Molybdenum(V) Species in Alkali Chloride Melts: An Electronic Absorption Spectroscopy Study

Molybdenum in chloride melts can form chloro-species containing the metal in the oxidation states +3, +4 and +5.  In the highest oxidation state, +6, molybdenum can form oxygen-containing ions.  Amongst the molybdenum chloro-species those of the higher oxidation states (+5 and +4) are the least studied.  In the present work the behavior of Mo(V) chloride was studied in a range of alkali chloride melts employing electronic absorption spectroscopy.  The experiments were performed in LiCl-KCl, NaCl-CsCl, NaCl-KCl-CsCl eutectics and NaCl-KCl equimolar mixture between 450 and 750 ^oC, depending on the melt composition.  Molybdenum(V) ions were introduced into the melt by feeding MoCl₅vapor using the procedure described earlier (1).  Analysis of the melt samples taken after the experiments showed that the mean oxidation state of molybdenum in the salt phase was +5.

Electronic absorption spectra recorded upon dissolution of MoCl₅ in an alkali chloride melt contained a broad band with a maximum around 13000 cm^-1 (770 nm).  Resolution of the spectra into individual bands, however, showed that this band represents a superposition of two bands with maxima around 15500 and 12500 cm^-1, Fig. 1. 

The ratio between the intensities of these two component bands depends on temperature.  At low temperatures the spectrum contains essentially one band (marked as 1 in Fig. 1) around 767 nm (13100 cm^-1) corresponding to ²T_2g→²E_g transition in the octahedral MoCl₆^- ion.  The energy of this transition agrees with that observed for WCl₆^- ion, isoelectronic with MoCl₆^- (2).  On increasing temperature the second band (marked as 2 in Fig. 1) around 650 nm (15500 cm^-1) becomes more prominent.  The ratio of intensities of bands 1 and 2 changes with temperature, e.g. in LiCl-KCl melt it decreases from 1 : 0.2 at 450 ^oC to 1 : 2.6 at 850 ^oC.

The band around 650 nm corresponds to the absorption of molecular MoCl₅ (3).  In the present study the absorption spectra of gaseous MoCl₅ were measured at various temperatures and an example is shown in Fig. 2.  Resolution of the spectrum of gaseous MoCl₅ into individual bands, Fig. 3, shows that in the visible region there are two bands, a very intense one above 17000 cm^-1 and a medium intensity band around 15000 cm^-1.  These bands can account for the changes in the spectra of Mo(V) solutions in the chloride melts occurring upon increasing temperature. 

Analysis of the spectroscopic data therefore shows that there are two species present in alkali chloride melts containing the products of MoCl₅ dissolution, MoCl₆^- and MoCl₅.  Most likely that these species are present in the equilibrium:

MoCl₆^- ↔ MoCl₅ + ½ Cl₂

and this equilibrium is shifted to the right with temperature.

1.  V. A. Volkovich, I. B. Polovov, R. V. Kamalov, B. D. Vasin and T. R. Griffiths, in Molten Salts Chemistry and Technology, M. Gaune-Escard and G. M. Haarberg, Editors, p. 489, John Wiley & Sons Inc., Hoboken (2014).

2.  V. A. Volkovich, D. A. Danilov, I. B. Polovov, B. D. Vasin, T. R. Griffiths, D. E. Aleksandrov, O. A. Tropin and D. V. Tsarevskii, ECS Transactions, 3(35), 555 (2007).

3.  R. F. W. Bader and A. D. Westland, Can. J. Chem., 39, 2306 (1961).