Metallocenes of Nb and Ta with η 2 ‐Se 2 H, SeH, and Se Ligands – Crystal Structures of [Cp′ 2 Ta(SeH)H 2 ] and [Cp′ 2 Ta(=Se)H · W(CO) 5 ] (Cp′ = t BuC 5 H 4 )

The reactions of [Cp # 2 NbBH 4 ] ( 1 ) [Cp # = C 5 Me 5 (Cp*) or C 5 Me 4 Et] and [Cp′ 2 TaH 3 ] (Cp′ = t BuC 5 H 4 ) with elemental selenium give complexes of the general composition [Cp 2 MSe 2 H]. These exist in three isomeric forms, A–C , depending on the metal and the chalcogen. [Cp # 2 NbSe 2 H] ( 2 ) incorporates the novel η 2 ‐Se 2 H ligand ( C ), which is labile when exposed to light. Solutions of 2 (Cp*) can be converted into [Cp* 2 NbSe 2 H] ( 3 ), which exists in two isomeric forms A or B , along with [Cp* 2 NbSe 3 H] ( 4 ) and [Cp* 2 NbO 2 H] ( 5 ). [Cp′ 2 Ta(η 2 ‐Se 2 )H] ( 6 ) is the only isomer found for the Ta system. Se abstraction by means of P(OEt) 3 gives [Cp′ 2 Ta(=Se)H] ( 7 ). Complex 7 can be reduced with Na/Hg and subsequently hydrolysed to give [Cp′ 2 Ta(SeH)H 2 ] ( 8 ), which has also been observed during the formation of 6 . Coordination 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 1 H‐ and, in some cases, 77 Se‐NMR spectroscopy. Products 8 and 9 have also been subjected to X‐ray diffraction analysis.