Despite their close structural similarity, the potassium and lithium salts of dipyridylamine and diphenylamine[(C 5 H 5 N) 2 N -,Ph 2 N -]reactwith(TMEDA) 2 -Nb 2 Cl 5 Li(TMEDA) in THF to afford two different compounds arising from the cooperative oxidative addition of the two metal centers on either the C-H or C-N bond of the amide ligand.

Reaction of the diamagnetic cluster (TMEDA)2Nb2Cl5Li(TMEDA) with the (3,5-Me2Ph)(Ad)NLi (Ad = adamantyl) amide led to a rare case of C−N bond activation and formation of tetravalent, dinuclear, and diamagnetic {(3,5-Me2Ph)(Ad)N]Nb(3,5-Me2Ph)}2(μ-NAd)2·ether (1) containing a Nb−Nb single bond. Conversely, a similar reaction carried out with [Cy2NLi]4 (Cy = cyclohexyl) yielded dinuclear {[(Cy2N)2Nb]2(μ-N)}(μ3-N)Li(TMEDA) (2).

In spite of the short Nb-Nb distance (2.268 Å) and the presumable existence of an Nb identical withNb bond, the paddle-wheel-shaped diniobium(II) complex 1 is paramagnetic. Theoretical calculations indicate that the presence of LiCl moieties on the intermetallic axis lowers the Nb-Nb bond order and is responsible for the observed paramagnetism.

Reaction of the dinuclear and divalent (TMEDA)2Nb2Cl5Li(TMEDA) with Ph2NK led to the formation of three different compounds. The first complex contains no amide ligands. It is a mixed-valence dinuclear and paramagnetic species with a typical face-sharing bioctahedral structure Cl2Nb2(μ-Cl)3(TMEDA)2 (1). The second complex, a trivalent and dinuclear {[(Ph2N)2Nb]2[μ-NPh(μ-η1:η2-C6H4)](μ-H)}{Li(TMEDA)2}·toluene (2) with one hydride and one cyclometalated aromatic ring, is the result of oxidative addition of the two metal centers to the C−H bond. In a subsequent reaction step, the oxidative addition is followed by reductive elimination to restore the C−H bond to form the neutral, dinuclear, and diamagnetic (Ph2N)2Nb{[μ-NPh(η6-C6H5)]2Nb}·ether (3) containing the two metals in formal oxidation states of +4 and 0. Ab initio and EHMO calculations have been carried out in an attempt to clarify the nature of the Nb−Nb interaction in the three complexes.

A nitrogen center was abstracted from a pyrrolyl ring to form the dinuclear nitrido- and dienyl-bridged complex 1 during the reaction of [{(tmeda)Nb Cl} (μ-Cl) Li(tmeda)] with the lithium salt of 2,5-dimethylpyrrole (tmeda=N,N,N',N'-tetramethylethylenediamine). A second product from this reaction is the amido-carbene-hydride niobium complex 2, which likewise forms under C-N bond cleavage.