Dismutational and global-minimum isomers of heavier 1,4- dimetallatetrasilabenzenes of group 14

Article abstract of DOI:10.1002/anie.201310475

Aromatic species with heavier Group 14 elements show remarkable differences in terms of stability, structure, and reactivity. Herein we report our experimental and theoretical investigations regarding isomers of germanium- and tin-containing benzene analogues E₂Si₄R₆ (E=Ge, Sn). The germanium-substituted dismutational isomer with a tricyclic six-membered scaffold is isolable, but unlike the homonuclear Si₆ analogue slowly rearranges even at room temperature to give the propellane-type global minimum isomer. In case of E=Sn the dismutational isomer may be an intermediate on the pathway to the propellane-type species obtained, but cannot be detected even at low temperature. Unprecedentedly large chemical shift anisotropies in the ²⁹Si NMR spectra that increase from the Si ₆ species through Ge₂Si₄ to Sn ₂Si₄ are rationalized by progressively larger paramagnetic-term contributions to the chemical shift tensor as a result of diminishing HOMO-LUMO gaps, which are also reflected in the absorption spectra, as well as by appearance and symmetry of these frontier orbitals. Heavy benzenes: The reduction of the product from disilenide and GeCl ₂×dioxane or SnCl₂×dioxane, respectively, gives Ge- and Sn-containing heavier isomers of benzene. With Ge, the dismutational isomer of a 1,4-digermatetrasilabenzene (A; Tip=2,4,6-iPr ₃C₆H₂) is formed. Thermal rearrangement gives the presumed global minimum isomer B. With Sn, B is formed directly. Unprecedented downfield shifts in the ²⁹Si NMR spectra are rationalized by paramagnetic shielding-term contributions.

Full text of DOI:10.1002/anie.201310475

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DOI: 10.1002/anie.201310475
Aromatic Group 14 Compounds
Dismutational and Global-Minimum Isomers of Heavier
1,4-Dimetallatetrasilabenzenes of Group 14**
Anukul Jana, Volker Huch, Michal Repisky, Raphael J. F. Berger,* and David Scheschkewitz*
Dedicated to Prof. Robert West on occasion of his 85th birthday
Abstract: Aromatic species with heavier Group 14 elements
show remarkable differences in terms of stability, structure, and
reactivity. Herein we report our experimental and theoretical
investigations regarding isomers of germanium- and tin-
containing benzene analogues E2Si4R6 (E = Ge, Sn). The
germanium-substituted dismutational isomer with a tricyclic
six-membered scaffold is isolable, but unlike the homonuclear
Si6 analogue slowly rearranges even at room temperature to
give the propellane-type global minimum isomer. In case of
E = Sn the dismutational isomer may be an intermediate on the
pathway to the propellane-type species obtained, but cannot be
detected even at low temperature. Unprecedentedly large
chemical shift anisotropies in the 29Si NMR spectra that
increase from the Si6 species through Ge2Si4 to Sn2Si4 are
rationalized by progressively larger paramagnetic-term con-
tributions to the chemical shift tensor as a result of diminishing
HOMO–LUMO gaps, which are also reflected in the absorp-
tion spectra, as well as by appearance and symmetry of these
frontier orbitals.
up to three silicon atoms in a six-atom carbon scaffold.[5]
Experimentally, benzene or naphthalene analogues contain-
ing up to two heavier elements are known, for example,
silabenzene 1,[6] 1,2-disilabenzene,[6] 1,4-disilabenzene,[7] ger-
mabenzene 2,[8] and 2-stannanaphthalene 3[9] (Scheme 1).
Scheme 1. Stable heavier Hꢀckel aromatics (Tbt=2,4,6-(CH-
(SiMe3)2)3C6H2) and dismutational (4) and global minimum (5) isomer
of Si6R6 (R=Tip=2,4,6-iPr3C6H2).
The dismutational aromatic isomer of a hexasilabenzene
(4)[10] and its rearrangement to the alleged global minimum
isomer 5[11] shed light on the particularities of silicon in this
context. In addition, Kyushin et al. very recently reported
a polycyclic hexasilabenzvalene derivative.[12] We therefore
became interested in the partial substitution of the silicon
atoms in isomers of hexasilabenzene by germanium and tin.
