Chalcogenide-bridged ditungsten (M-M) complexes: an experimental and theoretical study of the electronic structure and bonding in W-2(mu-E)(mu-(OCH2Bu)-Bu-t)(2)((OCH2Bu)-Bu-t)(6), where E = O, S, Se or Te

The series of compounds W-2(mu -E)(mu -(OCH2Bu)-Bu-t)(2)((OCH2Bu)-Bu-t)(6), where E=O, S, Se, or Te, has been isolated from the reactions between [W-2 ((OCH2Bu)-Bu-t)(8)](n) and pyridine N-oxide, elemental sulfur, elemental se lenium and tri-n-butylphosphine telluride. The reactions were carried out i n hexane or toluene solutions, and the new compounds were isolated as cryst alline products by recrystallization from hexanes at low temperatures (E=O, S, Se) or from hot toluene (E=Te). Each compound displays temperature-depe ndent H-1 NMR spectra indicative of bridge reversible arrow terminal alkoxi de group exchange which is frozen out at ca. -65 degreesC. The low-temperat ure spectra are consistent with the observed molecular structures in the so lidstate as determined by single-crystal X-ray studies. There is a central O3M(mu -O)(2)(mu -E)MO3 skeleton having virtual C-2 nu symmetry. The W-W in ternuclear distances range from 2.55 to 2.66 Angstrom and gradually increas e along the series O -->S --> Se --> Te. The compounds are colored (E=O: vi olet; E=S: blue; E=Se: blue-green; and E=Te: dark yellow/amber) as a result of absorptions in the visible region of the spectrum. The electronic struc tures of these compounds have been investigated using density functional th eory (DFT) by examining the model compounds W-2(mu -E)(mu -OCH3)(2)(OCH3)(6 ) (E=O, S, Se, or Te). These B3LYP(SDD,6-31G*) calculations reveal that the HOMO is a M-M sigma -type molecular orbital,and with increasing mass of E, the orbital energies of the Ep(pi) lone-pair orbitals approach this level. A tentative assignment of the electronic absorption spectra is made on the basis of time-dependent DFT (TDDFT) calculations.