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-), XAl3, Ga-4(2-), ln(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 Na2Al4 and Na2Ga4 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 Na2Al4 and 21 kcal/mol (B3LYP/6-311 +G*) for Na2Ga4 compared to 20 kcal/ mol in benzene. We believe that the high resonance energies in Na2Al4 and in Na2Ga4 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.