Electron transfer .130. Reductions with indium(I)

Aqueous solutions of the hypovalent state indium(I) have been prepared by treatment of In(Hg) with silver triflate in acetonitrile, followed by dilution with oxygen-free water. These solutions are stable for over 5 h at 25 OC. In-I(aq) reacts with oxidants of the type [(NH3)(5)Co-III(Lig)](2+) (In-I + 2Co(III) --> In-III + 2Co(II)), and kinetic profiles are consistent with a two-step sequence proceeding with formation of the metastable state In-II, which reacts rapidly with Co-III. Rate ratios for reductions of halogeno-substituted oxidants point to predominance of halide-bridged paths for the chloro, bromo and iodo complexes. Reductions of carboxylato-substituted derivatives are slow but appear to entail inner-sphere precursors if aided by an O-donor group in a position favorable for chelation. In no case is there evidence for reaction via initial reduction of the ligand (the radical-cation mechanism) although the potential of the In(I,II) couple (-0.40 V) allows this path for carbonyl-substituted oxidants. Reductions by In(I), like those by Eu(II), make no significant use of bridging by heterocyclic donor nitrogen centers in pyridine and pyrazine complexes.