Direct time-resolved study of the gas-phase reactions of germylene with ethyl- and diethylgermane: Absolute rate constants, temperature dependences, and mechanism

Article abstract of DOI:10.1021/jp0661948

Time-resolved studies of germylene, GeH₂, generated by the 193 nm laser flash photolysis of 3,4-dimethyl1 -germacyclopent-3-ene, have been carried out to obtain rate constants for its bimolecular reactions with ethyl- and diethylgermanes in the gas phase. The reactions were studied over the pressure range 1 -100 Torr with SF₆ as bath gas and at five temperatures in the range 297-564 K. Only slight pressure dependences were found for GeH₂ + EtGeH₃ (399, 486, and 564 K). The high pressure rate constants gave the following Arrhenius parameters: for GeH ₂ + EtGeH₃, log A = -10.75 ±0.08 and E_a = -6.7 ±0.6 kJ mol^-1; for GeH₂ + Et ₂GeH₂, log A = -10.68 ±0.11 and E_a = -6.95 ±0.80 kJ mol^-1. These are consistent with fast, near collision-controlled, association processes at 298 K. RRKM modeling calculations are, for the most part, consistent with the observed pressure dependence of GeH₂ + EtGeH₃. The ethyl substituent effects have been extracted from these results and are much larger than the analogous methyl substituent effects in the SiH₂ + methylsilane reaction series. This is consistent with a mechanistic model for Ge - H insertion in which the intermediate complex has a sizable secondary barrier to rearrangement.

Full text of DOI:10.1021/jp0661948

1434
J. Phys. Chem. A 2007, 111, 1434-1440
Direct Time-Resolved Study of the Gas-Phase Reactions of Germylene with Ethyl- and
Diethylgermane: Absolute Rate Constants, Temperature Dependences, and Mechanism
Rosa Becerra
Instituto de Quimica-Fisica “Rocasolano”, CSIC, C/Serrano 119, 28006 Madrid, Spain
Sergey E. Boganov, Mikhail P. Egorov, Irina V. Krylova, and Oleg M. Nefedov
N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky Prospekt 47, 117913
Moscow, Russian Federation
Robin Walsh*
Department of Chemistry, UniVersity of Reading, Whiteknights, P. O. Box 224, Reading, RG6 6AD, U.K.
ReceiVed: September 21, 2006; In Final Form: December 11, 2006
Time-resolved studies of germylene, GeH2, generated by the 193 nm laser flash photolysis of 3,4-dimethyl-
1-germacyclopent-3-ene, have been carried out to obtain rate constants for its bimolecular reactions with
ethyl- and diethylgermanes in the gas phase. The reactions were studied over the pressure range 1-100 Torr
with SF6 as bath gas and at five temperatures in the range 297-564 K. Only slight pressure dependences
were found for GeH2 + EtGeH3 (399, 486, and 564 K). The high pressure rate constants gave the following
Arrhenius parameters: for GeH2 + EtGeH3, log A ) -10.75 ( 0.08 and Ea ) -6.7 ( 0.6 kJ mol-1; for
GeH2 + Et2GeH2, log A ) -10.68 ( 0.11 and Ea ) -6.95 ( 0.80 kJ mol-1. These are consistent with fast,
near collision-controlled, association processes at 298 K. RRKM modeling calculations are, for the most
part, consistent with the observed pressure dependence of GeH2 + EtGeH3. The ethyl substituent effects
have been extracted from these results and are much larger than the analogous methyl substituent effects in
the SiH2 + methylsilane reaction series. This is consistent with a mechanistic model for Ge-H insertion in
which the intermediate complex has a sizable secondary barrier to rearrangement.
Introduction
stand these effects more fully, we decided to investigate the
further insertion reactions of GeH2 with EtGeH3 and Et2GeH2.
This enables us to make the complete comparison of reactions
1-4.
