Kinetics and mechanism of oxidation of [Mo(CN)₈]^4-[Mn(cdta)^- complex ion: Application of the Marcus relationship in support of an outer-sphere mechanism

Abstract

857-862Oxidation of [Mo(CN)8]4-by [Mn(cdta)]-, studied as a function of disappearance of [Mn(cdta)]- at 510 nm, is a first order reaction with respect to both the ions. The linear plot between kobs and [H+] with intercept indicates two paths; one independent (k) of and the other (kH) dependent on [H+]. These paths are ascribed to the protonation equilibrium between [Mn(cdta)]- and H[Mn(cdta)],  by analogy to the similar equilibrium existing in [Mn(III)edta)]-complex, and is more likely than the equilibrium involving the protonation of [Mo(CN)84- ion. The alkali metal cations (M+) catalyze the observed rate in the order K+> Na+ > Li+ ion. The data provide linear correlations between kobs[Mo(CN)84-] and [M+], between kobs/[Mo(CN)84-] and polarisability of alkali metal cations beside a linear plot between kobs/[Mo(CN)84-] and  √m/(1+√m) with a predictable slope ~4 for the reaction between [Mn(cdta)]- and [Mo(CN)84-]. The first correlation, indicative of the ion-pair formation, is not favoured because the kobs value did not show sign of saturation. The second correlation indicates the formation of a bridge by the alkali metal ion between the oxidant and reductant species. The rate constants k and kH at 25° C are (7.32 ± 0.17)x103 dm6 mol-2 s-1 and (1.3 ± 0.02)X107 dm6 mol-2 s-1 respectively. The related values of the enthalpy and entropy are ΔHk#= 25 ± 2 kJ mol-1, ΔSk# = -70 ± 6 J K-1 mol-1, and ΔHkH# = 28 ± 2 kJ mol-1, & ΔSkH# = 2 ± 5 J K-1 mo-1 respectively. The use of the Marcus theory supports the outer-sphere nature of the reactio