Metal aromaticity

HOMO is calculated to be a doubly occupied delocalized pi system making it obey Hückel's rule. Finally a match exists between the calculated values and the experimental photoelectron values for the energy required to remove the first 4 valence electrons. The first fully metal aromatic compound was a cyclogallane with a Ga₃^2- core discovered by Gregory Robinson in 1995.^[3]

metal clusters generated by laser vaporization of the pure metals in the presence of oxygen in a helium stream.^[4] 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 M
₃O
₉ compound. This energy turns out to be comparable to that of bulk tungsten trioxide and molybdenum trioxide. The photoelectric signal is also broad which suggests a large difference in conformation between the anion and the neutral species. Computational chemistry shows that the M
₃O⁻
₉ anions and M
₃O²⁻
₉ dianions are ideal hexagons with identical metal-to-metal bond lengths. Tritantalum oxide clusters (Ta₃O₃⁻) also are observed to exhibit possible D-orbital aromaticity.^[3]

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 atoms and water molecules.^[5] The experimental evidence is supported by computed NICS aromaticity