Novel Extensions of the Electrostatic Covalent Approach and

Chapter 12

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Novel Extensions of the Electrostatic Covalent Approach and Calorimetric Measurements to Organometallic Systems Russell S. Drago Department of Chemistry, University of Florida, Gainesville, FL 32611

Attempts to understand chemical reactivity in organometallic systems often involve comparison of some measured observable with pK or other one parameter criteria of sigma donor strength. Deviations from the sigma donor trends are often interpreted in terms of unusual bonding effects in the organometallic system. In this article, the pitfalls associated with selection of a one parameter criteria of donor strength are discussed. An alternative approach based on the ECW model is offered for both the interpretation and design of experiments. Several examples are presented which illustrate both the ways in which the model should be applied and the additional information that can be obtained from the data. Utilization of the approach to reactions in polar solvents and in heterogenous systems is also described. B

Most c h e m i s t s c a r r y o u t t h e r m o d y n a m i c m e a s u r e m e n t s i n o r d e r t o u n d e r s t a n d t r e n d s i n c h e m i c a l r e a c t i v i t y . To a c c o m p l i s h t h i s objective, quantitative c r i t e r i a are required t o provide the basis f o r what i s t o be e x p e c t e d u n d e r normal c i r c u m s t a n c e s where s i g m a b o n d i n g d o m i n a t e s r e a c t i v i t y . F o r e x a m p l e , t h e ρ Κ β s c a l e h a s been u s e d o f t e n t o p r o v i d e t h e b a s i s f o r t h e s i g m a bond r e a c t i v i t y o f o r g a n i c b a s e s . In t h e l a t e 1950's, i t was r e c o g n i z e d t h a t no s i n g l e s c a l e o f sigma donor s t r e n g t h e x i s t e d (1^3). As t h e Lewis a c i d i s v a r i e d , changes i n t h e order o f donor s t r e n g t h occur. 0097-6156/90A)428-0175$06.00A) © 1990 American Chemical Society

Marks; Bonding Energetics in Organometallic Compounds ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

176

BONDING ENERGETICS IN ORGANOMETALLIC COMPOUNDS

Some o f t h e most d r a m a t i c r e v e r s a l s o c c u r w i t h t h e a c i d s i o d i n e and p h e n o l (3). E a r l y , f i r s t row t r a n s i t i o n m e t a l c o m p l e x e s a l s o g i v e r i s e t o o r d e r s d i f f e r e n t f r o m d ^ t h i r d row s y s t e m s (1). Q u a l i t a t i v e e x p l a n a t i o n s o f t h e s e r e v e r s a l s i n b a s i c i t y were b a s e d on t h e M u l l i k e n (4) d e s c r i p t i o n o f b o n d i n g i n c h a r g e - t r a n s f e r complexes:

r

B A

= *·

β1

+ *-

(υ

cov

V a r i a t i o n s i n t h e i m p o r t a n c e o f c o v a l e n t , 0 ° » and e l e c t r o s t a t i c , ^ ° - | , b o n d i n g were p r o p o s e d t o a c c o u n t f o r t h e r e v e r s a l s i n s t r e n g t h o f oxygen and s u l f u r d o n o r s t o w a r d i o d i n e ( R S > R 0) and phenol (R 0 > R S ) . N i t r o g e n donors, w i t h both a l a r g e lone p a i r d i p o l e moment and low i o n i z a t i o n e n e r g y , e . g . , ( C ^ ^ N , t e n d t o be s t r o n g e r t h a n s u l f u r d o n o r s t o w a r d I and a l s o s t r o n g e r t h a n o x y g e n donors toward p h e n o l . These examples i l l u s t r a t e the q u a l i t a t i v e way i n w h i c h t h e M u l l i k e n o r P a u l i n g (5) e l e c t r o s t a t i c - c o v a l e n t , ( E C ) , model i s u s e d t o r a t i o n a l i z e d i f f e r e n t d o n o r o r d e r s . I f a q u a l i t a t i v e r a t i o n a l i z a t i o n o f b o n d i n g has any b a s i s i n f a c t , a t t h e v e r y l e a s t one s h o u l d be a b l e t o f i t bond s t r e n g t h s , e.g. adduct f o r m a t i o n e n t h a l p i e s , t o e m p i r i c a l parameters t h a t r e l a t e t o the e f f e c t s used i n the q u a l i t a t i v e d e s c r i p t i o n . I f a d d i t i o n a l p a r a m e t e r s a r e needed f o r a good q u a n t i t a t i v e f i t , t h e n an added e f f e c t i s a l s o needed f o r t h e c o m p l e t e q u a l i t a t i v e r a t i o n a l i z a t i o n . The e q u a t i o n p r o p o s e d (6^8) f o r t h e q u a l i t a t i v e e l e c t r o s t a t i c - c o v a l e n t model i s : c o v

e

2

2

2

2

2

-ΔΗ = E E A

B

+ C C A

B

-W

(2)

The e n t h a l p y o f c o o r d i n a t e bond f o r m a t i o n , -ΔΗ, i s g i v e n b y a n e l e c t r o s t a t i c t e r m , E^Eg, and a c o v a l e n t t e r m , C^Cg where A r e f e r s t o a L e w i s a c i d and Β a L e w i s b a s e . The W t e r m accommodates (8) any c o n s t a n t c o n t r i b u t i o n t o t h e e n t h a l p y w h i c h f o r a n a c i d ( o r b a s e ) i s i n d e p e n d e n t o f t h e b a s e ( o r a c i d ) e m p l o y e d . The W t e r m i s u s u a l l y z e r o b u t w o u l d be f i n i t e , f o r example, f o r t h e h e a t o f d i s s o c i a t i o n o f A1 C1 t o f o r m a B-AICI3 a d d u c t . Enthalpy data f o r r e a c t i o n s o f the type: 2

6

A + Β * A-B


(3)

12.

