N6 ring as a planar hexagonal ligand in novel M(η6-N6) species

The possibility of stabilizing the N6 species as a planar hexagonal ring in M(η6-N6) (M=Ti, Zr, Hf, Th) systems was studied theoretically. The M(η6-N6) systems were found to have minima at C6v geometry, which corresponds to an η6 complex of a metal with planar N6 ring. They lie approximately 70–130 kcal/mol above the energy of a metal atom and three N2 molecules and substantially below the energy of any naked N6 isomer. The stabilization of the planar N6 is attributed to the interaction of the ring π system with the metal d and f orbitals. A five-fold bond is observed between the N6 ring and a metal. At the ionic limit, the systems can be regarded as complexes of an M4+ cation and an N64− ligand. Further planar aromatic Nnq (n=3–8) rings are briefly discussed. Of them, the π-electron-rich anions could be a viable ηn ligands in transition metal complexes.