Kinetic studies of group VI metal carbonyl complexes. VI. Substitution reactions of the group VI metal hexacarbonyls

Article abstract of DOI:10.1021/ic50057a034

Hexacarbonyl complexes of the group VI transition metals react with phosphine and phosphite ligands, L, in decalin solvent according to the equation: M(CO)₆ + L → M(CO)₅L + CO. The rate of reaction is governed by the two-term rate law: rate = k₁[M(CO)₆] + k₂[L][M(CO)₆]. SN1 and SN2 types of reaction mechanisms are proposed to account for the two terms. Positive ΔS₁* values for the k₁ paths and negative ΔS₂* values for the k₂ paths support the proposed mechanisms. The values of ΔH₁* for the k₁ path decrease in the order: Cr(CO)₆ ～ W(CO)₆ > Mo(CO)₆. The magnitude of the second-order rate constants, k₂, was found to be a function of the basicity of the ligand, L, and increases as follows: As(C₆H₅)₃ < P(OC₆H₅)₃ < P(C₆H₅)₃ < P(OCH₂)₃CC₂H₅ < P(OC₂H₅)₃ < P(n-C₄H₉)₃. A comparison of k₂/k₁ ratios for the reaction of the hexacarbonyl complexes with P(n-C₄H₉)₃ suggests that the SN2 path becomes more preferred as the size of the metal atom increases from Cr to Mo to W.