DRAGO

Novel Extensions of Electrostatic Covalent Approach

177

i n s o l v e n t s w i t h m i n i m a l s o l v a t i o n c o n t r i b u t i o n s a r e used t o e m p i r i c a l l y d e t e r m i n e t h e Ε and C p a r a m e t e r s . T h e r e s u l t i n g d a t a f i t i s e x c e p t i o n a l . T h e most r e c e n t r e p o r t ( 9 ) o f v a l u e s f o r t h e s e p a r a m e t e r s e m p l o y e d 500 e n t h a l p i e s f o r 48 b a s e s and 43 a c i d s t o s o l v e f i v e h u n d r e d e q u a t i o n s f o r 185 unknown p a r a m e t e r s . (Seven a c i d s have W v a l u e s and a l l o t h e r a c i d s and b a s e s have W=0.) T h e EC p a r a m e t e r s t h a t r e s u l t f r o m t h e f i t r e p r e s e n t t h e t e n d e n c y o f t h e a c i d o r base t o u n d e r g o e l e c t r o s t a t i c o r c o v a l e n t b o n d i n g , r e s p e c t i v e l y , when f o r m i n g an a d d u c t . When t h e r e p o r t e d ( 9 ) e m p i r i c a l p a r a m e t e r s f o r t h e a c i d and base a r e s u b s t i t u t e d i n t o E q u a t i o n 2, t h e c a l c u l a t e d e n t h a l p y i s f o u n d t o a g r e e w i t h t h e e x p e r i m e n t a l r e s u l t t o w i t h i n 0.1 t o 0.2 k c a l m o l e " ^ . S y s t e m s i n w h i c h s t e r i c e f f e c t s e x i s t and t h o s e i n w h i c h t h e r e i s m e t a l - l i g a n d π - b a c k b o n d i n g show d e v i a t i o n s between t h e c a l c u l a t e d and m e a s u r e d v a l u e s t h a t p r o v i d e an e s t i m a t e o f t h e m a g n i t u d e o f t h e s e e f f e c t s . In c o n t r a s t t o t h e c l a i m ( 1 0 ) t h a t t h e ECW model " d i s g u i s e s t h e r e l a t i o n s h i p between r e a c t i v i t y and p e r i o d i c e l e m e n t a l p r o p e r t i e s " , elementary a p p l i c a t i o n o f f r o n t i e r molecular o r b i t a l t h e o r y ( H ) c a n be used t o u n d e r s t a n d t h e t r e n d s . U s i n g q u a l i t a t i v e trends i n i o n i z a t i o n energies, inductive effects, e l e c t r o n e g a t i v i t i e s and p a r t i a l c h a r g e / s i z e r a t i o s , o n e c a n e s t i m a t e t r e n d s i n t h e H0M0-LUM0 s e p a r a t i o n o f t h e d o n o r and a c c e p t o r . I n c r e a s i n g t h e s e p a r a t i o n d e c r e a s e s t h e c o v a l e n t and increases the e l e c t r o s t a t i c nature o f the i n t e r a c t i o n . Decreasing the s e p a r a t i o n has t h e o p p o s i t e e f f e c t . Trends i n t h e r e p o r t e d a c i d and base p a r a m e t e r s a s w e l l a s i n t h e Ε Ε and C C g p r o d u c t s c a n be u n d e r s t o o d i n t h i s way. T h e r e have been s e v e r a l a t t e m p t s i n r e c e n t i n o r g a n i c c h e m i s t r y t e x t b o o k s t o r e l a t e t h e ECW a p p r o a c h t o o t h e r a c i d - b a s e t h e o r i e s . F i n s t o n and Rychtman ( 1 2 ) have done an o u t s t a n d i n g j o b i n t h e i r r e c e n t t e x t and t h e r e a d e r i s r e f e r r e d t o t h i s s o u r c e f o r t h i s t o p i c . A s i m p l e t e s t c a n be used t o j u d g e o t h e r a p p r o a c h e s . A n y o n e o f f e r i n g o r c o n s i d e r i n g a d i f f e r e n t q u a l i t a t i v e model f o r d o n o r - a c c e p t o r r e a c t i v i t y s h o u l d e x p r e s s i t u s i n g an e m p i r i c a l e q u a t i o n , a s was done w i t h E q u a t i o n 2 f o r t h e c o v a l e n t e l e c t r o s t a t i c m o d e l . T h e 500 e n t h a l p i e s c o m p i l e d ( 9 ) s h o u l d be f i t to t h e proposed equation. I f t h e q u a n t i t a t i v e e x p r e s s i o n o f t h e model l e a d s t o a p o o r f i t b u t r e p r o d u c e s t h e c o r r e c t t r e n d s w i t h p a r a m e t e r s c o n s i s t e n t w i t h t h e model imposed t h e n i t c a n be c o n c l u d e d t h a t t h e q u a l i t a t i v e model i s a c c e p t a b l e . I f t h e q u a n t i t a t i v e e x p r e s s i o n does n o t reproduce t h e t r e n d s , then t h e q u a l i t a t i v e model i s n o t a c c e p t a b l e . I f t h e new model r e q u i r e s Α

β

A


178


t h r e e o r more e f f e c t s t o e x p l a i n t h e t r e n d s i n s t e a d o f t h e two o f t h e EC model i t c a n a l s o b e d i s r e g a r d e d . Though t h e a b o v e statements appear obvious, they are mentioned here because the f a i l u r e of various q u a l i t a t i v e i n t e r p r e t a t i o n s of chemical r e a c t i v i t y t o f i t q u a n t i t a t i v e d a t a have been j u s t i f i e d b y t h e argument t h a t " i t i s o n l y a q u a l i t a t i v e model." PHILOSOPHY OF THE ECW MODEL The ECW model p r o v i d e s a b a s i s f o r d e t e r m i n i n g what i s normal ( E q u a t i o n 1) i n sigma bond, d o n o r - a c c e p t o r i n t e r a c t i o n s . A s s u c h i t c a n b e u s e d i n t h e s t u d y o f t h e c o o r d i n a t i o n c h e m i s t r y o f new a c i d s or bases t o determine the e x i s t e n c e , o r l a c k t h e r e o f , o f u n u s u a l b o n d i n g e f f e c t s . The e n t h a l p y o f c o m p l e x a t i o n o f a s e r i e s o f b a s e s ( o r a c i d s ) i n t h e Ε and C c o r r e l a t i o n (9) c a n b e s t u d i e d t o w a r d t h e new a c i d ( o r b a s e ) and t h e s e r i e s o f s i m u l t a n e o u s e q u a t i o n s o f t h e f o r m o f E q u a t i o n 2 s o l v e d f o r t h e two ( o r t h r e e i f W i s n e e d e d ) unknown p a r a m e t e r s . A r e c e n t s t u d y (13) o f m e t a l m e t a l bonded a c i d s , M ( 0 C R ) , (where M = R h ( I I ) , M o ( I I ) , C r ( I I ) and R u ( I I ) R u ( I I I ) ) i l l u s t r a t e s t h e i n s i g h t p r o v i d e d b y t h i s t y p e o f a n a l y s i s . An u n u s u a l l y l a r g e π - b a c k b o n d s t a b i l i z a t i o n was o b s e r v e d i n t h o s e s y s t e m s where t h e π o r b i t a l s o f t h e m e t a l - m e t a l bond a r e o c c u p i e d . The m e t a l s i n a m e t a l - m e t a l bond i n t e r a c t i n a s y n e r g i s t i c way t o e n h a n c e t h e a b i l i t y o f t h e m e t a l c e n t e r t o π b a c k d o n a t e . The d i s c o v e r y o f t h i s s y n e r g i s m has f u n d a m e n t a l i m p l i c a t i o n s f o r understanding the r e a c t i v i t y o f metal c l u s t e r s . O t h e r e x a m p l e s o f t h e s e a p p l i c a t i o n s have been r e v i e w e d . (7-8) The u t i l i z a t i o n o f t h e ECW model need n o t b e r e s t r i c t e d t o bond s t r e n g t h s . I f one m e a s u r e s t h e s p e c t r a l s h i f t s , Δι/, o f a s e r i e s o f o r g a n o m e t a l l i c b a s e a d d u c t s , one c a n a t t e m p t t o f i t t h e s e to the expression: 2

2

A

4

E

E

" • A* B

+

C

W

A* B " * C

W

The a s t e r i s k i n d i c a t e s t h e s e a r e n o t e n t h a l p y b a s e d p a r a m e t e r s and c o n t a i n c o n v e r s i o n u n i t s t o g i v e E E g t h e same u n i t s as Δι/. F r e e e n e r g i e s , i n f r a r e d , nmr, e p r , uv, e t c . s h i f t s , r a t e c o n s t a n t s , c o n t a c t a n g l e s , g . c . r e t e n t i o n t i m e s , e t c . , can a l s o b e a n a l y z e d by s u b s t i t u t i n g t h e p r o p e r t y , Δ χ , f o r Δι/ i n t o E q u a t i o n 3 and a f i t A

it

"k

k

a t t e m p t e d . I f t h e f i t i s s u c c e s s f u l , ( i . e . t h e E , C and W p a r a m e t e r s d e t e r m i n e d c a l c u l a t e Δ χ t o e x p e r i m e n t a l e r r o r ) , one c a n c o n c l u d e t h a t t h e phenomenon i s b e i n g d o m i n a t e d b y n o r m a l , sigma A

A


12.

