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

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 (C_2v), C₄H₄EH^- (C_s and C_2v; C, D_5h), C₄H₄EH⁺ (singlet, C_2v), C₄H₄EHLi (C_s; C, C_5v), and C₄H₄ELi₂ (C_2v). In addition, structural trends are established for C₄H₄ELi^- (C_s) and for C₄H₄E^2- (C_2v) as well as for the singlet and triplet C₄H₄E (C_2v) sets. The increased pyramidality at E down group 14 results in strongly decreased aromaticity of metallolyl anions C₄H₄EH^- (C_s). In contrast, all planar C₄H₄EH^- (C_2v) geometries are significantly more aromatic. Although all C₄H₄EH⁺ (C_2v) 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 (C_s) to reduced aromaticity, but stabilizing Li−H interactions are apparent in these structures. The metallole dianions and their Li⁺ complexes (e.g. C₄H₄ELi₂, C_2v) 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.