Synthesis and molecular structures of N,N-dimethylhydroxyl-amino-trichlorosilane and -germane

Article abstract of DOI:10.1039/a906688h

The compounds Cl₃SiONMe₂ and Cl₃GeONMe₂ have been prepared by reacting HONMe₂ with SiCl₄ and GeCl₄, respectively, in the presence of the auxiliary base 2,6-dimethylpyridine. Their identity was proven by gas-phase IR and solution NMR spectroscopy of the nuclei ¹H, ¹³C, ¹⁵N, ¹⁷O, ²⁹Si, by mass spectrometry and elemental analyses. The solid-state structure of Cl₃SiONMe₂ was determined by low-temperature X-ray crystallography. The molecular structures of Cl₃SiONMe₂ and Cl₃GeONMe₂ in the gas phase have been determined by analysis of electron diffraction data augmented by restraints derived from ab initio calculations (MP2/6-31G*). The molecules adopt C_s symmetry. Important gas-phase geometry parameter values for Cl₃SiONMe₂ are: Si-O 1.623(3), Si-Cl_in-plane 2.022(4), Si-CU_out-of-plane 2.024(2), O-N 1.479(6) A, Si-O-N 105.6(8), O-Si-Cl_in-plane 104.2(3), O-Si-Cl_out-of-plane 113.7(2)°, for Cl₃GeONMe₂: Ge-O 1.759(6), Ge-Cl_in-plane 2.104(4), Ge-Cl_out-of-plane 2.106(2), O-N 1.484(9) A, Ge-O-N 104.0(11), O-Ge-Cl_in-plane 108.9(20), O-Ge-Cl_out-of-plane 111.6(12)°. The structural data are interpreted in terms of weak attractive interactions between the nitrogen donor and the silicon/germanium acceptor atoms. The results are discussed in comparison with other structural data from the literature: the donor-acceptor interaction in Cl₃SiONMe₂ is weaker than those in H₃SiONMe₂ or ClH₂SiONMe₂, but stronger than that in Me₃SiON(CF₃)₂. Both compounds reveal stronger donor-acceptor interactions than the methyl analogues Me₃SiONMe₂. The Royal Society of Chemistry 1999.

Full text of DOI:10.1039/a906688h

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Synthesis and molecular structures of N,N-dimethylhydroxyl-
amino-trichlorosilane and -germane†
Udo Losehand,a Norbert W. Mitzel*a and David W. H. Rankinb
a Anorganisch-chemisches Institut, Technische Universität München, Lichtenbergstraße 4,
85747 Garching, Germany. E-mail: N.Mitzel@lrz.tum.de
b Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh,
UK EH9 3JJ
Received 18th August 1999, Accepted 13th October 1999
The compounds Cl3SiONMe2 and Cl3GeONMe2 have been prepared by reacting HONMe2 with SiCl4 and GeCl4,
respectively, in the presence of the auxiliary base 2,6-dimethylpyridine. Their identity was proven by gas-phase IR
and solution NMR spectroscopy of the nuclei 1H, 13C, 15N, 17O, 29Si, by mass spectrometry and elemental analyses.
The solid-state structure of Cl3SiONMe2 was determined by low-temperature X-ray crystallography. The molecular
structures of Cl3SiONMe2 and Cl3GeONMe2 in the gas phase have been determined by analysis of electron
diffraction data augmented by restraints derived from ab initio calculations (MP2/6-31G*). The molecules adopt Cs
symmetry. Important gas-phase geometry parameter values for Cl3SiONMe2 are: Si–O 1.623(3), Si–Clin-plane 2.022(4),
Si–Clout-of-plane 2.024(2), O–N 1.479(6) Å, Si–O–N 105.6(8), O–Si–Clin-plane 104.2(3), O–Si–Clout-of-plane 113.7(2)Њ, for
Cl3GeONMe2: Ge–O 1.759(6), Ge–Clin-plane 2.104(4), Ge–Clout-of-plane 2.106(2), O–N 1.484(9) Å, Ge–O–N 104.0(11),
O–Ge–Clin-plane 108.9(20), O–Ge–Clout-of-plane 111.6(12)Њ. The structural data are interpreted in terms of weak attractive
interactions between the nitrogen donor and the silicon/germanium acceptor atoms. The results are discussed in
comparison with other structural data from the literature: the donor–acceptor interaction in Cl3SiONMe2 is weaker
than those in H3SiONMe2 or ClH2SiONMe2, but stronger than that in Me3SiON(CF3)2. Both compounds reveal
stronger donor–acceptor interactions than the methyl analogues Me3SiONMe2.
The determination of interactions of this type is important
Introduction
to understand specific reactivity of Si–O–N compounds,
Silanes and germanes reveal Lewis acidic character depending
such as their rearrangements into O–N–Si units,7 the use of
on the nature of their substituents. The hypervalent base
catalytic amounts of R2NOH in the alcoholysis reaction
adducts have been intensively investigated1 and some of them
of Si–H functions8 (also known in a stoichiometric version)9 or
are of pharmaceutical interest.2 Compounds with intra-
their use as so-called cold-curing catalysts in silicone polymer
molecular donor–acceptor bonds form an important subset,
synthesis.10
and the best known class of compounds in this respect is the
The principal concept of attractive interactions between
silatranes.3 Driving the intramolecular donor–acceptor bond
geminal centres can be seen in a more general context. For
formation to the extreme of only one linker atom between
example, interactions of this type have also been postulated for
donor and acceptor, three-membered rings can be formed or, as
an alternative, oligomerisation can occur to give six-membered
or larger ring systems.
We have studied internal donor–acceptor bond formation
between E and N in E–O–N fragments (E = Si, Ge, Sn) and
found a wide spectrum of strength of such interactions ranging
E–C–N linkages (E = Si, Ge, Sn; e.g. in Me3SnCH2NMe2), and
were used to explain the unusually low basicities of such amines
(also named the α-effect).11 However, they could not be proved
on the basis of experimental structure determination to date.12
Other examples from the literature in p-block element chem-
istryincludethecompounds(F C) BCPh NMe ,RN᎐C{Al[CH-
᎐
3
2
2
2
from predictably weak ones e.g. in Me3SiONMe2 [Si–N:
2.566(8) Å, Si–O–N: 107.9(6)Њ]4 and its germanium analogue,
through medium strength interactions e.g. in H2Si(ONMe2)2
[Si–N: 2.32 Å, Si–O–N: 95Њ],5 to strong interactions as in the
solid state of ClH2SiONMe2 [Si–N: 2.028(1) Å, Si–O–N:
79.7(1)Њ].6 In none of these compounds do we observe aggre-
gation to six-membered ring systems, but instead we find a
preference for formation of three-membered rings with one
weak bond, which may be described as a donor–acceptor
interaction with a large electrostatic contribution.
