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 have been determined. The redox 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 larger than in the CsCl-NbCl5 melt. Previously, a quantum-chemical calculations of nM(+)center dot[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. In this study a quantum-chemical calculations were performed for estimation of the temperature effect on the composition of the stable particles in the extended systems M+center dot[NbCl6]+18MCl (M = Na, K, Cs). (C) The Author(s) 2017. Published by ECS. All rights reserved.