HALIDE-BRIDGED ARSINE-CAPPED AND PHOSPHINE-CAPPED DIRUTHENIUM COMPLEXES, AS)(3)RU(MU-X)(3)RU(ASR3)(3)](-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() AND [(R3P)(3)RU(MU)

A series of six tertiary-arsine-capped binuclear complexes, [L3Ru(mu-X )(3)RuL3][CF3SO3] (L = AsMe3, AsMe2Ph or AsMePh2; X = Cl or Br) togeth er with a full range of purely PR3-capped analogues and the mixed liga nd complex )(Me3As)(2)Ru(mu-Cl)(3)Ru(AsMe3)(2)(PPh3)][CF3SO3] have bee n characterised. The previously neglected arsine-capped compounds shar e the well defined electrochemical behaviour of their phosphine congen ers. Stepwise reversible oxidations connect the Ru-2(II,II) closed-she ll 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+) spe cies can be characterised through electrogeneration in CH2Cl2 at -60 d egrees C. Unexpectedly the Ru-2(II,III) arsine complexes strongly rese mble the classical ruthenium 'blues' where L = NH3 or H2O. For such va lence-delocalised systems the visible region ordinarily contains an in tense sigma --> sigma band (the source of the intense blue colour) to gether with a much weaker, near-infrared delta pi --> sigma* band. Bo nding within the {RuX3Ru}(2+) core can then be monitored directly by v (sigma --> sigma). The distinctly different spectral appearance of th e more familiar PR3-capped mixed-valence compounds has been a long-sta nding puzzle, but the twenty electrogenerated 11-e binuclear systems a ssembled here with various AsR3 or PR3 terminal ligands are all deloca lised, and clearly belong within a continuum of electronic behaviour w ith steadily decreasing metal-metal interaction. In all, v(sigma --> s igma) 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 > PR 3 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 potentia ls which shrinks from 1.2 to 0.45 V. For the PR3 complexes the decreas e in v(sigma --> sigma) is accompanied by progressive intensity trans fer to the delta(pi --> sigma)* band. The anticipated Ru ... Ru separ ation is of the order of 2.9 and 3.0 Angstrom for the mixed-valence As Me3/mu-Cl and PMe3/mu-Cl systems respectively, markedly longer than th e crystallographic value of 2.75 Angstrom in [(NH3)(3)Ru(mu-Cl)(3)Ru(N H3)(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 sho w still greater visibre/near-infrared spectral deviations.