(SiMe ) ]} and (Me Si) C᎐N–N{Al[CH(SiMe ) ]} with B–C–
᎐
3
2
2
3
2
3
2
2
N,3 Al–C᎐N4 and Al–N–N5 linkages forming three-membered
᎐
ring systems in the solid state. In contrast to the limited number
of examples of β-donor interactions in p-block chemistry, such
a bonding situation is much more common in transition metal
compounds like Ti(ONR2)4.13 In these cases this type of bond-
ing is also described as η2-coordination.
For the bonding situation in Si–O–N units we know that the
nucleophilic character of the nitrogen centre is of crucial
importance, as has been shown in two recent studies of com-
14
pounds with reduced basicity at nitrogen: Me3SiON(CF3)2
and (Mes)F2SiON(SiMe3)2.15 We now wanted to evaluate the
importance of backbonding from substituents to the acceptor
atoms in Si–O–N and Ge–O–N compounds. In comparison to
ClH2SiONMe2, with its exceptionally strong Si ؒ ؒ ؒ N inter-
action, we aimed to prepare and study compounds with a larger
number of Cl substituents at Si and Ge. Here we report on the
synthesis of Cl3SiONMe2 and Cl3GeONMe2, and studies of
their spectroscopic and structural properties.
† Dedicated to Professor H. Schmidbaur on the occasion of his 65th
birthday.
Supplementary data available: rotatable 3-D crystal structure diagram
in CHIME format. See http://www.rsc.org/suppdata/dt/1999/4291/
Also available: correlation matrix elements for Cl3EONMe2 (E = Si,
Ge). For direct electronic access see http://www.rsc.org/suppdata/dt/
1999/4291/, otherwise available from BLDSC (No. SUP 57668, 2 pp.) or
the RSC Library. See Instructions for Authors, 1999, Issue 1 (http://
www.rsc.org/dalton).
J. Chem. Soc., Dalton Trans., 1999, 4291–4297
This journal is © The Royal Society of Chemistry 1999
4291

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Table 1 NMR chemical shifts (ppm) of different O-silylated and O-
germylated hydroxylamines
Table 2 Important crystal structure parameter values (distances/Å,
angles/Њ) for the two independent molecules of Cl3SiONMe2 in the unit
cell
Compound
δ29Si
δ15N
δ17O
Parameter
Molecule 1
Molecule 2
Cl3SiONMe2
Cl3GeONMe2
Me3SiONMe2
Me3GeONMe2
H3SiONMe2
Ϫ42.4
Ϫ17.8
Ϫ40.0
Ϫ229.4
Ϫ226.7
Ϫ247.8
Ϫ250.8
Ϫ234.0
161
187
Ϫ13
146
112
Si–O
Si–Cl4
Si–Cl5
Si–Cl6
O–N
N–C7
N–C8
Si–O–N
O–Si–Cl4
O–Si–Cl5
O–Si–Cl6
O–N–C7
O–N–C8
1.618(2)
2.018(1)
2.015(1)
2.013(1)
1.491(2)
1.462(4)
1.455(4)
103.4(1)
104.2(1)
113.5(1)
113.3(1)
104.2(2)
104.5(2)
1.622(2)
2.018(1)
2.022(1)
2.018(1)
1.493(2)
1.461(3)
1.457(3)
102.6(1)
104.3(1)
113.3(1)
113.5(1)
104.7(2)
104.7(2)
Results and discussion
Synthesis and characterisation of Cl3SiONMe2 and
Cl3GeONMe2
The compounds Cl3SiONMe2 and Cl3GeONMe2 were prepared
by the reaction of N,N-dimethylhydroxylamine and the corre-
sponding Group 14 tetrachloride in pentane in the presence of
2,6-dimethylpyridine as a non-coordinating hydrogen chloride
acceptor.
ECl4 ϩ HONMe2 ϩ 2,6-Me2C5H3N →
Cl3EONMe2 ϩ 2,6-Me2C5H3NؒHCl, E = Si, Ge
They were isolated as colourless liquids by fractionation
through a series of cold traps. Both Cl3SiONMe2 and Cl3-
GeONMe2 are extremely sensitive to moist air. Cl3GeONMe2 is
thermally unstable and decomposes rapidly at ambient tem-
perature to give GeCl4 and the thermodynamically favoured
Cl2Ge(ONMe2)2. The chemical properties and molecular struc-
ture of Cl2Ge(ONMe2)2 have been described elsewhere.16
Fig. 1 Crystal structure of Cl3SiONMe2 as determined by low-
temperature crystallography with numbering scheme for the two
independent molecules in the asymmetric unit. The molecules are
shown in their relative orientation towards each other in the crystal
lattice.
2 Cl3GeONMe2 → GeCl4 ϩ Cl2Ge(ONMe2)2
parameter values are listed in Table 2. Both molecules show a
distorted tetrahedral geometry at the silicon centre due to an
intramolecular donor–acceptor interaction between silicon and
nitrogen (Fig. 1). These Si ؒ ؒ ؒ N β-donor–acceptor interactions
lead to small Si ؒ ؒ ؒ N distances of 2.441(2) and 2.432(2) Å and
markedly compressed Si–O–N angles of 103.4(1) and 102.6(1)Њ.
Surprisingly, the Si ؒ ؒ ؒ N β-donor–acceptor interactions in this
case and in H3SiONMe2 [Si–O–N 102.6(1)Њ, Si ؒ ؒ ؒ N 2.456(1)
Å] are very similar, whereas in ClH2SiONMe2 the Si–O–N angle
is 24Њ less and the Si ؒ ؒ ؒ N distance is 0.41 Å shorter. Moreover,
this is despite the fact that H3SiONMe2 and ClH2SiONMe2 are
both compounds with a less electrophilic silicon centre, or in
other words with a less positive charge at the silicon atom,
which could be expected to lead to a stronger electrostatic
attraction between Si and N atoms in Cl3SiONMe2.
