Molecular structures of the digermanes Me2XGeGeXMe2 (X = Me, Cl, and H): An ab initio study combined with experimental investigation by raman spectroscopy and gas electron diffraction

Article abstract of DOI:10.1021/ic7018764

Ab initio calculations were carried out to investigate the potential-energy surface for internal rotation of the methylated digermanes hexamethyldigermane Me₃GeGeMe₃ (1), dichlorotetramethyldigermane Me ₂ClGeGeClMe₂ (2), and tetramethyldigermane Me ₂HGeGeHMe₂ (3). Different basis sets [6-31+G(d), SDD, aug-cc-pVTZ] were employed at the DFT and MP2 levels of theory to optimize structures and to calculate energies and vibrational frequencies. For 1, one minimum representing a staggered structure was located on the potential-energy surface. For 2 and 3, antiperiplanar conformations with C₂ symmetry were found to be the global minima. Additionally, synclinal minima were located for 2 and 3 when certain basis sets were employed. Determination of structural parameters in the gas phase by gas electron diffraction confirmed the computed predictions for all three compounds. For 2 and 3, the ratios of antiperiplanar to synclinal conformer were detected to be 90:10 (328 K) and 72:28 (293 K), respectively, by gas electron diffraction. The experimentally determined GeGe bond lengths in 1, 2, and 3 in the gas phase are 241.4(1), 242.7(2) (averaged for antiperiplanar and synclinal), and 241.7(1) pm (equal for antiperiplanar and synclinal). Only averaged structures were observed, using Raman spectroscopy, for 2 and 3 because the wavenumber differences are small between conformers and there is only a small contribution from the second conformer in each case. For 2, the crystal structure was also determined by X-ray diffraction. An anticlinal structure (with Cl atoms eclipsing the C atoms) was found with a GeGe bond length of 242.1 pm.

Full text of DOI:10.1021/ic7018764

Inorg. Chem. 2008, 47, 3023-3033
Molecular Structures of the Digermanes Me2XGeGeXMe2 (X ) Me, Cl,
and H): An Ab Initio Study Combined with Experimental Investigation
by Raman Spectroscopy and Gas Electron Diffraction
Margit Hölbling,† Sarah L. Masters (née Hinchley),*,‡ Michaela Flock,† Judith Baumgartner,†
Karl Hassler,† Heather E. Robertson,‡ and Derek A. Wann‡
Institut für Anorganische Chemie, Technische UniVersität Graz, Stremayrgasse 16,
A-8010 Graz, Austria, and School of Chemistry, UniVersity of Edinburgh, West Mains Road,
Edinburgh, EH9 3JJ, Scotland
Received September 21, 2007
Ab initio calculations were carried out to investigate the potential-energy surface for internal rotation of the methylated
digermanes hexamethyldigermane Me3GeGeMe3 (1), dichlorotetramethyldigermane Me2ClGeGeClMe2 (2), and
tetramethyldigermane Me2HGeGeHMe2 (3). Different basis sets [6–31+G(d), SDD, aug-cc-pVTZ] were employed
at the DFT and MP2 levels of theory to optimize structures and to calculate energies and vibrational frequencies.
For 1, one minimum representing a staggered structure was located on the potential-energy surface. For 2 and 3,
antiperiplanar conformations with C2 symmetry were found to be the global minima. Additionally, synclinal minima
were located for 2 and 3 when certain basis sets were employed. Determination of structural parameters in the gas
phase by gas electron diffraction confirmed the computed predictions for all three compounds. For 2 and 3, the
ratios of antiperiplanar to synclinal conformer were detected to be 90:10 (328 K) and 72:28 (293 K), respectively,
by gas electron diffraction. The experimentally determined GeGe bond lengths in 1, 2, and 3 in the gas phase are
241.4(1), 242.7(2) (averaged for antiperiplanar and synclinal), and 241.7(1) pm (equal for antiperiplanar and synclinal).
Only averaged structures were observed, using Raman spectroscopy, for 2 and 3 because the wavenumber
differences are small between conformers and there is only a small contribution from the second conformer in
each case. For 2, the crystal structure was also determined by X-ray diffraction. An anticlinal structure (with Cl
atoms eclipsing the C atoms) was found with a GeGe bond length of 242.1 pm.
Introduction
kJ mol-1 (MP2/6–31G*//6–31G*), which is almost the same
as for disilane.3 However, recent NMR spectroscopic rein-
vestigations found a higher rotational barrier for Me3SiSiMe3
of ∼7 kJ mol-1.4
The barriers to internal rotation around bonds between
elements in group 14 decrease within the series C-C, Si-Si,
Ge-Ge, and further to Sn-Sn and Pb-Pb. Although the
most investigated system in terms of conformations and
barriers is ethane and its derivatives, disilane has also been
extensively studied. For instance, while the rotational barrier
in ethane is ∼12.2 kJ mol-1,1 for disilane, it was determined
to be only 5.1 kJ mol-1.2 For hexamethyldisilane,
Me3SiSiMe3, Schleyer et al. reported a barrier of only ∼4
More complex disilanes, such as Me3SiSiH3,
MeX2SiSiX2Me with X ) H, F, Cl, Br, and I, ClMe2SiSi-
Me2Cl,5–7 and disilanes substituted with bulky Bu groups,
t
for example, tBuH2SiSiH2 Bu, tBu2HSiSiHtBu2, and
t
8–10
tBuX2SiSiX2 Bu,
have been studied computationally, as
t
(3) Schleyer, P.v.R.; Kaupp, M.; Hampel, F.; Bremer, M.; Mislow, K.
J. Am. Chem. Soc. 1992, 114, 6791–6797.
* To whom correspondence should be addressed. E-mail: s.masters@
ed.ac.uk.
(4) Aksnes, D. W.; Kimtys, L. Acta Chem. Scand. 1995, 49(10), 722–
727.
†
Technische Universität Graz.
University of Edinburgh.
(5) Mohamed, T. A. J. Mol. Struct. (THEOCHEM) 2003, 635, 161–172.
(6) Jähn, A.; Schenzel, K.; Zink, R.; Hassler, K. Spectrochim. Acta Part
A 1999, 55, 2677.
‡
(1) For instance, see: Eliel, E. L.; Wilen, S. H. Stereochemistry of Organic
Compounds; Wiley: New York, 1994.
(7) Kveseth, K. Acta Chem. Scand. 1979, A33(6), 453–462.
(8) Hnyk, D.; Fender, R. S.; Robertson, H. E.; Rankin, D. W. H.; Brühl,
M.; Hassler, K.; Schenzel, K. J. Mol. Struct. 1995, 346, 215–229.
