Electron transfer. Part 169 . Delayed reduction of mononuclear manganeses(IV) using vanadium(III)

The tris-chelate formed by biguanide, H₂NC(NH)NHC(NH)NH₂, and Mn(IV) is one of the rarely encountered examples of water-stable mononuclear complexes of this oxidation state. This cation is reduced in aqueous acid by both V(II) and V(III). In contrast to the kinetically straightforward bimolecular reduction by V(II) in 0.5 M HClO₄ (k = 7.4 × 10³ M⁻¹ s⁻¹ at 22 °C), reductions by excess V(III) yield profiles which are linear (zero-order in Mn^IV) until the last few percent reaction. Analyses of these composite curves are consistent with the sequences