Metalloaromaticity

Abstract

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 π‐electron delocalization, aromaticity in these new species is much more complex. These compounds have σ‐, π‐, δ‐, and ϕ‐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. © 2012 John Wiley & Sons, Ltd. This article is categorized under: Structure and Mechanism > Molecular Structures

Quantifying aromaticity with electron delocalisation measures

“…17 Interestingly, metallabenzene species were proposed first by Thorn and Hoffmann three years before from theoretical calculations. 18 More recently, in 2001 Boldyrev, Wang, and coworkers 19 detected a series of bimetallic clusters containing Al4 2-, the first all-metal aromatic cluster known, face-capped by an M + cation (M = Li, Na, Cu).…”

Why is Benzene Unique? Screening Magnetic Properties of C6H6Isomers

“…IUPAC recommends four criteria – structural, energetic, electronic, and magnetic (SEEM) – to characterize aromatic species . Extensive reviews have been published on each of these criteria ,,,,. In spite of its popularity among experimental chemists, aromaticity still lacks a generally accepted definition among theoreticians whom develop aromaticity probes ,.…”

Why Aromaticity Is a Suspicious Concept? Why?

“…Other indices are simply not good enough to provide reliable results. This is particularly the case if we want to quantify the aromaticity of metalloaromatic compounds (Feixas et al, 2013) because most of the methods to measure aromaticity were developed for the classical aromatic organic molecules and they cannot be directly applied to metallic clusters. For these species, the electronic indices and magnetic measures are likely the best choices (Feixas et al, 2010).…”