Reactions of metal carbonyl complexes. VIII. Kinetics and mechanisms of the substitution reactions of cis-Mn(CO)4LBr (L = PPh3, AsPh3, SbPh3) with group VA bidentate ligands

Abstract

The substitution reactions of cis-Mn(CO)₄LBr (L = PPh₃, AsPh₃, SbPh₃) with diphos, diarsine, and dipy (A—A) take place with the loss of one CO group and L to form fac-Mn(CO)₃(A—A)Br. The observed reaction rates in CHCl₃ solution are first order in substrate but depend on the nature of L and A—A, and in some cases the concentration of A—A as well. When L = PPh₃ and A—A = diphos, and when L = AsPh₃ and A—A = diphos, diarsine, or dipy, the rates are independent of [A—A]. For these reactions, an S_N1 dissociative mechanism involving the rupture of one of the Mn—CO bonds as the rate-determining step is proposed. The observed positive entropies of activation (∼10 e.u.) for these reactions are supportive of the proposed mechanism. The reactions of cis-Mn(CO)₄(SbPh₃)Br with A—A are essentially independent of both the nature and the concentration of A—A. However, for these reactions, an S_N1 dissociative mechanism involving the rupture of the Mn—SbPh₃ bond as the rate-determining step is proposed; the observed positive ΔS^≠ values (∼12 e.u.) are in accord with such a mechanism. The rates when L = PPh₃ and A—A = diarsine or dipy are dependent on both the nature and the concentration of A—A. A mechanism involving rapid pre-equilibrium steps between the parent molecule and the intermediates, Mn(CO)₃(PPh₃)Br and Mn(CO)₄Br, is proposed to account for the complex kinetic behaviour found for these reactions. These results are particularly noteworthy because octahedral first-row transition metal complexes generally undergo substitution reactions which are independent of the concentrations of the entering nucleophiles.