Structure,spectral properties and interconversions of bis-niobocenes and their dianions. The crystal and molecular structure of [(η5-C5H4SiMe2OSiMe2-η5 : η1-C5H3)2Nb2H2] [Na(OEt2)2]2

Electron-acceptor properties of bis-niobocenes (η⁵-C₅H₄Y)(H)$\text{Nb(η}{}^5\text{: η}{}^1\text{-C}{}_5\text{H}{}_3\text{X)}{}_2\text{N}$b(H)(η⁵-C₅H₄Y) with X  Y  H and XY  SiMe₂OSiMe₂have been investigated. Bis-niobocenes are shown to readily add two electrons forming stable salts of the corresponding dianions (η⁵-C₅H₄Y)(H)Nb(η⁵ : η¹- C₅H₃X)₂Nb(H)(η⁵-C₅H₄Y)]^2-. The surplus electrons can be removed to give quantitative regeneration of initial neutral bis-niobocenes. The crystal and molecular structure of the title compound has been determined; R  0.044, interatomic distance are Nb…Nb 3.93, NbH 1.62, average NbC(π) 2.36, NbC(σ) 2.31 Å, other distances correspond to the usually observed values. Unlike the neutral bis-niobocenes, there is no direct metalmetal bond in the dianionic structures. This conclusion is supported by electronic spectra of neutral and dianionic species.