Homogeneous Catalytic Reduction of Benzaldehyde with Carbon

action (1)] and methanol synthesis [reaction (2)], which can be .... between 100 and 1200 psi, the H2 0 c o n c e n t r a t i o n between 55 and 167 m...
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9 Homogeneous Catalytic Reduction of Benzaldehyde with Carbon Monoxide and Water Applications of the Water Gas Shift Reaction 1

WILLIAM J. THOMSON and RICHARD M. LAINE Physical Organic Chemistry Department, SRI International, Menlo Park, CA 94025

With the advent of the energy crisis, the study of homogeneous catalytic CO hydrogenation has become very popular because CO re­ presents one of the cheapest, readily available C building blocks for production of synfuels and because homogeneous catalysts can be very efficient hydrogenation catalysts. To date, the majority of the research has been devoted to catalytically hydrogenating CO directly to hydrocarbons (Fischer-Tropsch synthesis) with l i t t l e attention paid to other CO reduction reactions that could also be useful for synfuel production (1,2,3,4). For example, two areas that could be more thoroughly explored are CO homologation [re­ action (1)] and methanol synthesis [reaction (2)], which can be catalyzed homogeneously (5,6,7,8). 1

With the recent development of zeolite catalysts that can effi­ ciently transform methanol into synfuels, homogeneous catalysis of reaction (2) has suddenly grown in importance. Unfortunately, aside from the reports of Bradley (6), Bathke and Feder (7), and the work of Pruett (8) at Union Carbide (largely unpublished), very l i t t l e is known about the homogeneous catalytic hydrogenation of CO to methanol. Two p o s s i b l e mechanisms f o r m e t h a n o l f o r m a t i o n are suggested by l i t e r a t u r e d i s c u s s i o n s o f F i s c h e r - T r o p s c h c a t a ­ l y s i s ( 9 - 1 0 ) . T h e s e a r e shown i n Schemes 1 and 2.

1

Οn s a b b a t i c a l leave f r o m t h e University o f I d a h o .

0097-6156/81/0152-0133$05.00/0 © 1981 American Chemical Society

134

C A T A L Y T I C

/y° ΜΗ + CO

MH

O F

C A R B O N

M O N O X I D E

H —

• M-C + CH3OH

^MH

A C T I V A T I O N

2

i

^ —

MH + C H 0 2

M-CH 0H

^

2

= homogeneous m e t a l h y d r i d e c o m p l e x . Scheme 1

MH ++ CO M n (JO

v X

Ρ

2H^——••

M-c' M-C



3

'.

MH + CH3OH -

2

2

„. HM=C

H M = C

y

X

M-CH 0H

\

^

2

Scheme 2 We a r e i n t e r e s t e d i n homogeneous c a t a l y t i c m e t h a n o l s y n t h e s i s b e c a u s e o f o u r p r e v i o u s w o r k on t h e Reppe r e a c t i o n s ( 1 1 , 1 2 , 1 3 ) : R-CH=CH + 3C0 + 2 H 0 2

R

h

2

e

^ ^

1

6

/

K

Q

H

»

RCH CH CH 0H + 2C0 2

2

(3)

2

2 / Κ ° Η ^ R ( C H ) - N R + 2 C 0 (4) MeOH A common theme i n r e a c t i o n s ( l ) - ( 4 ) and Schemes 1 and 2 i s t h e hydrogénation o f c a r b o n - o x y g e n m u l t i p l e bonds t o s p e c i e s w i t h c a r b o n - o x y g e n o r c a r b o n - n i t r o g e n s i n g l e bonds. In undertaking t h e w o r k p r e s e n t e d h e r e i t was o u r i n t e n t i o n t o d e t e r m i n e w h e t h e r o r n o t i n f o r m a t i o n o b t a i n e d i n t h e s t u d y o f t h e hydrogénation o f a l d e h y d i c c a r b o n - o x y g e n d o u b l e bonds i s a p p l i c a b l e t o u n d e r s t a n d i n g (a) r e d u c t i o n o f c a r b o n m o n o x i d e t o m e t h a n o l , (b) CO r e d u c t i o n to F i s c h e r - T r o p s c h p r o d u c t s , (c) h o m o l o g a t i o n as i n r e a c t i o n ( 1 ) , and (d) t h e v a l i d i t y o f e i t h e r Scheme 1 o r 2. We c h o s e t o i n v e s t i g a t e t h e CO/H 0/KOH and Rh 6 (CO)i 6 c a t a l y z e d r e d u c t i o n o f b e n z a l d e h y d e and s u b s t i t u t e d b e n z a l d e h y d e s b e c a u s e :

R-CH=CH + 3C0 + H 0 2

2

+ R NH

R u ?