The reactions of germylene, GeH2, are of interest both because
of their involvement in the breakdown mechanisms of germanes
leading to solid germanium (chemical vapor deposition)1,2 and
also because of their participation in germane3 and organoger-
mane decompositions.4 In 1996 we reported the first directly
measured rate constants for GeH2 reactions5 carried out by time-
resolved means in the gas phase. Since then we have undertaken
a series of gas-phase studies of the kinetics of GeH2 in order to
throw light onto the mechanisms of its various reactions.6-15
The group of King and Lawrance have also begun similar
studies.16-18 Among the principal reactions of interest are the
insertion processes of GeH2 into Ge-H bonds6,7,11,15-17 and
Si-H bonds8,10,16,17 which have been shown to proceed via
association complexes rather similar to those implicated in the
insertion reactions of SiH2.19-25 The results show that, although
they are quite fast under the conditions of study, GeH2 reactions
occur more slowly than their SiH2 counterparts.
GeH2 + GeH4 f Ge2H6
GeH2 + EtGeH3 f EtGeH2GeH3
GeH2 + Et2GeH2 f Et2GeHGeH3
GeH2 + Et3GeH f Et3GeGeH3
(1)
(2)
(3)
(4)
Reactions 2 and 3 have not previously been studied. The
ethylgermanes were selected here (rather than the methylger-
manes) simply for comparison purposes. Originally,6 Et3GeH
was chosen as a substrate rather than Me3GeH, because of its
more ready availability.
Among useful probes of reactivity are substituent effects. In
studies of SiH2 with the methylsilanes Baggott et al.21 and
Becerra et al.26 have found, inter alia, that at 298 K SiH2 reacts
more slowly with Me3SiH than with SiH4 (although the per
Si-H bond reactivities were the other way round). For GeH2,
by contrast, the reaction with Et3GeH6 is faster than that with
GeH47 both overall and per Ge-H bond. The reaction of GeH2
+ Et3GeH also has a more negative activation energy than that
of SiH2 + Me3SiH. It is assumed here that the differences
between Et and Me substitution are not significant. To under-
Experimental Section
Germylene kinetic studies have been carried out by the laser
flash photolysis/laser absorption technique, details of which
have been published previously.5,7 Only essential and brief
details are therefore included here. GeH2 was produced by the
193 nm flash photolysis of 3,4-dimethyl-1-germacyclopent-3-
ene (DMGCP) by use of a Lamba Physik (Coherent) Compex
100 excimer laser. GeH2 concentrations were monitored in real
time by means of a Coherent 699-21 single-mode dye laser
10.1021/jp0661948 CCC: $37.00 © 2007 American Chemical Society
Published on Web 02/06/2007

GeH2 Gas-Phase Reactions with EtGeH3 and Et2GeH2
J. Phys. Chem. A, Vol. 111, No. 8, 2007 1435
pumped by an Innova 90-5 argon ion laser and operating with
Rhodamine 6G. Experiments were carried out in a variable
temperature spectrosil quartz vessel with demountable windows
which were regularly cleaned. Photolysis laser pulse energies
were typically 50-70 mJ with a variation of (5%. The
monitoring laser beam was multipassed 32 times through the
reaction zone to give an effective path length of ca. 1.0 m. The
laser wavelength was set by the combined use of a wavemeter
(Burleigh WA-20) and reference to a known coincident transi-
tion in the visible spectrum of I2 vapor and was checked at
frequent intervals during the experiments.
The monitoring laser was tuned to 17 111.31 cm-1 corre-
sponding to a known strong vibration-rotation transition (A˜ 1B1-
(0,1,0) r X˜ 1A1(0,0,0) band) discovered by us previously,5 and
now assigned as the pQ1(6) line by intracavity laser absorption
spectroscopy by Campargue and Escribano.27 Light signals were
measured by a dual photodiode/differential amplifier combina-
tion, and signal decays were stored in a transient recorder
(Datalab DL 910) interfaced to a BBC microcomputer. This
was used to average the decays of typically five laser shots (at
a repetition rate of 1 or 0.5 Hz). Signal decays were found to
be exponential up to 90% and were fitted by a nonlinear-least-
squares procedure to provide values for the first-order rate
coefficients, kobs, for removal of GeH2 in the presence of known
partial pressures of substrate, either EtGeH3 or Et2GeH2.
The gas mixtures for photolysis were made up consisting of
2.5-9 mTorr DMGCP, variable pressures of substrate between
4.5 and 220 mTorr, and inert diluent bath gas, SF6, up to total
pressures between 1 and 100 Torr, although most experiments
were done at 10 Torr. Pressures were measured with capacitance
manometers (MKS Baratron).
