Electronically-Coupled Tungsten−Tungsten Quadruple Bonds:  Comparisons of Electron Delocalization in 3,6-Dioxypyridazine and Oxalate-Bridged Compounds

The preparation of the 3,6-dioxypyridazine-bridged tungsten complex, [W₂(O₂C^tBu)₃]₂(μ-H₂C₄N₂O₂), I, is described, along with its single-electron oxidized cation, I⁺, formed in the reaction between I and Ag⁺PF₆^-. Compound I has been structurally characterized as a PPh₃ adduct, and I⁺PF₆^- as a THF solvate, by single-crystal X-ray studies. The geometric parameters of these compounds compare well with those calculated for the model compounds [W₂(O₂CH)₃]₂(μ-H₂C₄N₂O₂) and [W₂(O₂CH)₃]₂(μ-H₂C₄N₂O₂)⁺ by density functional theory employing the Gaussian 98 and 03 suite of programs. The calculations indicate that the two W₂ centers are strongly coupled by M₂ δ-to-bridge π-bonding, and further coupled by direct M₂···M₂ bonding. Compound I is purple and shows an intense absorption in the visible region due to a metal-to-bridge charge transfer and, with excitation within this absorption, compound I exhibits pronounced resonance Raman bands associated with symmetric vibrations of the bridge and the M₄ unit. The cyclic voltammogram of I in THF, the EPR spectrum of I⁺PF₆ in 2-MeTHF and the electronic absorption spectrum of I⁺PF₆^- in THF are consistent with electron delocalization over both W₂ units. These new data are compared with previous data for the molybdenum analogue, related oxalate-bridged compounds and closely related cyclic polyamidato-bridged Mo₄-containing compounds. It is proposed that, while the electronic coupling occurs principally by an electron-hopping mechanism for oxalate-bridged compounds, hole-hopping contributes significantly in the cases of the amidate bridges and that this is more important for M = Mo than for M = W. Furthermore, for Class III fully delocalized mixed-valence compounds, the magnitude of K_c, determined from electrochemical methods, is not necessarily a measure of the extent of electron delocalization.