Unusual Effect of the Second Coordination Sphere on the Standard Charge Transfer Rate Constants for the Nb(V)/Nb(IV) Redox Couple in Chloride-Fluoride Melts

摘要

Despite extensive studies dealing with molten salts, almost no information is available on standard charge transfer rate constants (k_s) for redox couples and on the effect of the second coordination sphere. Previously [1, 2], we showed that, for the redox couples Hf(IV)/Hf and Cr(III)/Cr(II), standard charge transfer rate constants decrease in going from molten NaCl-KCl to KCl and CsCl melts. According to the theory of elementary event of charge transfer, smaller and stronger complexes require higher energies to rearrange before the electron transfer process itself and, hence, the corresponding electrode reaction occurs at a lower rate [3]. The size of a complex and the metal-ligand bond length in it decrease and the stability of the complex increases in going from the NaCl-KCl melt to molten KCl and CsCl due to a decrease in the counter polarizing effect of outer-sphere cations in going from Na to Cs. Thus, a decrease in k_s for the Hf(IV)/Hf and Cr(III)/Cr(II) redox couples as the second coordination sphere changes from Na~+ to Cs~+ is in agreement with the statements of the theory of elementary event of charge transfer [3].