DMGCP was prepared as previously described.5 EtGeH3 and
Et2GeH2 were prepared by LiAlH4 reduction of EtGeCl3 and
Et2GeBr2, respectively, in dried n-Bu2O by standard procedures.28a
EtGeCl3 was made by direct synthesis from Ge and EtCl.28b
Et2GeBr2 was prepared from Et4Ge according to the procedure
of Mironov and Kravchenko.29 Et4Ge was made by the Grignard
method.30 The ethylgermanes were purified by low pressure
distillation, and characterized by IR and NMR spectroscopies.
Final purities, based on gas chromatographic (GC) analysis (2
m silicone oil column (OV101), Perkin-Elmer 8310 chromato-
graph operated at ambient temperature), were 98.2% (EtGeH3)
and 99.6% (Et2GeH2). SF6 was obtained from ICI and contained
no GC detectable impurities.
Figure 1. Second-order plots for reaction of GeH2 + EtGeH3 at 10
Torr (SF6) at different temperatures (indicated).
For each reaction it was independently verified during
preliminary experiments that, in a given reaction mixture, kobs
values were not dependent on the exciplex laser energy or
number of photolysis shots. Because static gas mixtures were
used, tests with up to 20 shots were carried out. The constancy
of kobs (five-shot averages) showed no effective depletion of
reactants in any of the systems. The sensitivity of detection of
GeH2 was high but decreased with increasing temperature.
Therefore, increasing quantities of precursor were required at
higher temperatures. However, at any given temperature precur-
sor pressures were kept fixed to ensure a constant (but always
small) contribution to kobs values.
For each substrate a series of experiments was carried out at
each of five temperatures in the range from room temperature
up to ca. 564 K. At 10 Torr total pressure (SF6 diluent), a
number of runs (usually five or more) at different substrate
partial pressures were carried out at each temperature. The
purpose of these experiments was to establish the second-order
nature of the kinetics. In addition to these experiments, another
set of runs was carried out at each temperature, in which the
total pressure (SF6) was varied in the range 1-100 Torr to test
the pressure dependence of the second-order rate constants. In
these runs the number of data points obtained was limited to
two or three, but second-order behavior was assumed, and the
constants were calculated by assuming a linear dependence of
kobs on substrate pressure. To keep errors to a minimum,
sufficient substrate was used to ensure kobs values in the range
(2-3) × 105 s-1 where reaction with substrate was at least 75%
of the total reaction. Allowance was made for reaction of GeH2
with precursor (measured directly for each total pressure but
found to be pressure independent). The total pressure range
investigated was determined by practical considerations. Below
1 Torr, decay traces tended to be noisy, and above 100 Torr,
GeH2 signals were rather small. (Although the reason for this
is unknown, it is probably due to quenching of the precursor
excited state.) The results of the work described here represent
measurements of some 140 decay constants (kobs values) overall.
Analysis of the decay constants in the absence of substrate gave
rate constants for GeH2 + DMGCP in the range (2-6) × 10-10
cm3 molecule-1 s-1 (generally decreasing with temperature).
Bearing in mind that these values are based on only one
DMGCP pressure at each temperature, they are in reasonable
agreement with the value of 3.5 × 10-10 cm3 molecule-1 s-1
obtained earlier at 295 K.5
The OV101 GC column, operated at 60 °C, was also used
for product analytical studies. The retention times (in minutes)
of compounds used or identified in this work under these
conditions are as follows: EtGeH3, 1.15; Et2GeH2, 2.60; 2,3-
dimethyl-1,3-butadiene (DMB), 2.68; EtGeH2GeH3, 3.26; Et2-
GeHGeH3, 10.8; DMGCP, 16.3.
Results
General Considerations. Prior to kinetic studies, checks were
made of the UV absorption spectra of EtGeH3 and Et2GeH2 at
room temperature. EtGeH3 showed no absorption at 193 nm
(cross section < 8 × 10-20 cm2), while Et2GeH2 had a small
absorption (cross section ) 4.3 × 10-19 cm2). This shows that
under experimental conditions (at 298 K) absorption of the
excimer laser pulse by the substrates either does not occur or is
negligible. For Et2GeH2, the calculated absorbance at 193 nm
in the reaction vessel at the highest partial pressure (30 mTorr)
was 0.0007. It is possible that increased absorption (due to peak
broadening) could occur at higher temperatures, but we have
no evidence of this.

