Synthesis and characterization of [{W(eta(5)-C5H5)(2)(CH2CH3)}(mu-O){W(eta(5)-C5H5)(O)(CH2CH3)}]PF6, a mixed valence oxo alkyl dimer in which d(0) and d(2) tungsten centers are connected by an asymmetric oxo bridge

Reaction of the ethylene hydride complex [W(eta(5)-C5H5)(2)(CH2 = CH2)H]PF6 (4PF(6)) with mild oxygen atom sources such as dimethyl sulfoxide or propylene oxide gives the terminal oxo ethyl complex [W(eta(5)-C5H5)(2)(O)(CH2CH3)]PF6 (3PF(6)). Reaction of this with PMe(2)Ph gives a strongly colored compound which crystallizes in the monoclinic space group C2/c with a = 19.483(4) Angstrom, b = 22.397(6) Angstrom, c = 14.289(3) Angstrom, beta = 133.59(1)degrees. A single crystal X-ray diffraction study (R(F) = 5.75%, R(w) = 5.31%) has established that the compound contains the cationic dimer [{W(eta(5)-C5H5)(2)(CH2CH3)}(mu-O){W(eta(5)-C5H5)(O)(CH2CH3)}](+) (1(+)). Metric parameters support formulation of If as a mixed valence dimer of W(IV) and W(VI) in which a mono-cyclopentadienyl tungsten oxo alkyl [W(eta(5)-C5H5)(O)(2)(CH2CH3)] is coordinated through an oxo ligand to a cationic tungstenocene alkyl ''[W(eta(5)-C5H5)(2)(CH2CH3)](+)'' The oxo bridge is asymmetric, with a double bond (1.78(1) Angstrom) to the mono-cyclopentadienyl tungsten center and a single bond (2.03(1) Angstrom) to the bis-cyclopentadienyl tungsten, while the terminal oxo ligand has a triple bond to tungsten (1.68(1) Angstrom). The strong absorbance of 1(+) at 583 nm is assigned to a metal to metal charge transfer transition. Formation of 1(+) is proposed to involve initial nucleophilic addition of PMe(2)Ph to a cyclopentadienyl ligand in 3(+) to give a transient cyclopentadiene ligand which is displaced by the oxo ligand of a second equivalent of 3(+). This sequence is supported by H-1 acid P-31 NMR studies which establish that there is a single P containing product formulated as the cyclopentadiene-substituted phosphonium salt [P(C5H5)Me(2)Ph](+) (5(+)), as confirmed by independent synthesis and spectroscopic characterization of 5(+) by addition of C5H5I to PMe(2)Ph. Photolysis of 1(+) results in dissociation of the monocyclopentadienyl oxo alkyl fragment [W(eta(5)-C5H5)(O)(2)(CH2CH3)](2), which has been independently characterized, and formation of the alkene hydride [W(eta(5)-C5H5)(2)(CH2 = CH2)H](+).