σ-Aromaticity and σ-Antiaromaticity in Alkali Metal and Alkaline Earth Metal Small Clusters

We extended the aromaticity concept to small σ-aromatic alkali metal and alkaline earth metal clusters. We performed ab initio calculations (B3LYP/6-311+G* and CCSD(T)/6-311+G*) on a selected group of triatomic and tetraatomic metal clusters. We have shown that the introduction of aromaticity and antiaromaticity in Li 3 + and Li 3 - ions, respectively, can be used to explain their relative stability and their structure. For the tetratomic 6σ-electron Li 2 Mg 2 species we predicted that the cyclic σ-aromatic structures of Li 2 Mg 2 are more stable than the classical linear Li−Mg−Mg−Li structure, thus showing the importance of the aromaticity in metal clusters. We believe that the further advancing of the aromaticity concept into metal clusters will be a useful tool in understanding chemical bonding in these species.