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
2
v
),
C4H4EH-
(C
s
and
C
2
v
; C,
D
5h
),
C4H4EH+
(singlet, C
2
v
),
C4H4EHLi
(C
s
; C,
C
5v
), and
C4H4ELi2
(C
2
v
). In addition,
structural trends are
established for C4H4ELi-
(C
s
) and for
C4H4E2-
(C
2
v
) as well as for the
singlet and triplet
C4H4E
(C
2
v
) 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
2
v
) geometries are
significantly more aromatic. Although all
C4H4EH+
(C
2
v
)
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
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.
