The tris(oxalato)cobaltate(III) complex [Co(C2O4)3]3−, EoCoIII/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 B12a
(aquacob(III)alamin, Eo
+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., SnIII and GeIII) which react rapidly with CoIII. Such a two-step sequence is in keeping with the observed behavior of the rare reductant, TiII, which is found to be oxidized by [Co(C2O4)3]3− more slowly than (independently prepared) Ti(III) under comparable conditions.
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