Metal hydrides as electron donors. The mechanism of oxidative cleavage with tris(phenanthroline) complexes of iron(III)

Article abstract of DOI:10.1021/ic50204a028

The group 4 metal hydrides HMR₃ (M = silicon, germanium, tin; R = alkyl, phenyl) react spontaneously with 2 equiv of tris(phenanthroline)iron(III) perchlorate, FeL₃(ClO₄)₃, in acetonitrile solutions. Although the second-order kinetics (first order in each reactant) indicate that the selective cleavage of only the hydrido-metal bond proceeds from a rate-limiting bimolecular process, there is no significant deuterium kinetic isotope effect. The free-energy dependence of the second-order rate constant k_H for the silicon and germanium hydrides follows the Marcus relationship with slope α close to the theoretical value of 8.5 for an outer-sphere electron-transfer process. The paramagnetic cation HMR₃⁺, similar to that formed by electron impact or photoionization of HMR₃, is postulated to be a metastable intermediate which undergoes spontaneous scission of the hydrogen-metal bond, followed by a further rapid oxidation of the fragment by a second equivalent of FeL₃³⁺. The rate-limiting, outer-sphere mechanism for HMR₃ accords with that previously established for electron transfer between the related series of peralkylmetals MR₄ and the same FeL₃³⁺ complexes. The electron-transfer rate constants k_H and k_R for HMR₃ and MR₄, respectively, are compared for their sensitivity to changes in the standard reduction potentials E° of FeL₃³⁺ and the gas-phase ionization potentials I_D of HMR₃ and MR₄. Polarization and solvation effects appear to be especially important in electron transfer from metal hydrides, especially those of tin.