The N–O bond length [1.492(ave) Å] is significantly elong-
ated with respect to the non-silylated HONMe2 [1.452(ave) Å],
but of the same length as in ClH2SiONMe2 [1.490(1) Å]. A
marked distortion of the coordination sphere of silicon is obvi-
ous from the different O–Si–Cl angles, with the angle in the
(pseudo-)plane of symmetry being about 104Њ, while the other
two are widened to more than 113Њ in both independent
molecules. This and the other parameter values justify the
silicon atom in Cl3SiONMe2 being regarded as (4 ϩ 1)-
coordinate.
The identity of Cl3SiONMe2 and Cl3GeONMe2 was proven
by NMR spectroscopy (1H, 13C, 15N, 17O, and 29Si for Cl3-
SiONMe2) and gas-phase IR spectroscopy. Owing to the high
air sensitivity and thermal instability of Cl3GeONMe2, results
from mass spectrometry and elemental analysis appeared only
to be satisfactory in the case of Cl3SiONMe2.
Table 1 contains important NMR data for Cl3SiONMe2 and
Cl3GeONMe2. The electronegative chlorine substituents in
Cl3SiONMe2 lead to a shift of the 29Si NMR signal to low
frequency (δ Ϫ42.4), compared to the hydrogenated and
methylated species H3SiONMe2 (δ Ϫ40.0) and Me3SiONMe2
(δ Ϫ17.8), respectively. The 15N NMR resonances of
Cl3SiONMe2 (δ Ϫ229.4) and Cl3GeONMe2 (δ Ϫ226.7) are
shifted about 20 ppm to high frequency relative to Me3-
SiONMe2 (δ Ϫ247.8) and Me3GeONMe2 (δ Ϫ250.8), which
may indicate a more intense interaction with the acceptor atoms
silicon and germanium, respectively. The similarity in the chem-
ical shifts gives further evidence of the similar electronic nature
of the nitrogen atoms in Cl3SiONMe2 and Cl3GeONMe2.
In contrast to the extremely different 17O NMR chemical
shifts of the methylated compounds Me3SiONMe2 and Me3-
GeONMe2 at δ Ϫ13 and 146, the 17O NMR resonances of
Cl3SiONMe2 and Cl3GeONMe2 appear at similar chemical
shifts (δ 161 and 187) and are the highest chemical shifts
obtained for O-silylated and O-germylated hydroxylamines.
Gas-phase structure determination
Crystal structure determination
The high volatility of both Cl3SiONMe2 and Cl3GeONMe2
allowed the determination of their molecular structures in the
gas phase by means of electron diffraction. In supporting
the refinement by restraints derived from ab initio geometry
optimisations the usual limitations of gas-phase structure
refinements can be overcome. This procedure is called the
SARACEN method17 and introduces an elaborate combination
of Bartell’s method of predicating values18 and Schäfer’s
MOCED method.19
In order to compare molecular geometries derived from gas-
phase electron diffraction with solid-state geometries a single
crystal of Cl3SiONMe2 was grown directly from the melt by
in situ methods. Attempts to perform an analogous experiment
for Cl3GeONMe2 failed due to problems with crystallisation.
Cl3SiONMe2 crystallises with two independent molecules in the
unit cell. There are no intermolecular contacts which would
normally be regarded as significant. Important geometrical
4292
J. Chem. Soc., Dalton Trans., 1999, 4291–4297

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Table 3 Geometrical parameter values and restraints for the GED refinements of Cl3SiONMe2 and Cl3GeONMe2. The parameter values from the
ab initio calculations (MP2/6-31G*) are listed for comparison. Distances are given in Å, angles and torsion angles in Њ
Cl3SiONMe2
Value
Cl3GeONMe2
Value
No. Parameter
Restraint
MP2/6-31G*
Restraint
MP2/6-31G*
p1
p2
p3
p4
p5
p6
p7
p8
p9
E–Cl4
E–Cl5/6
E–O
O–N
N–C7
C7–H9
C7–H10
C7–H11
O–E–Cl4
2.022(4)
2.024(2) p2 Ϫ p1 = 0.001(5)
1.623(3)
1.479(6)
1.445(4) p5 Ϫ p4 = 0.031(10)
1.142(9)
1.146(9) p7 Ϫ p6 = 0.004(5)
1.150(9) p7 Ϫ p6 = 0.008(5)
104.2(3)
105.6(8)
107.0(5)
114.9(7)
113.7(2)
107.9(9)
106.5(10)
110.2(10)
Ϫ198.1(31)
41.9(27)
Ϫ76.6(32)
60.6(5)
2.034
2.035
1.663
1.492
1.461
1.089
1.093
1.097
104.1
100.4
104.5
111.6
113.0
108.9
107.4
111.4
2.104(4)
2.106(2)
1.759(6)
1.484(9)
1.447(7)
1.095(21)
2.129
2.130
1.789
1.500
1.460
1.090
1.093
1.097
108.9
95.6
104.5
111.9
112.9
108.9
107.7
111.2
p2 Ϫ p1 = 0.001(5)
p5 Ϫ p4 = 0.040(10)
1.098(22) p7 Ϫ p6 = 0.003(5)
1.101(22) p7 Ϫ p6 = 0.007(5)
108.9(20)
104.0(11)
106.3(9)
114.1(11)
111.6(12)
108.1(10)
106.9(11)
110.5(11)
Ϫ197.5(89)
44.5(89)
p10 E–O–N
p11 O–N–C
2 × p11 ϩ p12 = 320.6(30)
p12 Ϫ p11 = 7.10(10)
2 × p11 ϩ p12 = 320.8(30)
p12 Ϫ p11 = 7.4 (10)
p12 C7–N–C8
p13 O–E–Cl5/6
p14 N–C7–H9
p15 N–C7–H10
p16 N–C7–H11
p17 τC–N–C–H9
p18 τC–N–C–H10
p19 τC–N–C–H11
p20 τN–O–E–Cl5
p14 = 108.9(10)
p15 Ϫ p14 = 1.5(5)
p16 Ϫ p14 = 2.5(5)
p14 = 108.9(10)
p15 Ϫ p14 = 1.2(5)
p16 Ϫ p14 = 2.4(5)
Ϫ176.8
Ϫ177.2
p18 Ϫ p17 = 241.9(30)
p19 Ϫ p17 = 121.4(30)
65.1
Ϫ55.4
62.5
p18 Ϫ p17 = 241.8(30)
p19 Ϫ p17 = 121.2(30)
64.6
Ϫ56.0
62.1
Ϫ77.0(90)
58.9(37)
Fig. 2 Molecular structure of Cl3SiONMe2 in the gas phase as deter-
mined by electron diffraction with numbering scheme. The structure of
Cl3GeONMe2 is similar, the numbering scheme analogous.
