B. Goldfuss et Pv. Schleyer, AROMATICITY IN GROUP-14 METALLOLES - STRUCTURAL, ENERGETIC, AND MAGNETIC CRITERIA, Organometallics, 16(8), 1997, pp. 1543-1552
Various structural (C-C bond length equalization, D), energetic (isode
smic stabilization energies, ISE), and magnetic (diamagnetic susceptib
ility exaltations, Lambda and nucleus-independent chemical shifts, NIC
S) criteria are employed (using B3LYP, CSGT, and GIAO ab initio method
s) 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 a
nd C-2v; C, D-5h), C4H4EH+ (singlet, C-2v), C4H4EHLi (C-s; C, C-5v), a
nd C4H4ELi2(C-2v). In addition, structural trends are established for
C4H4ELi- (C-s) and for C4H4E2- (C-2v) as well as for the singlet and t
riplet C4H4E (C-2v) sets. The increased pyramidality at E down group 1
4 results in strongly decreased aromaticity of metallolyl anions C4H4E
H- (C-s). In contrast, all planar C4H4EH-(C-2v) geometries are signifi
cantly more aromatic. Although all C4H4EH+ (C-2v) structures are plana
r, the antiaromaticity in singlet C5H5+ is much higher than that of th
e heavier congeners (E = Si to Pb). The four-pi-electron singlets C4H4
E exhibit nearly as localized geometries as the C4H4EH+ ions, but the
C4H4E triplets are more delocalized. As in the free anions, pyramidall
y coordinated E's lead in C4H4EHLi (C,) to reduced aromaticity, but st
abilizing Li-H interactions are apparent in these structures. The meta
llole dianions and their Li+ complexes (e.g. C4H4ELi2, C-2v) are the m
ost aromatic among the species studied. The aromaticity in these diani
onic metalloles is remarkably constant in going from E = C to E = Pb.