e d a r e t h e c o m p o u n d s of P t ( I I ) . kinetics and mechanisms
of s u b s t i t u t i o n r e a c t i o n s
investigated i n considerable detail.
of these s y s t e m s
have
The been
S t u d i e s o n c o m p l e x e s of t h e o t h e r m e t a l ions are
r a t h e r l i m i t e d , b u t t h e r e s u l t s o b t a i n e d suggest t h a t t h e i r r e a c t i o n m e c h a n i s m is s i m i l a r t o t h a t of t h e P t ( I I ) s y s t e m s .
T h i s p a p e r b r i e f l y s u r v e y s s o m e of t h e a v a i l
a b l e i n f o r m a t i o n , a n d presents t h e c u r r e n t v i e w o n t h e m e c h a n i s m of s u b s t i t u t i o n r e a c t i o n s of s q u a r e p l a n a r c o m p l e x e s .
Platinum
(II)
T r a n s Effect.
T h e a - ( c i s ) a n d 0 - ( t r a n s ) f o r m s of [ P t i N H . ^ C y
c o v e r e d m o r e t h a n a c e n t u r y ago. the c h e m i s t r y of P t ( I I ) c o m p l e x e s .
were dis
S i n c e t h e n there has been a c o n t i n u e d i n t e r e s t i n M o s t of t h e w o r k has d e a l t w i t h t h e syntheses
81 In Mechanisms of Inorganic Reactions; Kleinberg, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1965.
82
MECHANISMS OF INORGANIC
a n d p r o p e r t i e s of m a n y c o m p o u n d s .
O n t h e basis of these q u a l i t a t i v e o b s e r v a t i o n s
t h e n o w w e l l r e c o g n i z e d ( a l t h o u g h less w e l l u n d e r s t o o d ) was postulated.
REACTIONS
t r a n s effect
phenomenon
T h i s p h e n o m e n o n i s best d e s c r i b e d w i t h reference t o e q u a t i o n (1).
T h e t r a n s effect is a p r o p e r t y of t h e s u b s t r a t e a n d s h o u l d be d i s c u s s e d i n the s e c t i o n o n t h e n a t u r e of t h e s u b s t r a t e .
I t is t r e a t e d s e p a r a t e l y here because of i t s e m p h a s i s
i n t h e l i t e r a t u r e of P t ( I I ) c h e m i s t r y .
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+
Y
+
(D
χ
T h e influence of l i g a n d s A o n t h e r e a c t i v i t y of X i s s m a l l , b u t L has a p r o n o u n c e d effect, c a l l e d t r a n s effect, o n t h e r e p l a c e m e n t r a t e of X .
Q u a l i t a t i v e l y , the labiliz-
i n g i n f l u e n c e of L decreases i n t h e o r d e r : C H ~ N 0 ~ 0 C 0 ~ C N ~ > R P ~ H - ~ S C (NH ) > C H - > C H -> S C N - > 2
4
3
2
N0 ->
I">
2
2
3
e
6
Br~> Cl-> N H > 0 H - > 3
H 0.
(2)
2
S e v e r a l t h e o r i e s h a v e been p r o p o s e d t o e x p l a i n t h i s t r a n s effect, b u t a t p r e s e n t o n l y t h e p o l a r i z a t i o n a n d t h e 7r-bonding t h e o r i e s a r e c o m m o n l y u s e d . T h e t r a n s effect of m e t a l c o m p l e x e s w a s t h e s u b j e c t of a r e c e n t r e v i e w F u r t h e r d e t a i l s c a n be o b t a i n e d f r o m t h i s r e v i e w .
(6).
I t w i l l suffice here t o describe
b r i e f l y t h e t w o t h e o r i e s a n d t o g i v e s o m e d a t a t o s h o w t h e r e l a t i v e m a g n i t u d e of t h e t r a n s effect.
G r i n b e r g s p o l a r i z a t i o n t h e o r y e x p l a i n s t h e t r a n s effect of l i g a n d L a s
r e s u l t i n g f r o m i t s p o l a r i z a t i o n of P t ( I I ) so t h a t t h e o p p o s i t e P t - X b o n d i s w e a k e n e d . T h i s t h e n is r e s p o n s i b l e for t h e m o r e r a p i d r e p l a c e m e n t of X .
Some x-ray studies
s h o w t h a t t h e P t - X b o n d is l o n g e r w h e n i t is o p p o s i t e a g o o d t r a n s l a b i l i z i n g l i g a n d . I n f r a r e d d a t a p r o v i d e a d d i t i o n a l e v i d e n c e for a w e a k e n i n g of t h e P t - X b o n d . T h e 7r-bonding t h e o r y e x p l a i n s w h y l i g a n d s s u c h as C H , C O a n d P R s h a v e a n 2
e x t r e m e l y large t r a n s l a b i l i z i n g i n f l u e n c e .