The molecular framework and atom numbering scheme for
Cl3SiONMe2 and Cl3GeONMe2 are shown in Fig. 2. The
mathematical model for the least squares refinement of both
compounds was defined in Cs symmetry, with six bond lengths:
E–O, E–Cl4, E–Cl5, O–N, N–C and one C–H distance for the
methyl groups bound to nitrogen. The differences between the
C–H distances within these groups were restrained to the values
obtained from ab initio calculations at the MP2/6-31G* level of
theory. Parameters for refinement were the angles O–E–Cl4,
O–E–Cl5/6, E–O–N, O–N–C, C–N–C, and the angles N–C–H
as well as the torsion angles τNOECl5 and τCNCH, whereby
the differences between parameters for the different hydrogen
atoms were again restrained to calculated values. In all 20
geometry parameters were refined under the action of 10
restraints defining the differences of parameters of similar
nature or absolute values in the case of hydrogen-defining
parameters. The parameter and restraint definitions are given
in Table 3; the uncertainties are based on experience with the
reliability of these calculations. Also 21 amplitudes of vibration
were refined concurrently, representing all amplitudes belong-
ing to pairs of scatterers with a contribution of more than 5%
of the scattering of the E–Cl pair. Most of these amplitudes
were subject to restraints, either absolute or ratios between
related amplitudes, with the values taken from a force field
computed at the MP2/6-31G* level of theory and transformed
into amplitudes by means of the program ASYM4013 after scal-
ing it by an overall factor of 0.93, which is an established con-
stant in our laboratories. These amplitudes and restraints are
given in Table 4. The quality of the refinement can be assessed
from the residuals in the molecular scattering curves (Fig. 3)
and the radial distribution curves (Fig. 4). The final R factors
Fig. 3 Experimental molecular scattering intensity and final difference
curves (vs. model) as obtained by electron diffraction of (a) gaseous
Cl3SiONMe2 and (b) gaseous Cl3GeONMe2 (including the contribution
of GeCl4).
were 0.053 and 0.052 for Cl3SiONMe2 and Cl3GeONMe2. The
gas-phase structure of Cl3GeONMe2 was obtained from a mix-
ture of the compound with GeCl4, and the scattering contribu-
tion of GeCl4 was treated by including its known gas-phase
geometry in the model. The ratio Cl3GeONMe2 :GeCl4 was
found to be 52:48% by variation of the composition parameter
in the refinement and an uncertainty of 5% was estimated by a
Hamilton test at the 95% confidence level. Variation of the
J. Chem. Soc., Dalton Trans., 1999, 4291–4297
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Table 4 Distances, amplitudes of vibration and restraints for the GED refinements of the compounds Cl3SiONMe2 and Cl3GeONMe2. Distances
and amplitudes are given in Å
Cl3SiONMe2
Distance
Cl3GeONMe2
Distance
No.
Atom pair
Amplitude
Restraint
Amplitude
Restraint
d1
d2
d3
d4
d5
d6
d7
d8
E–Cl4
E–Cl5
E–O2
O2–N3
2.022(4)
2.024(2)
1.623(3)
1.479(6)
1.445(4)
1.142(9)
1.146(9)
1.150(9)
3.309(8)
3.228(18)
2.886(5)
4.218(9)
5.018(9)
3.061(3)
3.302(12)
4.623(11)
3.675(18)
2.473(12)
3.445(10)
2.351(8)
2.437(12)
4.601(27)
5.449(42)
5.980(25)
4.772(22)
5.343(22)
5.116(32)
3.068(38)
4.664(31)
4.125(57)
0.047(2)
0.047(1)
0.044(4)
0.049(4)
0.045(4)
0.083(6)
0.084(7)
0.085(7)
0.076(7)
0.076(9)
0.074(7)
0.098(9)
0.151(11)
0.072(6)
0.168(13)
0.139(9)
0.216(17)
0.131(10)
0.131(11)
0.075(6)
0.075(9)
0.241
2.104(4)
2.106(2)
1.759(6)
1.484(9)
0.043(3)
0.043(3)
0.040(3)
0.055(5)
0.050(5)
0.081(7)
0.081(8)
0.082(9)
0.107(32)
0.107(32)
0.096(10)
0.109(10)
0.145(12)
0.095(9)
0.160(16)
0.155(15)
0.223(21)
0.098(11)
0.130(11)
0.080(7)
0.069(7)
0.246
u2/u1 = 0.996(50)
u3/u1 = 0.949(95)
u2/u1 = 0.998(50)
u3/u1 = 0.917(46)
0.054(54)
u5/u4 = 0.908(45)
0.077(8)
u7/u6 = 1.005(50)
u8/u6 = 1.014(51)
N3–C7
u5/u4 = 0.928(46)
0.078(8)
u7/u6 = 1.005(50)
u8/u6 = 1.014(51)
1.447(7)
C7 ؒ ؒ ؒ H9
C7 ؒ ؒ ؒ H10
C7 ؒ ؒ ؒ H11
Cl4 ؒ ؒ ؒ Cl5
Cl5 ؒ ؒ ؒ Cl6
Cl4 ؒ ؒ ؒ O2
Cl4 ؒ ؒ ؒ N3
Cl4 ؒ ؒ ؒ C7
Cl5 ؒ ؒ ؒ O2
Cl5 ؒ ؒ ؒ N3
Cl5 ؒ ؒ ؒ C7
Cl5 ؒ ؒ ؒ C8
E ؒ ؒ ؒ N3
1.095(22)
1.098(22)
1.101(22)
3.440(72)
3.355(153)
3.150(40)
4.429(25)
5.245(31)
3.202(23)
3.372(27)
4.699(33)
3.733(37)
2.562(17)
3.537(17)
2.346(15)
2.428(18)
4.836(49)
5.652(142)
6.144(64)
4.855(51)
5.388(39)
5.150(79)
3.144(89)
4.671(79)
4.129(149)
d9
d10
d11
d12
d13
d14
d15
d16
d17
d18
d19
d20
d21
d22
d23
d24
d25
d26
d27
d28
d29
d30
u10/u9 = 0.998(50)
u10/u9 = 1.001(50)
0.102(10)
0.104(10)
0.137(14)
0.099(10)
0.156(16)
0.168(17)
0.216(22)
0.117(12)
0.123(12)
0.072(7)
0.069(7)
(fixed)
(fixed)
(fixed)
(fixed)
(fixed)
(fixed)
(fixed)
(fixed)
(fixed)
u14/u11 = 0.978(49)
0.161(16)
0.219(32)
E ؒ ؒ ؒ C7
0.124(12)
u20/u18 = 0.571(57)
u21/u18 = 0.577(58)
(fixed)
(fixed)
(fixed)
(fixed)
(fixed)
(fixed)
(fixed)
O2 ؒ ؒ ؒ C7
C7 ؒ ؒ ؒ C8
Cl4 ؒ ؒ ؒ H9
Cl4 ؒ ؒ ؒ H10
Cl4 ؒ ؒ ؒ H11
Cl5 ؒ ؒ ؒ H9
Cl5 ؒ ؒ ؒ H10
Cl5 ؒ ؒ ؒ H11
Cl6 ؒ ؒ ؒ H9
Cl6 ؒ ؒ ؒ H10
Cl6 ؒ ؒ ؒ H11
0.153
0.240
0.214
0.255
0.166
0.284
0.311
0.241
0.164
0.243
0.215
0.260
0.169
0.282
0.318
0.239
(fixed)
(fixed)
Cl3GeONMe2 :GeCl4 ratio about the optimised value in this
range did not affect the geometric parameters of Cl3GeONMe2
significantly.
