[1] The first experimental evidence for the existence of aromaticity in metals was found in aluminium cluster compounds of the type MAl−4 where M stands for lithium, sodium or copper.
Computational chemistry shows that these aluminium clusters consist of a tetranuclear Al2−4 plane and a counterion at the apex of a square pyramid.
In addition its HOMO is calculated to be a doubly occupied delocalized pi system making it obey Hückel's rule.
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.
X-ray crystallography showed that the sodium atoms are arranged in layers of hexagonal clusters akin to pentacenes.