STEPWISE LIGAND-ADDITIVITY EFFECTS ON ELECTRODE-POTENTIALS AND CHARGE-TRANSFER SPECTRA IN HEXAHALIDE, MIXED HALIDE NITRILE, AND HEXAKIS(NITRILE) COMPLEXES OF RUTHENIUM(IV), RUTHENIUM(III), AND RUTHENIUM(II)

This paper examines complete series of substituted ruthenium halide complexes ranging stepwise from [RuX6]2- through [RuX6-n(RCN)n]z to [Ru(RCN)6]2+ in order to define the variation in electrode potentials (Ru(V/IV), Ru(IV/III), and Ru(III/II)) and the accompanying trends in both halide-to-metal and metal-to-nitrile charge-transfer spectra. Three series are described: X = Cl-, RCN = PhCN; X = Cl-, RCN = MeCN; and X = Br-, RCN = PhCN. Voltammetric studies show that the metal-based electrode potentials are a linear function of stoichiometry: the effect on E1/2 of replacing each halide by nitrile is consistently +0.6 V for Ru(III/II) and Ru(IV/III) and +0.45 V for Ru(V/IV). For [MX6-n(RCN)n]z complexes in general, the same dependence of electrode potentials on stoichiometry is shared by congeneric 4d(n) and 5d(n) metal ions such as Ru and Os and by isovalent metal ions as diverse as Ru(IV) and Nb(IV). These measurements consolidate and extend the application of additive ligand electrochemical parameters. For the [MCl6]2- complexes of 4d and 5d transition metals, with M(IV) ranging from Mo to Rh and from W to Ir, the halide-to-metal charge-transfer energies are shown to be linearly dependent on the appropriate electrode potentials, E1/2(M(IV/III). For each complex in the [RuX6-n(RCN)n]z series, the charge-transfer spectra have been recorded in all accessible oxidation states (Ru(IV), Ru(III), and Ru(II)) by spectroelectrochemical measurements. The progressions in LMCT and MLCT band positions with stepwise substitution are complementary to the trend in the appropriate metal-based electrode potentials. Compilation of the electrochemical and spectroscopic data yields a conceptual representation of the relative frontier-orbital energies of ruthenium, halide, and nitrile as a function of stoichiometry.