Dominant role of solvent polarity in the reduction kinetics of cobalt(III)-ammine complex in mixed solvents

Abstract

Outer-sphere electron transfer reaction between Co(NH₃)₆³⁺ and Fe(CN)₆^4- has been studied as a function of relative permittivity of binary solvent mixtures of water-1,4-dioxane (5-30% (v/v) organic cosolvent). The electron transfer rate constants (<i style="mso-bidi-font-style:normal">k</i>_et) increase with increase in mole fraction (<i style="mso-bidi-font-style:normal">x</i>₂) of 1,4-dioxane in the mixture. The estimated rate constants have been linearly related to the reciprocal of relative permittivity of the medium (log <i style="mso-bidi-font-style:normal">k</i>_et versus 1/ɛ_r). They have also been correlated with the Grunwald Wistein empirical parameter, Y_GW, and Swain's polarity term (A+B). The results indicate the solvent participation in the formation and stability of ion-pair from Co(NH₃)₆³⁺ and Fe(CN)₆^4- leading to electron transfer. The rate constants have also been subjected to bi-and tri-parametric analyses to understand the specific and non-specific influence of binary solvent mixtures on reaction rates.