DRAGO


179

bond c o o r d i n a t i o n c h e m i s t r y . The more c o m p l e x t h e p r o p e r t y t h e g r e a t e r t h e c h a n c e t h a t t h e c o r r e l a t i o n w i l l n o t work b e c a u s e o t h e r f a c t o r s e x i s t and may make d o m i n a n t c o n t r i b u t i o n s t o t h e o b s e r v a b l e , Δ χ . T h e s e a p p l i c a t i o n s i l l u s t r a t e an i m p o r t a n t p h i l o s o p h i c a l p o i n t a b o u t t h e ECW a p p r o a c h . The ECW p a r a m e t e r s a r e b a s e d on s o l v e n t m i n i m i z e d e n t h a l p i e s o f a d d u c t f o r m a t i o n . Phenol h y d r o g e n b o n d i n g s h i f t s i n t h e i n f r a r e d were i n c l u d e d i n t h e f i t (9) o n l y a f t e r e x t e n s i v e s t u d i e s showed t h a t t h e y c o r r e l a t e d w i t h s e v e r a l known b a s e p a r a m e t e r s . T h u s , t h e ECW a p p r o a c h i s u n i q u e b e c a u s e i t i s b a s e d on t h e " r i g h t s t u f f " . O t h e r e m p i r i c a l a p p r o a c h e s u s e l a r g e q u a n t i t i e s o f more e a s i l y m e a s u r e d d a t a t o d e t e r m i n e t h e p a r a m e t e r s . The p h i l o s o p h y o f t e n i s t h e more d a t a t h e p a r a m e t e r s f i t , t h e b e t t e r t h e m o d e l . The EC p a r a m e t e r s w o u l d n o t be c h a n g e d t o f i t o n e - h u n d r e d e l e c t r o n i c t r a n s i t i o n s i f t e n good e n t h a l p i e s had t o be e l i m i n a t e d . I n s t e a d , t h e q u e s t i o n w o u l d be a s k e d , what e l s e i s o c c u r r i n g i n t h e s p e c t r o s c o p y t h a t i s n o t i n v o l v e d i n normal s i g m a bond f o r m a t i o n ? The ECW a p p r o a c h c a n be a p p l i e d t o o r g a n o m e t a l l i c s p e c t r a l and thermodynamic data i n p o l a r s o l v e n t s . I f a good c o r r e l a t i o n r e s u l t s , t h e phenomenon m e a s u r e d i s d o m i n a t e d by d o n o r - a c c e p t o r sigma bonding i n s t e a d o f s o l v a t i o n e f f e c t s , e n t r o p i e s , e t c . I f a c o r r e l a t i o n d o e s n o t r e s u l t , t h e phenomenon i s b e i n g d o m i n a t e d by e f f e c t s o t h e r t h a n normal s i g m a bond f o r m a t i o n . I t i s n o t c o r r e c t t o a t t r i b u t e t h e f a i l u r e t o t h e EC model and c l a i m o t h e r p a r a m e t e r s a r e b e t t e r i n h i g h l y c o o r d i n a t i n g s o l v e n t s ( 1 2 ) . Any s c a l e t h a t has s o l v a t i o n a s w e l l as c o o r d i n a t i o n p r o p e r t i e s i n c l u d e d i n t h e same p a r a m e t e r c a n n o t h a v e g e n e r a l a p p l i c a b i l i t y . T h e s e a r e d i f f e r e n t , i n d e p e n d e n t e f f e c t s t h a t need t o be u n d e r s t o o d and p a r a m e t e r i z e d s e p a r a t e l y . T h i s i s a l s o an i m p o r t a n t p h i l o s o p h i c a l p o i n t t h a t i s o f t e n n o t a p p r e c i a t e d . The g o a l o f ECW i s n o t t o c o r r e l a t e the universe but t o provide a tool f o r understanding i t .

PLOTS EMPLOYING REFERENCE ACIDS OR BASES

N e x t c o n s i d e r t h e i m p l i c a t i o n s o f t h e c o n c l u s i o n t h a t t h e r e i s no i n h e r e n t o r d e r o f d o n o r ( o r a c c e p t o r ) a b i l i t y . The m u l t i t u d e o f o r d e r s t h a t e x i s t i s n i c e l y i l l u s t r a t e d by t h e g r a p h i c a l p l o t o f C r a m e r and Bopp, ( 1 4 ) who f a c t o r e d t h e ECW e q u a t i o n ( w i t h W=0) t o obtain:


180


P l o t t i n g an e n t h a l p y normalized f o r a c i d s t r e n g t h , -AH/(C + E ) v e r s u s a " n o r m a l i z e d a c i d c o v a l e n t bond t e n d e n c y " ( C - E ) / ( E + C ) one o b t a i n s a s t r a i g h t l i n e f o r a g i v e n b a s e b o n d i n g t o a l l s i g m a a c c e p t o r s . S e l e c t a b a s e f r o m t h o s e r e p o r t e d , (9) s u b s t i t u t e i t s E g and C g i n t o E q u a t i o n 5, use any E and C v a l u e y o u w i s h , c a l c u l a t e Δ Η w i t h E q u a t i o n 2 and p l o t t h i s d a t a a c c o r d i n g t o E q u a t i o n 5. Any v a l u e s o f E and C c h o s e n , w i l l f a l l o n a s t r a i g h t l i n e f o r the base. F i g u r e 1 i s a p l o t o f such l i n e s f o r s e v e r a l b a s e s . I f you s e l e c t a v a l u e o f ( C - E ) / ( C + E ) o n t h e x - a x i s and move up p a r a l l e l t o t h e y - a x i s , t h e o r d e r o f i n c r e a s e d donor s t r e n g t h toward t h i s a c i d r e s u l t s . Every time the l i n e f o r a g i v e n b a s e c r o s s e s t h a t f o r a n o t h e r i n F i g u r e 1, t h e o r d e r o f d o n o r s t r e n g t h f o r t h e s e two b a s e s w i l l r e v e r s e f o r a c i d s o n o p p o s i t e s i d e s o f t h e i n t e r s e c t i o n . From t h e l a r g e number o f i n t e r s e c t i o n s i n F i g u r e 1, a l a r g e number o f d o n o r o r d e r s i s seen t o r e s u l t f o r the bases p l o t t e d as the a c i d i s v a r i e d . Thus, i t i s i m p o s s i b l e to f i n d a r e f e r e n c e a c i d t h a t w i l l i n d i c a t e the o r d e r o f sigma donor s t r e n g t h s f o r b a s e s . T h i s shows t h a t i t i s a f u n d a m e n t a l e r r o r i n c h e m i c a l r e a c t i v i t y t o m e a s u r e a Δχ, p l o t i t v s . pKg o r any o t h e r donor s c a l e based on a s i n g l e r e f e r e n c e a c i d (proton a f f i n i t y , S b C l ç , BF^, e t c . ) and i n t e r p r e t d e v i a t i o n s f r o m t h i s p l o t w i t h s t e r i c , π - b o n d i n g , e t c . t y p e a r g u m e n t s . The r e s u l t i n g d e v i a t i o n c o u l d r e s u l t simply because the r e f e r e n c e a c i d chosen i s not p r o p e r l y r e p r e s e n t i n g t h e c o v a l e n t and e l e c t r o s t a t i c c o n t r i b u t i o n to Δχ. I f t h e C / E r a t i o f o r some b a s e a d d u c t o b s e r v a b l e s , Δχ, i s t h e same a s t h a t f o r d o n o r o r d e r p a r a m e t e r s o f a r e f e r e n c e a c i d w i t h t h e same C / E r a t i o , a s t r a i g h t l i n e w i l l r e s u l t (15) when these base parameters (or e n t h a l p i e s ) are p l o t t e d vs. the o b s e r v a b l e , Δχ. D i v i d i n g both s i d e s o f Equation 2 by E produces: A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

A

ΔΗ C

B

+

E

=

B

B

k

(6)

For a c o n s t a n t C / E , the term i n b r a c k e t s i s a c o n s t a n t f o r each b a s e , B^, where k i s t h e C/E r a t i o , i . e . , when C / E = 0.01, BQ = 0.01 C g + Eg. We can v i e w t h e s e BQ p a r a m e t e r s a s a one A

A

A

A

Q 1


Q 1

DRAGO


τ

1

1

1

1

1

1

1

1

1

1

1

1

1

Γ

more covalent acids

F i g u r e 1. P l o t o f E q u a t i o n 5 i l l u s t r a t i n g t h e w i d e v a r i e t y o f d o n o r o r d e r s as t h e a c i d i s v a r i e d .


