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Aromaticity in Group 14 Metalloles:  Structural, Energetic, and Magnetic Criteria

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journal contribution
posted on 15.04.1997, 00:00 by Bernd Goldfuss, Paul von Ragué Schleyer
Various structural (C−C bond length equalization, D), energetic (isodesmic stabilization energies, ISE), and magnetic (diamagnetic susceptibility exaltations, Λ and nucleus-independent chemical shifts, NICS) criteria are employed (using B3LYP, CSGT, and GIAO ab initio methods) to assess the aromaticity and antiaromaticity of a variety of group 14 (E = C, Si, Ge, Sn, Pb) metalloles:  C4H4EH2 (C2v), C4H4EH- (Cs and C2v; C, D5h), C4H4EH+ (singlet, C2v), C4H4EHLi (Cs; C, C5v), and C4H4ELi2 (C2v). In addition, structural trends are established for C4H4ELi- (Cs) and for C4H4E2- (C2v) as well as for the singlet and triplet C4H4E (C2v) sets. The increased pyramidality at E down group 14 results in strongly decreased aromaticity of metallolyl anions C4H4EH- (Cs). In contrast, all planar C4H4EH- (C2v) geometries are significantly more aromatic. Although all C4H4EH+ (C2v) structures are planar, the antiaromaticity in singlet C5H5+ is much higher than that of the heavier congeners (E = Si to Pb). The four-π-electron singlets C4H4E exhibit nearly as localized geometries as the C4H4EH+ ions, but the C4H4E triplets are more delocalized. As in the free anions, pyramidally coordinated E's lead in C4H4EHLi (Cs) to reduced aromaticity, but stabilizing Li−H interactions are apparent in these structures. The metallole dianions and their Li+ complexes (e.g. C4H4ELi2, C2v) are the most aromatic among the species studied. The aromaticity in these dianionic metalloles is remarkably constant in going from E = C to E = Pb.