Carbon dual coordination

The idea (50, 5/) of dual coordination of CO implies the presence of two coordination centers in a Fischer-Tropsch catalyst system, i.e., a carbonyl carbon coordinating center, Ma, and a carbonyl oxygen coordinating center, M6 (14). It is this concept which has led at least two groups to examine transition metal carbonyl cluster compounds as homogeneous Fischer-Tropsch catalysts. 

A successful approach to reducing CO 1 the dual coordination of CO with a carbonyl carbon coordinating center M and a carbonyl oxygen coordinating center Mj followed by hydrogen transfer (27). 

Scheme 7.41). It seems to be reasonable to presume that the isomerization of propargylic alcohol to cinnamaldehyde proceeds via an intramolecular nucleophilic attack of coordinated water on an electropositive a-carbon of the allenylidene ligand (Scheme 7.42). Then, dicationicdiruthenium complexes work as Lewis acids to promote the aldol condensation between cinnamaldehyde and acetone. Thus, the dual catalytic activity of dicationic chalcogenolate-bridged diruthenium complexes is essential to promote the present novel reaction between propargylic alcohols and acetone. 

The dual activation of hgands is also of interest for catalytic reactions. Carbon monoxide is classified according to the HSAB concept as a very soft Lewis base. Thus activation occurs by coordination of the C atom to soft transition metals. The CO ligand can, however, react as a hard Lewis base via the oxygen atom. Sufficiently hard Lewis acids A can therefore coordinate to the oxygen atom and further weaken the C-0 bond [16] ... 

In this catalytic reaction the nickel plays a dual role first, it activates the aryl halide and makes its r uction more easy, and then it activates carbon dioxide. In such reactions, the carboxylation steps are found to occur within the coordination shell of the metal. 

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