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Stokes-Einstein products that were derived from electrochemical data for monomeric and polymeric anionic transition metal halide complexes in the basic aluminum chloride-l-methyl-3-ethylimidazolium chloride room-temperature ionic liquid and its bromide analog are shown to vary stepwise with the overall charge on the complex ion. These observations are reconciled in terms of ion-ion interactions between the melt cations and the anionic complexes which lead to diffusing entities with larger solvodynamic radii than expected. The number of associated cations appears to increase as the charge on the transition metal complex becomes more negative.

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Electronic absorption spectroscopy indicated that tantalum(V) is complexed as [TaCl~]-in the basic A1C13-1-methyl-3ethylimidazolium chloride molten salt. This anionic species can be reduced to [TaC10] 2-in a one-electron, reversible electrode reaction with a voltammetric half-wave potential of -0.49V in the 44.4/55.6 m/o melt vs. the A13+/A1 couple in the 66.7/ 33.3 m/o melt at 40~ Voltammograms of [TaC16]-solutions also exhibit a second reduction process with a voltammetric peak potential of about -1.70V vs. this same reference electrode. This second electrode reaction is complicated by coupled homogeneous chemistry at slow scan rates but appears to simplify to a one-electron reduction process at fast scan rates. The average Stokes-Einstein products for [TaC16]-and [TaC10] 2-are 3.5 • 10 -l~ and 1.9 • 10 -1~ g cm s 2 K-l, respectively. The former complex, but not the latter, appears to react slowly with oxide in the melt to form an oxide-containing complex. The conditional equilibrium constant for the reaction between oxide and [TaC10]-is approximately 1.4 _+ 0.3 x 103. The tantalum(V) oxide-containing species can be converted back to its chloride complex with COC12.