To date, there is only one example of a six-membered Hꢀckel
aromatic with two germanium atoms, which has been
obtained by Power et al. from the reaction of digermyne
with a nitrile.[13] The only stable, potentially aromatic ring
motif based exclusively on silicon and germanium/tin is a 1,2-
disila-3,4-digermacyclobutadiene dianion,[14] despite the fact
As the archetypical example of aromaticity, benzene is
ubiquitous in the chemical sciences and beyond.[1] With
regard to the design of extended conjugated systems,[2]
benzene analogues involving heavier Group 14 elements
and their isomers are of theoretical[3] and experimental[4]
interest. As recently calculated by Boldyrev and Ivanov,
Hꢀckel-aromatic isomers are possible in the incorporation of
ꢀ
that various methods for the synthesis of compounds with Si
Ge/Sn p-bonds have been known for some time.[15] Herein, we
focus on the formal replacement of the two unsubstituted
silicon atoms of 4 and 5 by germanium and tin. The resulting
Si4Ge2 and Si4Sn2 derivatives, 8, 9, and 11 show an increasing
electronic anisotropy resulting in unprecedented deshielding
of the NMR resonance signals at saturated silicon atoms of
the scaffolds.
[*] Dr. A. Jana, Dr. V. Huch, Prof. Dr. D. Scheschkewitz
Krupp-Chair of General and Inorganic Chemistry
Saarland University
66125 Saarbrꢀcken (Germany)
E-mail: scheschkewitz@mx.uni-saarland.de
Homepage: http://www.uni-saarland.de/fak8/scheschkewitz/
index.html
Priv.-Doz. Dr. R. J. F. Berger
Chemistry of Materials, Paris Lodron University
A-5020 Salzburg (Austria)
E-mail: raphael.berger@sbg.ac.at
For the synthesis of Si4Ge2 derivative 8 we chose disilenide
6[16] and GeCl2·dioxane[17a] as a source of silicon and germa-
nium, respectively. As any attempt to isolate the plausible
intermediate, disilenyl-substituted chlorogermylene 7, or
even the detection by NMR spectroscopy at low temperature
failed, we decided to investigate its in situ generation and
direct subsequent reduction to 8. Indeed, the 1:1 reaction of
disilenide and GeCl2·dioxane at ꢀ788C in toluene (in THF
tetrasilabutadiene Si4Tip6 is formed),[16a] and subsequent
addition of one equivalent of lithium/naphthalene (Li/
C10H8) in THF afforded 8, which was isolated as dark green
crystals (m.p. > 2258C) in 42% yield from n-hexane
Dr. M. Repisky
The Centre for Theoretical and Computational Chemistry
Department of Chemistry, The Arctic University of Norway
N-9037 Tromsø (Norway)
[**] Funding by the EPSRC (EP/H048804/1), the DFG (SCHE 906/4-1),
and the Krupp Foundation is gratefully acknowledged. R.B. thanks
Prof. Dr. R. Elsꢁsser, Dr. P. Zinterhof, and the Austrian Center for
Scientific Computing for the provision of computational resources.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201310475.
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metric structure is of tricyclic connectivity similar to that of
the Si6 analogue 4.[10] In the central four-membered ring, the
ꢀ
ꢀ
Si2 Ge1 (2.3878(9) ꢁ) bond is only slightly longer than Si2
ꢀ
Ge1’ (2.3601(9) ꢁ) and both are shorter than that of Si Ge
single bonds in 1-disilagermirene (2.415(1) ꢁ).[15a] As
expected the diagonal Ge1···Ge1’ (2.8624(7) ꢁ) distance is
Scheme 2. Synthesis of dismutational isomer of 1,4-digermatetrasila-
benzene 8 (Tip=2,4,6-iPr3C6H2).