181

182


p a r a m e t e r s c a l e o f b a s i c i t y (15b) g e n e r a t e d f r o m e n t h a l p i e s f o r a n a c i d w i t h E = 1 t h a t c a n be u s e d a s a s c a l e f o r o t h e r s y s t e m s whose C / E r a t i o i s 0.01. I f t h e C/E r a t i o f o r Δ χ i s 0.01, i t w i l l p l o t l i n e a r l y w i t h BQ v a l u e s i n d e p e n d e n t o f t h e E v a l u e o f the p r o p e r t y examined. Let's generate a s e t o f s y n t h e t i c Δ χ v a l u e s f o r a n a c i d whose C/E r a t i o i s 0.1 w i t h Ε = 2, i . e . , Δ χ j = .2 C g + 2 Eg. When Δ χ j i s p l o t t e d v s . B parameters, Figure 2 r e s u l t s . C e r t a i n base p o i n t s d e v i a t e f r o m t h e l i n e b e c a u s e t h e i r c o v a l e n t c o n t r i b u t i o n to the bonding i s g r e a t e r than t h a t f o r the B Q2 r e f e r e n c e a c i d whose C/E = 0.01. How many t i m e s have y o u seen some p r o p e r t y , Δχ, f o r m e t a l l i g a n d c o m p l e x e s p l o t t e d v s . pKg o r a p r o t o n a f f i n i t y o r a s i m i l a r one p a r a m e t e r b a s i c i t y s c a l e i n t h e l i t e r a t u r e ? How o f t e n a r e t h e d e v i a t i o n s o f p y r i d i n e and s u l f u r d o n o r s f r o m t h e s e p l o t s a t t r i b u t e d to π-backbonding? I t c o u l d j u s t as w e l l b e t h a t covalency plays a d i f f e r e n t r o l e i n the c o o r d i n a t i o n chemistry o f t h e c o m p l e x t h a n i n t h e b a s i c i t y s c a l e s e l e c t e d . The s t r a i g h t f o r w a r d a p p r o a c h t o t h e s t u d y o f o r g a n o m e t a l l i c and t r a n s i t i o n m e t a l c o m p l e x r e a c t i v i t y i s t o u t i l i z e some o f t h e 4 8 b a s e s whose Eg and C g v a l u e s a r e known (9) i n t h e e x p e r i m e n t a l d e s i g n and f i t t h e e x p e r i m e n t a l d a t a t o E q u a t i o n 2. B a s e s must b e s e l e c t e d whose Cg/Eg r a t i o v a r i e s . I f p y r i d i n e and d i e t h y l s u l f i d e were n o t u s e d i n F i g u r e 2, a good p l o t would have r e s u l t e d and one w o u l d have c o n c l u d e d i n c o r r e c t l y t h a t t h e two a c i d s (Δχ and t h e r e f e r e n c e a c i d used t o generate the b a s i c i t y s c a l e ) are s i m i l a r i n t h e i r e l e c t r o s t a t i c - c o v a l e n t b o n d i n g p r o p e r t i e s . D o n o r numbers (12) a r e r e p o r t e d f o r b a s e s w h i c h have a l i m i t e d r a n g e o f Cg/Eg r a t i o s . F o r t h i s reason they o f t e n appear to c o r r e l a t e with experimental r e s u l t s . The s u b t l e n a t u r e o f t h e above c o n c e p t s i s i l l u s t r a t e d i n o u r own r e s e a r c h . The c h a n g e i n t h e OH s t r e t c h i n g f r e q u e n c y o f p h e n o l upon a d d u c t f o r m a t i o n t o a s e r i e s o f b a s e s was p l o t t e d v s . the e n t h a l p y o f adduct formation (16). S u l f u r donors d i d not f a l l on t h e l i n e . I t was some t i m e l a t e r (17) b e f o r e we r e a l i z e d t h a t the C / E r a t i o f o r the phenol s h i f t i s d i f f e r e n t than C / E r a t i o f o r t h e e n t h a l p i e s . Both s e t s o f d a t a f i t E q u a t i o n 2 v e r y well f o r a l l bases. A

A

A

0 1

A

0

0

A

QQ 1

A

A

A

APPLICATION OF THE ECW APPROACH TO ORGANOMETALLIC SYSTEMS DIMER CLEAVAGE ENTHALPIES. A v a r i e t y o f a p p l i c a t i o n s i n w h i c h t h e ECW a p p r o a c h has been u s e d t o p r o b e u n u s u a l e f f e c t s i n c o o r d i n a t i o n c h e m i s t r y have been r e v i e w e d ( 7 , 8 , 1 3 ) . S y s t e m s i n w h i c h s t e r i c


12. DRAGO



183

184


e f f e c t s e x i s t have been s p o t t e d and t h e m a g n i t u d e o f t h e e f f e c t m e a s u r e d . In o t h e r e x a m p l e s , a q u a n t i t a t i v e m e a s u r e o f t h e a d d i t i o n a l s t a b i l i z a t i o n o f a m e t a l - l i g a n d bond e n e r g y f r o m π b a c k b o n d i n g h a s been d e t e r m i n e d and i n d u c t i v e i n f l u e n c e s o n t h i s e n e r g y p r o b e d ( 1 3 ) Here s y s t e m s a r e d i s c u s s e d t h a t p r o v i d e t h e b a s i s f o r f u t u r e a p p l i c a t i o n s o f t h e ECW model t o o r g a n o m e t a l l i c chemistry. T a b l e I c o n t a i n s a l i s t o f Ε β and C g p a r a m e t e r s f o r b a s e s commonly e n c o u n t e r e d i n o r g a n o m e t a l l i c c h e m i s t r y . A l s o l i s t e d a r e t h e BQ Q2 and BQ J p a r a m e t e r s . T h e s e c a n be u s e d a s r e f e r e n c e a c i d s i n p l a c e o f ρ Κ d a t a t o a t t e m p t one p a r a m e t e r p l o t s . T h e r e a d e r c a n c o n s u l t t h e l i t e r a t u r e (9.15b) f o r a more c o m p l e t e l i s t . β

Table I. Parameters f o r I n t e r p r e t i n g Experimental Data Base

E

NH (C H )NH (C H ) NH (C H ) N (CH ) NH HC(C H ) N NCH Im 3

2

5

2

2

5

2

2

5

3

2

5

2

4

3

3

CH CN CH C(0)CH CH C(0)0CH CH C(0)N(CH ) (C H ) 0 (CH ) 0 (C Hç) S (CH ) S (CH ) S0 C H N0 [(CH ) N] PO (C H 0) P0 (CH ) P 3

3

3

3

3

3

3

2

5

2

2

4

2

5

2

2

4

3

2

5

3

2

2

5

3

3

3

3

2

B

C

1.48 1.51 1.11 1.29 1.28 1.14 1.12 1.30 0.90 1.01 .92 1.32 1.08 1.06 0.55 0.58 1.36 1.40 1.52 1.37 1.11

3.32 5.91 8.59 10.83 9.00 12.71 9.30 6.69 1.34 2.38 1.79 2.48 3.08 4.12 7.40 7.70 2.78 4.40 3.80 1.84 6.51

B

B

0.01

1.52 1.58 1.22 1.43 1.39 1.30 1.29 1.38 .917 1.04 .942 1.35 1.12 1.11 .643 .677 1.40 1.45 1.64 1.39 1.19

B

0.1

1.81 2.11 1.97 2.37 2.18 2.61 2.05 1.96 1.03 1.24 1.10 1.57 1.39 1.49 1.29 1.29 1.64 1.84 1.88 1.55 1.76


12.

