Synthesis and Characterization of the Stable Cationic d2 Metal Acetylene Complexes [Nb(.eta.5-C5H4SiMe3)2(.eta.2(C,C)-RC.tplbond.CR')(NCMe)]+. X-ray Crystal Structures of [Nb(.eta.5-C5H4SiMe3)2(.eta.2(C,C)-MeO2CC.tplbond.CMe)(NCMe)][BPh4] and [(.eta.5-C5H4SiMe3)2(CO)Nb:C:C(CH3)(CH3)C:C:Nb(CO)(.eta.5