Halide-bridged arsine- and phosphine-capped diruthenium complexes, [(R3As)(3)Ru(mu-X)(3)Ru(AsR3)(3)](+) and [(R3P)(3)Ru(mu-X)(3)Ru(PR3)(3)](+) (X = Cl or Br), as precursors to confacial mixed-valence ruthenium 'blues': spectroelectrochemical studies spanning the binuclear oxidation states II,II, II,III and III,III

A series of six tertiary-arsine-capped binuclear complexes, [L3Ru(mu-X)(3)RuL3][CF3SO3] (L = AsMe3, AsMe2Ph or AsMePh2; X = Cl or Br) together with a full range of purely PR3-capped analogues and the mixed ligand complex [(Ph3P)(Me3As)(2)Ru(mu-Cl)(3)Ru(AsMe3)(2)(PPh3)][CF3SO3] have been characterised. The previously neglected arsine-capped compounds share the well defined electrochemical behaviour of their phosphine congeners. Stepwise reversible oxidations connect the Ru-2(II,II) closed-shell d(6)d(6) (=12-e) resting state with the d(5)d(6) (11-e) and d(5)d(5) (10-e) levels, and all the mixed-valence [L3Ru(mu-X)(3)RuL3](2+) species can be characterised through electrogeneration in CH2Cl2 at -60 degrees C. Unexpectedly the Ru-2(II,III) arsine complexes strongly resemble the classical ruthenium 'blues' where L = NH3 or H2O. For such valence-delocalised systems the visible region ordinarily contains an intense sigma --> sigma* band (the source of the intense blue colour) together with a much weaker, near-infrared delta pi* --> sigma* band. Bonding within the {RuX3Ru}(2+) core can then be monitored directly by v(sigma --> sigma)*. The distinctly different spectral appearance of the more familiar PR3-capped mixed-valence compounds has been a long-standing puzzle, but the twenty electrogenerated 11-e binuclear systems assembled here with various AsR3 or PR3 terminal ligands are all delocalised, and clearly belong within a continuum of electronic behaviour with steadily decreasing metal-metal interaction. In all, v(sigma --> sigma)* declines over a considerable range from 17 000 to below 5000 cm(-1) with the ligands ranked as follows: L = NH3 (and 1,4,7-trimethyl-1,4,7-triazacycrononane) > H2O > Cl, Pr (i e. nonahalides) > AsR3 > PR3 and mu-Cl > mu-Br. These changes are well correlated with systematic trends in the g(II) and g(perpendicular to) components of the axial g tensor, and also with the gap between the stepwise oxidation potentials which shrinks from 1.2 to 0.45 V. For the PR3 complexes the decrease in v(sigma --> sigma)* is accompanied by progressive intensity transfer to the delta*(pi --> sigma)* band. The anticipated Ru ... Ru separation is of the order of 2.9 and 3.0 Angstrom for the mixed-valence AsMe3/mu-Cl and PMe3/mu-Cl systems respectively, markedly longer than the crystallographic value of 2.75 Angstrom in [(NH3)(3)Ru(mu-Cl)(3)Ru(NH3)(3)](2+). The geometric distinction between the AsR3-and PR3-capped dimers is an unexpected consequence of selective crowding between the substituent R groups and the {mu-X-3} array. The present Ru-2(II,III) systems are electronically distinct from their PR3-containing osmium counterparts, such as [(Et3P)(3)Os(mu-Cl)(3)Os(PEt)(3)](2+), which show still greater visibre/near-infrared spectral deviations.