UV-visible absorption spectroscopy and amperometric titration experiments indicate that the ytterbium(III) chloride is complexed as the ytterbium(III) hexachloride, [YbCl6]3, in basic aluminum chloride-1-methyl-3-ethylimidazolium chloride molten salt, and [YbCl6]3- can be reduced to Yb(II) species via a quasi-reversible one-electron charge-transfer process. The standard heterogeneous rate constant and cathodic transfer coefficient for the reduction of [YbCl6]3- in 44.4/55.6 mole percent melt are 1.29 × 10-4 cm s-1 and 0.59, respectively. The formal potential, E°′', of this redox reaction is dependent upon the pCl of the basic melt, suggesting the loss of one chloride ion from [YbCl6]3- during reduction to produce [YbCl5]3-. In the basic melt, [YbCl5]3- is slowly oxidized by some components of the melt. In acidic melt, Yb(III) is stable and can be electrochemically reduced to produce stable Yb(II) via a reversible one-electron charge-transfer process. The formal potential of the Yb(III)/Yb(II) redox couple in acidic melt is about 2.5 V more positive than the corresponding reaction in basic melt, indicating a significant change in the coordination sphere. Values of the Stokes-Einstein product measured for the Yb(III) and Yb(II) species in these melts are comparable to those of transition metal chloro complexes with the same overall charge in the same melt.
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