Kinetic studies of reactions of hexaborane(10) with other binary boranes in the gas phase

Article abstract of DOI:10.1016/S0022-328X(00)00627-6

Cothermolysis reactions of B6H10 with the binary boranes B2H6, B4H10, B5H9, and B5H11 have been studied by a quantitative mass-spectrometric technique to gain insight into the role of B6H10 in borane interconversion reactions. Except in the B6H10-B5H9 system the initial rate of consumption of B6H10 was found to be considerably more rapid than in the thermolysis of B6H10 alone, indicating that interactions were occuring. Detailed kinetic studies of the B6H10-B2H6 and B6H10-B4H10 reactions showed that the rate of consumption of B6H10 was governed in each case by the rate-determining step in the decomposition of the co-reactant, the orders being 3/2 with respect to B2H6 and 1 with respect to B4H10; a considerable increase in the conversion of B6H10 to B10H14 at the expense of polymeric solids was also observed. Added hydrogen was found to have very little effect on the reaction rates and product distributions in the cothermolysis reactions, in marked contrast to its effect on the reactions of B2H6 and B4H10 alone. The kinetic results are entirely consistent with earlier suggestion, based on qualitative observations, that the reactive intermediates {B3H7} and {B4H8} are scavenged by reactions with B6H10, and suggest strongly that this borane, unlike B6H12, plays a pivotal role in the build-up to B10H14 and other higher boranes.

Full text of DOI:10.1016/S0022-328X(00)00627-6

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 614–615 (2000) 144–152
Kinetic studies of reactions of hexaborane(10) with other binary
boranes in the gas phase
Martin D. Attwood, Robert Greatrex *1, Norman N. Greenwood *2,
Christopher D. Potter
School of Chemistry, Uni6ersity of Leeds, Leeds LS2 9JT, UK
Received 31 March 2000
Dedicated to Professor Sheldon G. Shore on the occasion of his 70th birthday in appreciation of his outstanding contributions to the
chemistry of the boron hydrides.
Abstract
Cothermolysis reactions of B6H10 with the binary boranes B2H6, B4H10, B5H9, and B5H11 have been studied by a quantitative
mass-spectrometric technique to gain insight into the role of B6H10 in borane interconversion reactions. Except in the B6H10–B5H9
system the initial rate of consumption of B6H10 was found to be considerably more rapid than in the thermolysis of B6H10 alone,
indicating that interactions were occurring. Detailed kinetic studies of the B6H10–B2H6 and B6H10–B4H10 reactions showed that
the rate of consumption of B6H10 was governed in each case by the rate-determining step in the decomposition of the co-reactant,
the orders being 3/2 with respect to B2H6 and 1 with respect to B4H10; a considerable increase in the conversion of B6H10 to
B10H14 at the expense of polymeric solids was also observed. Added hydrogen was found to have very little effect on the reaction
rates and product distributions in the cothermolysis reactions, in marked contrast to its effect on the reactions of B2H6 and B4H10
alone. The kinetic results are entirely consistent with earlier suggestions, based on qualitative observations, that the reactive
intermediates {B3H7} and {B4H8} are scavenged by reaction with B6H10, and suggest strongly that this borane, unlike B6H12, plays
a pivotal role in the build-up to B10H14 and other higher boranes. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Hexaborane(10); Binary boranes; Thermolysis; Kinetic studies; Mechanism
1. Introduction
90% of the boron as a non-volatile solid hydride [1–3].
Hexaborane(12), on the other hand, decomposes
rapidly at this temperature in a first-order reaction
which generates, almost exclusively, the volatile prod-
ucts B5H9 and B2H6 [4]. Whereas B6H12 itself appears
not to be involved in the stepwise build-up to B10H14 [5]
it has been suggested [6] that B6H10 may play a crucial
role in the overall thermolysis of B2H6, by virtue of its
known tendency (albeit under somewhat different con-
ditions) to react as a Lewis base [7,8] with acidic borane
intermediates such as {BH3}, {B3H7}, {B4H8}, B8H12
and {B9H13} to produce larger boranes such as B13H19,
B14H22 and B15H23. However, there have been no gas-
phase studies, even of a qualitative nature, to test these
ideas in any systematic way. We have therefore carried
out a comprehensive investigation of cothermolysis re-
actions of B6H10 with the small binary boranes B2H6,
B4H10, B5H9 and B5H11 to address this problem. The
The virtual absence of one or more species intermedi-
ate between the pentaboranes and B10H14 in the thermal
decomposition of B2H6 is an intriguing feature of this
complex reaction, and raises the question of the role of
the hexaboranes, B6H10 and B6H12, in borane intercon-
versions. Previous work in this laboratory has shown
that these two boranes differ markedly from one an-
other in their thermal stabilities and in the details of
their individual gas-phase decompositions. Hexabo-
rane(10), when pure, is comparable in stability to B2H6
itself, decomposing only slowly at 373 K in a second-
order process which results in the deposition of some
1 *Corresponding author. Fax: +44-113-2336565; e-mail:
r.greatrex@chem.leeds.ac.uk.
