Usefulness of the σ‐Aromaticity and σ‐Antiaromaticity Concepts for Clusters and Solid‐State Compounds

Abstract

In this Review we present examples of clusters, molecules, and solid-state compounds, for which the use of σ-aromaticity and σ-antiaromaticity concepts is essential for understanding of chemical bonding. We show that the bonding patterns in these σ-aromatic and σ-antiaromatic compounds are similar to those of the corresponding π-aromatic and π-antiaromatic chemical systems, respectively. Undoubtedly, σ-aromaticity helps us understand why the high symmetry isomers are the most stable among myriads of other potential structures. We also show that besides systems exhibiting either σ- or π-aromatic features, there are species, which can possess multiple aromaticity/antiaromaticity, or conflicting aromaticity patterns. We believe that the σ-aromaticity and σ-antiaromaticity concepts will be helpful in rationalizing chemical bonding, structure, stability, and molecular properties of chemical species in both organic and inorganic chemistry. We hope that they will also be useful for other areas of science such as material science, catalysis, nanotechnology, and biochemistry.

Stabilizing a metalloid {Zn12} unit within a polymetallide environment in [K2Zn20Bi16]6−

“…Thus, the porphine-related arrangement of atoms observed in 1a can be viewed as an electron-poor variant of the quoted 24-atom macrocycles. To examine the applicability of this thought experiment, we were interested to see whether the polymetallide unit would also show all-metal aromaticity, such as observed for several smaller polymetallic ring systems [40][41][42] . To study aromaticity based on the magnetic criterion, we calculated the magnetically induced current density of the [Zn 20 Bi 16 ] 8− cluster based on the magnetic criterion 43,44 .…”

Predicted Reversal in N-Methylazepine/N-Methyl-7-azanorcaradiene Equilibrium upon Formation of Their N-Oxides

“…Aromaticity (antiaromaticity) is a fundamental concept to account for structure and reactivity of electron delocalized cyclic compounds, intermediates and transition states. The earliest postulates applied to cyclic π-delocalized systems (Hückel Rule [ 19 , 20 , 21 , 22 ]) but more recently include cyclic delocalized s-systems as well [ 23 ]. Aromatic species are characterized by their thermodynamic properties of inherent stabilities [ 24 ], structures with hybrid bond lengths [ 25 ], specific reactions such as preferred substitution not encountered by acyclic or nonaromatic systems [ 26 ], and secondary magnetic fields generated by ring current effects detectable by magnetic resonance spectroscopy (NMR for example).…”

Nb2©Au6: a molecular wheel with a short NbNb triple bond coordinated by an Au6 ring and reinforced by σ aromaticity

“…2c , 3 On the basis of vibrationally-resolved photodetachment spectroscopy, Au 6 was initially deduced to have a D 6h ring structure, 3 b but was later confirmed to have a D 3h triangular structure. 2 The high electronic stability of Au 6 is due to σ aromaticity from its delocalized σ electrons, 4 as is also found in the isoelectronic Au 5 Zn + cluster. 5 Photoelectron spectroscopy (PES) of MAu 6 – (M = Ti, V, Cr) along with density functional theory (DFT) calculations found these doped-clusters to have planar structures featuring an Au 6 ring with a central transition metal atom (M©Au 6 – ) with variable magnetic moments.…”