On the resonance energy in new all-metal aromatic molecules

We have recently advanced the aromaticity concept into all-metal molecules containing Al(4)(2-), XAl(3)(-), Ga(4)(2-), In(4)(2-), Hg(4)(6-), Al(3)(-), and Ga(3)(-) aromatic units. All these systems are electron deficient species compared to the corresponding aromatic hydrocarbons. The electron deficiency results in an interesting new feature in all-metal aromatic systems, which should be considered as having both pi- and sigma-aromaticity, and that should result in their additional stability. In this work, we obtain crude evaluations of the resonance energies for Na(2)Al(4) and Na(2)Ga(4) all-metal aromatic molecules. The resonance energies were found to be unusually high: 30 kcal/mol (B3LYP/6-311+G*) and 48 kcal/mol (CCSD(T)/6-311+G(2df)) for Na(2)Al(4) and 21 kcal/mol (B3LYP/6-311+G*) for Na(2)Ga(4) compared to 20 kcal/mol in benzene. We believe that the high resonance energies in Na(2)Al(4) and in Na(2)Ga(4) are due to the presence of three completely delocalized bonds, one pi-bond and two sigma-bonds, thus confirming the presence of pi- and sigma-aromaticity.