ꢀ
substantially longer than a typical Ge Ge single bond
(2.498(2) ꢁ)[23] suggesting the absence of a direct trough-
space bonding interaction. In case of the corresponding
Si6Tip6 congener 4 (and 5 as well) an experimental charge-
density analysis confirmed the absence of direct bonding
between the bridgehead atoms.[24]
(Scheme 2).[18] The green color of 8 is due to the longest
wavelength absorption in the UV/Vis spectrum at lmax
=
669 nm (e = 340 Lmolꢀ1 cmꢀ1), which is considerably red-
shifted compared to the hexasila congener (4: lmax = 623 nm;
dark green crystals; Table 1). Additional absorptions in the
UV/Vis spectrum appear at lmax = 557 (e = 320 Lmolꢀ1 cmꢀ1)
The 29Si NMR spectrum of 8 in [D6]benzene shows two
resonance signals at d = 185.8 and ꢀ88.6 ppm in a 1:1 ratio
(Table 1). On the basis of the data of the Si6 analogue 4, the
upfield signal at d = ꢀ88.6 ppm is assigned to the two Tip2Si
units (d = ꢀ84.8 ppm), which was confirmed by a 2D 29Si/1H
correlation. The 29SiTip resonance signal at d = 185.8 ppm is
observed further downfield than in 4 (d = 124.6 ppm).
In contrast to 4, in solution the digerma-substituted 8
slowly rearranges to the alleged global minimum isomer 9
even at room temperature. Complete conversion is achieved
(either in THF, hexane, or benzene) after 12 h at 658C. The
silicon-bridged digerma propellane 9 was isolated as orange
crystals in 88% yield from THF (Scheme 3).[18] Unlike in the
Table 1: 29Si NMR and UV/Vis spectroscopy data of 4, 5, 8, 9, and 11.
Parameter
4
5
8
9
11
d29SiTip2
ꢀ84.8
–
174.6
14.8
ꢀ88.6
–
236.0
14.5
29.8
284.4
52.7
23.8
679
d29SiTip2
[a]
d29SiTip
lmax [nm]
124.6
623
ꢀ7.5
185.8
669
473
518
(629)[b]
(475)[b]
(679)[c]
(486)[c]
[a] SiTip2 unit tethering two “propeller blades”. [b] Calculated value with
Dip instead of Tip group. [c] Calculated value with Ph instead of Tip.
(Tip=2,4,6-iPr3C6H2, Dip=2,6-iPr2C6H3, Ph=C6H5).
and 439 nm (e = 3690 Lmolꢀ1 cmꢀ1). A coupled clusters
calculation on 8Ph (Ph instead of Tip substituents)[19–22]
shows that the HOMO–LUMO pair contribution dominates
!
the lowest vertical electronic transition (S1 S0). The calcu-
Scheme 3. Rearrangement of 8 to 9 (Tip=2,4,6-iPr3C6H2).
lated absorption band at 679 nm is in very good agreement
with experiment. The surprisingly stable 8 can be exposed to
air for days as a solid or for hours in solution without
detectable changes.
case of 5, no side products were detected during the
rearrangement.[25]
Structure analysis by X-ray diffraction on single crystals
confirms 9 as a Ge2Si4 propellane-type structure with two of
the “propeller blades” tethered by an additional SiTip2
unit (Figure 2).[18] The distance between the unsubstituted
The solid-state structure of 8 (Figure 1)[18] as determined
by X-ray diffraction analysis confirms that the centrosym-
Figure 2. Molecular structure of 9·2C6H6 in the solid state (thermal
ellipsoids at 30%; hydrogen atoms and C6H6 molecules are omitted
for clarity). Selected interatomic distances [ꢂ]: Ge1···Ge2 2.7820(2),
Ge1-Si3 2.4566(4), Ge1-Si2 2.4161(4), Ge1-Si1 2.4506(4), Si1-Si4
2.3722(5).
Figure 1. Molecular structure of 8 in the solid state (thermal ellipsoids
at 30%; hydrogen atoms are omitted for clarity). Selected interatomic
distances [ꢂ]: Si1-Si2 2.3402(12), Si1-Ge1 2.4297(10), Si2-Ge1
2.3878(9), Si2-Ge1’ 2.3601(9), Ge1···Ge1’ 2.8624(7).