2 *Corresponding author. Fax: +44-113-2336565; e-mail:
n.n.greenwood@chem.leeds.ac.uk.
0022-328X/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(00)00627-6

M.D. Attwood et al. / Journal of Organometallic Chemistry 614–615 (2000) 144–152
145
B6H10–B2H6 and B6H10–B4H10 systems were studied in
quantitative detail at various temperatures to obtain
kinetic parameters, and the effect of adding an excess of
hydrogen to each system was also investigated. Data
were also recorded for the thermolysis of pure B2H6,
which has been studied many times in the past [9] to
provide a self-consistent set of initial-rate data and rate
constants for direct comparison with the results ob-
tained in the B6H10–B2H6 cothermolysis study. The
results on these various systems, some of which have
been discussed elsewhere in a preliminary form [1]
provide new insights into the role of B6H10 in borane
interconversions.
ponent onto the lower surfaces. The inert-gas or inert-
gas–hydrogen mixture was introduced into the vessel
containing the frozen boranes at 77 K, before the
mixture was allowed to warm to room temperature.
The boranes were prepared by methods described in
the literature (B2H6 [12], B4H10 [13], B6H10 [14]) and
manipulated by standard grease-free vacuum-line tech-
niques. Purification was achieved by trap-to-trap frac-
tionation, followed where necessary by fractionation on
a low-temperature column.
3. Results and discussion
2. Experimental
3.1. Thermolysis of B2H6
The quantitative mass-spectrometric techniques used
in this work have been described in detail elsewhere
[2,5,10]. Thermolyses were carried out in a pre-condi-
tioned spherical Pyrex bulb (volume ca. 1 dm3) in the
presence of a large background of an inert-gas mixture
(partial pressure 100 mmHg) comprising helium, argon
and krypton in the relative proportions 98.0:1.0:1.0.
[11] [For conversion to SI, 1 mmHg :133.322 Pa.] The
gases were pre-mixed in a similar bulb at room temper-
ature and expanded into the preheated reaction vessel,
from where the mixture was continuously sampled. To
facilitate adequate mixing, and to minimise premature
interaction of the reactants in the liquid phase, the
heavier component (B6H10) was always frozen onto the
upper surfaces of the mixing bulb and the lighter com-
In agreement with earlier studies [9] the major prod-
ucts of the thermolysis of B2H6 at temperatures in the
range 398–453 K were found to be H2, B5H9, B5H11,
B10H14 and traces of B6H10, B6H12, B8H12 and B9H15.
Mass-balance calculations indicated that 40–50% of the
boron was deposited as involatile solid hydride. Initial-
rate data, obtained by the tangent method from plots of
the type shown later for the cothermolysis reactions
(Figs. 2–4), are recorded in Table 1 for the thermolysis
of B2H6 alone and in the presence of added H2. From
log–log plots of the initial rates of consumption of
B2H6 and production of H2 against the initial concen-
tration of B2H6, the order of the reaction at 398 and
422 K was shown to be consistent with the consensus
value [9] of 3/2. This value has therefore been used in
Table 1
Initial-rate data and 3/2-order rate constants for thermolysis of B2H6 alone and in the presence of added H2
b
b
c
c
T/K (90.5)
po(B2H6) (mmHg a)
−(d[B2H6]/dt)o
mmol m−3 s−1
(d[H2]/dt)o
104k3/2,B H
104k3/2,H
2
6
2
mol−1/2 m3/2 s−1
398.4
397.1
398.5
398.4
407.1
421.9
421.8
422.1
421.9
v423.6
1.72
3.45
6.90
13.95
3.50
1.76
3.53
7.00
14.00
3.52
3.49
3.49
3.46
3.0
6.3
15.8
1.2
1.66
1.21
0.68
d
d
7.5
21.8
4.1
4.5
12.7
34.1
1.08
0.51
0.52
0.80
2.65
2.58
2.49
d
9.3
14.1
42.1
91.0
214.7
27.9
8.1
1.81
8.24
8.57
6.63
5.54
5.75
1.69
16.72
28.88
107.1
2.76
d
d
e
d
d
423.6
437.7
451.1
76.8
125.0
51.9
112.9
6.80
26.17
a Initial-rates accurate to within ca. 10%.