2

(

5°^

2

2

3

2

2

2





Benzaldehydes a r e not s u b j e c t to base c a t a l y z e d a l d o l c o n d e n s a t i o n s , and u n d e r t h e r e a c t i o n c o n d i t i o n s Cannizarro r e a c t i o n s are not important. From thermodynamic c o n s i d e r a t i o n s CO/H 0 must be a b e t t e r r e d u c t a n t than H , thus m i l d e r r e a c t i o n cond i t i o n s m i g h t be p o s s i b l e . The c a t a l y s t d e r i v e d f r o m Rh 6 (CO)i 6 i s t h e most a c t i v e c a t a l y s t f o r a l d e h y d e r e d u c t i o n o f a l l t h e group 8 m e t a l s we have s t u d i e d ( 1 2 , 1 3 ) . £-Substitution o f t h e b e n z a l d e h y d e s s h o u l d p r o v i d e i n f o r m a t i o n about the e l e c t r o n d e n s i t y a t the c a r b o n y l carbon d u r i n g the a d d i t i o n of metal h y d r i d e to the c a r b o n - o x y g e n d o u b l e bond. 2

2





9.

THOMSON

A N D

Experimental

Reduction of Benzaldehyde

LAINE

135

Procedures

G e n e r a l M e t h o d s . M e t h a n o l u s e d i n k i n e t i c r u n s was d i s t i l l e d from sodium methoxide o r c a l c i u m h y d r i d e i n a n i t r o g e n atmosphere b e f o r e u s e . F r e s h l y d i s t i l l e d c y c l o h e x a n o l was added t o t h e m e t h a n o l i n t h e r a t i o 6.0 m l c y c l o h e x a n o l / 2 0 0 m l MeOH and was u s e d a s a n i n t e r n a l s t a n d a r d f o r g a s c h r o m a t o g r a p h i c (GC) a n a l y s i s . B e n z a l d e h y d e was d i s t i l l e d u n d e r vacuum and s t o r e d u n d e r n i t r o g e n a t 5 ° . O t h e r a l d e h y d e s ( p u r c h a s e d f r o m A l d r i c h ) were a l s o d i s ­ t i l l e d b e f o r e u s e . The c o r r e s p o n d i n g a l c o h o l s ( p u r c h a s e d f r o m A l d r i c h ) w e r e d i s t i l l e d and u s e d t o p r e p a r e GC s t a n d a r d s . A l l m e t a l c a r b o n y l c l u s t e r c o m p l e x e s w e r e p u r c h a s e d f r o m S t r e m Chem­ i c a l Company a n d u s e d a s r e c e i v e d . T e t r a h y d r o f u r a n (THF) was d i s ­ t i l l e d f r o m s o d i u m benzophenone u n d e r n i t r o g e n b e f o r e u s e . A n a l y t i c a l M e t h o d s . A l l t h e a n a l y s e s w e r e done b y g a s chroma­ tography. L i q u i d p r o d u c t s were a n a l y z e d on a H e w l e t t - P a c k a r d M o d e l 5711 g a s c h r o m a t o g r a p h e q u i p p e d w i t h F I D u s i n g a 4.0 m b y 0.318 cm c o l u m n p a c k e d w i t h 5% Carbowax a c i d - w a s h e d Chromosorb G. Gas p r o d u c t s w e r e a n a l y z e d w i t h a H e w l e t t - P a c k a r d M o d e l 5750 g a s c h r o m a t o g r a p h e q u i p p e d w i t h a 3 m b y 0.318 cm, 150/200 P o r o p a k Q c o l u m n a n d a 1.8 m b y 0.318 cm t y p e 13A m o l e c u l a r s i e v e c o l u m n . H y d r o g e n a n a l y s i s was a c h i e v e d b y i n j e c t i n g i n t o t h e m o l e c u l a r s i e v e c o l u m n o p e r a t e d w i t h a 8.5% h y d r o g e n - i n - h e l i u m c a r r i e