(2) Beagley, B.; Conrad, A. R.; Freeman, J. M.; Monaghan, J. J.; Norton,
B. G.; Holywell, G. C. J. Mol. Struct. 1972, 11, 371–380.
10.1021/ic7018764 CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 8, 2008 3023
Published on Web 03/18/2008

Hölbling et al.
well as by Raman spectroscopy in the liquid state and by
gas electron diffraction (GED) in the gaseous state. For
Me2ClSiSiClMe2, the Si analogue of Me2ClGeGeClMe2 in
the current investigation, the anti conformer was predicted
to be the global minimum by RHF/SBK calculations with
Erel of the gauche conformer being 5.4 kJ mol-1.7 This was
verified by Raman spectroscopy for both the solid and liquid
states (solid, anti conformer only; liquid, Erel,gauche ) 4.2 kJ
mol-1).6 In the gas phase, however, the gauche conformer
was found to be the most stable by GED.7
when electron correlation is included for all except the
Ge-Ge system. The B3LYP method gives agreeable results
for the Ge2H6 geometry and vibrational frequencies, but the
rotational barriers are underestimated.13
From a recent theoretical investigation of cyclohexager-
mane by Hölbling et al., it is known that the barriers for
ring inversion calculated with B3LYP are very reasonable
when relativistic effects for Ge are included.15 Without these,
the barriers are overestimated by a factor of 4.
The conformational properties and rotational barriers of
the ethane analogues of Si, Ge, Sn, and Pb are well studied
theoretically and, in the cases of Si and Ge, also experimen-
tally. There are also numerous papers concerned with the
investigation of substituted di-, tri-, and tetrasilanes and so
forth. However, there is still a lack of information regarding
more complex structures of substituted digermanes (not to
mention the distannanes and diplumbanes). As for the
substituted disilanes, more than one stable conformer is
expected for substituted digermanes, although neither a
quantum chemical nor experimental investigation of the
rotational conformers of these molecules has yet been
published. Therefore, we were interested in studying the
conformational equilibria of methylated digermanes to learn
about conformational equilibria and rotational barriers.
In this paper, we report the molecular structures and con-
formational properties of hexamethyldigermane, Me3GeGeMe3
(1), dichlorotetramethyldigermane, Me2ClGeGeClMe2 (2), and
tetramethyldigermane, Me2HGeGeHMe2 (3). The combination
of quantum chemical calculations with Raman spectroscopy
(solid, liquid, and solution phase), X-ray diffraction (solid
phase), and gas electron diffraction (gas phase) is used to gain
information about rotational barriers and structures in all the
phases.
In the study of the conformations and rotational barriers
of the whole series H3XYH3 (X, Y ) C, Si, Ge, Sn, and
Pb), Schleyer et al. predicted that the rotational barriers
decrease from ∼11 to ∼1 kJ mol-1 from C to Pb, but do not
vanish.3 They predicted the staggered conformer as the
minimum [HF level employing 3–21G(d,p) for C, Si, and H
and split-valence basis sets of Huzinaga for Ge, Sn, and Pb].
Song et al.11 calculated the barriers to rotation of the same
series of compounds and found them in agreement, with a
lowering of the barrier from ∼13 to ∼1 kJ mol-1 at the MP2
level [employing the basis sets 6–31G(d) for C and Si and
LanL2DZ(d) for Ge, Sn and Pb]. It was deduced that the
barriers are dominated by steric repulsion. The hyperconju-
gative interaction between the X-H bonding orbitals and
the vicinal Y-H antibonding orbitals plays a secondary
role.11 The internal rotation in digermane was also investi-
gated by Goodman and co-workers at the MP2 level of
theory, employing the 6–311G(3df,2p) basis set.12 Using
NBO analysis, they found that the “quantum superposition”
(hyperconjugation between the germyl groups, σGe-H f
σ*Ge-H) accounts for only ∼40% of the barrier. This effect
of hyperconjugation is smaller than the Boltzmann energy
at ambient temperature and therefore insufficient to prevent
digermane from free rotation about the Ge-Ge bond. The
rotational barrier of ∼3.3 kJ mol-1 results from bond
weakening caused by steric repulsion in combination with
the hyperconjugation.12
Experimental Section
Synthesis. Compound 1 was prepared according to the method
reported by Triplett and Curtis.16 Potassium was melted in
cyclohexane, and chlorotrimethylgermane was added dropwise over
a period of 30 min. The solution was refluxed for 2 h and then
decanted. The pure product was obtained by fractional distillation
in a yield of 60%.
In a recent study, Urban et al. investigated the molecular
structures, vibrational spectra and rotational barriers of C2H6,
Si2H6, SiGeH6, and Ge2H6 at the HF, MP2, B3LYP, and
CISD levels, employing polarized triple-ꢀ basis sets.13
Although the discrepancy between theory and experiment
in determination of the Ge-Ge bond length in H3GeGeH3
has been discussed before,14 geometries and rotational
barriers were determined with acceptable accuracy at the
MP2 and DFT level.13 They found that the geometries
compare well, although the central E-E bond is shortened
To synthesize 2, a pure sample of 1 was allowed to react with
concentrated sulfuric acid for 24 h at room temperature. When the
gas evolution had stopped, ammonium chloride was added in small
portions, while the mixture was cooled in an ice bath. The solution
was then stirred for 30 min, and the precipitate which had formed
was separated and dissolved in pentane. Pure 2 was obtained by
crystallization from pentane and subsequent sublimation (0.1 Torr/
40 °C) in a yield of 85%.16
(9) Hinchley, S. L.; Robertson, H. E.; Parkin, A.; Rankin, D. W. H.;
Tekautz, G.; Hassler, K. Dalton Trans. 2004, 759, 766.
(10) Hinchley, S. L.; Smart, B. A.; Morrison, C. A.; Robertson, H. E.;
Rankin, D. W. H.; Coxall, R. A.; Parsons, S.; Zink, R.; Siegl, H.;
Hassler, K.; Mawhorter, R. J. J. Chem. Soc., Dalton Trans. 2001, 19,
2916–2925.
To prepare 3, a procedure for Me3SiSiH3 (reduction of
Me3SiSiCl3 with LiAlH4 in di-n-butylether) was adapted.17
A
solution of 2 in dimethyltriglycol (triglyme, dried over a molecular
sieve) was added dropwise to a dispersion of NaBH4 in triglyme
at 0 °C. The reaction was then stirred overnight at room temperature.
Fractional distillation at 30 Torr/130 °C gave 3 as a colorless liquid
(11) Song, L.; Lin, Y.; Wu, W.; Zhang, Q.; Mo, Y. J. Phys. Chem. A 2005,
109, 2310–2316.
(12) Goodman, L.; Pophoristic, V.; Wang, W. Int. J. Quantum Chem. 2002,
90, 657–662.
(13) Urban, J.; Schreiner, P. R.; Vacek, G.; Schleyer, P.v.R.; Huang, J. Q.;
Leszczynski, J. Chem. Phys. Lett. 1997, 264, 441–448.
(14) Oberhammer, H.; Sundermeyer, W. J. Mol. Struct. 1994, 323, 125–
128.
(15) Hölbling, M.; Lobreyer, T.; Flock, M.; Baumgartner, J.; Hassler, K.
Eur. J. Inorg. Chem. 2007, 4952–4957.
(16) Triplett, K.; Curtis, M. D. J. Organomet. Chem. 1976, 107, 23–32.
