Niobium-mercury heterometallic compounds as sources of niobium(II): Radical paths to organoniobium species

The niobium-mercury compounds [Cp′₂Nb(L)]₂Hg (Cp′ = η⁵-C₅H₄SiMe₃, L = CO (4), PMe₃ (5), or CN^tBu (6)) serve as stable precursors to the short-lived Nb(II) radicals of general formula [Cp′₂Nb-L]. The homolysis of the Nb-Hg bond may result from a slow thermal reaction (generating low concentrations of radicals) or from a photochemical process in which mercury extrusion is more rapid. Since the Nb(II) species show no evidence for Nb-Nb bond formation, they are useful in the synthesis of a variety of Nb(III) and Nb(IV) species. Reactions of 4 with dimeric species such as [CpFe(CO)₂]₂, [CpNi(CO)]₂, Co₂(CO)₈, or RSe-SeR give rise to new Nb-M or Nb-Se compounds, while reactions with potential π donors such as formaldehyde or azobenzene lead to displacement of the ligand L and formation of the Nb(IV) complexes. Crystallographic and variable-temperature NMR studies on the Nb-Fe compound Cp′₂Nb(μ-CO)₂Fe(CO)Cp (10) are consistent with a low-energy fluxional process involving exchange of bridging and terminal carbonyl ligands.