Metallocenes of Nb and Ta with eta(2)-Se2H, SeH, and Se ligands - Crystal structures of [Cp ' Ta-2(SeH)H-2] and [Cp ' Ta-2(=Se)H center dot W(CO)(5)](Cp '=tBuC(5)H(4))

The reactions of [(Cp2NbBH4)-Nb-#] (1) [Cp-# = C5Me5 (Cp*) or C5Me4Et] and [Cp'2TaH3] (Cp' = tBuC(5)H(4)) with elemental selenium give complexes of th e general composition [Cp2MSe2H]. These exist in three isomeric forms, A-C, depending on the metal and the chalcogen. [(Cp2NbSe2H)-Nb-#] (2) incorpora tes the novel eta(2)-Se2H ligand (C), which is labile when exposed to light . Solutions of 2(Cp') can be converted into [Cp*2NbSe2H] (3), which exists in two isomeric forms A or B, along with [Cp*2NbSe3H] (4) and [Cp*2NbO2H] ( 5). [Cp'Ta-2(eta(2)-Se-2)H] (6) is the only isomer found for the Ta system. Se abstraction by means of P(OEt)(3) gives [Cp'Ta-2(= Se)H] (7). Complex 7 can be reduced with Na/Hg and subsequently hydrolysed to give [Cp'Ta-2(SeH )H-2] (8), which has also been observed during the formation of 6. Coordina tion of [W(CO)(5)THF] at the Se lone pair of 7 affords the heterobimetallic complex 9. All compounds have been characterized by means of 1H- and, in s ome cases, Se-77-NMR spectroscopy. Products 8 and 9 have also been subjecte d to X-ray diffraction analysis.