(17) Zink, R. PhD Dissertation, Universität Graz, Graz, Austria, 1997.
3024 Inorganic Chemistry, Vol. 47, No. 8, 2008

Molecular Structures of Digermanes
only (C). The MP2 and B3LYP levels employing the same basis
sets gave very similar results for geometry optimizations, as well
as for energy calculations. Therefore, calculations including triple-ꢀ
basis sets were only performed at the B3LYP level. Generally, all
DFT calculations were performed using the B3LYP functional. For
simplification, the methods will be referred to as DFT/basis set and
compared to MP2/basis set. Vibrational frequency calculations were
performed to determine the nature of the stationary points. Minima
possess all real frequencies, while transition structures have a single,
imaginary frequency. No symmetry restraints were used. Analytic
second derivatives of the energy with respect to nuclear coordinates
calculated for 1, 2, and 3 gave the force fields, which were used to
provide estimates of the amplitudes of vibration (uh1)22 and the
curvilinear corrections (kh1)22 for use in the GED refinements.
(Methods used for each compound are described in the Supporting
Information.)
Figure 1. Molecular structure of Me3GeGeMe3 (1) showing a perspective
view with atom numbering.
Gas Electron Diffraction Measurements. Data were collected
for 1–3 using the Edinburgh gas-phase electron diffraction ap-
paratus.23 An accelerating voltage of around 40 kV was used,
representing an electron wavelength of approximately 6.0 pm.
Scattering intensities were recorded on Kodak Electron Image films
at nozzle-to-film distances and sample and nozzle temperatures
given in Table S1. The weighting points for the off-diagonal weight
matrices, correlation parameters, and scale factors for both camera
distances for all compounds are given in Table S1. The electron
wavelengths as determined from the scattering patterns for benzene,
which were recorded immediately after the patterns for the sample
compounds, are also included. The scattering intensities were
measured using an Epson Expression 1680 Pro flatbed scanner and
converted to mean optical densities as a function of the scattering
variable, s, using an established program.24 The data reduction and
the least-squares refinement processes were carried out using the
ed@ed program25> employing the scattering factors of Ross et
al.26
Raman Spectroscopy. Raman spectra were recorded using a
Jobin Yvon T64000 spectrometer equipped with a triple mono-
chromator and a CCD camera. The samples were held in 1 mm
glass capillary tubes and irradiated by 532 nm green light from a
frequency-doubled Nd:YAG laser (DPSS model 532–20, 20 mW).
Spectra were recorded in the solid state for 2. Liquid spectra were
recorded for pure samples of 1, 2, and 3 and solutions of 1 and 2.
A continuous-flow cryostat (Oxford Instruments OptistatCF using
liquid nitrogen for cooling) was employed for the low-temperature
measurements.
in a yield of 50%. The purity of all compounds was checked with
1H and 13C NMR and Raman spectroscopy, and the information
was compared with literature data.16
Crystallography. The crystal structure of 2 was determined by
mounting a crystal on the tip of a glass fiber. Data collection was
performed with a BRUKER-AXS SMART APEX CCD diffracto-
meter using graphite-monochromated Mo KR radiation (0.71073
2
Å). The data were reduced to F0 and corrected for absorption
effects using SAINT18 and SADABS,19 respectively. The structure
was solved by direct methods and refined by the full-matrix least-
squares method (SHELXL97).20 All non-hydrogen atoms were
refined with anisotropic displacement parameters. All hydrogen
atoms were located in calculated positions to correspond to standard
bond lengths and angles.
Computational Methods. All geometry optimizations and
energy minimizations, as well as the calculations of IR and Raman
frequencies, were performed using the Gaussian0321 package at
the DFT and the MP2 levels of theory. Extensive searches of the
torsional potentials of all three compounds were undertaken at the
B3LYP and MP2 levels of theory, employing the SDD (hereafter
referred to as basis set A) and 6–31+G* (B) basis sets to locate all
minima. In some cases mixed basis sets were used, applying
6–31+G* to C, H, and Cl and SDD to Ge. These mixed basis sets
are referred to as A#. The aug-cc-pVTZ-PP basis set for Ge,
including quasi-relativistic Stuttgart Dresden effective-core poten-
tials (ECP), and aug-cc-pVTZ for C, H, and Cl were employed for
optimizations and energy calculations at the B3LYP level of theory
(18) SAINTPLUS: Software Reference Manual, version 6.45; Bruker-AXS:
Madison, WI, 1997–2003.
Results
(19) Blessing, R. H. Acta Crystallogr. A 1995, 51, 33–38; SADABS, version
2.1; Bruker AXS: Madison, WI, 1998.
Computational Methods. Me3GeGeMe3 (1). The PES
of 1 was probed by scanning the torsional angle around the
central Ge-Ge bond at the DFT/A and A#, DFT/B, MP2/A,
and MP2/B levels of theory. A geometry optimization at the
DFT/C level was also performed. In each case, a staggered
structure was found to be the most favorable conformation
(20) Sheldrick, G. M. SHELX97, Programs for Crystal Structure Analysis,
release 97–2; Universität Göttingen: Göttingen, Germany, 1998.
(21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin,
K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone,
V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene,
M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.;
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O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala,
P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;
Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas,
O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;
Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
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(22) Sipachev, V. A. J. Mol. Struct. (THEOCHEM) 1985, 22, 143–151.
(23) Huntley, C. M.; Laurenson, G. S.; Rankin, D. W. H. J. Chem. Soc.,
Dalton Trans. 1980, 6, 954–957.
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Inorganic Chemistry, Vol. 47, No. 8, 2008 3025

Hölbling et al.
Table 1. Calculated Geometric Parameters (Distances in pm, Angles in deg) of Ge2Me6 Obtained Employing Different Methods (See Theoretical
Methods Above) and Refined Geometric Parameters for Ge2Me6 (Distances in pm, Angles in deg) from the SARACEN27 GED studya b
,
method
parameter
DFT/Ac
DFT/A#c
DFT/B
MP2/A
MP2/B
MP2/C
GED (rh1)
GED restraintd
rGe-Ge
250.0
200.6
109.7
110.9
110.4
-51.1
250.0
200.6
109.5
110.4
110.3
-49.7
243.8
196.3
109.5
110.6
110.6
-58.4
249.0
202.1
110.6
110.8
110.5
-60.0
243.2
195.6
109.5
110.1
110.6
-60.0
246.4
198.1
109.0
110.8
110.5
-58.4
241.4(1)
195.7(1)
110.4(3)
110.3(1)
112.4(4)
-59.3(14)
0.3(3)
rGe-C
rC-Hav
∠Ge-Ge-C
∠Ge-C-Hav
ꢁC(4)-Ge-Ge-C(19)
Me tilt
-59.2(15)
0.3(3)
a Figures in parentheses are the estimated standard deviations of the last digits. b See text for parameter definitions. c The ꢁC-Ge-Ge-C values are
almost certainly artifacts of the computational method, with all other methods returning values of about -60°. d Obtained from the average torsion obtained
by DFT/B, MP2/A, MP2/B, and MP2/C.
of a second stable conformer is indicated by a flat region in
the potential curve. Employing MP2/A# led to the location
of a second synclinal minimum.
