Perfluoroterephthalate Bridged Complexes with M−M Quadruple Bonds:  (<i><sup>t</sup></i><sup></sup>BuCO<sub>2</sub>)<sub>3</sub>M<sub>2</sub>(μ-O<sub>2</sub>CC<sub>6</sub>F<sub>4</sub>CO<sub>2</sub>)M<sub>2</sub>(O<sub>2</sub>C<i><sup>t</sup></i><sup></sup>Bu)<sub>3</sub>, where M = Mo or W. Studies of Solid-State, Molecular, and Electronic Structure and Correlations with Electronic and Raman Spectral Data BurstenBruce E. ChisholmMalcolm H. J. H. ClarkRobin FirthSteven HadadChristopher M. WilsonPaul J. WoodwardPatrick M. ZaleskiJeffrey M. 2002 The compounds [(<i><sup>t</sup></i><sup></sup>BuCO<sub>2</sub>)<sub>3</sub>M<sub>2</sub>(μ-O<sub>2</sub>CC<sub>6</sub>F<sub>4</sub>CO<sub>2</sub>)M<sub>2</sub>(O<sub>2</sub>C<i><sup>t</sup></i><sup></sup>Bu)<sub>3</sub>], M<sub>4</sub>PFT, where M = Mo or W, are shown by model fitting of the powder X-ray diffraction data to have an infinite “twisted” structure involving M···O intermolecular interactions in the solid state. The dihedral angle between the M<sub>2</sub> units of each molecule is 54°. Electronic structure calculations employing density functional theory (Gaussian 98 and ADF2000.01, gradient corrected and time dependent) on the model compounds (HCO<sub>2</sub>)<sub>3</sub>M<sub>2</sub>(μ-O<sub>2</sub>CC<sub>6</sub>F<sub>4</sub>CO<sub>2</sub>)M<sub>2</sub>(O<sub>2</sub>CH)<sub>3</sub>, where M = Mo or W, reveal that in the gas phase the model compounds adopt planar <i>D</i><sub>2</sub><i><sub>h</sub></i> ground-state structures wherein M<sub>2</sub> δ to bridge π* back-bonding is maximized. The calculations predict relatively small HOMO−LUMO gaps of 1.53 eV for M = Mo and 1.22 eV for M = W for this planar structure and that, when the “conjugation” is removed by rotation of the plane of the C<sub>6</sub>F<sub>4</sub> ring to become orthogonal to the M<sub>4</sub> plane, this energy gap is nearly doubled to 2.57 eV for M = Mo and 2.18 eV for M = W. The Raman and resonance Raman spectra of solid M<sub>4</sub>PFT and of Mo<sub>4</sub>PFT in THF solution are dominated by bands assigned to the bridging perfluoroterephthalate (pft) group. The intensities of certain Raman bands of solid W<sub>4</sub>PFT are strongly enhanced on changing the excitation line from 476.5 nm (off resonance) to 676.5 nm, which is on resonance with the W<sub>2</sub> δ → CO<sub>2</sub> (pft) π* transition at ca. 650 nm. The resonance enhanced bands are δ<sub>s</sub>(CO<sub>2</sub>) (pft) at 518 cm<sup>-1</sup> and its first overtone at 1035 cm<sup>-1</sup>, consistent with the structural change to W<sub>4</sub>PFT expected on excitation from the ground to this π* excited state. The electronic transitions for solid Mo<sub>4</sub>PFT (lowest at 410 nm) were not accessible with the available excitation lines (457.9−676.5 nm), and no resonance Raman spectra of this compound could be obtained. For Mo<sub>4</sub>PFT in THF solution, it is the band at 399 cm<sup>-1</sup> assigned to ν(MoMo) which is the most enhanced on approach to resonance with the electronic band at 470 nm; combination bands involving the C<sub>6</sub>F<sub>4</sub> ring-stretching mode, 8a, are also enhanced.