Comparison of Electrochemical Data for Reduction of Mercury (II) Dihalide Diphosphine Complexes in Dichloromethane and ¹⁹⁹Hg and ³¹P NMR Data

Detailed electrochemical studies have been carried out on the reduction of a series of HgX₂P₂ complexes (X = Cl⁻, Br⁻, I⁻; P = tertiary phosphine) in CH₂Cl₂ solution. At a Hg electrode, the complexes exhibit reversible two-electron reductions of unusual stoichiometry: HgX₂P₂ + 2e⁻ ⇌ Hg + 2X⁻ + 2P. The reduction potentials correlate well with δ(³¹P), δ(¹⁹⁹Hg), and J_P,Hg NMR data. Electrochemical characterization of the halide-exchange reactions HgX₂P₂ + HgX₂P₂ ⇌ 2HgXX′P₂ is in excellent agreement with NMR observations. However, electrochemical data reveal that phosphine-exchange reactions are substantially different in nature from the halide-exchange reactions. The combined use of electrochemical and NMR measurements readily enables several features of the different halide- and phosphine-exchange reactions to be unraveled.