b Errors in initial pressures ca. 91%. To convert pressure in mmHg to concentration in mol m−3 multiply by 16.03/(T/K).
3/2
3/2
c Evaluated from the expressions k3/2,B H =−(d[B2H6]/dt)o/[B2H6]o and k3/2,H =(d[H2]/dt)o/[B2H6]o
.
2
6
2
d Data not recorded.
e Thermolysis carried out in presence of added H2, po(H2)=50.0 mmHg.

146
M.D. Attwood et al. / Journal of Organometallic Chemistry 614–615 (2000) 144–152
the pre-exponential factor. The corresponding values
from the H2-production data are 11297 kJ mol−1 and
2.0
e23.99 (ca. 2.4×1010) m3/2 mol−1/2 −1. The latter are
s
regarded as being of limited value because the hydrogen
data measure not only the homogeneous gas-phase
reaction but also a contribution from the decomposi-
tion of solid hydride deposited on the surface of the
reaction vessel during the reaction; the data are also
complicated by the fact that the initial composition of
the solid hydride is temperature dependent.
In agreement with earlier work, the thermolysis of
diborane was found to be inhibited by the addition of
hydrogen. This is apparent from Table 1 and Fig. 1 for
the pair of experiments carried out at 423.6 K. Addi-
tion of H2 at a partial pressure of 50 mmHg is seen to
retard the rate of decomposition of B2H6 (p0 3.5
mmHg) by a factor of 3.4; this run is represented by the
open circle in Fig. 1.
Fig. 1. Arrhenius plots for the thermolysis of pure B2H6 (ꢀ) and
B2H6 in the presence of B6H10
(
). For comparison, data points are
also shown for a single thermolysis of B2H6 (p0 3.49 mmHg) in the
presence of a background of H2 (p0 50 mmHg) (ꢁ), and for the
B6H10–B2H6 cothermolysis in the presence of a background of H2
(ꢂ); details are given in Table 3. The solid line is a least-squares fit
to the data obtained from the thermolysis of pure B2H6 (ꢀ).
3.2. Cothermolysis of B6H10 with B2H6
A typical concentration-time profile for the B6H10–
B2H6 reaction is shown in Fig. 2. In this example the
co-reactants were at the same partial pressure of ca. 3.5
mmHg. Profiles for the numerous other runs in this and
the other systems referred to later, in which the partial
pressures are varied, are available elsewhere [15].
The main boron-containing product in this reaction
is B10H14, which in some cases accounts for up to ca.
60% of the boron consumed. This compares with a
value of less than ca. 5% in the thermolyses of either
reactant alone [2,10]. The most efficient conversion to
B10H14 occurs in those runs in which the initial concen-
tration of B2H6 is greater than that of B6H10. Hydrogen
is produced in quantity, and B5H9 (B5%) is also
observed, together with trace amounts of B8H12, n-
B9H15 and the higher boranes B15H23 and B16H20. Non-
volatile solid is produced in all reactions, the amounts
depending on the temperature and the initial concentra-
tions of the reactants. Mass-balance calculations indi-
cate a composition of approximately BH1.5
.
Inspection of the initial-rate data in Table 2 reveals
that the rate of consumption of B2H6 at any given
temperature and initial pressure is, within experimental
error, the same as that observed in the thermolysis of
B2H6 alone (see Table 1), and essentially independent of
the concentration of B6H10. By contrast, the rate of
consumption of B6H10 is invariably faster than in the
thermolysis of B6H10 alone, and depends dramatically
on the concentration of B2H6.