r g a s . O t h e r g a s e o u s components w e r e a n a l y z e d i n t h e P o r o p a k c o l u m n u s i n g a helium c a r r i e r gas. I n t h e p r o c e d u r e u s e d f o r s t u d y i n g b e n z a l d e h y d e r e d u c t i o n , 6.0 m l o f t h e M e O H / c y c l o h e x a n o l s o l u t i o n d e s c r i b e d above w e r e u s e d a s s o l v e n t . A l d e h y d e was added t o t h e r e a c t o r v i a a 5-ml g l a s s s y r i n g e , a n d 1.0 m l o f 3 Ν KOH s o l u t i o n was added b y means o f a p i p e t t e . We w e r e c a r e f u l t o a d d e i t h e r t h e KOH o r t h e a l d e h y d e j u s t b e f o r e p r e s s u r i z i n g w i t h CO t o m i n i m i z e t h e a l d e h y d e r e d u c ­ t i o n r e s u l t i n g f r o m t h e n o n c a t a l y t i c C a n n i z a r r o r e a c t i o n . The r e ­ a c t o r was t h e n s e a l e d a n d f l u s h e d t w i c e w i t h 6 0 0 - p s i CO b e f o r e b r i n g i n g t h e r e a c t o r up t o t h e d e s i r e d CO p r e s s u r e . The r u n was i n i t i a t e d a t t h e t i m e t h e r e a c t o r was immersed i n t h e t e m p e r a t u r e b a t h . On c o m p l e t i o n o f t h e r u n , t h e r e a c t o r was quenched i n c o l d w a t e r and t h e p r e s s u r e a t 24°C was r e c o r d e d . The p r i m a r y a n a l y s i s was c o n d u c t e d o n t h e l i q u i d s o l u t i o n ; h o w e v e r , some r u n s w e r e a l s o s u b j e c t e d t o gas phase a n a l y s i s . I n every experiment, l i q u i d s a m p l e s w e r e c o l l e c t e d i n 1 0 - c c s a m p l e v i a l s and s t o r e d i n a r e ­ frigerator. I n most c a s e s c h r o m a t o g r a p h i c a n a l y s e s w e r e c o n d u c t e d w i t h i n 6 h o u r s o f t h e t e r m i n a t i o n o f t h e r u n . The maximum t i m e t h a t a n y sample was s t o r e d b e f o r e a n a l y s i s was 72 h o u r s . To have a b a s i s f o r c o m p a r i s o n , we e s t a b l i s h e d a s t a n d a r d r u n f o r aldehyde r e d u c t i o n c o n s i s t i n g o f 0.1 mmole R h ( C 0 ) 30 mmole C H C H 0 1 m l 3 Ν KOH 6 m l (MeOH + c y c l o h e x a n o l ) 6

6

5

i 6

136

CATALYTIC ACTIVATION OF CARBON MONOXIDE

Ρ = 800 p s i T = 125°C R e a c t i o n time = 1 hour C a t a l y t i c a c t i v i t y was m e a s u r e d a s a f u n c t i o n o f t u r n o v e r f r e ­ quency [moles p r o d u c t / ( m o l e c a t a l y s t ) ( h o u r ) ] . The s t a n d a r d r u n has a t u r n o v e r f r e q u e n c y o f 105±10. A l l t h e p a r a m e t e r s i n v e s t i ­ g a t e d w e r e p e r t u r b e d a b o u t t h i s s t a n d a r d and i n c l u d e d the e f f e c t s o f c a t a l y s t , a l d e h y d e , KOH a n d w a t e r c o n c e n t r a t i o n , i n i t i a l CO p r e s s u r e , and r e a c t i o n t i m e . I n a d d i t i o n , a few s e l e c t e d runs were a l s o conducted t o examine the e f f e c t s o f hydrogen i n the gas phase as w e l l as the r e l a t i v e ease w i t h w h i c h o t h e r aldehydes c o u l d be reduced. L U

