Relative reactivities of anionic transition-metal hydrides. Reduction of alkyl halides

Article abstract of DOI:10.1021/om00082a030

A comparison of the second-order rate constants for halide displacement from n-BuBr by H^- derived from anionic transition-metal hydrides established the following order of reactivity: cis-HW(CO)₄P(OMe)₃^- > cis-HCr(CO)₄P(OMe)₃^- > HW(CO)₅^- > CpV(CO)₃H^- > HCr(CO)₅^- > HRu(CO)₄^- ? HFe(CO)₄^- (no reaction). The lack of dependence of reactivity on degree of branching of the alkyl halide for HM(CO)₅^- (M = Cr, W) and a dramatic diminishing of reactivity for HM(CO)₄P(OMe)₃^-, 1° ? 2° > 3°, resulted in a different order of reactivity on the basis of t-BuBr: HCr(CO)₅^- > HW(CO)₅^- > cis-HCr-(CO)₄P(OMe)₃^- > CpV(CO)₃H^- > cis-HW(CO)₄P(OMe)₃^- ? HFe(CO)₄^- (no reaction). Mechanistic probes were applied to reactions of HW(CO)₅^- and cis-HW(CO)₄P-(OMe)₃^-, and preliminary results suggest the former to involve some radical character (single electron transfer) and the latter to react with considerable ionic character (H^- transfer).