1436 J. Phys. Chem. A, Vol. 111, No. 8, 2007
Becerra et al.
100
TABLE 1: Experimental Second-Order Rate Constantsa,b for GeH2 + EtGeH3 at Various Pressures (SF6)
P/Torr
T/K
1
3
10
30
297
338
399
486
564
2.4 ( 0.2
2.7 ( 0.3
2.76 ( 0.07
1.83 ( 0.03
1.50 ( 0.05
0.844 ( 0.025
0.793 ( 0.025
2.7 ( 0.3
1.45 ( 0.04
1.13 ( 0.03
0.514 ( 0.044
0.295 ( 0.016
1.79 ( 0.02
1.58 ( 0.16
0.755 ( 0.061
0.441 ( 0.030
1.64 ( 0.16
0.910 ( 0.09
1.82 ( 0.18
1.01 ( 0.10
0.87 ( 0.09
a Units: 10-10 cm3 molecule-1 s-1
.
b Errors are single standard deviations (10 Torr values) or 10% at most other pressures (see text).
Figure 3. Arrhenius plots for rate constants for GeH2 + EtGeH3 (b)
and GeH2 + Et2GeH2 (O). P ) 10 Torr (SF6).
Figure 2. Pressure dependences of rate constants for GeH2 + EtGeH3
(2, 399 K; O, 486 K; f, 564 K). Lines are RRKM fits at the three
temperatures [RRKM(1), s; RRKM(2), ---].
∞
If the theoretical values of k2 are taken at 399, 486, and 564
K, then the resulting equation is
Kinetics of GeH2 + EtGeH3. This reaction was investigated
over the temperature range 297-564 K. The second-order rate
plots at 10 Torr total pressure are shown in Figure 1 for the
five temperatures studied. As can be seen, reasonably linear
plots resulted and the second-order rate constants, obtained by
least-squares fitting, are collected in Table 1. The error limits
are single standard deviations. These are random errors:
systematic errors are harder to assess, but based on previous
experience we do not anticipate that they should be more than
(10%. The rate constants clearly decrease with increasing
temperature.
∞
log(k2 /cm3 molecule-1 s-1) )
(-10.73 ( 0.09) + (6.65 ( 0.65) kJ mol-1/(RT ln10)
Kinetics of GeH2 + Et2GeH2. This reaction was investigated
over the temperature range 297-561 K. The second-order rate
plots at 10 Torr total pressure are shown in Figure 4 for the
five temperatures studied. As can be seen, reasonably linear
plots resulted and the second-order rate constants, obtained by
least-squares fitting, are collected in Table 2. The error limits
are single standard deviations. The rate constants clearly
decrease with increasing temperature.
This reaction showed negligible pressure dependence. The
rate constants found at other pressures were within experimental
error the same as those obtained at 10 Torr, even at the highest
temperature. Infinite pressure values can safely be assumed to
be the same as those at 10 Torr. An Arrhenius plot of k3 values,
also shown in Figure 3, gives a reasonably linear fit, bearing in
mind the uncertainties. The resulting Arrhenius equation is
The pressure dependence of these rate constants was also
investigated, and the results are also shown in Table 1. Because
the rate constants at other pressures are mostly based on fewer
data points, we have assumed uncertainties of (10% in values
to cover both random and systematic errors. At the two lower
temperatures the results suggest little or no pressure dependence,
except possibly at 1 Torr. At 399 K variations in values are
still slight. Only at 486 and 564 K are there more significant
signs of falloff at lower pressures. The data are plotted in Figure
2, where they are compared with RRKM theoretical predictions
(see next section). This is a much less marked pressure
dependence than that for the reaction of GeH2 + GeH4 (found
by us earlier7). Because the theory suggested very little pressure
dependence, infinite pressure values were taken to be the same
as those obtained at 10 Torr. An Arrhenius plot of k2 values,
shown in Figure 3, gives a reasonably linear fit, bearing in mind
the uncertainties. The resulting Arrhenius equation is
log(k3/cm3 molecule-1 s-1) )
