Niobium(V) is complexed as [NbCI6]- in the basic aluminum chloride-l-methyl-3-ethylimidazolium chloride ionic liquid, and it can be reduced to [NbCl6]2~ and [NbCl6]3 4 N~ in successive, one-electron, reversible redox reactions at a glassy-carbon electrode with voltammetric half-wave potentials of 0.17 and -0.93 V, respectively, in the 44.4/55.6 mol % melt at 40 °C, referenced to aluminum in the 66.7/33.3 mol % melt. The average Stokes-Einstein products for [NbCl6]- and [NbCl6]2- in the ionic liquid are 3.5 X 10~10and 2.1 X 10–10cm s-2K-1, respectively. The addition of oxide ion to solutions of [NbCl6]- in basic melt produces [NbOCl5]2-. This species can be electrochemically reduced to a strongly paramagnetic monomeric niobium(IV) species, which is associated with a single oxide ion, possibly [NbOCl4]2- or [NbOCl5]3-. The niobium(V)/niobium(IV) oxide chloride electrode reaction exhibits quasireversible behavior with a standard heterogeneous rate constant of 9.0 X 10-7cm s-1and a cathodic transfer coefficient of ca. 0.8. The formal electrode potential of the niobium(V)/niobium(IV) oxide chloride redox couple is -0,521 V in the 44.4/55.6 mol % melt. The Stokes-Einstein products of [NbOCl5]2_and the related niobium(IV) oxide chloride complex are 1.9 X 10-I° and 1.4 X 10-10cm s~2K-1, respectively. The overall electrochemical scheme for niobium complexes in the AlCl3-MeEtimCl ionic liquid is summarized as follows.
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