Kinetics and substitution mechanism in the reaction of the seven-coordinate complexes bis-μ-diphenylphosphido- and bis-μ-dimethylphosphido-octacarbonyldimolybdenum (MoMo) with tri-n-butyl-phosphine

Abstract

The dinuclear phosphido-bridged molybdenum complexes [(CO) 4 Mo(μ-PR 2) 2M o(CO) 4] (R=Ph or Me) react with P(n-Bu) 3 to give the corresponding mono and bis-phosphine derivatives. A kinetic study of the first substitution in decalin indicates a CO-dissociative mechanism involving the coordinatively unsaturated intermediate [(CO 4) Mo(μ-PR 2) 2M o(CO) 3]. The overall substitution rate depends on the rate of CO dissociation, k 1, and on the rate of bimolecular attack by CO, k −1, and by P(n-Bu) 3, k 2, on the reactive intermediate. The nature of the substituents at the phosphido bridge markedly affects the value of k 1, which is higher for the phenyl compared with the methyl group. This is mainly due to a lower activation enthalpy (Δ H 1*=125 (Ph) versus 141 (Me) kJ mol −1), which may reflect a weakening of the MoCO bond in the presence of a more electron-withdrawing ligand in trans position. The values of the competition rate ratio k −1/ k 2, always largely greater than unity, show that attack of the small CO is favoured with respect to the large P(n-Bu) 3; this suggests that the steric crowding observed on the molecular structure of the starting seven-coordinate complexes should play an important role also on the reactivity of their six-coordinate intermediates.