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 (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 ...