Results C a t a l y s t s and C a t a l y s t C o n c e n t r a t i o n . The most a c t i v e c a t a ­ l y s t f o r b e n z a l d e h y d e r e d u c t i o n a p p e a r s t o b e r h o d i u m [Rh 6 (C0)i 6 p r e c u r s o r ] , b u t i r o n [as and r u t h e n i u m [as R u ( C 0 ) ] 3 2 w e r e a l s o e x a m i n e d . The r e s u l t s o f t h e s e e x p e r i m e n t s a r e shown i n T a b l e 1. C o n s i s t e n t w i t h e a r l i e r r e s u l t s ( 1 2 ) , t h e r h o d i u m c a t a ­ l y s t i s b y f a r t h e most a c t i v e o f t h e m e t a l s i n v e s t i g a t e d a n d t h e ruthenium c a t a l y s t has almost zero a c t i v i t y . The l a t t e r i s c o n ­ s i s t e n t w i t h the f a c t t h a t ruthenium produces o n l y aldehydes during hydroformylation. Note t h a t a s y n e r g i s t i c e f f e c t o f mixed m e t a l s does n o t a p p e a r t o b e p r e s e n t i n a l d e h y d e r e d u c t i o n a s c o n ­ t r a s t e d w i t h the n o t i c e a b l e e f f e c t s observed f o r the water-gas s h i f t r e a c t i o n (WGSR) a n d r e l a t e d r e a c t i o n s ( 1 3 ) . The e f f e c t o f t h e c o n c e n t r a t i o n o f R h ( C 0 ) o n t h e number o f t u r n o v e r s was e v a l u a t e d b y u s i n g 0.01 mmole t o 0.10 mmole o f c a t a ­ l y s t , a n d t h e s e r e s u l t s a r e shown i n F i g u r e 1. The t u r n o v e r s i n ­ crease with decreasing c a t a l y s t precursor. This i s i n d i c a t i v e o f c a t a l y s i s by c l u s t e r f r a g m e n t a t i o n . The r e s u l t s o f o u r e a r l i e r w o r k (12) a r e a l s o shown i n F i g u r e 1. A l t h o u g h t h e t u r n o v e r s a r e h i g h e r than those observed here, i t s h o u l d be noted t h a t a h i g h e r temperature and a s h o r t e r r e a c t i o n time were used p r e v i o u s l y . H i g h e r t e m p e r a t u r e s h o u l d p r o d u c e more t u r n o v e r s , b u t s h o r t e r r e ­ a c t i o n times would be e x p e c t e d t o produce l e s s , depending on the existent reaction orders. T h i s i s c o m p l i c a t e d f u r t h e r by t h e e f f e c t o f C 0 p r o d u c t i o n , w h i c h t e n d s t o consume OH", a n d a l s o b y t h e n o n i s o t h e r m a l h e a t - u p p e r i o d (5 m i n ) , w h i c h i s a s i g n i f i c a n t f r a c t i o n o f t h e 0.5-hr r e a c t i o n t i m e u s e d p r e v i o u s l y .

Fe (C0)i ]

3

6

1 2

1 6

2

KOH C o n c e n t r a t i o n S t u d i e s . The e f f e c t o f KOH c o n c e n t r a t i o n o n b e n z a l d e h y d e r e d u c t i o n was e x a m i n e d , and t h e r e s u l t s a r e shown i n F i g u r e 2 a l o n g w i t h our p r e v i o u s r e s u l t s f o r ruthenium c a t a l y z e d hydroformylation (12). The E f f e c t o f R e a c t a n t C o n c e n t r a t i o n . Several experiments were conducted t o q u a n t i f y the e f f e c t o f r e a c t a n t c o n c e n t r a t i o n on t h e a l d e h y d e r e d u c t i o n r a t e . The i n i t i a l CO p r e s s u r e was v a r i e d

THOMSON

AND

Table I.

137

Reduction of Benzaldehyde

LAINE

Benzaldehyde Reduction with Various Catalysts CATALYST

RH (C0)

1 6

FE (C0)

1

RU (C0)

1 2

6

3

RH (C0) 6

RU3(C0)

M0LES

B

0.05

OF

TURNOVERS

A

105

30

3

A

NO,

PRECURSOR

2

9

1 6

1 2

/FE (C0) 3

/FE3(C0)

93

B 1 2

A L C O H O L FORMED MOLES

OF

21

B 1 2

PER MOLE

OF T O T A L

CATALYST.

EACH.

700

0.02

0.04

m moles R h ( C O ) 6

Figure 1.

