Electrochemical and quantum-chemical studies of chromium(III,II) fluoride complexes in alkali chloride melts

The standard rate constants (ks) of charge transfer on a glass carbon electrode were determined for the Cr(III)/Cr(II) redox pair in the NaCl-KCl-K3CrF6, KCl-K3CrF6, and CsCl-K3CrF6 systems at 973–1173 K by cyclic voltammetry. The ks constant was found to increase at elevated temperatures and the following nonmonotonic dependence of ks on the nature of the outer-spheric cation was found: ks (CsCl) > ks (NaCl-KCl) > ks (KCl). On the basis of quantum-chemical data for the M3CrF6 + 18MCl (M = Na, K) model systems, it was shown that the complex chromium particles with four or five outer-spheric sodium or potassium cations had maximum thermodynamic stability. Quantum-chemical calculations were performed to interpret the experimental data on the effect of the second coordination sphere of the complexes on the standard charge transfer rate constants.