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, [W(2)(O(2)C(t)Bu)(3)](2)(mu-H(2)C(4)N(2)O(2)), I, is described, along with its single-electron oxidized cation, I(+), formed in the reaction between I and Ag(+)PF(6)(-). Compound I has been structurally characterized as a PPh(3) adduct, and I(+)PF(6)(-) 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 [W(2)(O(2)CH)(3)](2)(mu-H(2)C(4)N(2)O(2)) and [W(2)(O(2)CH)(3)](2)(micro-H(2)C(4)N(2)O(2))(+) by density functional theory employing the Gaussian 98 and 03 suite of programs. The calculations indicate that the two W(2) centers are strongly coupled by M(2) delta-to-bridge pi-bonding, and further coupled by direct M(2)...M(2) 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 M(4) unit. The cyclic voltammogram of I in THF, the EPR spectrum of I(+)PF(6) in 2-MeTHF and the electronic absorption spectrum of I(+)PF(6)(-) in THF are consistent with electron delocalization over both W(2) units. These new data are compared with previous data for the molybdenum analogue, related oxalate-bridged compounds and closely related cyclic polyamidato-bridged Mo(4)-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.