4
T h e t h e o r y suggests t h a t t h e presence of
a 7r-bonding l i g a n d h a s a greater s t a b i l i z i n g influence o n t h e t i o n state t h a n on the four-coordinated ground state.
five-coordinated
transi
B e c a u s e of 7r-bonding w i t h
L , t h e w i t h d r a w a l of e l e c t r o n d e n s i t y f r o m t h e v i c i n i t y of t h e e n t e r i n g Y a n d l e a v i n g X p r o m o t e s t h e d i s p l a c e m e n t of X .
B o t h x-ray and infrared d a t a available show
t h a t t h e P t - X b o n d is a l s o o f t e n w e a k e n e d i n these s y s t e m s .
T h i s implies that the
d o n a t i o n of e l e c t r o n s f r o m P t ( I I ) t o L b y 7r-bonding r e s u l t s i n L h a v i n g a greater tendency to return electrons t o P t ( I I ) v i a σ-bonding. S o m e k i n e t i c d a t a o n t h e t r a n s effect a r e n o w a v a i l a b l e .
M o r e detailed system
a t i c s t u d i e s are needed, b u t q u a n t i t a t i v e i n f o r m a t i o n does p r o v i d e t h e m a g n i t u d e of t h e t r a n s l a b i l i z i n g a b i l i t y of v a r i o u s l i g a n d s for s u b s t i t u t i o n r e a c t i o n s i n these systems.
F o r c o m p l e x e s o f ' t h e t y p e [ P t N H L C l ] ~ , w h e r e t h e l e a v i n g C l " " is t r a n s 3
2
t o L , t h e t r a n s effect o r d e r of L is a p p r o x i m a t e l y (28), C H :N0 -:Br-:Cl- 2
4
2
>
100:9:3:1
T h e increase i n r e a c t i o n r a t e i s a c c o m p a n i e d b y a decrease i n a c t i v a t i o n energy. T h e r e a c t i o n rates of t r a n s [ P t ( P E t ) L C l ] w i t h p y r i d i n e s h o w t h e a p p r o x i m a t e t r a n s 3
effect o r d e r
2
(3).
In Mechanisms of Inorganic Reactions; Kleinberg, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1965.
4.
BASOLO
Squarm Plmar Complexe*
83
Cr:^-CH OQHr:A~ClC Hr:C H -;CH --:PEt :H- 8
1 :
32
:
e
e
38
: 40
6
3
8
:165
:10,000:10,000
T h e large trans labilizing ability of H"~ is explained on the basis of the polarization theory, whereas the ir-bonding theory is involked to account for the large effect of PEts. R a t e L a w a n d M e c h a n i s m of R e a c t i o n . Only about ten years ago kinetic investigations on these systems were reported.
T h i s research was done indepen-
dently in the laboratories of Prof. Grinberg in Russia, Prof. M a r t i n at Iowa State
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University and ours at Northwestern University.
T h e initial studies showed that
the rates of reaction such as (1) are first-order in substrate concentration but either first-order or zero-order in reagent concentration.
Subsequently, more detailed
studies have shown that the reaction rates obey rate law (3), where k is a first-order B
rate constant for Rate - £.[PtA LX] + * [ P t A L X ] [Y] 2
y
2
(3)
a solvent-controlled reaction and k is a second-order rate constant for reaction with y
Y.
Such a rate law was first reported by Rich and Taube (22) for the exchange of
chloride ion with [ A u C l J , and it was suggested that this may be a general rate expression for square planar complexes.
In experiments containing an excess of Y ,
the pseudo first-order rate constant, fab*, is related to the individual rate constants by equation (4).
Linear plots of
vs. [Y] are obtained and
*o6. have the same nonzero intercepts
*. +
(4)
*y[Y]
but different slopes k for different Y (Figure 1). y
T h i s rate law applies for all of the square planar systems investigated, except the sterically hindered Pd(II) complex described later.
2
4
6
8
10
Figure 1. Rates of reaction of trans[Pt(py)2Ck] with various nucleophiles in methanol at 30°C.
In Mechanisms of Inorganic Reactions; Kleinberg, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1965.
84
M E C H A N I S M S O F I N O R G A N I C REACTIONS B o t h the s o l v e n t - c o n t r o l l e d p a t h a n d t h e d i r e c t reagent p a t h a p p e a r t o i n v o l v e
a n e x p a n s i o n i n c o o r d i n a t i o n n u m b e r of t h e m e t a l a s s h o w n i n F i g u r e 2.