Fig. 2. Product analysis as a function of time for the cothermolysis at
T=424.9 K of B6H10 (p0 3.51 mmHg) with B2H6 (p0 3.49 mmHg):
(ꢃ) B2H6, (ꢂ) B6H10, (ꢁ) H2, (ꢄ) B5H9, (ꢀ) B10H14
.
the calculation of all the rate constants listed in Table
1, though it is realised that this may not apply strictly
to the experiments involving added H2.
The 3/2-order rate constants for the thermolysis of
B2H6 are shown in the form of an Arrhenius plot in
Fig. 1. Least-squares analysis of these data yields a
value of 9296 kJ mol−1 for the activation energy, and
a value of e18.891.7 (ca. 1.49×108) m3/2 mol−1/2 s−1 for
In an attempt to gain a better understanding of the
nature of the interaction between B6H10 and B2H6,
reaction orders have been determined from plots of the
log of the initial rate of consumption of B2H6 against
the log of the initial concentrations of each reactant.

Table 2
a
Initial-rate data for cothermolysis of B2H6 and B6H10 with and without added H2
T/K
po(B2H6)
po(B6H10
)
po(H2)
−(d[B2H6]/dt)o
mmol m−3 s−1
−(d[B6H10]/dt)o
−(d[B6H10]/dt)ocorr c
(d[H2]/dt)o
(d[H2]/dt)coorr d
(d[B5H9]/dt)o
(d[B10H14]/dt)o
mmHg b
372.7
372.4
371.8
372.9
372.3
371.8
372.9
39891
39891
39891
39891
39891
39891
39891
39891
39891
39891
408.1
408.1
408.1
408.1
408.1
408.1
408.1
408.1
408.1
408.1
408.1
408.1
408.1
1.75
3.54
3.51
3.54
3.43
7.00
13.93
1.74
3.52
3.52
3.50
3.46
3.46
3.53
3.53
7.00
13.95
1.77
3.47
3.51
3.50
3.47
3.48
3.46
7.04
7.04
10.45
10.48
13.92
13.92
1.77
3.51
1.72
3.46
3.48
7.02
3.48
3.44
3.46
0.44
0.88
1.76
3.48
3.53
7.02
13.26
3.49
3.49
3.50
1.76
1.76
3.50
3.47
6.95
7.00
3.53
3.55
3.48
3.53
3.51
3.54
3.49
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
15.0
0.0
15.0
0.0
15.0
0.0
15.0
0.0
15.1
0.0
15.0
0.0
1.1
0.3
2.7
3.8
ꢀ0
1.0
2.7
2.1
5.0
7.2
4.0
2.5
5.8
3.9
6.2
3.6
4.6
8.5
3.0
3.3
2.3
5.0
3.1
3.1
2.8
3.9
5.3
2.9
4.6
7.0
10.0
8.7
15.2
14.1
25.0
27.6
30.6
19.9
22.9
25.7
32.2
35.3
38.0
39.5
33.0
64.4
60.6
109.9
84.0
18.6
2.2
1.9
3.6
3.4
6.5
5.5
9.0
18.2
9.6
11.8
17.8
27.9
21.3
44.0
72.0
57.5
109.9
63.3
50.2
66.5
68.2
66.8
141.9
137.3
112.8
107.4
196.0
210.9
256.5
246.1
40.6
1.5
1.7
2.9
2.7
3.7
4.8
8.3
15.0
9.5
11.6
17.0
24.7
17.9
30.9
25.7
54.0
106.7
57.9
48.8
65.1
62.8
61.5
120.6
115.7
107.3
101.8
190.7
205.5
251.1
240.6
28.0
/
4.8
7.8
8.0
5.6
6.1
5.0
13.3
12.0
28.3
60.4
28.3
30.9
36.0
21.3
24.3
31.1
37.5
56.6
59.6
45.0
38.6
69.7
66.0
115.3
89.5
31.2
3.4
31.0
52.2
10.3
23.7
14.2
18.2
17.3
17.8
18.6
55.7
45.9
61.1
72.5
75.4
74.2
10.8
0.8
4.3
614
2.8
615
5.8
(
2.6
3.9
4.1
144
10.6
9.1
152
9.9
16.3
21.6
19.3
3.7
423.7
0.7

Table 2 (Continued)
T/K
po(B2H6)
po(B6H10
)
po(H2)
−(d[B2H6]/dt)o
mmol m−3 s−1
−(d[B6H10]/dt)o
−(d[B6H10]/dt)ocorr c
(d[H2]/dt)o
(d[H2]/dt)coorr d
(d[B5H9]/dt)o
(d[B10H14]/dt)o
mmHg b
/
425.0
424.9
427.2
423.3
425.2
423.0
423.5
423.0
423.0
422.5
437.1
437.1
437.0
436.5
438.4
437.9
437.4
471.1
3.50
3.49
3.53
3.49
7.02
7.02
13.94
14.0
14.0
20.6
1.75
3.52
3.48
3.50
3.51
6.97
14.0
3.51
1.76
3.51
7.00
13.99
3.49
3.94
3.49
3.50
3.50
3.46
3.49
1.73
3.49
6.95
14.00
3.46
3.46
3.49
0.0
0.0
0.0
0.0
0.0
16.9
0.0
10.0
16.8
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
53.5
31.5
29.6
40.8
41.2
112.6
37.6
28.6
63.0
65.5
98.8
392.6
337.2
185.4
62.3
86.1
343.1
135.4
173.0
0.5
1.2
2.8
9.7
8.4
9.2
48.6
