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(O2CtBu)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
Kc, determined from electrochemical methods, is not necessarily a measure of the extent of electron
delocalization.