Reaction mechanisms of metal–metal bonded carbonyls. Part IV. The substitution reaction of µ-diphenylacetylene-bis(tetracarbonylcobalt) with tri-n-butylphosphine

Abstract

µ-Diphenylacetylene-bis(tetracarbonylcobalt) undergoes two successive substitution reactions with tri-n-butylphosphine in decalin and the kinetics of these reactions have been studied. Each step shows pseudo-first-order behaviour with k_obs=k₁+k₂[PBu₃] under an atmosphere of argon. Activation parameters have been obtained for each path in each step and the effect of carbon monoxide on the paths governed by k₁ is consistent with a CO-dissociative mechanism. Relative rate constants for bimolecular attack on the co-ordinatively unsaturated intermediates by carbon monoxide and tributylphosphine have been obtained. The rate parameters for the two dissociative paths are quite similar but the second bimolecular path is governed by a much higher value of ΔH^‡, and a much less negative value of ΔS^‡, than the first.