116.3
124.6
284.9
351.2
314.0
341.5
28.7
114.9
111.8
71.8
70.4
99.6
58.0
84.3
2.6
2.2
22.2
25.3
51.7
63.3
71.0
90.0
12.5
17.2
21.6
22.5
26.8
41.0
110.0
208.3
172.7
301.8
203.1
127.2
99.7
195.7
160.6
289.7
190.8
102.6
93.7
57.9
93.0
57.3
472.9
460.3
10.3
10.8
10.5
26.7
1.0
2.3
6.5
3.7
17.1
630.7
42.2
618.3
17.4
192.4
183.1
315.9
591.3
472.3
1095
186.2
158.4
218.8
195.0
447.7
1072
82.6
190.7
453.1
238.9
422.2
550.2
614
68.6
317.3
641.6
602.1
214.7
398.0
454.8
615
28.6
)
1008
913.0
a See appropriate footnote to Table 1.
b See appropriate footnote to Table 1.
144
c Initial rates corrected for B6H10 consumed and H2 produced in the second-order self-decomposition of B6H10 (see [2] for relevant data).
d Rate too low to be measured accurately.
152

M.D. Attwood et al. / Journal of Organometallic Chemistry 614–615 (2000) 144–152
149
The values obtained for the consumption of B2H6 with
respect to its own initial concentration were 1.4690.12
at 424 and 1.4990.15 at 437 K; i.e. the same as the
established order of 3/2 for the gas-phase thermolysis of
pure diborane [9] and strongly suggesting that B2H6 is
undergoing the same initial rate-determining sequence
of reactions leading to the production of the reactive
intermediate {B3H7}:
have not been used in the kinetic analysis, but are given
in Table 2 for completeness.
These results for the B6H10–B2H6 cothermolysis are
therefore consistent with the rate equation, ratecoth
=
k3/2,coth[B2H6]3/2. Values for the 3/2-order rate constant
at various temperatures have been evaluated from the
B2H6-consumption, B6H10-consumption and H2-pro-
duction data (denoted k3/2,B H , k3/2,B H and k3/2,H
,
2
6
6
respectively), and listed in Table 3. If10 the workin2g
hypothesis is correct, the three values at any given
temperature should agree, and this is in fact the case for
k3/2,B H and k3/2,B H . The k3/2,H values tend to be
B2H6 X 2{BH3}
(1)
(2)
slow
{BH3}+B2H6 X {B3H9} X {B3H7}+H2
2
6
10
6
2
As implied earlier, the order for the consumption of
B2H6 with respect to the initial concentration of B6H10
was found to be essentially zero, suggesting that the
initial rate-determining sequence is followed by the
rapid reaction of the intermediate {B3H7} with B6H10 to
give higher boranes (see later). The order-plots for the
consumption of B6H10 and production of hydrogen
were less informative, each showing curvature leading
to increased orders at the higher temperatures. How-
ever, once it was realised that these data would include
contributions from the competitive, second-order de-
composition of B6H10, more meaningful results were
forthcoming. Table 2 contains both the raw data and
the initial rates corrected for this process. The order
plots based on the corrected data now indicate zero-or-
der dependence on the initial concentration of B6H10,
consistent with the analysis of the B2H6 rate data. By
contrast, there is a definite dependence on the initial
concentration of B2H6; the order is less well defined,
but is not inconsistent with the value of 3/2 determined
from the more accurate B2H6-consumption data for
which no corrections need to be applied. Rates of
production of the minor products B5H9 and B10H14
somewhat higher, but for reasons mentioned earlier this
is perhaps not surprising.
