Intervalence Charge Transfer of the Nb(V)/Nb(IV) Redox Couple in Alkali Chloride Melts: Experiment and Quantum-Chemical Calculations

The standard rate constants of charge transfer (k_s) for the Nb(V)/Nb(IV) redox couple in KCl-NbCl₅ and CsCl-NbCl₅ chloride melts are determined. The Nb(V) electrochemical reduction process in the NaCl-KCl-NbCl₅ melt was shown being reversible; hence, the determination of k_s appeared impossible. Values of k_s in the KCl-NbCl₅ melt are higher than those in the CsCl-NbCl₅ one. Previously, in a quantum-chemical calculations of nM⁺·[NbCl₆] model systems (M = Na, K, Cs), containing a complex anion with cations outer-sphere (OS) shell (nM⁺) have been done. It was found that compositions with an intermediate number of OS cations n < n_lim, where n_lim is the limiting number of OS cations bound to a given complex, are thermodynamically most stable. The aim of this quantum-chemical study is an estimation of the temperature effect on the composition of the stable particles in the extended systems M⁺[NbCl₆]+18MCl (M = Na, K, Cs).