(-10.68 ( 0.11) + (6.95 ( 0.80) kJ mol-1/(RT ln10)
End Product Analyses. (i) GeH2 + EtGeH3. A mixture of
0.42 Torr DMGCP and 1.27 Torr EtGeH3 was subjected to 100
shots of 193 nm laser radiation (60 mJ/pulse) and then analyzed
by GC. Under these conditions two major product peaks were
observed, comprising 93% of the total products based on peak
area (at a conversion of ca. 50% of DMGCP). The largest peak
(71%) eluted at 2.68 min, and was readily identified as 2,3-
log(k2/cm3 molecule-1 s-1) )
(-10.75 ( 0.08) + (6.74 ( 0.61) kJ mol-1/(RT ln10)

GeH2 Gas-Phase Reactions with EtGeH3 and Et2GeH2
J. Phys. Chem. A, Vol. 111, No. 8, 2007 1437
TABLE 3: Molecular and Transition State Parameters for
RRKM Calculation for Ethyldigermane Decomposition at
564 K
ethyldigermane
ethyldigermane‡
ν˜/cm-1
2960(5)
2110(5)
1450(3)
1260(2)
1000(1)
880(3)
850(3)
760(2)
700(1)
650(1)
410(4)
350(1)
250(1)
180(1)
170(1)
140(1)
100(1)
2960(5)
2110(4)
1500(1)
1450(3)
1260(2)
1000(1)
880(2)
850(3)
760(1)
700(1)
650(1)
590(1)
500(1)
410(2)
250(1)
197(1)
190(1)
180(1)
170(1)
140(1)
100(1)
Figure 4. Second-order plots for reaction of GeH2 + Et2GeH2 at 10
Torr (SF6) at different temperatures (indicated).
TABLE 2: Experimental Second-Order Rate Constantsa for
GeH2 + Et2GeH2 at 10 Torr (SF6)
reaction coordinate/cm-1
path degeneracy
350
3
154.8
4.48
Eo(critical energy)/kJ mol-1
ZLJ/10-10 cm3 molecule-1 s-1
T/K
k/10-10 cm3 molecule-1 s-1
297
342
401
480
561
3.17 ( 0.08
2.58 ( 0.08
1.91 ( 0.06
1.26 ( 0.05
0.831 ( 0.025
unimolecular decomposition of ethyldigermane, EtGeH2GeH3,
because the pressure dependences of association and dissociation
reactions are the same,31 provided there are no other reaction
channels. To our knowledge there has been no kinetic study of
the decomposition of EtGeH2GeH3, and so the necessary
parameters for the calculation were estimated. It has been
assumed also that the potential side channel leading to EtGeH
and GeH4 is minor, on the basis of analogy with the known
decomposition pathways of MeSiH2SiH3.32 The following
a Errors are single standard deviations.
dimethylbuta-1,3-diene (DMB), the known photodecomposition
product of DMGCP.5 The other major peak (16%), eluting
slightly after DMB at 3.26 min, was identified as ethyldigermane
as follows (see also the Discussion). A mixture of 0.98 Torr
EtGeH3 and 2.04 Torr GeH4 was photolyzed in the presence of
a drop of Hg using 253.7 nm radiation (Hg resonance lamp)
for ca. 5 min. When analyzed, the product mixture contained a
GC peak with the same retention time (3.26 min) as that seen
in the first experiment (above). This peak (42% of the total
products based on peak area) was absent in a blank photolysis
of Hg*-EtGeH3.
(ii) GeH2 + Et2GeH2. A mixture of 0.46 Torr DMGCP and
1.32 Torr Et2GeH2 was subjected to 100 shots of 193 nm laser
radiation (60 mJ/pulse) and then analyzed by GC. Under these
conditions several new product peaks were observed, only one
of them substantial. The DMB peak (from DMGCP photode-
composition) coeluted with that of Et2GeH2, but the large new
peak (85% of the remaining products based on peak area),
eluting at 10.8 min, was identified as 1,1-diethyldigermane as
follows (see also the Discussion). A mixture of 0.89 Torr Et2-
GeH2 and 2.89 Torr GeH4 was photolyzed in the presence of a
drop of Hg using 253.7 nm radiation (Hg resonance lamp) for
ca. 5 min. When analyzed, the product mixture contained a GC
peak with the same retention time (10.8 min) as that seen in
the first experiment (above). This peak (11% of total products
based on peak area) was absent in a blank photolysis of Hg*-
Et2GeH2. It was also confirmed that no products were formed
from the 193 nm laser photolysis of Et2GeH2 alone (at 298 K).
