Spectroscopic and Electrochemical Studies of Tungsten(VI) and Tungsten(V) Chloride and Oxychloride Complexes in a Sodium Chloride Saturated Sodium Chloroaluminate Melt

Raman and UV–visible absorption spectroscopies indicate that the addition of WC1₆ or KWC1₆ to a sodium chloride saturated sodium chloroaluminate melt at 175°C produces the tungsten(V) hexachloride anion [WC1₆]~. The production of [WC1₆]“ from WC1₆ may be attributed to the chemical reduction of the latter by the free chloride ion in this melt. [WC1₆]“ can be oxidized to WC1₆ via a reversible one–electron charge–transfer process with a voltammetric half-wave potential of 2.077 V referenced to aluminum in a pure sodium chloride saturated sodium chloroaluminate melt. With fast scan rate voltammetry, [WC1₆]~ can be reduced to [WC1₆]²~ and [WC1₆]^3– via two consecutive one-electron reduction processes with half-wave potentials of 1.578 and 0.818 V, respectively. Roth [WC1₆]²“ and [WCl₆]³r form precipitates in this melt at this temperature. Using slow scan rate voltammetry the reduction processes are complicated by coupled chemical reactions. Tungsten(VI) oxychloride, WOCl₄₎ is stable in this melt, and can be reduced to tungsten(V) oxychloride via a reversible one-electron reduction process with a half-wave potential of 1.746 V. Absorption spectroscopy shows that the tungsten(V) chloride is [WOCl₅]^2–. Fast scan rate voltammetry indicates that tungsten(V) oxychloride exhibits a two-electron reduction process to produce a W(III) species. At slow scan rates, the reduction appears to be complicated by coupled chemical reactions. Definitive characterization of the reduction mechanisms is prevented by film formation on the electrode surface during these reductions.