Preparation and reaction chemistry of trimethylsilyl derivatives of niobium. Redox chemistry of (.eta.5-C5H5)2Nb(SiMe3)Cl and x-ray structures of (.eta.5-C5H5)2Nb(SiMe3)(.eta.2-C2H4) and [(.eta.5-C5H5)2Nb(CH2SiMe3)Cl]PF6