Abstract

A quasi-planarmember of the so-called 'Wankel motor' family, B-18(2-), is found. This boron cluster is an electronically stable dianion and a concentric doubly sigma- and pi-aromatic system. The inner B-6 unit in B-18(2-) undergoes quasi-free rotation inside the perimeter of the B-12 ring. The absence of any localized sigma-bond between the inner ring and the peripheral boron atoms makes the system fluxional.