The 3/2-order rate constants k3/2,B H for the cother-
2
molysis are shown in the form of an6Arrhenius plot,
together with the data for the thermolysis of pure B2H6
in Fig. 1. It can be seen that the two sets of data do
indeed lie close to the same line of best fit. The least-
squares analysis of these data yields a value of 99.49
2.5 kJ mol−1 for the activation energy, and a value of
e21.190.8 (ca. 1.5×109) m3/2 mol−1/2 s−1 for the pre-
exponential factor. The corresponding values from the
H2-production data recorded in Table 1 are 92.593.8
kJ mol−1 and e19.791.1 (ca. 3.6×108) m3/2 mol−1/2 s−1
and from the B6H10-consumption data, 82.995.3 kJ
mol−1 and e16.591.5 (ca. 1.5×107) m3/2 mol−1/2 s−1
,
.
Included in Fig. 1 and indicated by open triangles,
are data for two runs in which an excess of hydrogen
was added to the B6H10–B2H6 cothermolysis. Interest-
ingly, in marked contrast to the thermolysis of pure
B2H6, the addition of hydrogen to the B6H10–B2H6
cothermolysis was found to have little effect on the
rates (Table 2) or on the product distribution. The
significance of this is discussed later.
Table 3
a
3/2-Order rate constants for the B6H10–B2H6 cothermolysis with and without added H2
Added H2 p/mmHg
T/K b
No. of runs b
104k3/2,B H
104k3/2,B H
104k3/2,H
2
corr.
corr.
2
6
6
10
1
2
mol− m3/2 s−1
0.0
0.0
0.0
37291.7
39891.0
40890.5
40890.5
42491.4
42390.5
42390.5
43790.6
47190.5
7
10
7
7
8
1
2
7
1
0.3090.25
1.290.3
3.691.2
3.391.1
7.092.4
9.1
8.6
2094
147
0.4690.52
1.790.8
5.794.6
5.393.3
7.393.6
4.2
6.8
1693
111
0.4090.17
4.391.8
14.099.0
15911
15.0
0.0
10.0
ꢀ17.0
0.0
21919
34911
221
0.0
a Evaluated from the expressions k3/2,B H =−(d[B2H6]/dt)o/[B2H6]o3/2; k3/2,B H corr. =−d[B6H10]/dt)coorr./[B2H6]3o/2; and k3/2,H corr. =(d[H2]/
2
6
10
2
6
dt)coorr./[B2H6]o3/2; (see discussion and footnotes to Table 1 for definition of these corrected rates). Errors quoted are standard deviations. Initial
rates for these various runs are given in Table 2.
b Errors in temperature measurement ca.90.5 K; when values have been obtained from a series of runs, the error is the standard deviation from
the mean value quoted.

150
M.D. Attwood et al. / Journal of Organometallic Chemistry 614–615 (2000) 144–152
3.3. Cothermolysis of B6H10 with B4H10
As B4H10 is much less stable than B2H6, it was
possible to study the B6H10–B4H10 reaction over a
range of temperatures, 40–90°C, at which pure B6H10
does not undergo significant decomposition. This sim-
plified the analysis of the data because it was not
necessary to correct the reaction rates for the competi-
tive self-decomposition of B6H10. A typical run at 348
K is shown in Fig. 3. Initial rates for all the reactions
are given in Table 4. The cothermolysis showed paral-
lels with the B6H10–B2H6 system in the sense that the
rate of consumption of the co-reactant, B4H10 in this
case, was essentially the same as in the thermolysis of
the pure gaseous compound over the temperature range
studied [11,16] but the rate of decomposition of the
B6H10 was enhanced compared with that in the pure
state [2]. The main products are seen to be H2, B2H6,
pentaboranes (mainly B5H11 in the early stages) and
B9H15; B10H14 builds up slowly and B6H12 appears only
in the later stages of the reaction.