Interestingly, employing all-electron or triple-ꢀ basis sets
(B, C) also resulted in two minima being located on the PES
of 2. All calculated minima are summarized in Table 2. The
stable antiperiplanar and synclinal conformers, located
employing MP2/B, are shown in Figure 3.
As for 1, the calculated geometric parameters depended
Figure 2. Torsion potential for Me2ClGeGeClMe2 (2) obtained employing
DFT/A# (b) and MP2/A# (2).
on the method used. Selected parameters for 2 (antiperiplanar
being referred to as conformer 1, synclinal as conformer 2)
are given in Table 2. The SDD basis set overestimates the
GeGe bond by 7 and 5 pm with B3LYP and MP2,
respectively, compared to the experimental value of 243.7(5)
pm. Employing basis sets B and C yields bond lengths in
reasonable agreement with the experimental value.
[see Figure S1 (Supporting Information Figure 1)]. The
molecular structure of 1 with atom numbering is shown in
Figure 1.
Generally, the calculated geometric parameters depend on
the basis sets employed. Calculations at the DFT/C level gave
a structure very similar to that obtained by DFT/B. Methods
that include quasi-relativistic pseudopotentials for Ge (A, A#)
tend to overestimate the GeGe bond length by about 8 pm
compared to the gas-phase structure determined by GED. The
GeC bond lengths are also calculated to be ∼5 pm longer. The
use of relativistic triple-ꢀ basis sets (C) for Ge also tends to
overestimate GeGe and GeC bonds by about 5 and 2 pm,
respectively. The barrier to internal rotation was calculated
employing DFT/A and MP2/B. For DFT/A, an eclipsed
transition structure possessing D3h symmetry was found, with
a relative energy of 2.1 kJ mol-1, representing the barrier to
internal rotation. The barrier obtained with MP2/B is 5.8 kJ
mol-1, with the larger value resulting from the shorter computed
GeGe bond. A summary of selected geometry parameters from
different methods is given in Table 1.
The barrier for the synclinal to antiperiplanar transition is
very small. The transition structure has a ClGeGeCl dihedral
angle between 94 and 113°, depending on the method
employed. Using MP2/A#, we calculated the barrier to be
0.5 kJ mol-1, but with DFT/B, it is only 0.12 kJ mol-1. The
largest barrier is calculated with MP2/B as being 2.89 kJ
mol-1. This relatively large value results from the short GeGe
bond length found with this method (see Table 2).
At about 20 kJ mol-1, the barrier for the antiperiplanar to
antiperiplanar transition is surprisingly large (see Figure 2),
much larger than would be expected from the van der Waals
radius of the chlorine atom, which is slightly smaller than
for a methyl group (170–190 pm for Cl, 200 pm for Me).
Therefore, the barrier height must be attributable to the small
GeGeCl bond angle, which causes a large nonbonded Cl · · ·Cl
interaction in the transition state, characterized by ꢁClGeGeCl
) 0.0°. Depending on the method and basis set used, the
nonbonded Cl · · · Cl distances are calculated to be between
377 and 400 pm, and GeGeCl angles widen to about 108°.
The GeGe bond is somewhat lengthened in the transition
state (246–252 pm), but this does not contribute much to
the overall height of the energy barrier. The PES has been
probed with respect to the GeGe bond length, with results
shown in Figure 4. For both DFT/A and DFT/B, a distortion
Me2ClGeGeClMe2 (2). The PES for 2 was probed by
rotating about the central Ge-Ge bond using mixed basis
sets at the DFT/A# and MP2/A# levels. The antiperiplanar
conformer was found to be the global minimum. However,
a stable synclinal conformer was located, depending on the
method employed. In Figure 2 the torsion potentials obtained
with DFT/A# and MP2/A# are given. Although no synclinal
minimum can be located with DFT/A#, the possible existence
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(27) (a) Brain, P. T.; Morrison, C. A.; Parsons, S.; Rankin, D. W. H.
J. Chem. Soc., Dalton Trans. 1996, 24, 4589–4596. (b) Blake, A. J.;
Brain, P. T.; McNab, H.; Miller, J.; Morrison, C. A.; Parsons, S.;
Rankin, D. W. H.; Robertson, H. E.; Smart, B. A. J. Phys. Chem.
1996, 100, 12280–12287. (c) Mitzel, N. W.; Rankin, D. W. H. Dalton
Trans 1996, 24, 3650–3662.
(31) Weidenbruch, M.; Grimm, F.-T.; Herrndorf, M.; Schaefer, A.; Peters,
K.; von Schnering, H. G. J. Organomet. Chem. 1988, 341, 335–343.
(32) Griffiths, J. E.; Walrafen, G. E. J. Chem. Phys. 1964, 40, 321–328.
3026 Inorganic Chemistry, Vol. 47, No. 8, 2008

Molecular Structures of Digermanes
Table 2. Geometric Parameters (Distances in pm, Angles in deg) and Relative Energies (kJ mol-1) of Me2ClGeGeClMe2 (2) Obtained with Different
Methods
method
parameter
DFT/A
DFT/A#
DFT/B
MP2/A
MP2/A#
MP2/B
DFT/C
conformer 1
rGe-Ge
249.1
198.1
233.4
109.6
114.8
102.1
109.8
180.0
0.0/0.0
249.5
198.4
227.9
109.4
114.3
103.1
109.8
180.0
0.0/0.0
242.7
194.3
223.9
109.4
113.6
102.9
110.0
180.0
0.0/0.0
248.2
199.5
233.7
110.4
114.5
102.6
109.9
180.0
0.0/0.0
247.6
197.2
226.4
109.4
114.1
102.5
110.2
180.0
0.0/0.0
241.3
193.4
221.1
109.4
113.1
102.4
110.0
180.0
0.0/0.0
246.0
196.3
222.8
108.9
114.2
103.5
109.8
180.0
0.0/0.0
rGe-Cav
rGe-Cl
rC-Hav
∠Ge-Ge-Cav
∠Ge-Ge-Cl
∠Ge-C-Hav
ꢁCl-Ge-Ge-Cl
Erel/Grel
conformer 2
rGe-Ge
243.8
194.6
222.8
109.5
113.6
103.9
110.0
-78.0
7.3/7.3
248.7
197.5
225.3
109.4
114.0
104.0
110.1
-85.4
8.5/8.0
242.5
193.7
220.0
109.4
113.9
101.6
110.0
-69.4
6.4/7.9
rGe-Cav
rGe-Cl
rC-Hav
∠Ge-Ge-Cav
∠Ge-Ge-Cl
∠Ge-C-Hav
ꢁCl-Ge-Ge-Cl
Erel/Grel
Table 3. Experimental and Calculated Wavenumbers (cm-1) of the Most Intense Lines in the Raman Spectrum of Me2ClGeGeClMe2 (2) in the Range
of 200-650 cm-1
method
DFT/B
antiperiplanar
MP2/A#
antiperiplanar
MP2/B
antiperiplanar
mode
experiment
synclinal
synclinal
synclinal
νGeGe
νsGeCli-ph
νsGeC2
275
359
593
277
356
595
270
369
592
269
359
576
264
371
575
291
384
624
286
398
623
of the GeGe bond by 5 pm raises the energy by just 1 kJ
mol-1. An estimation of the potential energy with respect to
the torsion around the GeGe bond can be derived from the
geometry optimizations, where the variation of the ClGeGeCl
dihedral angle of the synclinal conformer by 20° either side
of the equilibrium position results in a maximum change of
the relative energy of about 2 kJ mol-1.
