Oxidation of Hypophosphite and Phosphite by Anderson‐Type Hexamolybdocobaltate(III) Anion

Abstract

ABSTRACTThe oxidation of hypophosphite and phosphite by the Anderson‐type hexamolybdocobaltate(III), [H6CoIIIMo6O24]3−, anion was investigated at pH 2 and 1, respectively, in aqueous medium. The reaction is found to occur through an outer‐sphere mechanism with a prior weak complex formation between the reactants. Under the reaction conditions, the oxidant exists in the [H5CoIIIMo5O20]2−, [H6CoIIIMo6O24]3−, and [H4CoIII2Mo10O38]6−(dimer) forms, and [H5CoIIIMo5O20]2− is the active species. Inhibition of the reaction by the oxidant anion and added molybdate ion kinetically indicates existence of prior equilibria between various forms of the oxidant. Both hypophosphite and phosphite exists in their protonated forms. The reaction involves direct electron transfer from the phosphorus center to the anion‐generating free radicals in a rate‐determining step. The effect of ionic strength and change in the solvent polarity did not affect the rate of the reaction. A probable mechanism was proposed leading to a complicated rate law as a result of involvement of prior equilibria between various forms of the oxidant. The activation parameters were also determined and are in support of the proposed mechanism.