DRAGO

185


In t h e a p p l i c a t i o n o f t h e s e p a r a m e t e r s t o o r g a n o m e t a l l i c s y s t e m s , t h e m e a s u r e d q u a n t i t y t h a t v a r i e s as t h e l i g a n d i s c h a n g e d , Δχ, i s s u b s t i t u t e d i n t o E q u a t i o n 4 a l o n g w i t h Eg and Cg l e a d i n g t o a s e r i e s o f s i m u l t a n e o u s e q u a t i o n s . The l e a s t s q u a r e f i t o f t h i s data produces E , C and W . In t h e a p p l i c a t i o n o f t h e ECW e q u a t i o n t o s i m p l e a d d u c t f o r m a t i o n , E q u a t i o n 3, W g e n e r a l l y has a v a l u e o f z e r o . E q u a t i o n s 7 and 8 d e s c r i b e r e a c t i o n s i n w h i c h t h e r e i s a c o n s t a n t e n e r g y c o n t r i b u t i o n to the r e a c t i o n enthalpy. A

A

AB + B' 1/2 A

> AB' 2

+ Β

*

+

Β

(7)

AB

(8)

E q u a t i o n 7 i s a b a s e d i s p l a c e m e n t r e a c t i o n . The e n t h a l p i e s f o r a s e r i e s o f d i f f e r e n t b a s e s B' d i s p l a c i n g Β c a n be f i t t o t h e ECW e q u a t i o n . In s u c h a f i t , W i s t h e e n t h a l p y o f a d d u c t f o r m a t i o n o f AB f r o m A and B, i f AB i s c o m p l e t e l y a s s o c i a t e d i n s o l u t i o n . I f AB i s p a r t i a l l y d i s s o c i a t e d , i t i s a f r a c t i o n o f t h e e n t h a l p y o f AB f o r m a t i o n c o r r e s p o n d i n g t o t h e f r a c t i o n o f AB t h a t i s a s s o c i a t e d i n solution. Equation 8 i s a general r e a c t i o n f o r a d i m e r i c a c i d i n which W r e f e r s t o t h e h e a t o f d i s s o c i a t i o n o f t h e d i m e r . The h e a t o f r e a c t i o n o f a s e r i e s o f bases with [ R h ( C 0 D ) C l ] i s measured, l e a d i n g f r o m E q u a t i o n 2 and T a b l e I t o t h e f o l l o w i n g e q u a t i o n s i n u n i t s o f k c a l m o l e " o f Rh, i . e . -ΔΗ f o r Β + l / 2 [ R h ( C O D ) C l ] > BRh(C0D)Cl: 2

1

2

Base C

H

N

5 5 CH C H N (CH ) S l-CH Im C H N (CH ) S0 HC(C H ) N 3

5

2

4

4

3

5

n

3

2

2

4

3

7, .6 8. ,2 5,,3 9, J 10, Λ 3.8 7,.0

= E (1.30) = E (1.30) = E (.58) = E (1.12) = E (1.28) = E (1.36) = E (1.14) A

A

A

A

A

A

A

+ + + + + + +

C C C C C C C

A

A

A

A

A

A

A

(6.69) (7.42) (7.70) (9.30) (9.00) (2.78) (12.71)

-

W W W W W W W

A l e a s t s q u a r e s s o l u t i o n o f t h e s e e q u a t i o n s f o r t h e b e s t f i t EC and W p a r a m e t e r s l e a d s t o E = 4.85, C = 1.07, W = 5.78 k c a l m o l e " . W i s one h a l f t h e e n t h a l p y v a l u e f o r c l e a v i n g t h e d i m e r . T h i s q u a n t i t y c a n n o t be o b t a i n e d as e a s i l y by any o t h e r means. C o m p u t e r programs are a v a i l a b l e from the author f o r the l e a s t squares 1

A

A


186


s o l u t i o n o f t h e s i m u l t a n e o u s e q u a t i o n s . The e n t h a l p y f o r [ R h ( C 0 D ) C l ] r e a c t i n g with a whole s e r i e s o f o t h e r bases i s r e a d i l y c a l c u l a t e d b y s u b s t i t u t i n g E , C and W w i t h E g and C g v a l u e s f r o m T a b l e I i n t o E q u a t i o n 2. The v a l u e f o r t h e d i s s o c i a t e d monomer Rh(C0D)Cl r e a c t i n g w i t h b a s e s i s o b t a i n e d b y s e t t i n g W = 0 and s u b s t i t u t i n g E , C , E g and C g i n t o E q u a t i o n 2. S i m i l a r s t u d i e s o n [ R h ( C O ) C l ] and [π-allyl P d C l ] l e a d t o E = 9.23, C = 1.71, W = 11.19 and E = 3.2, C = 1.0, W = 4.1 r e s p e c t i v e l y . The w e a k e r sigma b a s i c i t y o f CO v s . a l k e n e s i s m a n i f e s t e d i n t h e g r e a t e r a c i d i t y o f Rh(C0) Cl vs. Rh(C0D)Cl. In t h e n e x t two s e c t i o n s , t h e a n a l y s i s o f more c o m p l e x r e a c t i o n s i s i l l u s t r a t e d . These w i l l i n c l u d e systems i n which a l i t t l e i n g e n u i t y e n a b l e s one t o add o r s u b t r a c t a s e r i e s o f e q u a t i o n s w h i c h a r e composed o f c o n s t a n t e n e r g y p r o c e s s e s and adduct formation r e a c t i o n s to produce the equation f o r the d e s i r e d reaction. 2

A

A

2

A

A

2

2

A

A

A

A

2

COBALT-CARBON BOND ENERGIES. The c o b a l t - c a r b o n bond d i s s o c i a t i o n e n e r g y has been r e p o r t e d (18) f o r a s e r i e s o f base a d d u c t s o f alkyl-substituted bis (dimethylglyoximato) c o b a l t ( I I ) , R-Co(DH) -B

> Co(DH) -B + R

2

(9)

2

(where R i s C g H C H ( C H ) and DH i s d i m e t h y l g l y o x i m a t o ) . A t f i r s t g l a n c e i t l o o k s a s t h o u g h ECW i s n o t a p p l i c a b l e . The c o b a l t - c a r b o n bond d i s s o c i a t i o n e n e r g y v a r i e s when Β i s c h a n g e d , b u t e v e r y t i m e Β c h a n g e s we c h a n g e t h e ECW o f t h e c o b a l t c e n t e r t o w a r d R. F u r t h e r m o r e , t h e c o b a l t - c a r b o n bond d i s s o c i a t i o n ( o r f o r m a t i o n i f we c h a n g e t h e s i g n o f Δ Η ρ ) i s n o t c o o r d i n a t e bond f o r m a t i o n b u t a f r e e r a d i c a l r e a c t i o n . However, t h e f o r m a t i o n r e a c t i o n ( i . e . , t h e r e v e r s e o f E q u a t i o n 9) can b e b r o k e n up (9) i n t o t h e f o l l o w i n g steps: 5

3

C o ( D H ) + .R 2

SUM

>

Co(DH) R

>

Co(DH) + Β

2

W

(10)

Co

(Π)

+

BCo(DH)

+

Co(DH) R + Β

>

Β - Co(DH) R

" CoR

(12)

B C o ( D H ) + .R

>

BCo(DH) R

-AH

(13)

2

2

2

2

2

2

+ A H

A H

D


12.