A more
detailed representation would include i n the substrate coordinated solvent a n d / o r reagent, a t g r e a t e r d i s t a n c e s o n t h e ζ a x i s a b o v e a n d b e l o w t h e xy p l a n e . g e o m e t r y of t h e
five-coordinated
The
species i s n o t k n o w n , b u t p e r h a p s i t has a t r i g o n a l
bipyramidal or a tetragonal p y r a m i d a l structure.
Either structure permits sub
s t i t u t i o n w i t h c o n f i g u r a t i o n r e t e n t i o n , a n d t h i s is f o u n d e x p e r i m e n t a l l y . N a t u r e of t h e Substrate.
D a t a g a t h e r e d f r o m s e v e r a l sources p r o v i d e i n
f o r m a t i o n o n t h e effect of c e r t a i n changes i n t h e p l a t i n u m (I I) s u b s t r a t e o n t h e
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r e a c t i o n rates.
T h e s e results a r e s u m m a r i z e d here w i t h specific reference t o t h e
effect of (1) c h a r g e o n t h e c o m p l e x , (2) t h e n a t u r e of the d e p a r t i n g l i g a n d , (3) s t e r i c h i n d r a n c e of o t h e r l i g a n d s , a n d (4) b a s i c i t y a n d π - b o n d i n g of o t h e r l i g a n d s . T h e effect of c h a r g e o n r e a c t i o n r a t e s of p l a t i n u m (I I) c o m p l e x e s i s best i l l u s t r a t e d b y t h e r e s u l t s of t h e excellent i n v e s t i g a t i o n s of P r o f e s s o r M a r t i n (23) a n d his s t u d e n t s o n t h e series of c o m p l e x e s f r o m a n i o n i c [ P t C U l
- 2
through [ P t ( N H ) C l ] . 3
+
3
T h e r a t e c o n s t a n t s f o r a q u a t i o n , r e p l a c e m e n t of t h e first c h l o r i d e i o n , a n d f o r d i r e c t exchange of r a d i o c h l o r i d e i o n a r e s h o w n i n T a b l e I .
T h e m o s t s t r i k i n g feature of
these d a t a i s t h e s m a l l v a r i a t i o n i n a q u a t i o n r a t e c o n s t a n t s of t h e series, f o r s u b strates v a r y i n g i n charge from
—2 t h r o u g h + 1 .
T h i s argues a g a i n s t a s i m p l e
d i s s o c i a t i v e m e c h a n i s m ( S ^ l ) w h e r e charge s e p a r a t i o n i s i m p o r t a n t .
I t supports
t h e d i s p l a c e m e n t process (S^2) s h o w n i n F i g u r e 2 w h e r e b o t h charge s e p a r a t i o n a n d charge n e u t r a l i z a t i o n a r e i m p o r t a n t .
B e c a u s e of these c o m p e n s a t i n g o p p o s i n g
effects a change i n charge o n t h e c o m p l e x does n o t g r e a t l y a l t e r i t s r e a c t i v i t y .
I t is
s i g n i f i c a n t t h a t for the s i x - c o o r d i n a t e d s y s t e m s w h e r e charge s e p a r a t i o n i s i m p o r t a n t , t h e c o m p l e x charge c a n effect t h e r e a c t i o n r a t e . [Co(NH ) Cl ] 3
4
2
+
T h u s , t h e a q u a t i o n r a t e of t r a n s
i s a p p r o x i m a t e l y 1000 t i m e s faster t h a n t h a t of [ C o ( N H ) C l ] 3
6
+ 2
.
F i n a l l y , t h e results a l s o s h o w t h a t C l " r e l a t i v e t o H 2 O i s a b e t t e r reagent f o r zero valent a n d cationic substrates t h a n for anionic complexes.
T h i s m a y be expected
o n t h e basis of t h e n e g a t i v e charge o n c h l o r i d e i o n , b u t t h a t w o u l d n o t a c c o u n t for i t s b e i n g a b e t t e r reagent t o w a r d s / r a w s - t h a n a . s - [ P t ( N H ) 2 C l 2 ] .
Reagent reactivity
3
is d e s c r i b e d l a t e r .
Table I.
Rate Constants f o r t h e Hydrolysis a n d Chloride Exchange of C h l o r o a m m l n e p l a t i n u m (II) Complexes* T e m p . 25°C. μ - 0 . 3 1 8 M W k 20 seer
Complex
5
[PtClJ[PtNHjCl,]" m-[Pt(NH ) Cl ] *rafw-[Pt(NHj)aCl ] [Pt(NH,),Cl]+ 3
2
1
H
lO^kcCM'
1
3.9 3.6 2.5 9.8 2.6
2
2
2
seer
1
