Reactions of tris(oxalato)cobaltate(iii) with two-electron reductants

Abstract

The tris(oxalato)cobaltate(III) complex [Co(C₂O₄)₃]³⁻, E^o_Co^III/II = +0.57 V) is readily reduced by the 2e⁻ reagents, Sn(II) and Ge(II), in contrast to (NH₃)₅CoCl²⁺ and (NH₃)₅CoBr²⁺, which are unreactive toward these donors. Rates for the oxalato oxidant are only 10⁻³–10⁻² as great as those for vitamin B_12a (aquacob(III)alamin, E^o +0.35 V at pH 1), in accord with the suggestion that reductions of corrin-bound cobalt(III) by Sn(II) and Ge(II) occur predominantly through an additional path involving Co(I). Reductions of the oxalato complex by 2e⁻ donors are taken to proceed by initial formation of odd-electron intermediates (e.g., Sn^III and Ge^III) which react rapidly with Co^III. Such a two-step sequence is in keeping with the observed behavior of the rare reductant, Ti^II, which is found to be oxidized by [Co(C₂O₄)₃]³⁻ more slowly than (independently prepared) Ti(III) under comparable conditions.