Solution dynamics of the reversible carbonyl–phosphorus ligand exchange in [(OC)4Mo(µ-PEt2)2Mo(CO)4]: kinetic parameters as a measure of the relative steric hindrance of phosphines and phosphites

Abstract

Di-µ-diethylphosphido-bis(tetracarbonylmolybdenum)(Mo–Mo) undergoes reversible carbonyl sub stitution with phosphorus ligands in decalin, giving [(OC)₄M[graphic omitted](CO)₃L][L = P(OEt)₃, PMe₂Ph, PBuⁿ₃, PMePh₂, PH(C₆H₁₁)₂, PPh₃, or PPh₂(C₆H₁₁)]. Both the forward and reverse reactions occur by a dissociative mechanism involving the reactive intermediate [(OC)₄M[graphic omitted](CO)₃], which has a co-ordinatively unsaturated six-co-ordinate molybdenum atom. The reactivity towards substitution depends on the relative rates of bimolecular attack of CO, k_–1, and of the phosphorus ligand, k₂, on this intermediate. Values of the competition ratio k_–1/k₂ range, at 100 °C, from 4.38 [L = P(OEt)₃] to 2.09 × 10⁵[L = PPh₂(C₆H₁₁)], mainly as a consequence of a more favourable activation entropy for the smaller triethyl phosphite. The unexpected high sensitivity of the co-ordinatively unsaturated metal centre towards the steric hindrance of the incoming group is nicely demonstrated by plotting the differences in activation entropy ΔS_–1^‡–ΔS₂^‡ against Tolman's cone angle for the various phosphorus ligands. A good straight line is obtained, whose slope (4.6 J K^–1 mol^–1 degree^–1) can be assumed as the kinetic selectivity of the intermediate towards the size of the entering ligand. Activation parameters corresponding to the rate constants and competition ratios, together with equilibrium data, are reported.