0.08

0.06

0.10

16

The effect of catalyst concentration on benzaldehyde reduction: (O) 125°C, 1 h; ( ) 150°C, 0.5 h (\2)

CATALYTIC ACTIVATION OF CARBON MONOXIDE

138

b e t w e e n 1 0 0 a n d 1200 p s i , t h e H 0 c o n c e n t r a t i o n b e t w e e n 55 a n d 167 mmole, a n d t h e C H C H 0 c o n c e n t r a t i o n b e t w e e n 10 a n d 40 ramole. Because o f s o l u b i l i t y problems, the experiments a t the h i g h e s t H 0 and h i g h e s t C H C H 0 c o n c e n t r a t i o n s w e r e a n a l y z e d a f t e r a d d i n g 3 m l THF t o t h e m i x t u r e r e c o v e r e d a t t h e e n d o f t h e r u n . F i g u r e 3 shows t h e r e s u l t s o f v a r y i n g t h e CO p r e s s u r e . The maximum a c t i v i t y a p p e a r s t o l i e n e a r 600 p s i f o r b e n z a l d e h y d e r e duction. F i g u r e 3 i s an attempt t o e l u c i d a t e an apparent r e a c t i o n o r d e r w i t h r e s p e c t t o t h e a r i t h m e t i c a l l y a v e r a g e d CO p r e s s u r e . A t p r e s s u r e s l e s s t h a n 400 p s i , t h e o r d e r i s n e a r l y f i r s t o r d e r . The s i t u a t i o n a t h i g h e r p r e s s u r e s i s not c l e a r ; however, i t i s r e a s o n a b l e t o s p e c u l a t e t h a t the dominant a s p e c t s o f the k i n e t i c s s h i f t at these pressures. The d a t a s u g g e s t t h e s h i f t i s t o z e r o - o r d e r dépendance. S t u d i e s a n a l y z i n g the e f f e c t s o f the remaining r e a c t a n t s , H 0 and C H C H 0 i n d i c a t e t h a t t h e r e a c t i o n a p p e a r s t o be z e r o o r d e r w i t h respect t o both reactants. I t i s i n t e r e s t i n g that i n previous w o r k we a l s o f o u n d s i m i l a r b e h a v i o r f o r H 0 i n r u t h e n i u m c a t a l y z e d h y d r o f o r m y l a t i o n ( 1 2 ) , a s d i d Ungermann e t a l . w i t h t h e WGSR ( 1 4 ) . 2

6

5

2

6

s

2

6

5

2

The E f f e c t o f R e a c t i o n Time. The p r o b l e m a s s o c i a t e d w i t h t i m e v a r y i n g 0H~ c o n c e n t r a t i o n s h a s a l r e a d y b e e n m e n t i o n e d . The d i f f i c u l t y a s s o c i a t e d w i t h the i n f l u e n c e o f d i s s o l v e d C 0 can be a p p r e c i a t e d b y r e f e r r i n g t o F i g u r e 4, w h i c h shows t h e r e s u l t s o f two experiments. I n one, s a m p l e s w e r e t a k e n e v e r y h o u r a n d i n t h e o t h e r s a m p l i n g o c c u r r e d e v e r y two h o u r s . However, t h e i m p o r t a n t f a c t o r i s t h a t t h e r e a c t o r was r e c h a r g e d w i t h CO a f t e r e a c h s a m p l e . N o t e t h a t t h e e f f e c t i v e r e a c t i o n r a t e i s l o w e r when two h o u r s e l a p s e between samples, presumably due t o the b u i l d u p o f C 0 , w h i c h consumes 0H~. I n f a c t , o n e e x p e r i m e n t was c o n d u c t e d a t 94°C f o r 17 h o u r s a n d o n l y 27% c o n v e r s i o n t o a l c o h o l o c c u r r e d , t h e same c o n v e r s i o n e x p e r i e n c e d a f t e r 3 h o u r s when f r e s h CO was added h o u r ly. 2

2

R e d u c t i o n o f O t h e r A l d e h y d e s . We examined t h e r e d u c t i o n o f a n i s a l d e h y d e , p-CH 0C H CH0 and t o l u a l d e h y d e , p - C H ( C H ) C H 0 t o examine t h e e f f e c t o f e l e c t r o n d e n s i t y o n a l d e h y d e r e d u c t i o n . I n a d d i t i o n , we a l s o i n v e s t i g a t e d o n e k e t o n e , a c e t o p h e n o n e , C H C 0 C H . The r e s u l t s o f t h e s e e x p e r i m e n t s a r e g i v e n i n T a b l e 2. 3

