Aromaticity/antiaromaticity in "bare" and "ligand- stabilized" rings of metal atoms

Abstract

During the last decade, the aromaticity/antiaromaticity of "bare" and "ligand-stabilized" rings of metal atoms has flourished reaching a high level of sophistication. Compared to the aromaticity/antiaromaticity encountered in organic rings, primarily being of the traditional π-type, "bare" and "ligand-stabilized" rings of metal atoms can exhibit a multifold aromaticity/antiaromaticity and conflicting aromaticity, arising from the electron delocalization of s, p, d, and even f orbitals of the ring atoms. In this context we can speak for σ-, π-, δ-, and φ-aromaticity/antiaromaticity as well as for double (σ + π) and even triple (σ +π + δ) aromaticity (possible in rings of transition metal atoms) and any possible combination of the σ-,π-, δ-, and σ-types of aromaticity/antiaromaticity. The f-orbital and possibly φ-aromaticity can exist only in rings of lanthanide and actinide metal atoms. In view of these facts, in this review we have provided an overview and updated information about aromaticity/antiaromaticity of rings of metal atoms covering the most significant developments in this new emerging field. © Springer-Verlag Berlin Heidelberg 2010.