Observation of all-metal aromatic molecules

Aromaticity is a concept invented to account for the unusual stability of a n important class of organic molecules: the aromatic compounds. Here we rep ort experimental and theoretical evidence of aromaticity in all-metal syste ms. A series of bimetallic clusters with chemical composition MAl4- (M = Li , Na, or Cu), was created and studied with photoelectron spectroscopy and a b initio calculations. All the MAl4- species possess a pyramidal structure containing an M+ cation interacting with a square Al-4(2-) unit. Ab initio studies indicate that Al-4(2-) exhibits characteristics of aromaticity with two delocalized pi electrons (thus following the 4n + 2 electron counting rule) and a square planar structure and maintains its structural and electr onic features in all the MAl4- complexes. These findings expand the aromati city concept into the arena of all-metal species.