PNNL

Abstract

Aromatic molecules are usually planar species formed by main group elements, whose bonding is primarily from the s or p orbitals. Here we report experimental and theoretical evidence for aromaticity from d-orbitals. Two metal oxide clusters, W3O9- and Mo3O9-, are studied using photoelectron spectroscopy and theoretical calculations, which show that W3O9 and Mo3O9 both possess a D3h structure with a low-lying unoccupied molecular orbital formed from the metal d orbitals. Occupation of this orbital by one or two electrons gives rise to strong three-center one-electron and three-center two-electron metal-metal bonds, respectively, significantly reducing the M-M distances in M3O9- and M3O92-. The totally delocalized three-center bond renders aromaticity for both M3O9- and M3O92-, further evidenced by a large calculated resonance energy. The M3O9- and M3O92- species are highly stable and represent a new class of d-orbital aromatic molecules, which may be synthesized in the condensed phase.