RRKM Calculations. Because the GeH2 + EtGeH3 reaction
shows signs of pressure dependence characteristic of a third-
body assisted association reaction,31 as found previously for
reaction 1,7 we undertook RRKM calculations to model this.
This was carried out on the reverse reaction -2, viz. the
7
Arrhenius parameters were assumed based on those for Ge2H6
but with correction for path degeneracy: log(A-2/s-1) ) 14.42
and Ea(-2) ) 154.8 kJ mol-1. It is not possible to state with
complete certainty how reliable these parameters are, but on
the basis of the similarity of Arrhenius parameters for the
decompositions of the structurally similar methyldisilanes33 (four
compounds), it is reasonable to assume that the A factor is
reliable within a factor of 2 and the activation energy within
(8.4 kJ mol-1
.
The next stage was to assign the vibrational wavenumbers
of the molecule and its activated complex (transition state) at
the key temperatures of study. This was done for EtGeH2GeH3
34
using group frequencies based on those for Ge2H6 and those
of the C2H5 group.35 The activated complexes were assigned
by adjusting the wavenumbers of the key transitional modes,
principally those of the GeH3 group, until a match was obtained
with the entropy of activation and the A factor in the usual way.31
Whether precise values of all vibrational wavenumbers are
correct or not is not important provided the entropies of
activation are matched. The values of the molecular and
transition state parameters are shown in Table 3 for the
assignment at 564 K, the highest calculation temperature. The
table also includes the value of the Lennard-Jones collision
number, ZLJ (in SF6), calculated using the formulas recom-
mended by Troe36 using the following collision parameters: for
SF6, σ ) 5.13 Å and ꢀ/k ) 222 K; for EtGe2H5, σ ) 5.61 Å
and ꢀ/k ) 463 K. These were obtained or estimated from those
for similar molecules listed by Reid et al.37 Small changes (not
shown) were made to the EtGeH2GeH3 activated complex
wavenumbers and ZLJ values for the RRKM calculations at 486

1438 J. Phys. Chem. A, Vol. 111, No. 8, 2007
Becerra et al.
TABLE 4: Comparison of Arrhenius Parameters for Ge-H
and 399 K. We have assumed that geometry changes in the
decomposing EtGeH2GeH3 molecule do not lead to significant
changes in overall moments of inertia and abiabatic rotational
effects. This is an approximation, in view of the loose activated
complex structures, but we believe it will not lead to serious
errors. However, we have used a weak collisional (stepladder)
model for collisional deactivation,31 because there is over-
whelming evidence against the strong collision assumption.38
The average energy removal parameter, ∆E down, was taken as
9.6 kJ mol-1 (800 cm-1) for all reactions by analogy with earlier
studies7,12 of GeH2 reactions, although variations within the
range 8.4-12.0 kJ mol-1 had little effect on the fitting. The
results of these calculations, called RRKM(1), are shown
graphically in Figure 2, where they are compared with experi-
ment. Because the trends of these calculations were somewhat
less than those observed, we extended these calculations within
the possible uncertainties in the values of A and Eo for
decomposition (see above). Another set of calculations, called
RRKM(2), was based on values of log(A-2/s-1) ) 14.72 and
Ea(-2) ) 146.4 kJ mol-1. All these results are discussed below.
Insertion Reactions of GeH2
reaction
log Aa
Ea/kJ mol-1
ref
GeH2 + GeH4
-11.17 ( 0.10b
-10.75 ( 0.08
-5.2 ( 0.7b
-6.7 ( 0.6
7
GeH2 + EtGeH3
this work
GeH2 + Et2GeH2 -10.68 ( 0.11
GeH2 + Et3GeH
-6.95 ( 0.80 this work
-11.43 ( 0.15
-10.6 ( 1.1
6
a Units: cm3 molecule-1 s-1
.
b High pressure limiting values.
