Oxo Complexes of Tungstenocene via Oxidation of [W(.eta.5-C5H5)2(OCH3)(CH3)] and Related Reactions: Synthesis, Structural Characterization, and Photodisproportionation of the Spin-Paired d1-d1 Oxo-Bridged Dimer [{W(.eta.5-C5H5)2(CH3)}2(.mu.-O)]2+ and Synthesis and Characterization of the d0 Terminal Oxo Complex [W(.eta.5-C5H5)2(O)(CH3)]+