from earlier comparison of experiments with theory that an
improvement in the basis set leads to better values, but so far we
have not observed such large deviations. A summary of theor-
etical and experimental bond lengths and angles is given in
Table 3.
Cl3SiONMe2 and Cl3GeONMe2 adopt similar molecular
structures in the vapour. Each of the compounds shows a
compressed E–O–N angle, 105.6(8)Њ for Cl3SiONMe2 and
104.0(11)Њ for Cl3GeONMe2, which leads to short E ؒ ؒ ؒ N
distances [E = Si: 2.473(12) Å, E = Ge: 2.562(17) Å]. The N→E
donor–acceptor interaction results in a distortion of the tetra-
hedral coordination geometry, which is more pronounced in
Cl3SiONMe2 [O–Si–Cl angles 104.2(3) and 113.7(2)Њ] than in
Cl3GeONMe2 [O–Ge–Cl angles 108.9(20) and 111.6(12)Њ].
Angles and distances of relevance are given in Table 3.
Comparison of the results derived from gas-phase electron
diffraction and X-ray diffraction of a single crystal of Cl3-
SiONMe2 shows that both molecular structures are similar. The
distortion of the tetrahedral coordination geometry at the
silicon centre is, within experimental errors, the same for both
gas-phase and crystal structures. Cl3SiONMe2 adopts slightly
smaller Si–O–N angles in the crystal than in the vapour. This
was expected with respect to the known structure of the
anti-conformer of ClH2SiONMe2 [Si–O–N: gas phase 87.1(9)Њ;
crystal lattice 79.1(1)Њ].6 The compression of the Si–O–N angle
in the solid state is explained by the increase of the molecular
dipole moment due to the regular alignment of the molecules in
the crystal lattice.
Ab initio calculations at the MP2(fc)/6-31G* level of theory
overestimate the strength of the Si ؒ ؒ ؒ N interaction: Cl3-
SiONMe2 is predicted to have a significantly smaller E–O–N
angle (100.4Њ), implying a stronger donor–acceptor interaction
than found by experiment. This overestimation of the strength
of the E ؒ ؒ ؒ N β-donor interaction is even more pronounced for
Cl3GeONMe2, for which the Ge–O–N angle is computed 8Њ
smaller (95.6Њ) than the experimental value of 104.0(11)Њ and
the coordination tetrahedron at the germanium centre is less
distorted than anticipated by theory (MP2/6-31G*). It is known
In order to improve understanding of the bonding situation
in the title compounds, it is worth comparing them with other
compounds containing X3SiO linkages. Selected parameter
values for reference compounds are given in Table 5. The Si–O
bond in Cl3SiONMe2 [1.632(3) Å] is longer than in Cl3SiOSiCl3
[1.592(10) Å]20 or the fluorine analogue F3SiOSiF3 [1.580(25)
Å].21 This is consistent with the much narrower valence angle at
oxygen in Cl3SiONMe2 [105.6(8)Њ] as compared to these
disiloxanes [146(4) and 155.7(20)Њ], as the strength of a Si–O
bond is related to a contribution of the oxygen lone pairs of
electrons, which is more correlated with the width of the angle
at oxygen. However, compounds without electronegative sub-
stituents at silicon have longer Si–O bonds, as is the case in
H3SiOSiH3 and H3SiOCH3 (see Table 5). The variation in the
Si–O–X angle from Si–O–Si to Si–O–C to Si–O–N angle is
impressive and illustrates the importance of interactions
between geminal atoms, which is repulsive in Si–O–Si, about
neutral in Si–O–C and attractive in Si–O–N units.
