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

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 SN1 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 SN1 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.