Aromaticity and antiaromaticity in transition-metal systems

Abstract

Aromaticity is an important concept in chemistry primarily for organic compounds, but it has been extended to compounds containing transition-metal atoms. Recent findings of aromaticity and antiaromaticity in all-metal clusters have stimulated further research in describing the chemical bonding, structures and stability in transition-metal clusters and compounds on the basis of aromaticity and antiaromaticity, which are reviewed here. The presence of d-orbitals endows much more diverse chemistry, structure and chemical bonding to transition-metal clusters and compounds. One interesting feature is the existence of a new type of aromaticity-delta-aromaticity, in addition to sigma- and pi-aromaticity which are the only possible types for main-group compounds. Another striking characteristic in the chemical bonding of transition-metal systems is the multi-fold nature of aromaticity, antiaromaticity or even conflicting aromaticity. Separate sets of counting rules have been proposed for cyclic transition-metal systems to account for the three types of sigma-, pi- and delta-aromaticity/antiaromaticity. The diverse transition-metal clusters and compounds reviewed here indicate that multiple aromaticity and antiaromaticity may be much more common in chemistry than one would anticipate. It is hoped that the current review will stimulate interest in further understanding the structure and bonding, on the basis of aromaticity and antiaromaticity, of other known or unknown transition-metal systems, such as the active sites of enzymes or other biomolecules which contain transition-metal atoms and clusters.

A multicentre-bonded [ZnI]8 cluster with cubic aromaticity

“…Among them, four bonding orbitals with a 1g and t 1u symmetry are fully occupied, giving a ground-state electronic configuration of (a 1g ) 2 (t 1u ) 6 , whereas four antibonding orbitals with t 2g and a 2u symmetry are completely empty. The bonding pattern of the eight MOs is comparable to those of Au 3 þ and benzene molecules that have the famous planar s-and p-aromaticity 34 and obey Hückel's 4n þ 2 rule (Supplementary Fig. 15).…”

A Critical Assessment of the Performance of Magnetic and Electronic Indices of Aromaticity

“…Recently discovered [11], all-metal and semimetal aromatic clusters represent one of the new boundaries of material science (three recent reviews on this kind of clusters can be found in Refs. [12][13][14]). The unusual stability of all these clusters comes from their aromatic character.…”

“…Indeed, the aromaticity is a key property of these compounds since it explains their molecular and electronic structure, stability, and reactivity. At variance with the classical aromatic organic molecules that possess only π-electron delocalization, these compounds present σ-, π-, and δ-(involving d orbitals) [15][16][17] or even φ-(involving f orbitals) [18] electron delocalization, exhibiting characteristics of what has been called multifold aromaticity [12][13][14][19][20][21][22]]. …”

“…However, simple total electronic counts lead sometimes to incorrect results [14,[53][54][55]. In addition, electron counting does not provide a quantitative value, so comparisons of aromaticity from different compounds are not possible.…”

A quantum Monte Carlo study on electron correlation in all-metal aromatic clusters MAl4− (M = Li, Na, K, Rb, Cu, Ag and Au)

“…Further progress in this area involves theoretical and experimental studies of all-metal clusters with several chemical compositions. [4][5][6][7][8][9][10][11][12][13] The most commonly used theoretical method to deal with metal clusters is the single-particle density functional theory (DFT). It is well known that, due to approximate exchangecorrelation (XC) energy, DFT usually cannot predict results within the chemical accuracy.…”