Reaction of Metal Carbonyl Anions with Metal Carbonyl Dimers: Thermodynamic and Kinetic Factors That Control the Reactions

The reactions of mononuclear metal carbonyl anions, M⁻ (M⁻ = Co(CO)₄⁻, CpFe(CO)₂⁻, Re(CO)₅⁻, Mn(CO)₄L⁻, L = PPh₃, PBu₃, P(OPh)₃, CpM(CO)₃⁻ (M - Cr, Mo, W)) with metal carbonyl dimers, M′₂ (M′₂ = Co2(CO)₈, Cp₂Fe₂(CO)₄, Re₂(CO)₁₀, Mn₂(CO)₁₀, Cp₂M₂(CO)₆ (M = Cr, Mo, W), and Cp₂Ru₂(CO)₄)), are described: To determine the thermodynamic parameters, we have derived values for the two-electron-reduction potentials (M₂ + 2e⁻ → 2M⁻) and shown that these values correctly predict the direction of reaction. The order of these reduction potentials is (all are negative) Co₂(CO)₈ > Cp₂Cr₂(CO)₆ > Cp₂Mo₂(CO)₆ > Mn₂(CO)₁₀ > Re₂(CO)₁₀ > Cp₂Fe₂(CO)₄. In each case a clean reaction is observed with only M₂ and 2M′ produced. The kinetics show that the rate has a first-order dependence on [M⁻]; rate = k[M⁻][M′2]. All dimers that contain a cyclopentadienyl ligand react more rapidly than expected from the potential. Product distributions for reactions of heterobimetallic complexes are also consistent with a different mechanism for dimers with a cyclopentadienyl ligand.