DRAGO

187


We have r e w r i t t e n E q u a t i o n 8 a s c o b a l t - c a r b o n bond f o r m a t i o n a n d e x p r e s s e d i t a s t h e sum o f a c o n s t a n t e n e r g y p r o c e s s (W), an a d d u c t d i s s o c i a t i o n r e a c t i o n a n d an a d d u c t f o r m a t i o n r e a c t i o n t o p r o d u c e t h e d e s i r e d r e a c t i o n , E q u a t i o n 13. S t e p s 11 a n d 12 a r e t h e t y p e r e a c t i o n t r e a t e d by ECW l e a d i n g r e s p e c t i v e l y t o E q u a t i o n s 14 a n d 15. +

•

A H

A H

Co

E

-

CoR

=

E

E

+ C

E

+

Co B

CoR B

C

1 4

Co B C

( >

C

( 1 5 )

CoR B

The h e a t o f c o b a l t - c a r b o n bond f o r m a t i o n ( E q u a t i o n 13) i s g i v e n by summing W a n d E q u a t i o n s 14 a n d 15 t o p r o d u c e : A H

D

E

E

C

C

"

" Co B " Co B

=

ΔΕ Ε

+

E

E

CoR B

+

C

C

CoR B "

W

or ΔΗ

0

Α

Β

+ AC C A

B

(16)

- W

where ΔΕ = E - E a n d A C = CQ - C . S i x b a s e s w i t h d i f f e r e n t Cg/Eg r a t i o s s h o u l d have been s e l e c t e d f r o m T a b l e I a n d t h e i r i n f l u e n c e on c o b a l t - c a r b o n bond d i s s o c i a t i o n s t u d i e d . T h e s i x s i m u l t a n e o u s e q u a t i o n s o f t h e f o r m o f E q u a t i o n 16 c a n be s o l v e d f o r t h r e e unknowns ΔΕ , A C a n d W. We w o u l d know i f sigma bond f o r m a t i o n d o m i n a t e s t h e Co-R d i s s o c i a t i o n e n e r g y , t h e r e l a t i v e i m p o r t a n c e o f t h e base c o v a l e n t o r e l e c t r o s t a t i c bond f o r m i n g p r o p e r t i e s on t h e s t a b i l i t y and, i f s t e r i c e f f e c t s e x i s t o r i f π b a c k b o n d i n g i s p r e s e n t . I n a d d i t i o n , t h e c o b a l t - c a r b o n bond d i s s o c i a t i o n e n e r g y w i t h no base a t t a c h e d i s g i v e n by W a n d t h e c o b a l t - c a r b o n bond d i s s o c i a t i o n e n t h a l p y f o r t h e 48 a d d u c t s o f b a s e s i n ECW c a n be c a l c u l a t e d . How e l s e c a n o n e o b t a i n t h i s much i n f o r m a t i o n from experimental data? U n f o r t u n a t e l y , t h e experiment was r e p o r t e d (18) w i t h o n l y f o u r s i m i l a r b a s e s l e a d i n g t o t h e f o u r simultaneous equations. Α

C o R

C o

A

Α

-21.2 -20.1 -19.5 -17.9

qR

C o

A

= = = =

1.37 1.33 1.30 1.14

ΔΕ ΔΕ ΔΕ ΔΕ

Α

Α

Α

Α

+ + + +

7.99 7.24 6.69 3.89

AC AC AC AC

A

A

A

A

-W -W -W -W

The r e s u l t i n g t e n t a t i v e v a l u e s f r o m t h e l e a s t s q u a r e s s o l u t i o h n o f t h e f o u r e q u a t i o n s a r e ΔΕ = 4.27, A C = 0.54 a n d W = -10.7. The q u a l i t y o f t h e f i t i s shown i n T a b l e I I where e n t h a l p i e s Α

A


188


c a l c u l a t e d b y s u b s t i t u t i n g A E , A C and W v a l u e s i n t o E q u a t i o n 16 (AH ) a r e compared t o e x p e r i m e n t a l r e s u l t s ( A H ). A

A

c a l c

m e a s

Table II.

C o b a l t - C a r b o n Bond D i s s o c i a t i o n -AH, f o r R - C o ( D H ) - B A d d u c t s

Energies,

?

X - C H N 5

4-NH

A H

" calc

A H

" meas

4

20.8 20.3 19.9 17.7

21.2 20.1 19.5 17.9

2

4-CH3

Η 4-CN

The r a d i c a l p a i r i n g r e a c t i o n o f 0 t o c o b a l t ( I I ) i s s i m i l a r t o t h e r e a c t i o n o f c o b a l t ( I I ) w i t h a n a l k y l r a d i c a l ( 1 9 ) . The 0 b i n d i n g e n t h a l p i e s h a v e been a n a l y z e d (19) w i t h ECW t o g i v e A E = 2.9 A C = 0.6 a n d W = -1.5 where A r e f e r s t o C o 0 - C o ( I I ) . The l i g a n d s y s t e m a r o u n d c o b a l t i n t h e 0 b o n d i n g s t u d i e s i s a porphyrin. I t i s i n t e r e s t i n g t h a t b i n d i n g R* t o C o ( D H ) i s much more e x o t h e r m i c (-10.7 k c a l m o l e " * ) t h a n b i n d i n g 0 t o Co ( p o r ) (-.5 k c a l m o l e " ) . I t i s a l s o i n t e r e s t i n g t o n o t e t h a t b i n d i n g R^to C o ( D H ) has i n c r e a s e d t h e a c i d i t y o f t h e c o b a l t much more t h a n b i n d i n g 0 t o Co ( p o r ) ( A E i s much l a r g e r f o r C o ( D H ) and A C c o m p a r a b l e ) . T h i s i s c o n s i s t e n t w i t h more e x t e n s i v e e l e c t r o n t r a n s f e r i n t o the a l k y l group than i n t o 0 , s u p p o r t i n g our p o s i t i o n (19) t h a t c o b a l t bound 0 d o e s n o t r e s e m b l e i o n i c a l l y bound superoxide. Though t h e c o n c l u s i o n s on C o ( D H ) a r e t e n t a t i v e b e c a u s e o f t h e e x p e r i m e n t a l d e s i g n , t h e y a r e p r e s e n t e d h e r e t o show t h e power o f an ECW a n a l y s i s i n p r o v i d i n g i n s i g h t s i n t o c h e m i c a l r e a c t i v i t y t h a t c a n n o t b e o b t a i n e d b y o t h e r means. 2

2

A

A

2

2

2

2

1

2

2

A

2

A

2

2

2

2:1 BASE ADDUCTS. The e x t e n s i o n o f t h e ECW a n a l y s i s t o 2:1 a d d u c t s constitutes another challenge. Consider acids that react in t h i s fashion: A + Β * AB AB + Β ^ A B

(17) 2


(18)

12.

