Metalloaromaticity

In the last decade, the study of aromaticity has experienced enormous progress. The new discoveries, which include species such as the metallabenzenes, heterometallabenzenes, metallabenzynes, metallabenzenoids, metallacyclopentadienes, metallacyclobutadienes, and all-metal and semimetal clusters, have joined the classical organic aromatic molecules such as benzene, benzenoid and nonbenzenoid polycyclic aromatic hydrocarbons, and heteroaromatic species to conform the current aromatic zoo. These new molecules, which are potentially useful for certain purposes as specific and very efficient catalysts, molecular electronic devices, molecular magnets, drugs, and other as yet unimagined applications, have brought a complete revolution in the field. At variance with the classical aromatic organic molecules that possess only pi-electron delocalization, aromaticity in these new species is much more complex. These compounds have sigma-, pi-, delta-, and phi-electron delocalization. In addition, they can combine different types of aromaticity thus giving rise to double or triple aromaticity, the so-called multifold aromaticity. The new molecules can also have conflicting aromaticity, i.e., they can be aromatic in one component and antiaromatic in another. Moreover, most of the old indicators are not valid to discuss the complex aromaticity of these novel compounds. The lack of reliable measures of aromaticity for these systems has triggered the development of new and more general and reliable indices that can be applied to both classical organic and inorganic aromatic compounds. Among them, the use of multicenter electronic delocalization indices is advocated because they help to detect the different types of aromaticity and provide reasonable qualitative orderings of aromaticity. (C) 2012 John Wiley & Sons, Ltd.