The niobium-mercury compounds [Cp′2Nb(L)]2Hg (Cp′ = η5-C5H4SiMe3, L = CO (4), PMe3 (5), or CNtBu (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.