DRAGO

189


The f i r s t r e a c t i o n i s a s t r a i g h t f o r w a r d ECW p r o b l e m . The s e c o n d one r e q u i r e s o b t a i n i n g E and C v a l u e s f o r AB. When Β i s t h e same f o r b o t h s t e p s ( E q u a t i o n s 17 and 1 8 ) , e v e r y t i m e we s t u d y a d i f f e r e n t b a s e we g e n e r a t e a new a c i d f o r s t e p 18 whose E and C v a l u e s a r e n o t known. The unknowns mount up f a s t e r t h a n t h e knowns. U s u a l l y , we c a n n o t m e a s u r e e n t h a l p i e s f o r AB r e a c t i n g w i t h s e v e r a l d i f f e r e n t b a s e s B' t o o b t a i n t h e s i m u l t a n e o u s equations needed to s o l v e f o r E and C . In t h e m i x e d s y s t e m e x p e r i m e n t AB, AB', ABB', A B and A B ' f o r m . The a s s u m p t i o n i s made (13) that: A B

A B

A B

A B

2

A B

A B

2

E

-

A B

E

- kE

A

(19)

B

and C

=

f t B

C

- k'C

A

(20)

B

H e r e k and k' r e p r e s e n t t h e e x t e n t t o w h i c h c o o r d i n a t i o n o f Β t o A m o d i f i e s t h e a c i d i t y o f AB i n i t s s u b s e q u e n t r e a c t i v i t y t o f o r m AB . I t i s a m e a s u r e o f t h e i n d u c t i v e t r a n s f e r o f B's c o o r d i n a t i n g t e n d e n c y i n m o d i f y i n g t h e new a c i d AB. The model has been t e s t e d in metal-metal bonding systems i n v o l v i n g M ( R C 0 ) complexes (13). The p r o p o r t i o n a l i t y c o n s t a n t s m e a s u r e t h e e f f e c t i v e n e s s o f t h e M-M bond t o t r a n s m i t t h e i n d u c t i v e e f f e c t o f b a s e c o o r d i n a t i o n a t one m e t a l c e n t e r t o t h e o t h e r m e t a l . S u b s t i t u t i n g E q u a t i o n s 19 and 20 into Equation 2 with W = 0 leads to: 2

2

-ΔΗ

2 ; 1

= E E A

B

- kE

2

+ C C

B

A

- k'C

B

2

= AH

B

2

4

- kE

1 : 1

2

- k'C

g

2

(21)

B

T h u s a h a l f d o z e n e n t h a l p i e s can be s o l v e d f o r t h e two unknowns k and k' when AHJ.J can be m e a s u r e d s e p a r a t e l y . When o n l y t h e sum o f t h e 1:1 and 2:1 e n t h a l p i e s c a n be d e t e r m i n e d ( e . g . , K > K j ) t h e d a t a has t o be f i t t o : 2

-ΔΗ

Τ

=

2(E E A

B

+ C C ) - kE A

B

2 B

- k'C

2

(22)

g

In t h i s c a s e a v e r y e x t e n s i v e s e t o f d a t a i s n e e d e d t o s o l v e f o r t h e f o u r unknowns. REACTIONS OF CATIONS AND ANIONS. Many o f t h e o r g a n o m e t a l l i c e n e r g i e s i n the l i t e r a t u r e are o b t a i n e d from the r e a c t i o n : MX

+

HY

>

MY + HX


bond

(23)

190


The e n t h a l p y f o r M

+

+ Y"

> MY

(24)

+

can b e r e p r e s e n t e d r e l a t i v e t o M + X" > MX b e i n g z e r o . I f we had E g and C g v a l u e s f o r a n i o n s , an ECW a n a l y s i s w o u l d l e a d t o a b s o l u t e e n e r g i e s w i t h W b e i n g t h e v a l u e f o r M-X. The o n l y p l a c e t o o b t a i n s o l v a t i o n f r e e e n e r g i e s f o r i o n s i s t h e gas p h a s e . However, t h e ECW e q u a t i o n d o e s n o t a p p l y t o t h e l a r g e e n t h a l p i e s o f i n t e r a c t i o n a s s o c i a t e d w i t h gas p h a s e i o n m o l e c u l e r e a c t i o n s ( 2 0 , 2 1 ) . The g a s e o u s i o n i s a s t r o n g L e w i s a c i d and when i t i n t e r a c t s w i t h a b a s e , a s i g n i f i c a n t c o n t r i b u t i o n t o t h e m e a s u r e d e n t h a l p y o f i n t e r a c t i o n comes f r o m e x t e n s i v e e l e c t r o n t r a n s f e r t o t h e i o n ( 2 1 ) . One can v i e w t h i s a s a one c e n t e r e n e r g y t e r m i n w h i c h t h e c a t i o n has r e g a i n e d some f r a c t i o n o f i t s i o n i z a t i o n e n e r g y . The m a g n i t u d e o f t h i s e f f e c t , f o r a n e x t r e m e c a s e , c a n be a p p r e c i a t e d b y c o n s i d e r i n g t h e e n t h a l p y d i f f e r e n c e i n t h e r e a c t i o n o f two h y d r o g e n atoms t o f o r m H w i t h t h a t o f H r e a c t i n g w i t h H". R e c e n t work f r o m t h i s l a b o r a t o r y (22) shows t h a t 369 e n t h a l p i e s c a n b e f i t t o t h e e q u a t i o n : +

2

-ΔΗ = E E A

B

+ C C A

B

+ T T A

B

(25)

where T^Tg i s t h e t r a n s f e r t e r m . The s i g n i f i c a n t new f i n d i n g i s t h a t t h e r e p o r t e d (9) E g and C g v a l u e s f r o m s o l v e n t m i n i m i z e d weak a d d u c t e n t h a l p i e s can be u t i l i z e d i n E q u a t i o n 25. T w e n t y t h r e e c a t i o n s i n c l u d i n g t h e p r o t o n , e i g h t e e n new b a s e s and t w e n t y f i v e known b a s e s l e a d t o 148 unknown p a r a m e t e r s f o r t h e l e a s t s q u a r e s f i t . V a l u a b l e i n s i g h t s c o n c e r n i n g gas p h a s e and s o l u t i o n r e a c t i v i t y r e s u l t ( 2 3 ) . The T^Tg t e r m d o m i n a t e s gas p h a s e i o n c h e m i s t r y but i s l a r g e l y c a n c e l l e d o u t i n s o l u t i o n where t h e r e a c t i o n s are u s u a l l y displacement. C o n t r i b u t i o n s from t h i s e f f e c t must be c o n s i d e r e d when d i s p l a c e m e n t e n t h a l p i e s a r e p l o t t e d v s . gas p h a s e bond e n e r g i e s . The e x t e n s i o n o f t h e s e f i n d i n g s t o o r g a n o m e t a l l i c d i s p l a c e m e n t r e a c t i o n s , i s c u r r e n t l y underway. However, t h e r e s u l t s p r e s e n t l y a v a i l a b l e i n d i c a t e t h a t v a r i a t i o n s i n t h e c o v a l e n t and e l e c t r o s t a t i c c o n t r i b u t i o n s t o t h e b o n d i n g o c c u r and c a u s e v a r i a t i o n s i n t h e t r e n d s o f sigma bond e n e r g i e s f o r t h e d i f f e r e n t o r g a n o m e t a l l i c Lewis a c i d s . Thus, i t i s q u i t e i n a p p r o p r i a t e t o p l o t M-X v s . H-X d i s s o c i a t i o n e n e r g i e s and e x p e c t t o o b t a i n a l i n e a r p l o t . A l l o f the arguments p r e s e n t e d i n the s e c t i o n on P l o t s E m p l o y i n g R e f e r e n c e A c i d s and Bases a p p l y t o t h e a n a l y s i s o f


12.

