Correlation analysis of reactivity in the reduction of Co(en)2Br2+ by Fe(CN)64- in water-methanol/1,4-dioxane media

The reduction of [Co(en)(2)Br-2](+) by [Fe(CN)(6)](4-) in H2O-MeOH and H2O-1,4-dioxane mixtures has been studied over a range of solvent compositions [5-30% (v/v)]. The reduction of [Co(en)(2)Br-2](+) was monitored under second order conditions and was found to be rapid in the various solvent compositions investigated. The favoured mechanism is an outer-sphere electron-transfer process consisting of elementary steps, ion-pair formation (K-IP), electron-transfer (k(et)) and successor dissociation. Therefore, the overall rate constant, k(2) = K(IP)k(et)[Co(en)(2)- Br-2(+)][Fe(CN)(6)(4-)]. The rates increase as the proportion of organic cosolvent increases. The rates correlate with solvent properties, such as relative permittivity (epsilon (r)) and the Grunwald-Winstein parameter, Y-GW, which are used to explain the non-specific interaction upon solvation of mixture of solvents on the incipient reactants and on the ion-pair. In addition, they are also subjected to multiparametric analysis employing Swain's solvent vectors A and B also with Kamlet-Taft's solvatochromic parameters alpha, beta and pi*. The reduction rates show an excellent correlation with multiparametric equations and are susceptible to both specific and non-specific solvation effects. A quantitative estimation of the latter components has been attempted.