Hydride Transfer Reactions of Transition Metal Hydrides:  Kinetic Hydricity of Metal Carbonyl Hydrides

Hydride transfer from neutral transition metal hydrides (MH) to Ph₃C⁺BF₄^- gives M-FBF₃ and Ph₃CH. The rate law −d[Ph₃C⁺BF₄^-]/dt = k[Ph₃C⁺BF₄^-][MH] was established from kinetic measurements using stopped-flow methods. Second-order rate constants determined in CH₂Cl₂ solution at 25 °C range from k_H⁻ = 7.2 × 10^-1 M^-1 s^-1 to k_H⁻ = 4.6 × 10⁶ M^-1 s^-1. The order of increasing kinetic hydricity is (C₅H₄CO₂Me)(CO)₃WH < (CO)₅MnH < Cp*(CO)₃CrH < Cp(CO)₃WH < HSiEt₃ < cis-(CO)₄(PCy₃)MnH < cis-(CO)₄(PPh₃)MnH < (C₅H₄Me)(CO)₃WH < Cp(CO)₃MoH < Cp*(CO)₃WH < (indenyl)(CO)₃WH < (CO)₅ReH < Cp*(CO)₃MoH < cis-(CO)₄(PPh₃)ReH < Cp(NO)₂WH < trans-Cp(CO)₂(PCy₃)MoH < trans-Cp(CO)₂(PPh₃)MoH < trans-Cp(CO)₂(PMe₃)MoH (Cp = η⁵-C₅H₅, Cp* = η⁵-C₅Me₅, Cy = cyclohexyl). Ranges of activation parameters for hydride transfer from trans-Cp(CO)₂(PMe₃)MoH, trans-Cp(CO)₂(PCy₃)MoH, cis-(CO)₄(PPh₃)ReH, and Cp*(CO)₃MoH are ΔH = 3.0−5.9 kcal mol^-1 and ΔS = −18 to −24 cal K^-1 mol^-1. The rate constant for hydride transfer (k_H⁻) from cis-Cp(CO)₂(PCy₃)MoH at −55 °C is 3 orders of magnitude lower than that for trans-Cp(CO)₂(PCy₃)MoH. Phosphine substitution for CO generally enhances the kinetic hydricity, with trans-Cp(CO)₂(PMe₃)MoH being 10⁴ times as reactive as Cp(CO)₃MoH. The electronic effect of phosphine substitution is attenuated by steric factors when the phosphine is cis to the metal hydride. The hydride transfer kinetics reported here are interpreted to be single-step hydride transfers, rather than a multiple-step mechanism involving an initial electron transfer followed by hydrogen atom transfer. A distinction is made between hydricity and nucleophilicity of metal hydrides.