TABLE 5: Comparison of Room-Temperature Rate
Constantsa for Ge-H Insertion Reactions of GeH2
reaction
1010k(298 K)a,b
krel
krel(per Ge-H)
GeH2 + GeH4
0.547c
2.70
3.45
1
1.0
6.6
12.6
19.9
GeH2 + EtGeH3
GeH2 + Et2GeH2
GeH2 + Et3GeH
4.94
6.31
4.97
2.72
a Units: 10-10 cm3 molecule-1 s-1 b Values calculated from Arrhe-
.
nius equations. c High pressure limiting value.
TABLE 6: Comparison of Room-Temperature Rate
Constantsa,b for Si-H Insertion Reactions of SiH2
reaction
1010k(298 K)a
krel
krel(per Si-H)
Discussion
SiH2 + SiH4
4.6c
4.1
3.5
2.5
1
1.0
1.2
1.5
2.2
Product Identification and Overall Reactions. The iden-
tification of the unknown GC peaks was made by the following
argument. The technique of mercury (63P1) photosensitization
is known to proceed via X-H bond cleavage.39 The mechanism
for reaction in mixtures of EtGeH3 + GeH4 should be as follows:
SiH2 + MeSiH3
SiH2 + MeSiH2
SiH2 + Me3SiH
0.89
0.76
0.54
a Units: 10-10 cm3 molecule-1 s-1
limiting value.
.
b From ref 26. c High pressure
Hg* + GeH4 f GeH3 + H + Hg
Hg* + EtGeH3 f EtGeH2 + H + Hg
H + GeH4 f GeH3 + H2
data at all temperatures, although the fit at 564 K is still not
good. However, the problem with RRKM(2) is that at 10 Torr
at 564 K the rate constant is predicted to be 20% below the
high pressure limit. This cannot be ruled out given the scatter
of the data. This indicates that loosening the transition state in
these calculations does help with the fit. However, we still
suspect that there may be a systematic source of error in the
measurements at the low pressures, which we were unable to
pinpoint. Despite this, we believe, on the basis of the large
majority of measurements, that the data are reliable and, from
the RRKM calculations, that the reaction has very little pressure
H + EtGeH3 f EtGeH2 + H2
2GeH3 f Ge2H6
GeH3 + EtGeH2 f EtGeH2GeH3
2EtGeH2 f EtGeH2GeH2Et
∞
dependence. Use of k2 (10 Torr) or k2 in the Arrhenius plots
∞
makes very little difference, although k2 would be better if
there were less scatter in the data at 564 K.
Of the expected end products, Ge2H6 does not give a GC signal
(using FID)8 and EtGeH2GeH2Et was not seen, probably because
GeH4 was used in a 2-fold excess, leaving only EtGeH2GeH3,
which corresponded to the product of the GeH2 + EtGeH3 study.
Exactly similar and parallel arguments establish Et2GeHGeH3
as the product of the GeH2 + Et2GeH2 reaction. These results
confirm the formation of the expected products based on earlier
studies of GeH2 insertion into Ge-H bonds.40-42 This approach
was used previously to identify Me3SiGeH3 as the product of
GeH2 + Me3SiH.8
RRKM Calculations and Pressure Dependence. The ex-
perimental measurements of extent of pressure dependence and
calculated degrees of falloff for reaction 2, that of GeH2 +
EtGeH3, are compared in Figure 2. The results of the first set
of calculations, RRKM(1), are in reasonable agreement with
experiment at all pressures except 1 Torr at 399 and 486 K. At
564 K the agreement is less good. However, the calculations
do support the conclusion that at pressures of 10 Torr and above
the reaction is close to its high pressure limit. The calculations
RRKM(2) correspond to a slightly looser transition state with
a slightly lower critical energy, within the reasonable uncertainty
range of our estimates of these parameters. The increased
curvature in the plots reflects somewhat better the trends in the
Rate Constant Comparisons. To our knowledge, these are
the first measurements of gas-phase rate constants for reactions
2 and 3. The measured Arrhenius parameters are compared with
those of reactions 1 and 4 in Table 4. The values are very
comparable for all these reactions with A factors in the range
10-11.0(0.4 cm3 molecule-1 s-1 and negative activation energies
between -5 and -11 kJ mol-1. Although the A factors show
no systematic trend, there is a trend toward more negative
activation energies with increasing ethyl group substitution. To
gain further insight into these reactions, we have singled out
the rate constants at 298 K for comparison. These are shown in
Table 5. The rate constants for reactions 2-4 are significantly
larger than that for reaction 1, showing that the effect of ethyl