Calculation of vibrational frequencies was performed with
DFT/B, MP2/A#, and MP2/B. Although the absolute wave-
numbers vary with the different methods, the wavenumber
differences between the two conformers for each vibration
are almost identical (see Table 3). For νGeGe this difference
was calculated to be 5-7 cm-1 between antiperiplanar and
synclinal conformers. The difference in wavenumbers of the
in-phase (i-ph) GeCl stretching mode, νGeCli-ph, is the largest
Figure 3. Molecular structures of the two observed conformers of
Me2ClGeGeClMe2 (2) at the MP2/B level with atom numbering: (a)
ꢁCl-Ge-Ge-Cl ) 180.0 ° and (b) ꢁCl-Ge-Ge-Cl ) -69.4°.
Figure 4. Dependence of the potential energy of Me2ClGeGeClMe2 (2)
on the GeGe bond length obtained employing DFT/A (b) and MP2/B (2).
All other coordinates were allowed to optimize fully.
Inorganic Chemistry, Vol. 47, No. 8, 2008 3027

Hölbling et al.
(12 to 14 cm-1). Despite this large difference, no separate
bands are expected in the Raman spectrum because of the
small barrier between the conformers, which lies significantly
below the Boltzmann energy at room temperature (2.4 kJ
mol-1). Therefore only an averaged spectrum is expected.
The calculated wavenumber difference of the Raman-active
symmetric GeC stretching mode, νsGeC2, is only 1-3 cm-1
between antiperiplanar and synclinal conformers.
amplitudes of vibration and curvilinear corrections for the
GED refinement were performed using DFT/C#.
The barriers separating the minima are very small, as can
be seen from Figure 6, which shows some torsion potentials.
The barriers were calculated to be in the range from 0.6
(MP2/A) to 2.6 kJ mol-1 (DFT/C#). Again, these barriers
are in the range of the Boltzmann energy at room temperature.
The predicted GeGe bond lengths depend heavily on the
basis set employed. Using the all-electron basis set (B) gives
GeGe bond lengths of about 244 pm for both DFT and MP2.
Including quasi-relativistic effects and pseudopotentials with
the SDD basis set gives longer bonds of about 249 pm
(DFT/A and MP2/A). DFT/C, which includes relativistic
triple-ꢀ basis sets, gives GeGe bond lengths of 245.1 pm
for the antiperiplanar conformer and 245.3 pm for the
synclinal conformer. The GeC bond lengths exhibit the same
basis-set dependence as the GeGe bonds. Using SDD results
in an overestimation of ∼4 pm, while the all-electron basis
set predicts them to be a little shorter than determined in
the gas phase by GED. The GeH bonds are overestimated
slightly with all methods tested, except with basis set C,
which gives a GeH bond length of about 155 pm. Some
selected geometric parameters for the structures obtained with
different methods are given in Table 4.
Frequencies were calculated employing DFT/A, MP2/A,
and DFT/C# methods. The calculated wavenumbers of
νGeGe, νsGeC2, and νGeHi-ph are given in Table 5. These
three vibrations give the most intense lines in the Raman
spectrum. From the small differences in calculated wave-
numbers (∼3, 2, and 7 cm-1, respectively), as well as the
small barrier for conformer interconversion, no separated
bands of the antiperiplanar and synclinal conformers are
expected in the Raman spectrum.
Me2HGeGeHMe2 (3). The PES of 3 was probed with the
DFT/A, DFT/B, MP2/A, and MP2/B methods. Calculations
including the all-electron basis set B for Ge led to only one
stable conformer, which has an almost eclipsed structure. It
possesses C2 symmetry with a HGeGeH dihedral angle of
-110°. These results contradict the GED experiments, which
clearly prove the existence of synclinal and antiperiplanar
conformers in the gas phase.
The scans employing basis sets including relativistic effects
for Ge (A, A#, and C) led to two minima, antiperiplanar
and synclinal. With the DFT/C method, an antiperiplanar
conformer was located as the global minimum on the PES;
the synclinal conformer was found to be 2.3 kJ mol-1 higher
in energy. These two minima, with atom numbering, are
shown in Figure 5. Employing DFT with mixed basis sets,
6–31+G* for C and H and aug-cc-pVTZ-PP for Ge (in the
following referred to as C#), gave very similar geometries
and relative energies to those obtained with DFT/C. How-
ever, the computing time was considerably reduced. There-
fore the frequency calculations to provide estimates of the
GED Refinements. On the basis of the ab initio calculations
described above, electron-diffraction refinements were carried
out for 1–3. A model with overall pseudo-C2 symmetry to
describe the gaseous structure of 1, and models with two
conformers, both of C2 symmetry, were used to describe the
gas-phase structures of 2 and 3. See Supporting Information
for a full description of the parameters.
The final refinements for 1–3 provided good fits to the
data, with RG ) 0.069 (RD ) 0.058) for 1, RG ) 0.085
Figure 5. Molecular structures of the two conformers of Me2HGeGeHMe2
(3) at the DFT/C# level of theory with atom numbering: (a) ꢁH-Ge-Ge-H
) -179.9° and (b) ꢁH-Ge-Ge-H ) -67.3°.
Figure 6. Torsion potential of Me2HGeGeHMe2 (3) obtained at the DFT/A
(b), MP2/B (2), and DFT/C# level of theory (×, only the relative energies
of stationary points were calculated).
