ELECTROCHEMICAL AND SPECTROSCOPIC CHARACTERIZATION OF (TA6CL12)(Z+) CHLORIDE CLUSTERS IN ACETONITRILE AND IN THE ALUMINUM CHLORIDE-1-METHYL-3-ETHYLIMIDAZOLIUM CHLORIDE MOLTEN-SALT

The electrochemistry and electronic absorption spectroscopy of the anionic chloride complexes derived from the metal-metal bonded {Ta6Cl12}(2+) (z = 2-5) clusters were examined in CH3CN/[(n-Bu)(4)N]PF6 at 23 degrees C and in the room-temperature chloroaluminate molten salt, aluminum chloride-1-methyl-3-ethylimidazolium chloride (AlCl3-[MeEtim]Cl) at 40 degrees C. The following electrochemical reactions were identified in the former solvent (potentials versus Ag/0.10 M Ag+): [(Ta6Cl12)Cl-6](2-) (red-orange) p [(Ta6Cl12)Cl-6](-) (unstable) +e, E(1/2) = 1.19 V; [(Ta6Cl12)Cl-6](2-) + e(-) = [(Ta6Cl12)Cl-6](3-) (yellow), E(1/2) = -0.28 V; [(Ta6Cl12)Cl-6](3-) (yellow) + e(-) = [(Ta6Cl12)Cl-6](4-) (green), E(1/2) = -0.84 V; [(Ta6Cl12)Cl-6](4-) + CH3CN ' [(Ta6Cl12)Cl-5(CH3CN)](3-) (green) + Cl-. The replacement of chloride ion by CH3CN in the [(Ta6Cl12)Cl-6](4-) complex in the last step shown above takes place in a slow homogeneous reaction that is coupled to the [(Ta6Cl12)Cl-6](3-/4-) electrode reaction. This homogeneous reaction is observed on the time scale of bulk controlled-potential electrolysis in CH3CN/[(nBu)(4)N]PF6 without added chloride ion, but not during voltammetric experiments. The addition of a large excess oi chloride ion to a solution containing [(Ta6Cl12)Cl-5(CH3CN)](3-) converts this complex back to [(Ta6Cl12)Cl-6](4-). In the acidic AlCl3-[MeEtim]Cl melt, two reversible redox reactions, analogous to the {Ta6Cl12}(4+/3+) and {Ta6Cl12}(3+/2+) reactions seen in CH3CN, were found at E(1/2) = 1.16 and 0.79 V (versus Al/Al(III) in the 66.7-33.3 mol % melt), respectively. However, intersolvent comparisons of the E(1/2) of these reactions referenced to the Fe(cp)(0/+) couple indicated that; both are shifted positively by more than 1 V in the molten salt relative to CH3CN/[(n-Bu)(4)N]PF6, suggesting replacement of labile chloride ions on the [(Ta6Cl12)Cl-6]((6-z)-) (z = 2-4) clusters with [AlCl]- during dissolution in;acidic melt to produce species with the general formula, [(Ta6Cl12)Cl-y(AlCl4)(6-y)]((6-z)-) (y less than or equal to 6). The Stokes-Einstein products of the [(Ta6Cl12)Cl-6]((6-z)-) (z = 3 and 4) clusters in CH3CN/[(nBu)(4)N]PF6 were nearly the same with an average value of ca. 10 x 10(-11) g cm s(-2) K-1. This value is very close to that found for the corresponding anionic niobium clusters in a previous investigation conducted in basic AlCl3-[MeEtim]Cl.