Reaction mechanisms of metal–metal-bonded carbonyls. Part 13. Substitution reactions of µ-diphenylacetylene-bis(tricarbonylcobalt)-(Co–Co)

Abstract

The kinetics have been studied of substitution (in decalin at 55 °C) of one carbonyl ligand in the complex [Co₂(C₂Ph₂)(CO)₆] by several phosphorus-donor ligands covering a wide range of nucleophilic character {L = PBuⁿ₃, PPh₃, PEt₂Ph. PEtPh₂, P(OEt)₃, P(OPh)₃, and 4-ethyl-2.6,7-trioxa-l-phosphabicyclo[2.2.2]octane (etpb)}. Reactions under an atmosphere of argon follow the rate equation k_obs, =k₁+k₂[L], and the dependence of log k₂ on the basicity of L shows that bond making (probably to the metal) plays an important part in the transition state for the bimolecular path. The effect of carbon monoxide on the path governed by the limiting first-order rate constant k₁ has been studied and found to be consistent with. but not to prove, a simple CO-dissociative mechanism for substitution by this path. If this is the mechanism, the data for reactrons under carbon monoxide provide relative rate constants for bimolecular attack by the various ligands at the vacant co-ordination site. This would be the first example where relative nucieophilicities have been determined towards a fully co-ordinated complex and a co-ordinatively unsaturated form of the same complex.