Metal aromaticity

In metal aromaticity the concept of aromaticity found in many hydrocarbons is extended to metals. The first experimental evidence for the existence of aromaticity in metals was found in aluminium cluster compounds of the type MAl₄^- where M stands for lithium, sodium or copper ^[1]. These anions can be generated in a helium gas by laser vaporization of an aluminium / lithium carbonate composite or a copper or sodium / aluminium alloy, separated and selected by mass spectroscopy and analyzed by photoelectron spectroscopy. The evidence for aromaticity in these compounds is based on several considerations. Computational chemistry shows that these aluminium clusters consist of a tetranuclear Al₄^2- plane and a counterion at the apex of a square pyramid. The Al₄^2- unit is perfectly planar and is not perturbed the presence of the counterion or even the presence of two counterions in the neutral compound M₂Al₄. In addition its 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.

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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 ^[2]. 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 M₃O₉^2- 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 ^[3]. The experimental evidence is supported by computed NICS aromaticity values.

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