Kinetic studies of the substitution reactions of the cis-dihalotetracarbonylmanganate(I) ions

The substitution reactions of (C 2 H 5 ) 4 [cis-Mn(CO) 4 X 2 ], where X = Br or I, with a variety of ligands, L, were observed to take place with the loss of halide ion, X − , to form cis-Mn(CO) 4 LX and (C 2 H 5 ) 4 NX. The reaction rates in s-tetrachloroethane were first order in substrate, independent of both the nature and the concentration of L, and decreased with increasing atomic weight of the halogen. A dissociative mechanism involving the breaking of a manganese–halogen bond as the rate-determining step is proposed. It is postulated that the order of reactivity of the substrates is most probably the result of an entropy rather than an enthalpy effect. The substitution reactions of (C 2 H 5 ) 4 N[cis-Mn(CO) 4 BrI] in s-tetrachloroethane were also studied kinetically and found to be first order in substrate and independent of the nature and concentration of L. The products of these reactions, however, were mixtures of cis-Mn(CO) 4 LBr and cis-Mn(CO) 4 LI, and (C 2 H 5 ) 4 NBr and (C 2 H 5 ) 4 NI. A solvent study of the reaction between (C 2 H 5 ) 4 N[cis-Mn(CO) 4 Br 2 ] and P(C 6 H 5 ) 3 indicated that the rate of reaction tended to increase with increasing polarity of the solvent.