Niobium isocyanide complexes, Nb(CNAr)6, with Ar = 2,6-dimethylphenyl (Xyl), a diamagnetic dimer containing four reductively coupled isocyanides, and Ar = 2,6-diisopropylphenyl (Dipp), a paramagnetic monomer analogous to the highly unstable hexacarbonylniobium(0).

Treatment of bis(mesitylene)niobium(0) with 6-7 equivalents of 2,6-dimethylphenyl isocyanide (CNXyl) affords two products with the empirical formula Nb(CNXyl)n (n = 7 or 6), which have been shown to be the diamagnetic dimers bis[μ-N,N',N'',N'''-tetrakis(2,6-dimethylphenyl)squaramidinato(2-)]bis[pentakis(2,6-dimethylphenyl isocyanide)niobium(I)], [Nb2(C9H9N)10(C36H36N4)] or [Nb(CNXyl)5]2[μ-C4(NXyl)4]·xSolvent, 1, and bis[μ-N,N',N'',N'''-tetrakis(2,6-dimethylphenyl)squaramidinato(2-)]bis[tetrakis(2,6-dimethylphenyl isocyanide)niobium(I)] tetrahydrofuran trisolvate, [Nb2(C9H9N)8(C36H36N4)]·3C4H8O or [Nb(CNXyl)4]2[μ-C4(NXyl)4]·3THF (THF = tetrahydrofuran), 2. Each contains NbI bound to either five or four terminal isocyanides, respectively, and to an unprecedented bridging tetraarylsquaramidinate(2-) unit, coordinated as a bidentate ligand to each niobium center, symmetrically due to the crystallographic inversion center that coincides with the centroid of the central C4 unit. Thus, in the presence of CNXyl, the bis(mesitylene)niobium(0) is oxidized to niobium(I), resulting in the facile loss of both mesitylene groups and the reductive coupling of two CNXyl groups per niobium to provide the first examples of tetraarylsquaramidinate(2-) ligands, [cyclo-C4N4Ar4]2-, coordinated to metals. In contrast, bis(mesitylene)niobium(0) reacts with the more crowded 2,6-diisopropylphenyl isocyanide (CNDipp) to afford the paramagnetic monomer hexakis(2,6-diisopropylphenyl isocyanide)niobium(0), [Nb(C13H17N)6] or Nb(CNDipp)6, 3, the first zero-valent niobium isocyanide analog of the highly unstable Nb(CO)6, which is presently only known to exist in an argon matrix at 4.2 K.