3028 Inorganic Chemistry, Vol. 47, No. 8, 2008

Molecular Structures of Digermanes
Table 4. Geometric Parameters (Distances in pm, Angles in deg) and Relative Energies (kJ mol-1) of Me2HGeGeHMe2 (3) Obtained with Different
Methods
method
parameter
DFT/A
DFT/B
MP2/A
MP2/B
DFT/C
DFT/C#
conformer 1
rGe-Ge
249.2
200.2
156.8
109.7
111.5
109.2
110.3
-179.8
0.0/0.0
248.7
201.9
157.3
110.5
111.2
109.3
110.4
-179.8
0.2/0.0
245.3
197.8
154.3
108.9
111.6
108.8
110.3
-179.9
0.0/0.0
245.1
197.8
155.1
109.5
111.7
108.8
110.5
-179.9
0.0/0.0
rGe-C
rGe-H
rC-Hav
∠Ge-Ge-C
∠Ge-Ge-H
∠Ge-C-Hav
ꢁH-Ge-Ge-H
Erel/Grel
conformer 2
rGe-Ge
249.4
200.3
156.7
109.7
111.7
108.8
110.3
-68.3
0.8/4.2
243.9
195.7
157.2
109.6
110.6
108.3
110.7
-110.8
0.0/0.0
248.8
201.9
157.2
110.5
111.0
109.8
110.4
-73.5
0.0/2.4
244.2
195.1
157.9
109.5
111.1
108.6
110.6
-110.6
0.0/0.0
245.4
197.9
154.8
108.9
111.8
108.6
110.3
-67.3
0.9/2.3
245.3
197.9
155.0
109.5
111.8
108.6
110.5
-67.3
1.0/2.4
rGe-C
rGe-H
rC-Hav
∠Ge-Ge-C
∠Ge-Ge-H
∠Ge-C-Hav
ꢁH-Ge-Ge-H
Erel/Grel
Table 5. Experimental and Calculated Wavenumbers (cm-1) of the Most Intense Lines in the Raman Spectrum of Me2HGeGeHMe2 in the Range of
200-2100 cm-1
method
DFT/A
antiperiplanar
MP2/A
antiperiplanar
DFT/C#
antiperiplanar
mode
experiment
synclinal
synclinal
synclinal
νGeGe
νsGeC2
νGeHi-ph
268
584
2016
249
556
1980
245
558
1987
260
558
2011
257
560
2018
251
563
2054
248
565
2060
(RD ) 0.059) for 2, and RG ) 0.091 (RD ) 0.058) for 3.
The radial-distribution curves for 1–3 are given in Figures
7-9 The molecular-scattering intensity curves are given
in the Supporting Information (Figures S2-S4). Final
refined parameters are listed in Tables 1, 6, and 7. The
interatomic distances and corresponding rms amplitudes
of vibration are given as supporting material in Tables
S2-S4. The least-squares correlation matrices are given
in Tables S5-S7, and the coordinates of the final refined
structures from the GED investigation are given in Tables
S8-S10.
Raman Spectroscopy. Me3GeGeMe3 (1). The vibrational
frequencies and IR and Raman intensities were calculated
at the DFT/A# and DFT/B levels, where the wavenumbers
of the GeGe stretching mode, νGeGe, are calculated as
249/267 cm-1, respectively. The Raman-active symmetric
and asymmetric GeC stretching modes, νsGeC3 and
νasGeC3, are 528/570 and 551/592 cm-1, respectively. In
the Raman spectrum, these bands are observed at 273,
573, and 588 cm-1, in excellent agreement with the DFT/B
Figure 8. Experimental and difference (experimental - theoretical) radial-
distribution curves, P(r)/r, for Me2ClGeGeClMe2 (2). Before Fourier
Figure 7. Experimental and difference (experimental - theoretical) radial-
distribution curves, P(r)/r, for Me3GeGeMe3 (1). Before Fourier inversion
the data were multiplied by s exp(-0.00002s2)/(ZGe - fGe)(ZC - fC).
inversion the data were multiplied by s exp(-0.00002s2)/(ZGe - fGe)(ZCl
fCl).
-
Inorganic Chemistry, Vol. 47, No. 8, 2008 3029

Hölbling et al.
243.7 pm (R ) 0.033, RT).29 A shorter GeGe bond of 241.9
pm was found in Ph3GeGeMe3 (R ) 0.036, RT).30 The
longest GeGe bond has been detected in Ge2But6 (271.3 pm,
R) 0.028, RT).31
For 2, the GeGeCl angles are 104.8 and 105.8°, respec-
tively. These small angles, which are also observed in the
gas phase, may be explained by the tendency of germanium
to become pentacoordinated, causing an attractive intramo-
lecular interaction between a germanium atom and the vicinal
chlorine atom. The CGeCl angles are also smaller than the
tetrahedral angle (109.5°) with an average of 104.5°.
Interestingly, the Cl(1)Ge(1)Ge(2)Cl(2) dihedral angle in the
crystal is 124.4°, while the C(1)Ge(1)Ge(2)Cl(2) dihedral
angle is only about 10°.
Figure 9. Experimental and difference (experimental - theoretical) radial-
distribution curves, P(r)/r, for Me2HGeGeHMe2 (3). Before Fourier
Discussion
inversion the data were multiplied by s exp(-0.00002s2)/(ZGe - fGe)(ZC
fC).
-
Quantum chemical investigations of the conformational
properties of methylated digermanes Me2XGeGeXMe2 (X
) Me, Cl, or H) (1, 2, and 3) have been performed at
different levels of theory. The potential-energy curves for
internal rotation of all three compounds are characterized
by shallow minima, which make accurate location and
optimization difficult. However, sufficiently accurate results
were obtained by probing the internal rotation with different
methods and basis sets. Calculations employing basis sets
that include quasi-relativistic or relativistic ECP’s (basis sets
A, A# and C, C#) generally give longer Ge-Ge, Ge-C,
and Ge-Cl bond lengths than all-electron basis sets for the
digermanes 1–3. However, calculations employing triple-ꢀ
basis sets (C, C#) give good agreement with experimentally
determined values for these bond lengths. The calculated
barriers for internal rotation of 1 and 3 are in the range of
2-5 kJ mol-1. This is in reasonable agreement with the
predictions for the barrier in digermane of about 6 kJ mol-1
estimated with vibrational spectroscopy.32 For 2, the
antiperiplanar-synclinal barrier is in the same range. This
isindirectcontrasttothebarrierfortheantiperiplanar-antiperi-
planar interconversion, via an eclipsed ClGeGeCl torsion,
which is ∼20 kJ mol-1. Despite the longer GeGe bond, the
barrier is much larger than that reported for Si2Cl6, which is
4.2 kJ mol-1.33 The reason for this increased value is the
tendency of fourth and higher row elements to become
pentacoordinated in the presence of halogen atoms, in this
case caused by an attractive intramolecular Ge · · ·Cl interac-
tion. It can be seen that, for these compounds, calculated
structural parameters depend much more on the basis sets
employed than is the case for the related C and Si systems,
where no relativistic effects have to be considered.
results. Raman spectra of the pure liquid, as well as of
solutions in THF and benzene, in the range of 200 –>700
cm-1 are shown in Figure 10. No differences between
these spectra were observed.
Me2ClGeGeClMe2 (2). Raman spectra of the pure crystal-
line compound, as well as solutions in hexane and benzene,
were recorded at different temperatures. The Raman spectrum
of 2 in benzene solution is shown in Figure 11. The spectrum
does not change at all within the temperature range of 265–293 K
because of the small barriers separating the conformers.
An anticlinal conformation (also observed in the crystalline
sample, see below) was observed by Raman spectroscopy. Four
bands were observed in the range of GeC stretching vibrations
from 560 to 640 cm-1 (see Figure 12), indicating the absence
of an inversion center in the molecule. For the eclipsed structure
(C2, no inversion center), the selection rules predict four Raman-
active bands in this region, while for the anti conformer (C2h,
including an inversion center), only two Raman-active bands
are expected.
