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: C 4 H 4 EH 2 ( C 2 v ), C 4 H 4 EH - ( C s and C 2 v; C, D 5 h ), C 4 H 4 EH + (singlet, C 2 v ), C 4 H 4 EHLi ( C s; C, C 5 v ), and C 4 H 4 ELi 2 ( C 2 v ). In addition, structural trends are established for C 4 H 4 ELi - ( C s ) and for C 4 H 4 E 2 - ( C 2 v ) as well as for the singlet and triplet C 4 H 4 E ( C 2 v ) sets. The increased pyramidality at E down group 14 results in strongly decreased aromaticity of metallolyl anions C 4 H 4 EH - ( C s ). In contrast, all planar C 4 H 4 EH - ( C 2 v ) geometries are significantly more aromatic. Although all C 4 H 4 EH + ( C 2 v ) structures are planar, the antiaromaticity in singlet C 5 H 5 + is much higher than that of the heavier congeners (E = Si to Pb). The four-π-electron singlets C 4 H 4 E exhibit nearly as localized geometries as the C 4 H 4 EH + ions, but the C 4 H 4 E triplets are more delocalized. As in the free anions, pyramidally coordinated E's lead in C 4 H 4 EHLi ( C s ) to reduced aromaticity, but stabilizing Li−H interactions are apparent in these structures. The metallole dianions and their Li + complexes (e.g. C 4 H 4 ELi 2, C 2 v ) 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.