A Mechanistic Investigation of Phosphine Migration and Subtitution in (-)

Article abstract of DOI:10.1021/ja00191a022

The kinetics of phosphine migration from a metal to a carbon site in a trimetallic cluster have been investigated.The rate of ligand migration is significantly decreased by bulky phosphines and is relatively insensitive to changes in phosphine basicity.Bridging phosphine and carbonyl ligands are proposed for the transition state, and pairwise exchange of these ligands is favored.Activation parameters for PR3 = PEt3, PEt2Ph, and PMePh2 are ΔH(excit.) = +16.8 to +17.3 kcal/mol and ΔS(excit.) = -17 to -21 cal/mol K.Reactions of small phosphines contain an additional term in the rate law due to phosphine substitution for CO in , which gives .This reaction is in competition with ligand migration.Competition experiments and the observed steric barrier to substitution lead to the proposal that two phosphine ligands initially coordinate to the Co metal center and this is followed by a rapid intramolecular migration of one phosphine to the capping carbon atom.In the course of these mechanistic studies has been isolated and characterized.