All-metal aromaticity and antiaromaticity.

Since its introduction by August Kekulé in 1865,1 the concept of aromaticity has been continuously conquering new territories in chemistry (refs 2-20 are only a small fraction of the published articles, reviews, books, and conference proceedings on this subject). Initially, aromaticity was developed for the following classes of organic compounds: (i) monocyclic planar conjugate hydrocarbons and their ions with (4n + 2) π electrons; (ii) polycyclic conjugate hydrocarbonssbenzenoid hydrocarbons built of fused benzene rings; (iii) polycyclic conjugated carbocyclic hydrocarbons based on nonbenzenoid systems such as azulene and other conjugate hydrocarbons with four-, five-, seven-, and eight-membered rings.2,4-17 Today, aromaticity has been extended to a highly diverse number of heterocyclic compounds,3,8,13,15,17,21-23 as well as three-dimensional (3D) compounds, such as ferrocenes and related sandwich systems,24-29 * E-mail (A.I.B.) [email protected]; (L.-S.W.) [email protected]. 3716 Chem. Rev. 2005, 105, 3716−3757