The kinetics and stoicheiometry of silver(III) reduction by the octacyano-complexes of molybdenum(IV) and tungsten(IV)

Abstract

The reactions of [Ag(OH)₄]^– with [Mo(CN)₈]^4– and [W(CN)₈]^4– involve an initial one-electron reduction of Ag^III to Ag^II with second-order rate constants of 80 and 2.9 × 10⁴ dm³ mol^–1 s^–1(25 °C, I= 1.2 mol dm^–3), respectively. The tungstate reaction is quantitative with a stoicheiometry of 1 : 1. The reaction with [Mo(CN)₈]^4–, however, does not go to completion. There is apparently an equilibrium of oxidation states. From the value of the equilibrium constant a formal potential of 0.87 V ([OH^–]= 1.2 mol dm^–3, 25 °C) is estimated for the reduction of [Ag(OH)₄]^– to dimeric Ag^II. The rate of the initial one-electron redox step is strongly correlated with E_M/M⁺ for the series of reductants [Fe(CN)₆]^4–, MnO₄^2–, [W(CN)₈]^4–, and [Mo(CN)₈]^4–. The results are interpreted in terms of a general mechanism which involves silver(II) polymerisation following the initial redox step.