Alkyl-Transfer Reactions from Transition Metal Alkyl Complexes to CpFe(CO)₂⁻: Rate and Mechanistic Studies

We have examined reactions of several alkyl complexes (Mn(CO)₅Me, Mn(CO)₅CH₂Ph, Mn(CO)₅Ph, CpMo(CO)₃Me, CpMo(CO)₃Et, and CpMo(CO)₃CH₂Ph) with CpFe(CO)₂⁻. Each of these reactions results in transfer of the alkyl group to the iron with formation of Mn(CO)₅⁻ or CpMo(CO)₃⁻. The reactions were first order in the concentration of CpFe(CO)₂⁻ and first order in the alkyl complex. The dependence on the group transferred, H⁺ > CH₂Ph⁺ > Me⁺ > Et⁺ > Ph⁺, is consistent with a nucleophilic attack mechanism. For methyl transfer the rate correlates with the difference in nucleophilicity between the reactant and product anions. We have also evaluated the self-exchange between CpFe(CO)₂Me and CpFe(CO)₂⁻ by line-broadening experiments. Our attempt to estimate the self-exchange rate constant for CpMo(CO)₃Me and CpMo(CO)₃⁻ shows this reaction to occur very slowly through a methyl migration sequence.