Me2HGeGeHMe2 (3). Raman spectra of pure liquid 3
were recorded at different temperatures, with the room
temperature spectrum shown in Figure 13. The GeGe
stretching mode, νGeGe, is observed at 268 cm-1 as a sharp
line that shows no asymmetry, which would be expected if
it consisted of two lines originating from antiperiplanar and
synclinal conformers. Only an averaged spectrum, which
does not change with temperature, is observed.
Crystal Structure Determination. Me2ClGeGeClMe2
(2). Two crystal structures of 2, with R ) 0.028 and 0.033,
have been determined at temperatures of 100 and 223 K,
respectively. Crystal data, structure refinement, and structural
parameters are given in Table S11. An anticlinal structure
was found at both temperatures, as shown in Figure 14. The
central GeGe bond is found to be 242.1(1) pm long;
the average GeCl bond length is 220.7(1) pm, and the average
GeC bond length is 193.4(9) pm. In comparison, the GeGe
bond length in Ge6Ph12 has been determined to be 246.6 pm,
and the dihedral angles are 49.2 and -48.9° [R ) 0.071,
room temperature (RT)].28 In Ge2Ph6, the bond length is
The dependency of the results upon the basis set used is
highlighted by the investigation of the potential-energy surface
of 3. Calculations with basis set A result in two minima,
antiperiplanar and synclinal, in agreement with the GED
experiment. The use of basis set B results in only one eclipsed
conformer being observed, and the antiperiplanar conformer is
predicted to be a maximum on the PES, albeit only ∼1 kJ mol-1
(33) (a) Swick, D. A.; Karle, I. L. J. Chem. Phys. 1955, 23, 1499–1504.
(b) Morino, Y.; Hirota, E. J. Chem. Phys. 1958, 28, 185–197.
3030 Inorganic Chemistry, Vol. 47, No. 8, 2008

Molecular Structures of Digermanes
Table 6. Refined and Calculated Geometric Parameters for Me2ClGeGeClMe2 (2) (Distances in pm, Angles in deg) from the SARACEN27 GED
Studya b
,
number
parameter
MP2/B (re)
GED (rh1)
restraint
109.4(5)
p1
p2
p3
p4
p5
p6
p7
p8
rC-H
109.4
193.4
241.9
1.2
220.6
1.1
109.9
113.6
2.2
2.8
102.0
0.8
106.6
105.9
106.3
62.8
108.7(3)
192.0(2)
242.7(2)
1.2(5)
219.6(2)
0.9(5)
109.8(4)
112.7(6)
2.2(5)
rGe-C
rGe-Geav
rGe-Gediff
1.2(5)
220.6(10)
1.1(5)
rGe-Clav
rGe-Cldiff
∠Ge-C-H
∠Ge-Ge-Cav
∠Ge-Ge-Cdiff1
∠Ge-Ge-Cdiff2
∠Ge-Ge-Clav
∠Ge-Ge-Cldiff
109.9(5)
p9
2.2(5)
2.8(5)
p10
p11
p12
p13
p14
p15
p16
p17
p18
p19
p20
p21
2.8(5)
103.1(3)
0.7(5)
0.8(5)
∠C-Ge-Cl (conformer 1)
105.4(7)
105.8(10)
106.2(10)
62.7(24)
-65.3(25)
68.0(25)
179.9(20)
-69.5(25)
0.90(fixed)
∠C-Ge-Cl (conformer 2)
105.9(10)
106.3(10)
62.8(25)
-65.3(25)
68.0(25)
180.0(20)
-69.4(25)
∠C-Ge-Cl (conformer 2)
ꢁH(6)-C(3)-Ge(1)-Ge(2) (conformer 1)
ꢁH(10)-C(7)-Ge(1)-Ge(2) (conformer 2)
ꢁH(6)-C(3)-Ge(1)-Ge(2) (conformer 2)
ꢁCl(19)-Ge(1)-Ge(2)-Cl(20) (conformer 1)
ꢁCl(15)-Ge(1)-Ge(2)-Cl(20) (conformer 2)
-65.3
68.0
180.0
-69.4
0.95
c
fraction of conformer 1 in vapor
a Figures in parentheses are the estimated standard deviations of the last digits. b See text for parameter definitions. c It was found that varying the torsion
from -69.4° (MP2/B) to -85.4° (MP2/A#) made no difference to the goodness-of-fit factor; 0.90 was found to be the optimal amount of conformer 1 in
the vapor. This was determined by a loop of the parameter value from 0.6 to 0.95 in steps of 0.05. This information, combined with the restraints needed
to refine parameters relating to conformer 2, indicates that there is little information about conformer 2 contained within the data.
Table 7. Refined and Calculated Geometric Parameters for Me2HGeGeHMe2 (3) (Distances in pm, Angles in deg) from the SARACEN27 GED Studya b
,
number
parameter
DFT/C# (re)
GED (rh1)
restraint
p1
p2
p3
p4
p5
p6
p7
p8
rC-H
109.5
197.8
245.1
155.1
110.5
111.7
108.8
107.7
59.7
109.7(7)
196.5(2)
241.7(1)
154.9(5)
111.7(7)
111.6(2)
109.3(9)
108.7(6)
59.7(3)
-179.8(19)
58.5(19)
-67.3(19)
0.72(fixed)
rGe-C
rGe-Ge
rGe-H
155.1(5)
110.5(10)
∠Ge-C-H
∠Ge-Ge-C
∠Ge-Ge-H
C-Ge-H
ꢁH-C-Ge-Ge (conformer 1)
ꢁH-Ge-Ge-H (conformer 1)
ꢁH-C-Ge-Ge (conformer 2)
ꢁH-Ge-Ge-H (conformer 2)
fraction of conformer 1 in the vapor
108.8(10)
107.7(10)
p9
p10
p11
p12
p13
-179.9
-179.9(20)
58.5(20)
-67.3(20)
58.5
-67.3
0.72
a Figures in parentheses are the estimated standard deviations of the last digits. b See Supporting Information for parameter definitions.
Figure 10. Raman spectra of Me3GeGeMe3 (1) at room temperature. The
GeGe and GeC stretching vibrations between 200 and 700 cm-1 are shown.
Figure 11. Raman spectra of Me2ClGeGeClMe2 (2) in benzene solution
higher in energy, which is well within the Boltzmann energy
distribution at room temperature. Experimental structure data
for compounds composed of fourth and higher row elements
therefore are important for assessing the validity and quality of
quantum chemical calculations.
at different temperatures. The GeGe, GeCl, and GeC stretching vibrations
between 250 and 750 cm-1 are shown.
barriers for internal rotation are much smaller than those for
their sila analogues. Contrary to the conformational analyses
of SiC- and SiSi-containing systems, Raman spectroscopy is
not able to resolve bands originating from different conformers
Experimental investigations of 1–3 have been performed with
Raman spectroscopy. As predicted by the calculations, the
Inorganic Chemistry, Vol. 47, No. 8, 2008 3031

Hölbling et al.
conformers could not be resolved by Raman spectroscopy, the
conformational properties of 1–3 have also been probed by gas
electron diffraction. Because GED has a time scale ∼107 times
faster than Raman spectroscopy, any additional conformers
should be observable. One staggered structure is found in the
gas phase for 1. For 2, two conformers were located by GED,
antiperiplanar and synclinal, although the vapor consisted mainly
of the antiperiplanar conformer (90%). For 3, a mixture of
synclinal and antiperiplanar conformers is also found in the gas
phase, this time with 72% of the antiperiplanar conformer.