substitution in the substrate germane is to increase markedly
the Ge-H insertion reactivity, although the trend is not
monotonic. Particularly of note are the per Ge-H rate constants
showing the steady increase in values with successive ethyl
substitution. Another useful comparison here is with the silylene
counterpart reactions. Table 6 shows the analogous set of SiH2
rate constants for its Si-H insertion reactions with the meth-
ylsilanes.26 One clear feature of contrast is apparent. The effect
of methyl substitution in the substrate silane actually reduces

GeH2 Gas-Phase Reactions with EtGeH3 and Et2GeH2
J. Phys. Chem. A, Vol. 111, No. 8, 2007 1439
carried out ab initio calculations on these reactions, we may
make some inferences based on findings in the SiH2
+
methylsilane reaction systems.26 In these latter reactions the
effect of methyl substituents was to stabilize the intermediate
complexes (LMs) and their accompanying transition states (TSs)
by ca. 6-9 kJ mol-1 per Me group. If the effect for ethyl
substitution on the Ge-H insertion process were of similar
magnitude, we can infer that the inhibiting effect of the
secondary barriers will diminish with increasing ethyl substitu-
tion, and become quite small for Et3GeH. Since the effect of
the secondary barriers on the reaction rate constant is almost
negligible for the SiH2 + SiH4 reaction,23 whereas it fairly large
for the GeH2 + GeH4 reaction7 (i.e., slowing it down), alkyl
substituents should have a larger accelerating effect in the latter
system, as we have found. This accelerating effect will however
be temperature dependent, and as temperature increases it should
become less. This is to say that the inhibiting effects of the
secondary barriers become more marked at higher temperatures.
The actual barrier magnitudes will determine the size of this
effect, but clearly the sharper decline in rate constant values
with temperature (i.e., the more negative activation energies)
as ethyl substitution increases shows that secondary barriers are
important for all the GeH2 + ethylgermane reactions.
Figure 5. Generic potential energy surface for GeH2 insertion reactions
with germanes. LM is local minimum; TS is transition state.
the value of the rate constants for Si-H insertion, although the
per Si-H rate constants show a small but monotonic increase
with successive methyl substitution. Thus the alkyl substituent
effect is much less marked for the Si-H insertion reactions (of
SiH2) than for the Ge-H insertion reactions (of GeH2). It is
also interesting to note that in the case of reaction 4, GeH2 +
Et3GeH, the rate constant actually exceeds that for SiH2 + Me3-
SiH. The values for these rate constants are approaching the
collision theory maximum (ca. 3 × 10-10 cm3 molecule-1 s-1),
but it is unusual for GeH2 reactions to occur faster than their
SiH2 counterparts.15 A further comparison, which does dem-
onstrate the lower reactivity of GeH2 compared with SiH2, is
that of its rate constants for Si-H insertion. For GeH2 + SiH4,
In a future publication43 we plan to publish the results of
experiments and quantum chemical calculations on the effects
of methyl substituents in both germylene and substrate germane
on the Ge-H insertion reaction.
Acknowledgment. We thank the following: INTAS-RFBR
(Project No. IR-97-1658) and NATO (Project No. PST-
.CLG975368). S.E.B, M.P.E, I.V.K., and O.M.N also thank
RFBR (Project No. N 07-03-00693), the President of the Russian
Federation (Presidential Program for Support of Leading
Research Schools, Grant NSh-6075.2006.03), and the Russian
Academy of Sciences (Programs P-09 and OX-01). R.B. thanks
the Spanish DGI for support under Project No. BQU2002-03381.
k(298 K) ) 1.24 × 10-11 cm3 molecule-1 s-1 10
, and for GeH2
+ Me3SiH, k(298 K) ) 7.64 × 10-11 cm3 molecule-1 s-1 8
.
These are significantly less than their SiH2 counterparts shown
in Table 6. They are also significantly less than their Ge-H
insertion analogues shown in Table 5, which supports the general
proposition that GeH2 will insert less readily in stronger bonds
than in weaker ones. One final comparison is worth making.
For GeH2 + Me2GeH2, k(298 K) ) 2.38 × 10-10 cm3
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