Detailed kinetic analysis of the data indicated that
the rate of consumption of B4H10 was accurately first-
order with respect to B4H10 (i.e. the same as in the
thermolysis of the pure compound) and zero order with
respect to B6H10. The order determined from the pro-
duction of H2 was close to unity, 0.8990.07, with
respect to the initial concentration of B4H10, but its rate
of production was essentially independent of the initial
Fig. 3. Product analysis as a function of time for the cothermolysis at
T=348.5 K of B6H10 (p0 3.51 mmHg) with B4H10 (p0 3.51 mmHg):
(
) B4H10, (ꢂ) B6H10, (ꢁ) H2, (ꢃ) B2H6, (ꢅ) B5H11, (") B9H15,
(ꢀ) B10H14
.
Table 4
a
Initial-rate data for cothermolysis of B4H10 and B6H10 with and without added H2
90.5
T/K
po(B4H10
)
po(B6H10
)
po(H2)
−(d[B4H10]/dt)o
mmol m−3 s−1
−(d[B6H10]/dt)
(d[H2]/dt)o
(d[B2H6]/dt)o
(d[B5H11]/dt)o
mmHg b
c
c
c
c
c
c
c
c
313.2
323.3
338.3
342.9
348.5
347.9
348.5
348.5
347.7
348.2
348.7
348.4
348.3
348.4
348.5
363.2
3.48
3.51
3.52
3.52
1.75
3.50
3.51
3.50
3.50
3.48
3.50
5.18
7.01
10.63
14.04
3.47
3.49
3.51
3.50
3.52
3.52
1.75
3.51
5.25
7.00
6.97
10.55
3.48
3.50
3.50
3.48
3.50
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.0
50.0
0.0
0.0
0.0
0.0
0.0
0.0
6.8
12.7
18.2
61.4
55.9
106.7
93.6
122.0
117.4
51.1
140.5
123.5
173.9
229.4
469.2
286.0
10.9
9.1
10.9
39.2
32.7
5.9
20.8
44.9
120.6
108.7
190.4
173.4
205.9
12.9
14.4
24.0
38.9
8.1
42.0
27.2
10.7
40.5
51.8
122.3
104.6
46.2
127.7
69.7
153.7
43.0
18.7
c
c
c
233.1
273.9
473.1
553.9
639.0
630.6
29.8
31.3
63.0
28.7
21.3
36.4
82.7
90.7
79.9
107.4
168.1
138.0
77.8
95.2
c
c
a See appropriate footnote to Table 1.
b See appropriate footnote to Table 1.
c Rate too low to be measured accurately.

M.D. Attwood et al. / Journal of Organometallic Chemistry 614–615 (2000) 144–152
151
By contrast, in the cothermolysis of B6H10 and B5H11
(Fig. 4) the two boranes were consumed rapidly, and at
essentially identical initial rates. The main products were
B2H6 and H2, together with higher boranes B8H12, B9H15
and B10H14 in smaller but significant amounts. Small
amounts of B4H10 were also observed in the early stages
of the reaction. The rate of consumption of B5H11 is
comparable with that observed in the thermolysis of
B5H11 alone [18] but the rate of consumption of B6H10
is increased by a massive 300/400-fold compared with its
rate of decomposition when heated alone.
3.5. Mechanistic implications
In the cothermolysis reactions of B6H10 separately with
B2H6, B4H10 and B5H11 the rate of consumption of B6H10
was considerably more rapid than in the thermolysis of
B6H10 alone, indicating clearly that interactions were
occurring. In contrast, no reaction was observed between
B6H10 and B5H9 under the conditions studied. The 3/2
order observed in the B6H10–B2H6 reaction suggests that
B6H10 must react with the {B3H7} intermediate formed
from B2H6, and not with either B2H6 itself or with {BH3}.
{B3H7}+B6H10“B9H15+H2
(3)
Fig. 4. Product analysis as a function of time for the cothermolysis at
T=347.3 K of B6H10 (p0 3.50 mmHg) with B5H11 (p0 3.50 mmHg):
Such a step has been proposed in the past without direct
evidence [19] and it has been shown that B6H10 reacts
with [10] B3H7.THF at 273 K in the presence of BF3 to
give a labelled B9H15 [8a]. Step (3) presumably competes
successfully with the reaction of {B3H7} with B2H6 that
is thought to lead to B4H10 in the absence of B6H10 [20].