The high flexibility of these systems is underlined by the most
interesting anticlinal solid-state molecular structure of 2. In this
structure, each chlorine ligand is eclipsed by a methyl group, a
structure that was found to be a maximum on the potential-
energy surface for the gaseous molecules. Therefore some strong
intermolecular interactions must be present to outweigh the
unfavorable atomic arrangement. Examination of the crystal
structure indicates a widening of the angle between the methyl
groups attached to germanium. This widening occurs at both
ends of the molecule. The crystal packing indicates a reasonably
close interaction between the chlorine and germanium atoms
of adjacent molecules; the distance is 380 pm, compared to 375
pm for the sum of the van der Waals radii. There is also a strong
intramolecular interaction between the chlorine and germanium
atoms within the same molecule. The combined effect of these
interactions may explain the eclipsed structure observed in the
solid state.
Figure 12. Raman spectrum of crystalline Me2ClGeGeClMe2 (2) at room
temperature in the range of the GeC stretching vibrations (560-640 cm-1).
A similar anticlinal molecular structure was found for solid
Me2ClSnSnClMe2 (113 K, R ) 0.071) with ꢁClSnSnCl )
128.0° The CSnSnCl dihedral angles are 21.5 and 122.1°,
and at 99.4° and 102.3°, the ClSnSn angles are significantly
smaller than tetrahedral.34 Because of the intermolecular
Sn · · · Cl interactions [324.0(3) and 329.2(3) pm], a double-
chain structure with pentacoordinated Sn atoms is present
in the crystal. However, no interactions stronger than van
der Waals contacts were detected for 2, and no crystal
structure of the silicon analogue could be found in the
literature but it may be presumed that no interactions stronger
than van der Waals contacts would be observed for it either.
One may expect that this is purely caused by steric reasons,
with the shorter Si-Si and Ge-Ge bonds preventing
interaction between the chlorine of another molecule and the
silicon or germanium. However, in the solid-state structures
of far less crowded systems, such as Me3SiCl, no interactions
between Cl and Si are observed,35 whereas the Sn analogue
forms aggregates.36 Hypercoordination around silicon and
germanium is known, for example, H3SiCl forms a Si · · ·O
interaction with dimethylether,37 and Me3GeCN forms
infinite chains with Ge · · · N interactions.38 Thus pentacoor-
dination about silicon and germanium is clearly possible but
Figure 13. Raman spectrum of pure liquid Me2HGeGeHMe2 (3), recorded
at room temperature. The region containing the GeGe, GeC, and GeH
stretching vibrations (268/583/2015 cm-1) is shown.
Figure 14. ORTEP plot (30% probability ellipsoids) of the molecular
structure of Me2ClGeGeClMe2 (2), including atom numbering. Hydrogen
atoms have been omitted for clarity.
(34) Adams, S.; Draeger, M.; Mathiasch, B. Z. Anorg. Allg. Chem. 1986,
532, 81–89.
of digermanes 1–3. In these cases, the low barriers to rotation
prevent the observation of separate conformers. However, the
conformational equilibrium of permethylcyclohexagermane,
where the barrier for the ring inversion is significantly larger
because of concerted rotation about six Ge-Ge bonds, has been
recently resolved with Raman spectroscopy.15 Because the
(35) Buschmann, J.; Lentz, D.; Luger, P.; Rottger, M. Acta Crystallogr.,
Sect.C: Cryst.Struct. Commun. 2000, 56, 121–122.
(36) Lefferts, J. L.; Molloy, K. C.; Hossain, M. B.; van der Helm, D.;
Zuckerman, J. J. J. Organomet. Chem. 1982, 240(4), 349–361.
(37) Blake, A. J.; Cradock, S.; Ebsworth, E. A. V.; Franklin, K. C. Angew.
Chem. 1990, 102(1), 87–89.
(38) Schlemper, E. O.; Britton, D. Inorg. Chem. 1966, 5, 511–514.
3032 Inorganic Chemistry, Vol. 47, No. 8, 2008

Molecular Structures of Digermanes
is not observed in the solid-state structure of 2, with merely
a close interaction between the germanium and silicon atoms
of slightly more than the van der Waals radii.
BP/RSE Research Fellowship and the EPSRC for funding
for the electron diffraction (grant number EP/D057167).
D.A.W. and H.E.R. thank the EPSRC for funding (EP/
C513649).
One explanation for the minimal interactions in the solid-
state structure of 2 (and the presumed lack of interaction in
the silicon analogue) is the reduced atomic radii for silicon
and germanium (∼120 pm) in comparison with that of tin
(∼160 pm). The small atomic radii of silicon and germanium
make them poorer acceptors than tin in general, and the
acceptor properties are not enhanced by the presence of the
electron-donating methyl groups. Another reason is that
the donor to the coordination sphere is chlorine, which is
not a good donor. Replacing chlorine with, for example,
nitrogen or oxygen enhances the donor capabilities and leads
to far stronger intermolecular interactions being observed.
Further work on the conformational properties of the
related digermane MeCl2GeGeCl2Me is currently being
undertaken to observe the effect of replacing methyl groups
with chlorine atoms.
Supporting Information Available: Tables listing the nozzle-
to-plate distances, sample and nozzle temperatures (K), weighting
functions (nm-1), correlation parameters, scale factors, and electron
wavelengths used in the gas electron diffraction, interatomic
distances and amplitudes of vibration for the GED structures of
1-3, least-squares correlation matrix for the GED refinement of
1-3, experimental GED coordinates from the refinement of the
pseudo-C2 structure of 1-3, and crystal data, structure refinement,
and geometry parameters for 2 at 100 K and figures showing the
torsional potential of 1, experimental and final weighted difference
molecular scattering intensities for 1-3, and the GED parameter
description for 1-3. This material is available free of charge via
the Internet at http://pubs.acs.org. Crystallographic data (excluding
structure factors) for 2 have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication no.
CCDC 647136. Copies of the data can be obtained free of charge
on application to The Director, CCDC, 12 Union Road, Cambridge,
CB2 1EZ, U.K. [fax (internat.) +44 (0)1223 336033; e-mail
deposit@ccdc.cam.ac.uk].
Acknowledgment. M.H. and K.H. gratefully acknowledge
the financial support by the “Fonds zur Förderung der
wissenschaftlichen Forschung”, Vienna (project P-17435).
S.L.M. is grateful to the Royal Society of Edinburgh for a
IC7018764
Inorganic Chemistry, Vol. 47, No. 8, 2008 3033