A study of high-oxidation-state complexes of the type [W(.eta.5-C5Me5)Me2X]2(.mu.-N2), including x-ray structures of [W(.eta.5-C5Me5)Me2(OC6F5)]2(.mu.-N2) and [W(.eta.5-C5Me5)Me2(S-2,4,6-C6H2Me3)]2(.mu.-N2)