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

The dinuclear phosphido-bridged molybdenum complexes [(CO)4o(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 [(CO4)o(CO)3]. The overall substitution rate depends on the rate of CO dissociation, k1, and on the rate of bimolecular attack by CO, k−1, and by P(n-Bu)3, k2, on the reactive intermediate. The nature of the substituents at the phosphido bridge markedly affects the value of k1, which is higher for the phenyl compared with the methyl group. This is mainly due to a lower activation enthalpy (ΔH1*=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/k2, 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.