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WIREs Comput Mol Sci
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Metalloaromaticity

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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

Bond‐energy decomposition (in kcal/mol) of Al42− constructed from two equivalent rigid fragments, along the localization distortion (in deg.) from delocalized to localized structure as defined in Figure 2, computed at BP86/TZ2P.

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Construction and distortion of Al42− and their frontier orbitals in terms of two rigid [Al2]••• fragments.

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The low‐lying occupied and unoccupied molecular orbitals of Al42−.

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Absolute nucleus‐independent chemical shift (NICS) values at r Å above the M3 ring, as a function of the distance between X and the center of the M3 ring (X‐M3). The horizontal blue line shows the NICS(1) values, whereas the straight vertical red lines separate the three tuning regions.

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Nucleus‐independent chemical shift (ppm) and 3c‐ESI (electrons) as a function of the distance (in Å) between Na and the center of the ring for Na2Mg3 species (D3h).

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Valence molecular orbital plots for (a) singlet cyclo‐[Mg3]2− (D3h), (b) NaMg3 (C3v), and (c) Na2Mg3 (D3h). Isosurfaces values are at 0.03 a.u.

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Canonical molecular orbital contribution to NICS(0)rcp (ppm) for the series Al42−, Al3Ge, Al2Ge2, AlGe3+, and Ge42+.

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Comparison between dissected nucleus‐independent chemical shift (ppm) indices calculated at the ring center and at the ring critical point (RCP, dotted line) along the series Al42−, Al3Ge, Al2Ge2, AlGe3+, and Ge42+.

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Comparison between nucleus‐independent chemical shift (ppm) indices calculated at the ring center and at the ring critical point (RCP, dotted line) along the series Al42−, Al3Ge, Al2Ge2, AlGe3+, and Ge42+.

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Variation of multicenter indices MCI, MCIπ, and MCIσ (in electrons) along the series Al42−, Al3Ge, Al2Ge2, AlGe3+, and Ge42+.

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Change in selected overlap integrals ΔS along the localization distortion ζ (in deg.) between [B2]••• fragment orbitals relative to D4h (90°)

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Schematic molecular orbital (MO) interaction diagram of Al42− constructed from two [M2]••• fragments in their quadruplet valence configuration, emerging from quantitative Kohn–Sham MO analyses at BP86/TZ2P.

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