SYNTHESIS AND ELECTRONIC-PROPERTIES OF TRIPLY BONDED HEXAKIS(FLUOROALKOXY)DIMOLYBDENUM COMPLEXES - STRUCTURE OF MO2[OCME(CF3)2]6 AND INVESTIGATION OF THE NATURE OF THE FRONTIER ORBITALS IN TRIPLY BONDED M2X6 COMPOUNDS

The compound "MoCl3(dme)" (dme = 1,2-dimethoxyethane) was prepared in pure form and formulated as the dimeric Mo2Cl6(dme)2 (1). This material proved an excellent starting material for the metathetical syntheses of metal-metal triply bonded Mo2X6 complexes, including the hexaalkyls Mo2(CH2MMe3)6 (M = C (6), Si (7)), the hexaalkoxides Mo2(OCMe2CH2R) (R = H (2), Me (3)), and the new hexakis(fluoroalkoxide) dimers Mo2[OCMe3-x(CF3)x]6 (x = 1 (4), 2 (5)). The structure of 5 was determined by single-crystal X-ray diffraction: T = 253 K, R3BAR (No. 148), a = 10.894 (8) angstrom, c = 29.255 (9) angstrom, V = 3006.8 angstrom3, R = 0.071, R(w) = 0.094, GOF = 0.50 for 747 observed reflections varying 113 parameters. The molecule exhibits disorder, some of which could be resolved, and typical Mo=Mo (2.230 (3) angstrom) and Mo-O (1.88 (1) angstrom) bond lengths. The former observation is consistent with theoretical and experimental studies indicating that the triple bond is exclusively comprised of metal-based orbitals giving rise to a limiting sigma-2-pi-4 description; the latter observation is somewhat at odds with previous suggestions that decreasing the pi-donor ability of the alkoxide ligands in these systems increases the metal-oxygen distance. The electronic absorption spectra of the congeneric 2, 4, and 5 are essentially equivalent; this surprising result suggests that either the HOMO and LUMO are largely comprised of metal-based orbitals and contain little metal-ligand delta-character or that these orbitals contain the same amount of ligand character and are equally affected by changes in the electronic properties of the ligands.