Mercury dications: linear form is more stable than aromatic ring.

Analysis of the molecular orbitals and the nucleus independent chemical shift showed that the D(4h) and D(6h) symmetry Hg(4)(2+) and Hg(6)(2+) rings are aromatic. However, accurate quantum chemical methods indicated that the linear forms are more than 100 kJ mol(-1) lower in energy. This surprising case, where the non-aromatic species are considerably more stable than the aromatic rings, was explained by the charge distribution and the ring strain. Electronic structures of these species were consistently described using the phenomenological shell model of metal clusters.