The possibility of stabilizing the N-6 species as a planar hexagonal ring in M(eta(6)-N-6) (M = Ti, Zr, Hf, Th) systems was studied theoretically. The M(eta(6)-N-6) systems were found to have minima at C-60 geometry, which corresponds to an eta(6) complex of a metal with planar N-6 ring, They lie approximately 70-130 kcal/mol above the energy of a metal atom and three N-2 molecules and substantially below the energy of any naked N-6 isomer. The stabilization of the planar N-6 is attributed to the interaction of the ring pi system with the metal d and f orbitals. A five-fold bond is observed between the N-6 ring and a metal. At the ionic limit, the systems can be regarded as complexes of an M4+ cation and an N-6(4-) ligand. Further planar aromatic N-n(q) (n = 3-8) rings are briefly discussed. Of them, the pi-electron-rich anions could be a viable eta(n) ligands in transition metal complexes. (C) 2002 Elsevier Science B.V. All rights reserved.