Reaction of di-µ-diethylphosphido-bis(tetracarbonylmolybdenum)(Mo–Mo) with tri-n-butylphosphine: kinetics and mechanism of a reaction involving seven-co-ordinate complexes

Abstract

In the absence of light, di-µ-diethylphosphido-bis(tetracarbonylmolybdenum)(Mo–Mo), (I), undergoes a two-step carbonyl substitution reaction with tri-n-butylphosphine in decalin, giving [(Bu₃P)(OC)₃[graphic omitted]o(CO)₄], (II), and [(Bu₃P)(OC)₃[graphic omitted]o(CO)₃(PBu₃)], (III). The substitution reaction does not go to completion in the presence of carbon monoxide and the kinetics of the forward and reverse reaction for each step have been studied. All the substitutions occur by a dissociative mechanism involving the reactive intermediate [L(OC)₃[graphic omitted]o(CO)₃](L = CO or PBu₃) which has a co-ordinatively unsaturated six-co-ordinate molybdenum atom. Values of the competition ratio k_co/k_PBus for bimolecular attack on this intermediate, at 80 °C, range from 67.0 for L = CO to 2.62 × 10⁴ for L = PBu₃. Thus the co-ordinatively unsaturated metal centre shows an unexpected high sensitivity to the nature of the incoming ligand and to steric and/or electronic variations on the adjacent metal atom. The substitution of one CO group by PBu₃ in (I) does not affect the rate of dissociation of CO, whereas (II) has a different rate of dissociation of PBu₃ compared to (III). Activation parameters for the rate constants and competition ratios, together with equilibrium data, for these reactions are reported.