DRAGO

191


o r g a n o m e t a l l i c bond e n e r g i e s . The CH^-X, H-X and K-X s y s t e m s v a r y s u b s t a n t i a l l y i n t h e c o v a l e n t and e l e c t r o s t a t i c c o n t r i b u t i o n s t o t h e s i g m a b o n d s . P l o t s o f M-X bond e n e r g i e s v e r s u s a l l t h r e e r e f e r e n c e s y s t e m s may p r o v i d e some i n s i g h t s . However, d e v i a t i o n s f r o m t h e p l o t s a r e d i f f i c u l t t o i n t e r p r e t and i n a d d i t i o n t o n o v e l b o n d i n g c o n t r i b u t i o n s d e v i a t i o n s may o c c u r b e c a u s e t h e c o v a l e n t e l e c t r o s t a t i c c o n t r i b u t i o n t o sigma b o n d i n g i s n o t p r o p e r l y r e p r e s e n t e d i n t h e r e f e r e n c e compound. HETEROGENOUS CATALYSTS. The c o o r d i n a t i o n o f s u b s t r a t e o r b i n d i n g o f a small molecule i s o f t e n i n v o l v e d i n t h e mechanism o f heterogeneous c a t a l y z e d r e a c t i o n s . Thermodynamic s t u d i e s which use t y p i c a l o r g a n i c b a s e s as l i g a n d s t o t h e c a t a l y t i c m e t a l c e n t e r s have t h e p o t e n t i a l o f c h a r a c t e r i z i n g t h e i r c o o r d i n a t i o n c h e m i s t r y and e n a b l i n g us t o compare t h e s e h e t e r o g e n e o u s s y s t e m s t o c o n v e n t i o n a l c o o r d i n a t i o n compounds. K n o w l e d g e o f t h e s t e r i c and e l e c t r o n i c c h a r a c t e r i s t i c s ( E g , Cg) o f t h e l i g a n d s w i l l a i d i n t h e c h a r a c t e r i z a t i o n o f t h e a c i d i t y o f t h e m e t a l c e n t e r and i n u n d e r s t a n d i n g c a t a l y s t p o i s o n i n g . In o r d e r t o o b t a i n d a t a c o n s i s t e n t w i t h t h e ECW d a t a s e t , t h e s o l i d c a t a l y s t s h o u l d be s l u r r i e d i n a p o o r l y s o l v a t i n g s o l v e n t and t h e e n t h a l p y o f i n t e r a c t i o n determined a f t e r c o r r e c t i n g f o r the enthalpy o f s o l u t i o n o f b a s e . When t h e c a t a l y s t s u p p o r t i n t e r a c t s w e a k l y w i t h t h e b a s e , t h e c a l o r i m e t r i c d a t a a l s o has t o be c o r r e c t e d f o r t h i s c o n t r i b u t i o n . The e q u a t i o n f o r t h e r e a c t i o n o f 5% Pd/C r e a c t i n g with p y r i d i n e i s thus w r i t t e n as: P d

C

/ (sl)

+

P

*(sol)

>

P

*

P d

C

26

/ (sl)

where s i r e f e r s t o t h e s o l i d s l u r r y and s o l t o s o l u t i o n . equilibrium constant i s written as:

< > The

PyPd/C Κ =

(27) [Py][n.

s

- PyPd/C]

s

where n ^ i s t h e number o f a c t i v e s i t e s p e r gram o f c a t a l y s t ; PyPd/C t h e number o f grams o f p y r i d i n e c o o r d i n a t e d p e r gram o f c a t a l y s t ; [Py] i s the s o l u t i o n c o n c e n t r a t i o n o f p y r i d i n e i n m o l e l i t e r ' ; and Κ has u n i t s o f 1 m o l e " . The h e a t l i b e r a t e d i n t h e t i t r a t i o n h' i s g i v e n by (24)· 1

1


192


h'

n^KJPy] 1

g Pd/C

1

1+

AHj

K^Py]

(28)

With p r a c t i c a l c a t a l y s t s that o f t e n c o n t a i n m u l t i p l e a c i d s i t e s a d d i t i o n a l t e r m s a r e added f o r e a c h s i t e w i t h t h e s u b s c r i p t s c h a n g i n g f r o m 1 t o 2, 3 e t c . i n e a c h s i t e a d d e d . The i n f o r m a t i o n a v a i l a b l e f r o m a c a l o r i m e t r i c t i t r a t i o n o f a b a s e and s l u r r i e d c a t a l y s t i s n o t s u f f i c i e n t t o d e t e r m i n e a l l t h e unknown q u a n t i t i e s i n E q u a t i o n 28. The p r o b l e m i s a l l e v i a t e d b y c o u p l i n g the c a l o r i m e t r i c t i t r a t i o n with the d e t e r m i n a t i o n o f an a b s o r p t i o n i s o t h e r m . E q u a t i o n 26 can b e r e a r r a n g e d t o g i v e t h e f a m i l i a r Laugmuir equation: [Py] PyPd/C

1

[Py]

n^Kj

(29)

n ^

F o r two s i t e s we can w r i t e ( 2 4 ) : PyPd/C

s

n^KJPy]

n K [Py] 2

= - ^ - i

2

+ - i — i

1 + KjiPy]

(30)

1 + K [Py] 2

Knowing t h e amount o f p y r i d i n e added and m e a s u r i n g t h e e q u i l i b r i u m c o n c e n t r a t i o n i n s o l u t i o n (by t i t r a t i o n , g . c , u v - v i s i b l e e t c . ) we know b o t h PyPd/C and [Py] o f E q u a t i o n 30. The b e s t v a l u e s o f n - , and ΔΗ^ t h a t s i m u l t a n e o u s l y f i t t h e d a t a t o E q u a t i o n 30 and 28 ( w i t h t h e added t e r m s f o r m u l t i p l e s i t e s ) a r e d e t e r m i n e d . This a n a l y s i s g r o u p s s i m i l a r s i t e s i n t o one and g i v e s t h e minimum number o f d i f f e r e n t types of s i t e s r e q u i r e d by the data. Independent c o n f i r m a t i o n o f t h e c o n c l u s i o n a b o u t t h e number o f d i f f e r e n t s i t e s by s u r f a c e s c i e n c e t e c h n i q u e s i s d e s i r a b l e . F o r t h e t i t r a t i o n o f 5 % Pd/C w i t h p y r i d i n e , t h e e x p e r i m e n t showed (24) two d i f f e r e n t s i t e s , n^ = 2.5 mmole/g and n = 3.2 mmole/g. V a l u e s o f K j = 2.5 χ 1 0 M" and K = 2.9 χ 1 0 M" r e s u l t e d w i t h AHj = 13 k c a l m o l e " and Δ Η = 10 k c a l m o l e ' . ESCA s t u d i e s showed t h e e x i s t e n c e o f two d i s t i n c t s i t e s o n t h e s u r f a c e , one b e i n g P d ( I I ) and t h e o t h e r P d ( 0 ) . T h e r m o g r a v i m e t r i c studies and d i f f e r e n t i a l s c a n n i n g c a l o r i m e t r y were n o t a b l e t o d i s t i n g u i s h t h e two s i t e s but p r o d u c e d a v e r a g e v a l u e s i n a g r e e m e n t w i t h t h e average found i n the t i t r a t i o n . T h i s p r o c e d u r e has many p o t e n t i a l a p p l i c a t i o n s i n c a t a l y s i s and m a t e r i a l s c i e n c e . In t h e l a t t e r a r e a Fowkes and c o w o r k e r s (25) s

2

4

1

2

1

2

1

1

2


12.

DRAGO


193

and Chen ( 2 6 ) have r e p o r t e d s e v e r a l e x a m p l e s w h e r e i m p o r t a n t i n s i g h t s i n t o a d h e s i o n a t i n t e r f a c e s h a v e been o b t a i n e d by a p p l i c a t i o n o f ECW c o n s i d e r a t i o n s . V e r i f i c a t i o n o f some o f t h e s e c o n c l u s i o n s w i t h c a l o r i m e t r i c s t u d i e s i s underway. These d i v e r s e examples i n d i c a t e t h e p h i l o s o p h y o f a p p l y i n g t h e ECW m o d e l . I t i s a v a l u a b l e t o o l f o r u n d e r s t a n d i n g t h e v e r y complicated area o f chemical r e a c t i v i t y because i t provides q u a n t i t a t i v e c r i t e r i a f o r what i s t o be e x p e c t e d when normal s i g m a bond f o r m a t i o n d o m i n a t e s a r e a c t i o n .

LITERATURE CITED

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

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19.

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