| The electrochemistry of several chromium subgroup transition metal solutes was studied in the basic AlCl(,3)-1-methyl-3-ethylimidazolium chloride (MEIC) ionic liquid at 40.0(DEGREES)C by using stationary and rotating disk electrode voltammetry, chronoamperometry, coulometry, and UV-vis spectrophotometry. Chromium(III) was present in the melt as the chloro complex {lcub}CrCl(,6){rcub}('3-). This complex underwent an essentially irreversible reduction, apparently as a result of slow charge-transfer, to a chromium(II) species at ca. -1.08 V versus an aluminum wire immersed in 66.7 mol % melt.; Molybdenum(V) chloride, MoCl(,5), was reduced to the {lcub}MoCl(,6){rcub}('2-) complex upon addition to the melt. This species could be electrochemically reduced via a reversible, one-electron, uncomplicated charge-transfer to {lcub}MoCl(,6){rcub}('3-) at -0.138 V. The molybdenum(IV) chloro complex reacted with oxide in the melt (from the dissolution of Li(,2)CO(,3)) to form an oxychloro complex, probably {lcub}MoOCl(,4){rcub}('2-). The first reported UV-vis solution spectrum of the {lcub}MoCl(,6){rcub}('2-) species was obtained.; Tungsten(VI) was reduced by the melt immediately upon dissolution to give {lcub}WCl(,6){rcub}('-), which was further reduced by the melt to {lcub}WCl(,6){rcub})('2-) over several hours. Voltammetry indicated that {lcub}WCl(,6)('-){rcub} could undergo two one-electron reductions. The first voltammetric wave (E(, 1/2) = 0.45 V) corresponded to the reversible, uncomplicated {lcub}WCl(,6){rcub}('-)/WCl(,6){rcub})('2-) redox couple. The second process (E(, 1/2) = -0.80 V) was identified as the reversible, uncomplicated reduction of {lcub}WCl(,6){rcub})('2-) to {lcub}WCl(,6){rcub})('3-). Coulometric reduction of tungsten(V) to tungsten(IV) was readily accomplished. However, hydrogen ion impurities interfered with the exhaustive reduction of the tungsten(IV) chloro complex.; The metal-metal bonded tungsten(III) cluster ion, {lcub}W(,2)Cl(,9){rcub}('3-), exhibited two one-electron voltammetric oxidation waves. The first wave (E(, 1/2) = -0.07 V) was reversible and uncomplicated by homogeneous chemistry on the voltammetric time interval and resulted in the mixed valence complex, {lcub}W(,2)Cl(,9){rcub}('2-). The second wave with a peak potential of 0.46 V corresponded to the irreversible oxidation of this species. The coulometric oxidation of {lcub}W(,2)Cl(,9){rcub}('2-). The second wave with a peak potential of 0.46 V corresponded to the irreversible oxidation of this species. The coulometric oxidation of {lcub}W(,2)Cl(,9){rcub}('3-) at an applied potential of 0.15 V consumed 2.0 faradays of charge per mol of the tungsten(III) dimer and resulted in a solution containing only {lcub}WCl(,6){rcub}('2-). An ECE mechanism was proposed to account for these results. This mechanism involves the disproportionation of electrogenerated {lcub}W(,2)Cl(,9){rcub}('2-) to {lcub}WCl(,6){rcub}('2-) and an unidentified tungsten(III) species, followed by oxidation of the tungsten(III) species to {lcub}WCl(,6){rcub}('2-) at the same applied potential. |
