Organolanthanoids. 9. Electrochemical reduction of tris(cyclopentadienyl)samarium(III), -ytterbium(III), and -europium(III) compounds in tetrahydrofuran

The cyclopentadienyl complexes (C₅H₅)₃M (M = Sm, Yb, and Eu) and (MeC₅H₄)₃Sm undergo chemically reversible electrochemical reduction in tetrahydrofuran at platinum, gold, and mercury electrodes under purified nitrogen to give [(C₅H₅)₃M]^- species, (C₅H₅)₃M + e^- → [(C₅H₅)₃M]^-. As required by this reaction, reversible electrochemical oxidation of KSm(C₅H₅)₃ to give (C₅H₅)₃Sm has also been observed. The reversible half-wave potentials, E^r_1/2 (approximately equal to the standard redox potential E°), are considerably more negative than for reduction of M(III) to M(II) in water, indicating that both solvent and ligand terms are important in lanthanoid redox chemistry. The E^r_1/2 values show that thermodynamic stability of the divalent species toward oxidation falls in the sequence M = Eu > Yb > Sm. The kinetic reactivity with adventitious moisture and/or oxygen follows the thermodynamic sequence with [(C₅H₅)₃Sm]^- being particularly sensitive to traces of water. Reaction of (C₆F₅)₂Yb with cyclopentadiene and (C₆F₅)₂Hg provides a new synthesis of (C₅H₅)₃Yb.