We also wanted to compare our new experimental results
with data for related Group 14 hydroxylamino compounds. For
this purpose we prepared Table 6, which contains a collection
of geometrical data and the corresponding values of the sums
of Bartell’s one angle radii [these radii define the (non-bonded)
distance between geminal atoms, i.e. the A ؒ ؒ ؒ C distance in an
A–B–C unit enclosing one angle],22 giving an estimate of the
non-bonded distance between two geminal atoms bound to a
common centre. The table shows that, depending on the substi-
tution pattern, it is possible to achieve an E ؒ ؒ ؒ N distance in
E–O–N units which is identical to the sum of Bartell’s radii
indicating the absence of any attractive interaction, as in
MesF2SiON(SiMe3)2. However, it is also possible to achieve
4294
J. Chem. Soc., Dalton Trans., 1999, 4291–4297

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Table
5
Important gas-phase geometrical parameter values
Table
6
Important geometrical parameter values (distances/Å,
(distances/Å, angles/Њ) for Cl3SiONMe2, Cl3GeONMe2 and other E–O–
E and E–O–C reference compounds (GED values all ra)
angles/Њ) for Cl3SiONMe2, Cl3GeONMe2 and E–O–N reference
compounds (ave = averaged value; GED values all ra)
Compound
E–O
E–O–X
Σ Bartell’s
radii
Compound/method
E–O–N
E ؒ ؒ ؒ N
Cl3SiONMe2
Cl3SiOSiCl3
F3SiOSiF331
F3SiOCH3
H3SiOSiH3
H3SiOCH3
1.623(3)
105.6(8)
146(4)
1.592(10)
1.580(25)
1.580(assumed)
1.634(2)
1.640(3)
1.759(6)
Cl3SiONMe2/GED
Cl3SiONMe2/XRD
Cl3GeONMe2/GED
Me3SiONMe2/GED
Me3GeONMe2/GED
Me3SnONMe2/GED
Me3SiON(CF3)2/GED
MesF2SiON(SiMe3)2/XRD
ClH2SiONMe2 (anti)/XRD
ClH2SiONMe2 (anti)/GED
ClH2SiONMe2 (gauche)/GED
H3SiONMe2/XRD
105.6(8)
103.0(ave)
104.0(11)
107.9(6)
108.9(7)
102.5(8)
113.4(19)
117.66(11) 2.692
79.7(1)
87.1(9)
2.473(12)
2.437(ave) 2.69
2.562(17)
2.566(8)
2.682(11)
2.731(14)
2.66
2.69
155.7(20)
131.4(32)
144.1(8)
120.6(10)
104.0(11)
126.5(3)
2.72
2.69
2.72
3.02
2.69
2.69
2.69
2.69
2.69
2.69
Cl3GeONMe2
H3GeOGeH3
1.766(4)
2.028(1)
2.160(7)
2.468(25)
2.453
104.7(11)
102.6
H2Si(ONMe2)2/XRD
95.2(ave)
2.318(ave) 2.69
between the gauche substituents at E and the nitrogen substitu-
ents (Me).
In Table 7 a number of computational results are compiled
including the E–O–N angles, the atomic charges at E, N and O
and the shortest contacts between the atoms in silyl and germyl
groups on one side and in the nitrogen substituents on the other
side. For the compounds listed in this table, the charges on their
N and O atoms vary only slightly, which is to be expected, as all
compounds contain the same ONMe2 group. The charge at the
acceptor centre E is largely dependent on the nature of the
substituents, with F3SiONMe2 being the most extreme case
with q(Si) = 1.80 e. Although F3SiONMe2 is predicted to show
the strongest E ؒ ؒ ؒ N attraction, there is no obvious dependence
of the strength of interaction and the charge on the acceptor
centre, as Cl3SiONMe2 bears a markedly larger charge on
silicon but has a larger Si–O–N angle than ClH2SiONMe2. This
can be used as an argument against purely electrostatic attrac-
tion between E and N atoms as an explanation for the effect
under investigation.
However, the compounds with the largest negative charge on
nitrogen are those with the strongest E ؒ ؒ ؒ N interactions. This
accumulation of negative charge at nitrogen is inconsistent with
the picture of a nitrogen centre donating electrons towards
the acceptor atom, or as described in terms of natural orbital
analysis with lp(N)→σ*(E–X) electron delocalisation. How-
ever, calculations for aminoboranes also predict the negative
charge at the donor atom (nitrogen) to be slightly higher than in
the free amine.23
The shortest distances between the gauche substituent at
silicon and gemanium (Cl in the case of Cl3SiONMe2, H in
the case of anti-ClH2SiONMe2) to the hydrogen atoms of the
methyl groups at nitrogen should give an indication of the
occurrence of repulsive forces between silyl or germyl groups on
one side and methyl groups on the other, which would prevent
shortening of the E ؒ ؒ ؒ N distance, despite an attractive inter-
action between these centres. This could explain the large
difference in the Si ؒ ؒ ؒ N distances in Cl3SiONMe2 and ClH2-
SiONMe2. However, the data in Table 7 show that the Cl ؒ ؒ ؒ H
and H ؒ ؒ ؒ H distances in Cl3SiONMe2 and ClH2SiONMe2 are
considerably larger than the respective sums of van der Waals
radii: the experimental value for the shortest Cl ؒ ؒ ؒ H contact in
Cl3GeONMe2 is 3.068(38) Å (the ab initio value is 2.994 Å),
whereas the sum of the van der Waals radii is 2.80 Å. The
repulsion in Cl3SiONMe2 could even be less pronounced if the
possibility of weak hydrogen bonding between Cl and H is
taken into consideration.
Fig. 4 Radial distribution and difference curve for the electron diffrac-
tion refinement of (a) Cl3SiONMe2 and (b) Cl3GeONMe2 (with the
contribution of GeCl4 subtracted). Before Fourier inversion the data
were multiplied by sؒexp[(Ϫ0.002 s2)/(ZE Ϫ fE)(ZCl Ϫ fCl)], E = Si, Ge.
Vertical lines indicate atom pairs with their height being proportional to
their scattering contribution.
E ؒ ؒ ؒ N distances almost as short as the sum of the covalent
radii of E and N, as is found in solid ClH2SiONMe2.
In Table 6 we see that the Si–O–N and Ge–O–N angles of
Cl3SiONMe2 and Cl3GeONMe2 are in the same range as those
of Me3SiONMe2 and Me3GeONMe2, the latter two having only
marginally wider angles and slightly longer E ؒ ؒ ؒ N distances.
One might expect that the electrophilicity of the Cl3Si and
Cl3Ge groups would lead to much smaller E ؒ ؒ ؒ N distances
associated with smaller Si–O–N and Ge–O–N angles. However,
even in H3SiONMe2 a stronger attraction between Si and N
atoms was observed.
Although most of the molecules under consideration here are
comparatively small, a number of different effects play import-
ant roles in determining the strength of the E ؒ ؒ ؒ N attractions.