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ꢀ
bridgehead germanium atoms Ge1 Ge2 (2.7820(2) ꢁ) is in-
between the corresponding distances in homo- and hetero-
nuclear propellanes (Ge5Mes6 2.869(2) ꢁ,[26a,e] Ge2Si3Mes6
2.767(1) ꢁ;[26b,e] Mes = 2,4,6-Me3C6H2). The redshift of the
UV/Vis absorption at lmax = 518 nm (e = 690 Lmolꢀ1 cmꢀ1)
compared to that of Ge2Si3Mes6 (415 nm) is attributed to the
lifted degeneracy of the HOMO resulting from the reduction
in idealized symmetry (C2 for 9 vs. D3h for Ge2Si3Mes6).[26b] In
addition a more intense absorption at lmax = 381 nm is
observed (e = 12040 Lmolꢀ1 cmꢀ1). The calculated value of
ꢀ1893 ppm),[26d,e] and similar to Breherꢂs Sn2Si3Mes6 hetero-
nuclear
propellane
(d = ꢀ1348 ppm;
Mes = 2,4,6-
Me3C6H2).[26b] The two 117Sn satellites with a coupling constant
of J(119Sn–117Sn) = 3910 Hz are comparable to that observed
for Sn2Si3Mes6.[26b] This is in qualitative agreement with the
computational value (for details see below) obtained for the
parent compound (11H, R = H, J(119Sn1–117Sn2) = 5879 Hz).
An additional pair of 29Si satellites in the 119Sn spectrum of 11
(1J = 326 Hz) presumably originates from unresolved cou-
pling to the different 29Si nuclei. According to the calculations,
the 1J(Sn–Si) coupling constants of 11H are indeed predicted
to be of similar magnitude [1J(119Sn–29Si1) = 1J(119Sn–29Si2) =
!
the lowest vertical transition S1 S0 for 9Ph (Ph instead of
Tip)[19] is 486 nm in satisfactory agreement with the exper-
imental value of 518 nm for 9.
1
2
ꢀ434 Hz, J(129Sn–29Si4) = ꢀ467 Hz]. The J(Sn–Si) value is
apparently too small to be resolved experimentally [11H:
2J(119Sn–29Si3) = 45 Hz].
The 29Si NMR spectrum of 9 in [D6]benzene shows three
signals at d = 236.1, 29.8, and 14.6 ppm in a 1:2:1 ratio in
accordance with the C2 symmetry (Table 1). The downfield
resonance is assigned to one of the SiTip2 units by 2D 29Si/1H
and thus to a tetracoordinate silicon atom. By analogy with
the Si6 analogue 5,[11] we concluded that this most deshielded
silicon atom should originate from the untethered “propeller
blade” (Si3). Very recently, a cluster with two spiro-connected
SiGe5 motifs of similar geometry was reported that showed
29Si NMR shift of d = 91.6 ppm for the SiR2 bridge.[27]
To experimentally address the 1,4-distannatetrasilaben-
zene isomers, we employed SnCl2·dioxane[17b] as a tin source
under analogous reaction conditions as during the synthesis of
8. The 1:1 in situ Li/C10H8 reduction of the reaction mixture
generated from 6 and SnCl2·dioxane in toluene directly
affords the global minimum isomer of 1,4-distannatetrasila-
benzene, 11, in 22% isolated yield as dark-brown crystals
(Scheme 4).[18] The anticipated intermediate 10 could not be
detected even at ꢀ808C suggesting a much lower stability of
the dismutational isomer for the Sn2Si4 scaffold and confirm-
ing the trend observed for Ge2Si4.
An X-ray diffraction study on single crystals confirms the
constitution of the Sn2Si4 propellane-type structure 11
(Figure 3).[18] The distance between the tin atoms Sn1–Sn2
Figure 3. Molecular structure of 11 in the solid state (thermal ellip-
soids at 30%; hydrogen atoms and co-crystallized solvent are omitted
for clarity).Selected interatomic distances [ꢂ]: Sn1···Sn2 3.1099(3), Sn1-
Si1 2.6382(8), Sn1-Si2 2.6017(8), Sn1-Si4 2.6565(8), Si1-Si3
2.3856(10).
(3.1099(3) ꢁ) is slightly longer than the corresponding
distance in Breherꢂs heteronuclear propellane Sn2Si3Mes6
(3.097(1) ꢁ).[26b] The UV/Vis absorption of 11 at lmax
=
679 nm (e = 770 Lmolꢀ1 cmꢀ1) is again red-shifted compared
to that of Sn2Si3Mes6 (lmax = 550 nm) due to the loss of
degeneracy of the HOMO resulting from lower symmetry (C2
for 11 vs. D3h for Sn2Si3Mes6).[26b] Additional absorptions
appear at lmax = 555 (e = 1250 Lmolꢀ1 cmꢀ1) and 456 nm (e =
11630 Lmolꢀ1 cmꢀ1).
Scheme 4. Synthesis of global minimum isomer of 1,4-distannatetrasi-
labenzene 11 (Tip=2,4,6-iPr3C6H2).
