Reaction of bis-µ-diethylphosphido-bis(tetracarbonylmetal)(M–M)(M = Cr or W) with tri-n-butylphosphine : kinetics and mechanism of a reaction involving seven-co-ordinate complexes

Abstract

The complexes [(OC)4[graphic omitted](CO)4](M = Cr or W) undergo a two-step CO–PBu3 exchange, in decalin, giving [(Bu3P)(OC)3[graphic omitted](CO)4] and [(Bu3P)(OC)3[graphic omitted](CO)3(PBu3)]. All the substitutions for the forward and reverse reactions occur by a dissociative mechanism involving the reactive intermediate [L(OC)3[graphic omitted](CO)3](L = CO or PBu3). Values of the competition ratio k(CO)/k(PBu3)(up to 105) indicate that bimolecular attack of CO on this intermediate is largely favoured compared with that of PBu3. This unexpected sensitivity of the co-ordinatively unsaturated metal centre towards the nature of the incoming ligand is higher for chromium than for tungsten derivatives and increases from the first to the second substitution. Activation parameters corresponding to the rate constants and competition ratios, together with equilibrium data, for these reactions are reported.