ABSTRACT
The aromaticity concept is used in chemistry to distinguish some special types of molecules, which are particularly stable and present similar chemical reactivity. The name is derived from early known examples such as benzene and its derivatives that present pleasant aroma. Aromaticity is associated with high stability, uniform bond lengths intermediate between single and double bonds, and a ring current that produce a chemical shift in the NMR spectra. However, like many other chemical concepts, its origin is empirical and qualitative. Like many important concepts in chemistry, aromaticity has been generalized. It was used at the beginning only for planar π-conjugated hydrocarbons, but very soon it was also used for hetero atoms, and for noncompletely planar molecules, such as biphenyl. Later it was also generalized for nonplanar molecules, especially for caged molecules like fullerenes [1,2]. Over the last decade, a new very interesting generalization was proposed. The aromaticity concept in all-metal clusters and the existence of a σ aromaticity [3], which open the possibility of conicting aromaticity, that is, σ aromatic and π antiaromatic or vice versa [4].