The 29Si NMR spectrum acquired in solution for 11 shows
an even more downfield-shifted resonance signal at d =
284.46 ppm that corresponds to the tin-bridging SiTip2 unit
(Table 1). To our knowledge, the 29Si NMR chemical shifts
assignable to the untethered “propeller blades” of 9 and 11
represent the by far most deshielded 29Si NMR signals of
tetracoordinate silicon atoms in a molecular environment of
a diamagnetic compound to date. In fact, such extreme
deshieldings are not even found in cationic three-coordinate
Compounds 5, 9, and 11 show exceedingly deshielded 29Si
NMR resonance signals for the tetracoordinate SiTip2 atoms
of the untethered propeller blades (see Table 1). The
deshielding effect is enhanced with increasing atomic
number of the unsubstituted bridgehead atoms E (5, E = Si:
d = 174.6; 9, E = Ge: d = 236.0; 11, E = Sn: d = 284.5 ppm)
and much more pronounced than in the closely related
untethered propellane species Si2Si3Mes6,[29] Ge2Si3Mes6,[26b]
Sn2Si3Mes6.[26b] (E = Si: d = 25.5; E = Ge: d = 66; E = Sn: d =
98 ppm). This differing behavior is even more remarkable in
view of the fact that the tethering SiTip2 moiety and thus the
symmetry-breaking motif is located at the opposite end (with
respect to the untethered SiTip2 bridge) of the molecular
structures of 5, 9, and 11.
silylium ions (e.g. Mes3Si+B(C6F5)4 in [D6]benzene: d =
ꢀ
225.5 ppm).[28] The 119Sn NMR resonance signal of 11 at d =
ꢀ1260.19 ppm is downfield-shifted compared with that
of the unbridged homonuclear propellanes Sn5(2,6-
Et2C6H3)6 (d = ꢀ1751 ppm),[26c,e] Sn5{(2,6-iPrO)2C6H3}6 (d =
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To clarify the reasons for this, we have analyzed the
physical origins of the chemical shieldings using first-princi-
ples methods with the program ReSpect.[30,31] We extended
the series of bridged and unbridged propellanes beyond the
experimentally accessible (5, 9 and 11) to include the lead-
substituted species as well. The calculations revealed that the
diamagnetic terms are virtually constant within each of the
two homologous series of species (for details see Supporting
Information). This implies that shielding effects caused by
magnetically induced diatropic currents are also almost
identical as confirmed by the calculated, very similar induced
current strengths and topologies in each series. Moreover, the
integrated currents in both series are also virtually equal (ca.
10 nATꢀ1 diatropic molecular current). According to our
findings regarding the current topology[10b,11] these currents
are mainly caused by the cluster-like electron pair at the
bridgehead atoms resulting in a two-electron–3D aromatic
cluster bonding.
A comparison between non-relativistic and fully relativ-
istic levels of computation reveals that for the Si, Ge, and Sn
substituted species the relativistic corrections are negligible.
In case of the experimentally inaccessible lead-substituted
species, relativity enhances the downfield shift even more (by
almost d = 100 ppm). A closer look at the paramagnetic
electronic P1e term[31b,c] shows that in all cases the HOMO–
LUMO excitation pair provides the dominating contribution
greater than 90% (Figure 4). In both compound series, the
virtual as well as the occupied orbital are characterized by the
presence of three main nodal planes,[32] which are related by
a rotation about 908 with respect to the y-axis (left–right
direction in the Figure). This geometrical relationship
between virtual and occupied MO of the important virtual
excitation is required for a contribution to the P1e terms of the
chemical shielding tensors. Since the MO energy differences
appear in the denominator of the corresponding expression
for the paramagnetic part of shielding tensor the contribution
decreases with increasing energy difference. The HOMO–
LUMO gaps of the bridged propellane species are signifi-
cantly lower than those of the untethered propellanes. This
fact as well as the localization of the orbitals at the silicon
atom of the untethered “propeller blade” leads to the larger
paramagnetic terms in this case. Substitution of the Si atoms
by Ge, Sn, and Pb narrows the MO energy differences in both
series of compounds and the 29Si resonances are therefore
progressively shifted downfield.
