D-orbital aromaticity is found in trinuclear tungsten W3O9- and molybdenum Mo3O9- metal clusters generated by laser vaporization of the pure metals in the presence of oxygen in the helium stream . In these clusters the three metal centers are bridged by oxygen and each metal has two terminal oxygen atoms. The first signal in the photoelectron spectrum corresponds to the removal of the valence electron with the lowest energy in the anion to the neutral M3O9 compound. This energy turns out to be comparable to that of bulk tungsten trioxide and molybdenum trioxide. The photoelectron signal is also broad which suggests a large difference in conformation between the anion and the neutral species. Computational chemistry shows that the M3O9- anions and M3O92- dianions are ideal hexagons with identical metal to metal bond lengths.
The molecules discussed thus far only exist diluted in the gas phase. A study exploring the properties of a compound formed in water from sodium molybdate (Na2MoO4.2H2O) and iminodiacetic acid also revealed evidence of aromaticity but this compound is actually isolated. X-ray crystallography showed that the sodium atoms are arranged in layers of hexagonal clusters akin pentacenes. The sodium to sodium bond lengths are unusually short (327 pm versus 380 pm in elemental sodium) and like benzene the ring is planar. In this compound each sodium atom has a distorted octahedral molecular geometry with coordination to molybdenum and water oxygen atoms . The experimental evidence is supported by computed NICS aromaticity values.