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

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)3Mo(μ-PEt2) 2Mo(CO)4], (II), and [(Bu3P)(OC)3Mo(μ-PEt2) 2Mo(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)3Mo(μ-PEt2)2Mo(CO)3] (L = CO or PBu3) which has a co-ordinatively unsaturated six-co-ordinate molybdenum atom. Values of the competition ratio kco/kPBus 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.