We considered the possibility that the B2H6 decomposi-
tion might proceed as far as the production of {B4H8}
[reactions (4) and (5)] before the interaction with B6H10
occurs,
(ꢅ) B5H11, (ꢂ) B6H10, (ꢁ) H2, (ꢃ) B2H6, ( ) B4H10, (ꢅ) B8H12
(") B9H15, (ꢀ) B10H14
,
.
concentration of B6H10. The activation energy for the
decomposition of B4H10 was found to be 88.496.1
compared with 99.290.8 kJ mol−1 for B4H10 alone
[11,16].
The effect of added H2 on the reaction can be seen from
the rates recorded in Table 4 for the pair of experiments
in which the initial pressures of B4H10 and B6H10 were
ca. 3.5 and 7.0 mmHg, respectively. The rate of decom-
position of B4H10 was reduced by half in the presence of
H2 at 50 mmHg . The retarding effect of added H2 is
therefore considerably less than that observed in the
thermolysis of B4H10 alone, but greater than that ob-
served in the B6H10–B2H6 cothermolysis [16].
{B3H7}+B2H6“B4H10+{BH3}
B4H10 X {B4H8}+H2
(4)
(5)
but the lack of inhibition in the presence of a large excess
of added H2 demonstrates that B6H10 must react very
rapidly with {B3H7} [(reaction (3)] before the latter can
be re-converted to B2H6 by reaction (2).
In the B6H10–B2H6 cothermolysis the temperature is
too high for B9H15 to survive, and it must react further
to generate the substantial amounts of B10H14 that are
observed. This reaction is known to occur, but isotope
studies have shown that it probably takes place in a
two-stage process involving B8H12 and {B9H13} as inter-
mediates [21]. In the B6H10–B4H10 and B6H10–B5H11
reactions, the temperatures are low enough to allow
B9H15 to survive, and it is actually observed as an
important product in the early stages of these reactions.
A similar result has been observed in the reaction at even
lower temperatures between the unstable adduct
B4H8CO and B6H10 [22] and it seems likely that the
reaction involving {B4H8} is responsible in each case.
3.4. Cothermolysis of B6H10 with B5H9 and with B5H11
When an equimolar mixture of B6H10 and B5H9 was
heated at 408 K, the partial pressure of the latter
increased very slightly from ca. 3.5 to 3.7 mmHg over a
period of 60 min, whereas the initial rate of decomposi-
tion of the B6H10 was found to be the same as that
observed [2] in the thermolysis of this borane alone. As
B5H9 is known to be relatively stable in the gas phase at
this temperature [17] the slight increase in its concentra-
tion can be attributed to the decomposition of B6H10, and
it is apparent that little or no interaction occurs between
these two boranes under the conditions employed.

152
M.D. Attwood et al. / Journal of Organometallic Chemistry 614–615 (2000) 144–152
{B4H8}+B6H10“B9H15+{BH3}
(6)
and Physical Sciences Research Council, the Royal
Society and the US Army Research and Standardiza-
tion Group (Europe).
In both the B6H10–B2H6 and B6H10–B4H10 cothermoly-
ses the production of B9H15 [reactions (3) and (6),
respectively] is presumed to be in competition with the
reaction leading to the formation of B5H11 [reaction
(7)]. Accordingly, as observed,
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(7)
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compared with its rate of formation in the thermolysis
reactions of B2H6 and B4H10 alone.
The B6H10–B5H11 system has not been studied in
depth, but the results indicate that the kinetics of this
reaction are also governed by the proposed [18] rate-de-
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the elimination of {BH3} to give {B4H8}. The observa-
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rate suggests that the {B4H8} is mopped up in a very
efficient reaction with B6H10. The substantial produc-
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concentrations of {BH3} expected to be produced from
the rate-determining dissociation of B5H11 and from the
dissociation of the B9H15.
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both the B2H6 and B4H10 decompositions are retarded
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Acknowledgements
We thank Mr D. Singh for invaluable experimental
assistance. The work was supported by the Engineering
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.