CBe5E- (E = Al, Ga, In, Tl): planar pentacoordinate carbon in heptaatomic clusters

A series of clusters with the general formula CBe5E- (E = Al, Ga, In, Tl) are theoretically shown to have a planar pentacoordinate carbon atom. The structures show a simple and rigid topological framework-a planar EBe4 ring surrounding a C center, with one of the ring Be-Be bonds capped in-plane by a fifth Be atom. The system is stabilized by a network of multicenter sigma bonds in which the central C atom is the acceptor, and pi systems as well by which the C atom donates charge to the Be and E atoms that encircle it.