hydroformylation and finally until experimental evidence for catalytic binuclear
elimination was obtained.
In both the unmodified cobalt and rhodium carbonyl cases, the β step will be fast.
Indeed, Ungvary [72] extensively studied molecular hydrogen activation on
Co 2 (CO) 8 to give HCo(CO) 4. The stoichiometric sequence involves the formation
of the hepta-carbonyl, followed by oxidative addition of hydrogen on the dinuclear
centre resulting in the mononuclear species. In the case of mixtures of rhodium
carbonyls in the presence of molecular hydrogen, it is clear that system Rh 4 (CO) 12 ,
Rh 2 (CO) 8 and HRh(CO) 4 redistribute to a new equilibrium on a very fast timescale
when the hydrogen partial pressure is changed, even under considerable CO [59].
Returning to the monometallic non-disjoint mechanism in Fig. 8 and starting
with the α step, the following step takes place: the bimolecular reaction between
mononuclear species (HCo(CO) 4 + RCOCo(CO) 3 or HRh(CO) 4 + RCORh(CO) 3 ) to
give aldehyde and the coordinately unsaturated species Co 2 (CO) 7 and Rh 2 (CO) 7
(which are in equilibrium exchange with their respective coordinately saturated
carbonyls M 2 (CO) 8 ). Then in the sequence involving r 2 , hydrogen activation on the
dinuclear species occurs and the mononuclear species HCo(CO) 3 /HCo(CO) 4 and
HRh(CO) 3 /HRh(CO) 4 are generated at the β step. The sequence involving r 1
possesses only the coordinately saturated hydrides HM(CO) 3 /HM(CO) 4 . The
sequence involving r 3 possesses HM(CO) 3 /HM(CO) 4 , the coordinated alkene H
(π-alkene)M(CO) 3 , the alkyls RM(CO) 3 /RM(CO) 4 and finally the acyls RCOM
(CO) 3 /RCOM(CO) 4 . At the branch point, either the acyl proceeds to the α step,
or molecular hydrogen activation directly occurs along the r 4 sequence yielding
aldehyde and HM(CO) 3 /HM(CO) 4 . All sequences are at this point connected. It is
worth emphasizing again that in the CBER mechanism, each mononuclear
sequence picks up or carries with it a moiety which will eventually become part
of the organic product.
2.3.2 Heterobimetallic Case
A non-disjoint mechanism, which at least in principle allows simultaneous linear–
bilinear kinetics, is shown in Fig. 9. This arises exclusively in the bimetallic case
{M} 6 ¼ {M
0 } when the core CBER structure is shared with a unicyclic mechanism.
In this case, there are two pathways for product formation, and this arises from three
interconnected cycles. There is one set of all important steps α and β, and there are
four sequences of intermediates. Three of these sequences are mononuclear and one
is dinuclear.
In a non-disjoint heterobimetallic [M ¼ M
f g, M
0
È É
, M À M
0
È
É
] CBER+UNI mechanism, the relationships between rates are (1) r 1 ¼ r 2 (2) r 3 ¼ r 2 + r 4 .
2 If r 3 ( r 4 , the
unicyclic mechanism dominates and the rate of product formation becomes linear in
2 Taking a metal-centric perspective, the rate relationships take the form (1)
M r 1 ¼
M
r 2 and
(2)
M
0
r 3 ¼
M
0
r 2 +
M
0
r 4 .
The Catalytic Binuclear Elimination Reaction: Importance of Non-linear. . .
205
elimination was obtained.
In both the unmodified cobalt and rhodium carbonyl cases, the β step will be fast.
Indeed, Ungvary [72] extensively studied molecular hydrogen activation on
Co 2 (CO) 8 to give HCo(CO) 4. The stoichiometric sequence involves the formation
of the hepta-carbonyl, followed by oxidative addition of hydrogen on the dinuclear
centre resulting in the mononuclear species. In the case of mixtures of rhodium
carbonyls in the presence of molecular hydrogen, it is clear that system Rh 4 (CO) 12 ,
Rh 2 (CO) 8 and HRh(CO) 4 redistribute to a new equilibrium on a very fast timescale
when the hydrogen partial pressure is changed, even under considerable CO [59].
Returning to the monometallic non-disjoint mechanism in Fig. 8 and starting
with the α step, the following step takes place: the bimolecular reaction between
mononuclear species (HCo(CO) 4 + RCOCo(CO) 3 or HRh(CO) 4 + RCORh(CO) 3 ) to
give aldehyde and the coordinately unsaturated species Co 2 (CO) 7 and Rh 2 (CO) 7
(which are in equilibrium exchange with their respective coordinately saturated
carbonyls M 2 (CO) 8 ). Then in the sequence involving r 2 , hydrogen activation on the
dinuclear species occurs and the mononuclear species HCo(CO) 3 /HCo(CO) 4 and
HRh(CO) 3 /HRh(CO) 4 are generated at the β step. The sequence involving r 1
possesses only the coordinately saturated hydrides HM(CO) 3 /HM(CO) 4 . The
sequence involving r 3 possesses HM(CO) 3 /HM(CO) 4 , the coordinated alkene H
(π-alkene)M(CO) 3 , the alkyls RM(CO) 3 /RM(CO) 4 and finally the acyls RCOM
(CO) 3 /RCOM(CO) 4 . At the branch point, either the acyl proceeds to the α step,
or molecular hydrogen activation directly occurs along the r 4 sequence yielding
aldehyde and HM(CO) 3 /HM(CO) 4 . All sequences are at this point connected. It is
worth emphasizing again that in the CBER mechanism, each mononuclear
sequence picks up or carries with it a moiety which will eventually become part
of the organic product.
2.3.2 Heterobimetallic Case
A non-disjoint mechanism, which at least in principle allows simultaneous linear–
bilinear kinetics, is shown in Fig. 9. This arises exclusively in the bimetallic case
{M} 6 ¼ {M
0 } when the core CBER structure is shared with a unicyclic mechanism.
In this case, there are two pathways for product formation, and this arises from three
interconnected cycles. There is one set of all important steps α and β, and there are
four sequences of intermediates. Three of these sequences are mononuclear and one
is dinuclear.
In a non-disjoint heterobimetallic [M ¼ M
f g, M
0
È É
, M À M
0
È
É
] CBER+UNI mechanism, the relationships between rates are (1) r 1 ¼ r 2 (2) r 3 ¼ r 2 + r 4 .
2 If r 3 ( r 4 , the
unicyclic mechanism dominates and the rate of product formation becomes linear in
2 Taking a metal-centric perspective, the rate relationships take the form (1)
M r 1 ¼
M
r 2 and
(2)
M
0
r 3 ¼
M
0
r 2 +
M
0
r 4 .
The Catalytic Binuclear Elimination Reaction: Importance of Non-linear. . .
205