The decreasing electronegativity going from Si to Pb
results in a stabilization of MOs with antibonding contribu-
tions, in particular the LUMO. Experimentally, this energetic
lowering of the LUMO with the increasing atomic number of
the hetero atoms is also reflected in a red-shift of the longest
wavelength absorption in the UV/Vis spectra (see Table 1). It
is noteworthy that virtual transitions of rotational symmetry
are also responsible for magnetically induced paratropic
molecular ring/cluster currents. In heavy atoms, however, the
contributions from diatropic currents are orders of magnitude
smaller than those from the paratropic terms. Nevertheless,
both mechanisms occur inherently parallel.
In conclusion, we have prepared digerma- and distanna-
substituted heteronuclear derivatives of two recently reported
hexasilabenzene isomers. Very notably, the dismutationally
aromatic digermanium-derivative 8 is stable towards air and
moisture even in solution. Upon mild heating, however, 8
readily rearranges into the thermodynamically more stable,
but kinetically more labile propellane-type isomer 9. This
trend is reflected in the fact that the bridged distanna-
propellane 11 is accessible by the same methodology, while
the corresponding dismutational isomer cannot even be
observed at low temperature. Clear-cut trends in the aniso-
tropic NMR spectroscopic properties from silicon- to tin-
substituted derivatives confirm the extremely unusual elec-
tronic situation present in the entire class of compounds. To
our knowledge, the 29Si NMR chemical shifts assignable to the
untethered “propeller blades” of 9 and 11 represent the by far
most deshielded 29Si NMR resonance signals of tetracoordi-
nate silicon atoms in a molecular environment to date. The
enhanced reactivity of the hetero-substituted propellanes 9
and 11 raises interesting prospects for their application in the
construction of extended staffane-type materials,[33] a possi-
bility which is currently being investigated in our laboratories.
Received: December 3, 2013
Published online: February 26, 2014
Keywords: aromaticity · germanium · ring currents · clusters ·
.
silicon · tin
Figure 4. Molecular orbitals and MO energy differences (in Hartree) of
untethered propellane model species a) Si2Si3H6, b) Ge2Si3H6,
c) Sn2Si3H6, d) Pb2Si3H6, and bridged propellane model species
e) Si2Si4H6, f) Ge2Si4H6, g) Sn2Si4H6, h) Pb2Si4H6. HOMO(b1) denotes
the highest occupied orbital of b1 symmetry and HOMO(e’) the
highest occupied orbital of e’ symmetry.
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[30] The Tip-substituents were modeled as hydrogen atoms. Struc-
ture optimization was carried out using DFT with the PBE
density functional and cc-pVDZ basis sets on all elements except
Sn for which the SVP all-electron basis set was used. Further
non-standard settings were $denconv 1.0D-8 and $scfconv 7, and
$grid m4. The NMR signals were modeled with the ReSpect
program package[31] employing the PBE density functional
(PBEPBE, GGA) and uncontracted cc-pVDZ basis sets on
light elements along with Dyallꢂs VDZ basis on hetero atoms.
[31] a) ReSpect, version 3.3.0, 2013; Relativistic Spectroscopy DFT
program of authors M. Repisky, S. Komorovsky, V. G. Malkin,
O. L. Malkina, M. Kaupp, K. Ruud, with contributions from R.
Bast, U. Ekstrom, S. Knecht, I. Malkin, E. Malkin; b) S.
´
´
Komorovsky, M. Repisky, O. L. Malkina, V. G. Malkin, I. M.
[18] Experimental details are supplied in the Supporting Informa-
tion. CCDC 939615 (8), 939616 (9), 948751 (11) contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[19] All calculations have been performed with the TURBOMOLE
program package[20] version 6.3. The aryl groups were modeled
as Ph groups. The structure optimization of 8 and 9 was carried
out using RI-DFT with the BP86 density functional and def2-
SV(P) basis sets on all elements. The excitation energies were
Ondik, M. Kaupp, J. Chem. Phys. 2008, 128, 104101 – 104116;
´
´
c) S. Komorovsky, M. Repisky, O. L. Malkina, V. G. Malkin, J.
Chem. Phys. 2010, 132, 154101 – 154109.
[32] Orbitals (e)–(g) in Figure 4 show a minor additional lobe giving
rise to an additional nodal plane. However, this is of no
consequence for the qualitative description given herein.
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