These include (i) electrostatic attraction between positively
charged E and negatively charged N atoms, (ii) dipole–dipole
and dipole–charge interactions, (iii) electron delocalisation of
the types lp(N)→σ*(E–X) and lp(O)→σ*(E–X), (iv) the angle-
bending potential at the oxygen atom and (v) steric repulsion
It should finally be noted that a more detailed consideration
of orbital interaction is consistent with our experimental
results. We performed a natural bond orbital analysis (NBO),
despite the obvious deviations of theory from the experi-
J. Chem. Soc., Dalton Trans., 1999, 4291–4297
4295

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Table 7 Computed Mulliken charges q (e), Si–O–N angles (Њ) and non-bonded distances (Å) between the silyl/germyl groups and the methyl units in
Cl3SiONMe2, Cl3GeONMe2 and E–O–N reference compounds. For consistency all values are taken from fully optimised MP2(fc)/6-31G*
calculations
Compound
SiON
q(Si/Ge)
q(N)
q(O)
Shortest contact
Σ v.d.W. radii
Cl3SiONMe2
Cl3GeONMe2
F3SiONMe2
ClH2SiONMe2 (anti)
ClH2SiONMe2 (gauche)
H3SiONMe2
100.4
95.6
86.5
88.9
102.1
99.9
1.18
0.96
1.80
1.06
1.04
0.92
Ϫ0.30
Ϫ0.26
Ϫ0.36
Ϫ0.33
Ϫ0.28
Ϫ0.29
Ϫ0.60
Ϫ0.60
Ϫ0.59
Ϫ0.61
Ϫ0.63
Ϫ0.65
Cl ؒ ؒ ؒ H
Cl ؒ ؒ ؒ H
F ؒ ؒ ؒ H
H ؒ ؒ ؒ H
Cl ؒ ؒ ؒ H
H ؒ ؒ ؒ H
3.012
2.8
2.8
2.4
2.0
2.8
2.0
2.994
2.65
2.70
3.08
2.92
Table 8 Experimental conditions (camera distances (mm), electron wavelengths (Å), nozzle and sample temperatures (ЊC)), data ranges and
weighting functions (ÅϪ1), correlation parameters, scale factors and final R factors for the GED experiments and refinements of the compounds
Cl3SiONMe2 and Cl3GeONMe2
Compound/
data set
Camera
distance
T
nozz
T
samp
Correlation
parameter
Scale
factor
Wavelength
∆s
smin
s1
s2
smax
R1
Rg
Cl3SiONMe2/1
Cl3SiONMe2/2
Cl3GeONMe2/1
Cl3GeONMe2/2
285.47
128.08
285.47
128.08
0.06016
0.06016
0.06016
0.06016
20
20
20
20
0
0
0
0
0.2
0.4
0.2
0.4
2.0
6.0
2.0
6.0
4.0
8.0
4.0
8.0
11.2
28.8
11.2
30.8
13.2
33.2
13.2
32.4
0.3650
0.4116
0.4842
0.2903
0.850(5)
0.890(14)
0.840(5)
0.850(10)
0.0388
0.0700
0.0574
0.0469
0.0533
0.0522
mentally obtained geometries. These calculations reveal many
small contributions to electron delocalisation in these com-
pounds. The most important to mention are those of the type
lp(O)→σ*(E–X), which are common for Si–O and Ge–O
compounds. A comparatively strong remote type of negative
hyperconjugation, lp(N)→σ*(E–X), as was used to rationalise
the nature of bonding in anti-ClH2SiONMe2, with its
extremely small Si–O–N angle, is predicted to be present in
Cl3SiONMe2 and Cl3GeONMe2, but in the latter two cases it
makes a substantially smaller contribution. Although it is
difficult to take all the contributions into concurrent con-
sideration, it might be argued that the significant contribu-
tions of the type lp(Cl)→σ*(E–X) saturate the electrophilicity
of the σ*(E–X) orbital positioned anti relative to the nitrogen
lone pair, or to describe it in simpler terms, “backbonding”
from the chlorine lone pairs to silicon reduces its electrophilic
character.
m/z = 192 [Mϩ Ϫ 1], 158 [Mϩ Ϫ Cl], 133 [Mϩ Ϫ ON(CH3)2].
H6C2Cl3NOSi (194.52 g molϪ1) calc.: C, 12.34; H, 3.11; N, 7.20.
Found: C, 12.34; H, 3.66; N, 6.52%.
(N,N-Dimethylhydroxylamino)trichlorogermane. A solution
of N,N-dimethylhydroxylamine (2.7 g, 45 mmol) and 2,6-
dimethylpyridine (4.8 g, 45 mmol) in pentane (30 mL) was
added dropwise to a solution of germanium tetrachloride (9.6
g, 45 mmol) in pentane (50 mL) at Ϫ55 ЊC. The mixture was
slowly warmed to ambient temperature and all volatile products
were separated from the remaining hydrochloride by repeated
condensation. The product was separated in good yield (6.2 g,
32 mmol, 71%) by condensation through a series of cold traps
held at Ϫ20, Ϫ60 and Ϫ196 ЊC, with the product retained at
Ϫ60 ЊC. (N,N-Dimethylhydroxylamino)trichlorogermane is a
colourless liquid, extremely sensitive to moist air and thermally
unstable. 1H NMR: δ 2.28 (s, 6H, H3C). 13C-{1H} NMR: δ 49.84
(s, CH3). 15N-{1H} NMR: δ Ϫ226.7 (s). 17O-{1H} NMR: δ 187
(s). IR: 2990, 2870 ν(CH), 1144 cmϪ1
.
Experimental
General
Crystal structure determination of Cl3SiONMe2
The experiments were carried out using a standard Schlenk line
or a vacuum line with greaseless PTFE stopcocks (Young’s
taps), which was directly attached to the gas cell of an FTIR
spectrometer (Midac Prospect FTIR). All NMR spectra were
recorded at 21 ЊC on a JEOL JNM-LA400 spectrometer in
sealed tubes with C6D6 as a solvent directly condensed onto the
sample from K/Na alloy.
A single crystal of Cl3SiONMe2 was grown in situ by slowly
cooling the melt in a sealed capillary below the melting point
after generation of a suitable seed crystal. C2H6Cl3NOSi,
Mr = 194.5, crystal system triclinic, space group P21/c, Z = 8,
a = 11.585(1), b = 7.487(1), c = 19.069(2) Å, β = 98.37(1)Њ,
V = 1636.4(3) Å3 at 149(2) K, µ = 1.184 mmϪ1. 2θmax = 56Њ,
ω-scan, 3545 independent reflections [Rint = 0.032]. 187 param-
eters, R1 = 0.0355 for 3532 reflections with Fo > 4σ(Fo) and
wR2 = 0.1029 for all 3532 data.
Syntheses
CCDC reference number 186/1696.
See http://www.rsc.org/suppdata/dt/1999/4291/ for crystallo-
graphic files in .cif format.