6

A

3

6

A

6

5

Discussion Because o f the c o m p l e x i t y o f the r h o d i u m - c a t a l y z e d r e d u c t i o n o f b e n z a l d e h y d e t o b e n z y l a l c o h o l w i t h CO a n d H 0 , i t i s n o t p o s s i b l e t o f u l l y e l u c i d a t e t h e mechanism o f c a t a l y t i c r e d u c t i o n g i v e n the extent o f the k i n e t i c s t u d i e s performed t o date. Howe v e r , t h e r e s u l t s do a l l o w u s t o draw s e v e r a l i m p o r t a n t c o n c l u s i o n s a b o u t t h e r e a c t i o n mechanism f o r b e n z a l d e h y d e hydrogénation and s e v e r a l r e l a t e d r e a c t i o n s . We r e c e n t l y d e s c r i b e d (12,15) t h e u s e o f c a t a l y s t c o n c e n t r a 2

3

THOMSON

AND

LAINE

0

0.5

Reduction of Benzaldehyde

1.0

1.5 2.0 mmoles KOH

2.5

3.0

3.5

Figure 2. The Effect of KOH: (--O --) KOH; (A) 1.56 mmol K COy, (hydroformylation (12) 2

ι—ι—I

100

200

I I I I

500 p

co ~

P

1000

2000

si

Figure 3. Benzaldehyde reduction turnovers vs. P o C

140

CATALYTIC ACTIVATION OF CARBON MONOXIDE

Table II.

Reduction of Other Reactants

REACTANTS

1

TURNOVER

C H CH0 6

PCH C H 3

6

Z 4

PCH 0C H 3

6

CH0

Î 4

C H C0CH 6

5

FREQUENCY

105

5

78

CH0

63

j

3

24

ι (STANDARD OF

Figure 4.

CONDITIONS,

30

MMOLES

REACTANTS)

Time varying benzaldehyde reduction Τ = 94°C: (O) purge every 1 h, (A) purge every 2 h

9.

THOMSON

A N D

141

Reduction of Benzaldehyde

LAINE

t i o n studies t o help i d e n t i f y a c t i v e c a t a l y s t species, e s p e c i a l l y where c l u s t e r c a t a l y z e d r e a c t i o n s a r e s u s p e c t e d . I n the present work, c h a n g e s i n t h e amount o f r h o d i u m added t o t h e r e a c t i o n s o l u t i o n have dramatic e f f e c t s on t h e t u r n o v e r f r e q u e n c y . A s shown i n F i g u r e 1, d e c r e a s i n g t h e r h o d i u m c o n c e n t r a t i o n r e s u l t s i n c o n s i d e r able increases i n turnover frequencies. These changes a r e i n d i c a t i v e o f e q u i l i b r i a i n v o l v i n g rhodium c l u s t e r complexes t h a t fragment r e v e r s i b l y t o s m a l l e r c l u s t e r complexes o r mononuclear complexes w h e r e i n t h e a c t i v e s p e c i e s a r e t h e fragments. I n t h e r e a c t i o n s o l u t i o n s where b e n z a l d e h y d e r e d u c t i o n o c c u r s we h a v e o b s e r v e d ( 1 2 ) , b y I R , b o t h t h e R h ( C O ) o ~ and R h ( C 0 ) " complexes. R e c e n t w o r k b y C h i n i and c o w o r k e r s (16) s u g g e s t s a t l e a s t one p l a u s i b l e e q u i l i b r i u m : 2

1 2

Rh (CO) 6

1 6

+ OH"

6

6

6

2 l s

6

2 1 5

1 2

2

3 O

" + 15C0

3 0

1 5

(5)

2

" + H 0

(6)

2

2

• Rh (C0)

1 5

- + /.Rh (CO) 12

1 5

• Rh (C0)

1 5

2Rh (C0) H~ Rh (C0)

5

• Rh (C0) H~ + C0

R h ( C 0 ) H ~ + OH" 6

3

"" + H

(7)

2

6Rh (C0) " 5

(8)

1 5

V i d a l a n d W a l k e r (17) h a v e o b s e r v e d t h a t Rh(CO)