Yang Zhiyong, Gould Edwin S
Department of Chemistry, Kent State University, Ohio 44242, USA.
Dalton Trans. 2004 Nov 7(21):3601-3. doi: 10.1039/B408758P. Epub 2004 Sep 16.
The tris(oxalato)cobaltate(III) complex Co(C(2)O(4))(3), E(o)(Co)(III/II)=+0.57 V) is readily reduced by the 2e(-) reagents, Sn(II) and Ge(II), in contrast to (NH(3))(5)CoCl(2+) and (NH(3))(5)CoBr(2+), 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(C(2)O(4))(3) more slowly than (independently prepared) Ti(III) under comparable conditions.
三（草酸根）钴（III）配合物[Co(C₂O₄)₃]³⁻（E₀(Co)(III/II) = +0.57 V）很容易被二电子试剂Sn(II)和Ge(II)还原，这与(NH₃)₅CoCl²⁺和(NH₃)₅CoBr²⁺形成对比，后者对这些供体没有反应。草酸根氧化剂的反应速率仅为维生素B₁₂a（水合钴（III）钴胺素，在pH 1时E₀ +0.35 V）的10⁻³ - 10⁻²，这与以下观点一致，即Sn(II)和Ge(II)对钴胺素结合的钴（III）的还原主要通过涉及Co(i)的额外途径发生。二电子供体对草酸根配合物的还原被认为是通过最初形成奇数电子中间体（例如Sn(III)和Ge(III)）来进行的，这些中间体与Co(III)快速反应。这样的两步序列与稀有还原剂Ti(II)的观察行为一致，发现在可比条件下，[Co(C₂O₄)₃]³⁻氧化Ti(II)的速度比（独立制备的）Ti(III)慢。