(N,N-Dimethylhydroxylamino)trichlorosilane. A solution of
N,N-dimethylhydroxylamine (2.4 g, 40 mmol) and 2,6-dimethyl-
pyridine (4.3 g, 40 mmol) in pentane (30 mL) was added drop-
wise to a solution of silicon tetrachloride (7.1 g, 42 mmol) in
pentane (50 mL) at Ϫ55 ЊC. The mixture was slowly warmed to
ambient temperature and all volatile products were separated
from the remaining hydrochloride by repeated condensation.
The product was separated in good yield (7.6 g, 32 mmol, 71%)
by condensation through a series of cold traps held at Ϫ35, Ϫ60
and Ϫ196 ЊC, with the product retained at Ϫ60 ЊC. (N,N-
Dimethylhydroxylamino)trichlorosilane is a colourless liquid
Gas-phase electron diffraction
Electron scattering intensity data for Cl3SiONMe2 and Cl3-
GeONMe2 were recorded on Kodak Electron Image plates
using the Edinburgh electron diffraction apparatus.24 The sam-
ples were held at 0 ЊC and the inlet nozzle at 20 ЊC during the
experiments. Scattering data for benzene were recorded concur-
rently and used to calibrate the electron wavelength and camera
distances. Three exposures were taken at each camera distance.
Data were obtained in digital form using the microdensitometer
at the Royal Greenwich Observatory at Cambridge.25 The
experimental conditions and data ranges are listed in Table 8.
1
(mp Ϫ55 ЊC) and extremely sensitive to moist air. H NMR:
1
δ 2.33 (s, 6H, H3C). 13C NMR: δ 49.2 (qq, JCH = 136.9 Hz,
2
3JCNCH = 5.1 Hz). 15N NMR: δ Ϫ229.4 (sep, JNCH = 2.1 Hz).
17O-{1H} NMR: δ 161 (s). 29Si NMR: δ Ϫ42.4 (s). GC-MS:
4296
J. Chem. Soc., Dalton Trans., 1999, 4291–4297

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13 (a) K. Wieghard, I. Tolksdorf, J. Weiss and W. Swiridoff, Z. Anorg.
The data analysis followed standard procedures, using estab-
lished data reduction and least-squares refinement programs26
and the scattering factors established by Fink and co-workers.27
The refined molecular parameters, their definition and the
applied restraints, a list of selected interatomic distances includ-
ing vibrational amplitudes and applied restraints, and elements
of the correlation matrix are given Tables 3, 4 and supplemen-
tary data.
Allg. Chem., 1982, 490, 182; (b) N. W. Mitzel, S. Parsons, A. J. Blake
and D. W. H. Rankin, J. Chem. Soc., Dalton Trans., 1996, 2089.
14 T. Hertel, J. Jakob, R. Minkwitz and H. Oberhammer, Inorg. Chem.,
1998, 37, 5092.
15 R. Wolfgramm, U. Klingebiel and M. Noltemeyer, Z. Anorg. Allg.
Chem., 1998, 624, 856.
16 U. Losehand and N. W. Mitzel, Eur. J. Inorg. Chem., 1998, 2023.
17 (a) A. J. Blake, P. T. Brain, H. McNab, J. Miller, C. A. Morrison,
S. Parsons, D. W. H. Rankin, H. E. Robertson and B. A. Smart,
J. Phys. Chem., 1996, 100, 12280; (b) N. W. Mitzel, B. A. Smart,
A. J. Blake, H. E. Robertson and D. W. H. Rankin, J. Phys. Chem.,
1996, 100, 9339.
18 L. S. Bartell, D. J. Romenensko and T. C. Wong, Mol. Struct. Diffr.
Methods, 1975, 3, 72.
19 V. J. Klimkowski, J. D. Ewbank, C. Van Alsenoy, J. N. Scarsdale and
L. Schäfer, J. Am. Chem. Soc., 1982, 104, 1476.
20 (a) W. Airey, C. Glidewell, A. G. Robiette and G. M. Sheldrick,
J. Mol. Struct., 1971, 8, 413; (b) M. Yokoi, Bull. Chem. Soc. Jpn.,
1957, 30, 100; (c) K. Yamasaki, A. Kotera, M. Yokoi and Y. Ueda,
J. Chem. Phys., 1950, 18, 1414.
21 W. Airey, C. Glidewell, D. W. H. Rankin, A. G. Robiette, G. M.
Sheldrick and D. W. J. Cruickshank, Trans. Faraday Soc., 1970, 66,
551.
Ab initio calculations
Ab initio molecular orbital calculations were carried out using
the Gaussian 98 program.28 Geometry optimisations and
vibrational frequency calculations were performed from
analytic first and second derivatives at the SCF and MP2 levels
of theory. Calculations were undertaken at the SCF level using
the standard 3-21G*29 and 6-31G*30 basis sets, while the larger
was used for calculations at the MP2 level of theory. NBO
calculations were undertaken with the NBO 3.0 facilities built
into Gaussian 98.
22 (a) L. S. Bartell, J. Chem. Phys., 1960, 32, 827; (b) C. Glidewell,
Acknowledgements
Inorg. Chim. Acta, 1975, 12, 219.
23 A. Haaland, Angew. Chem., Int. Ed. Engl., 1989, 28, 992.
24 C. M. Huntley, G. S. Laurenson and D. W. H. Rankin, J. Chem.
Soc., Dalton Trans., 1980, 945.
25 J. R. Lewis, P. T. Brain and D. W. H. Rankin, Spectrum, 1997, 15, 7.
26 N. W. Mitzel, P. T. Brain and D. W. H. Rankin, ED96, Version 2.0,
1998. A program developed on the basis of formerly described ED
programs: A. S. F. Boyd, G. Laurenson and D. W. H. Rankin,
J. Mol. Struct., 1981, 71, 217.
This work was supported by the Bayerisches Staatsministerium
für Wissenschaft, Forschung und Kunst (Bayerischer Habilit-
ationsförderpreis 1996 for N. W. M.), the Deutsche
Forschungsgemeinschaft, the Fonds der Chemischen Industrie,
the Leonhard-Lorenz-Stiftung and EPSRC (Grant No. GR/
K44411). Generous support by Professor H. Schmidbaur
(Garching) is gratefully acknowledged.
27 A. W. Ross, M. Fink and R. Hilderbrandt, International Tables for
X-Ray Crystallography, ed. A. J. C. Wilson, Kluwer Academic
Publishers, Dordrecht, Boston, 1992, vol. C, p. 245.
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