Abstract

Could more than two atoms be bound in such a way that a pi-bond is the major bonding force without involving sigma-bond at all? To answer this question, we have shown that 3, 4 or 5 metal atoms can be held together by a single pi-bond in the Mg(3)(-), NaMg(3)(-) and Na(2)Mg(3) species. In both the NaMg(3)(-) and Na(2)Mg(3) species, the basic structural unit is an unusual Mg(3)(2-) trigonal planar cluster, which exhibits properties of aromaticity, because it satisfies the 4n + 2 rule for pi-electrons and it is an equilateral triangle. Natural population analysis shows that the Na atomic orbitals contribute to formation of the pi-bonding MO and thus all four in NaMg(3)(-) or all five in Na(2)Mg(3) atoms are involved in chemical bonding. In these clusters, pi-aromaticity occurs without initial formation of the sigma-framework. (C) 2004 Elsevier B.V. All rights reserved.