Some chemical properties of monochlorogallane: Decomposition to gallium(I) trichlorogallate(III), Ga+[GaCl3H]-, and other reactions

Article abstract of DOI:10.1021/ic010128x

Thermal decomposition of monochlorogallane, [H₂GaCl]_n, at ambient temperatures releases H₂ and results in the formation of gallium(I) species, including the new compound Ga[GaHCl₃], which has been characterized crystallographically at 100 K (monoclinic P2₁/n, a = 5.730(1), b = 6.787(1), c = 14.508(1) A, β = 97.902(5)°) and by its Raman spectrum. The gallane suffers symmetrical cleavage of the Ga(μ-Cl)₂Ga bridge in its reaction with NMe₃ but unsymmetrical cleavage, giving [H₂Ga(NH₃)₂]⁺Cl^-, in its reaction with NH₃. Ethene inserts into the Ga-H bonds to form first [Et(H)GaCl]₂ and then [Et₂GaCl]₂.

Full text of DOI:10.1021/ic010128x

Inorg. Chem. 2001, 40, 4755-4761
4755
Some Chemical Properties of Monochlorogallane: Decomposition to Gallium(I)
Trichlorogallate(III), Ga+[GaCl3H]-, and Other Reactions
Emma Johnsen, Anthony J. Downs,* Michael J. Goode, Timothy M. Greene,
Hans-Jo1rg Himmel, and Matthias Mu1ller
Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QR, U.K.
Simon Parsons and Colin R. Pulham
Department of Chemistry, The University of Edinburgh, King's Buildings, West Mains Road,
Edinburgh, EH9 3JJ, U.K.
ReceiVed January 30, 2001
Thermal decomposition of monochlorogallane, [H2GaCl]n, at ambient temperatures releases H2 and results in the
formation of gallium(I) species, including the new compound Ga[GaHCl3], which has been characterized
crystallographically at 100 K (monoclinic P21/n, a ) 5.730(1), b ) 6.787(1), c ) 14.508(1) Å, â ) 97.902(5)°)
and by its Raman spectrum. The gallane suffers symmetrical cleavage of the Ga(µ-Cl)2Ga bridge in its reaction
with NMe3 but unsymmetrical cleavage, giving [H2Ga(NH3)2]+Cl-, in its reaction with NH3. Ethene inserts into
the Ga-H bonds to form first [Et(H)GaCl]2 and then [Et2GaCl]2.
Introduction
Scheme 1. Formation, Decomposition, and Some Reactions
of Monochlorogallane5-8,21
Unlike many other base-free monosubstituted derivatives of
gallane, [H2GaX]n, where X is not very bulky or incorporates
appropriate donor centers,1 monochlorogallane is comparatively
long-lived and tractable at ambient temperatures.2-4 In a
previous article2 we have described the synthesis and some of
the physical properties of the compound, which is a labile liquid
at room temperature and vaporizes to give dimeric molecules,
H2Ga(µ-Cl)2GaH2. Here we report some of its chemical proper-
ties including (i) its thermal decomposition to give mixed
valence gallium compounds, (ii) selected metathesis reactions
which make the compound important as a source of other GaH2
derivatives, (iii) addition reactions with the nitrogen bases NH3
and NMe3, and (iv) the insertion of ethene into the Ga-H bonds.
Results and Discussion
The studies we have undertaken have revealed several aspects
of the chemistry of monochlorogallane, [H2GaCl]n, as sum-
marized in Scheme 1. There are four features meriting discus-
sion: (i) the decomposition of the compound with the formation
of elemental gallium, hydrogen, and mixed valence gallium
derivatives; (ii) metathesis reactions with appropriate nucleo-
philes X- at low temperatures to afford other gallanes of the
- 5,6
- 5,7
- 5,8
type [H2GaX]n, e.g., X- ) GaH4
,
BH4
,
and B3H8 ;
* To whom correspondence should be addressed.
(1) See, for example, Campbell, J. P.; Hwang, J.-W.; Young, V. G., Jr.;
Von Dreele, R. B.; Cramer, C. J.; Gladfelter, W. L. J. Am. Chem.
Soc. 1998, 120, 521. Baxter, P. L.; Downs, A. J.; Rankin, D. W. H.;
Robertson, H. E. J. Chem. Soc., Dalton Trans. 1985, 807. Elms, F.
M.; Koutsantonis, G. A.; Raston, C. L. J. Chem. Soc., Chem. Commun.
1995, 1669. Veith, M.; Faber, S.; Wolfanger, H.; Huch, V. Chem.
Ber. 1996, 129, 381. Cowley, A. H.; Gabba¨ı, F. P.; Isom, H. S.;
Decken, A. J. Organomet. Chem. 1995, 500, 81.
(iii) cleavage of the Ga(µ-Cl)2Ga unit by a nitrogen base either
(5) Downs, A. J.; Pulham, C. R. AdV. Inorg. Chem. 1994, 41, 171; Chem.
Soc. Rev. 1994, 23, 175.
(6) Pulham, C. R.; Downs, A. J.; Goode, M. J.; Rankin, D. W. H.;
Robertson, H. E. J. Am. Chem. Soc. 1991, 113, 5149.
(7) Pulham, C. R.; Brain, P. T.; Downs, A. J.; Rankin, D. W. H.;
Robertson, H. E. J. Chem. Soc., Chem. Commun. 1990, 177. Downs,
A. J.; Parsons, S.; Pulham, C. R.; Souter, P. F. Angew Chem., Int. Ed.
Engl. 1997, 36, 890.
(2) Johnsen, E.; Downs, A. J.; Greene, T. M.; Souter, P. F.; Aarset, K.;
Page, E. M.; Rice, D. A.; Richardson, A. N.; Brain, P. T.; Rankin, D.
W. H.; Pulham, C. R. Inorg. Chem. 2000, 39, 719.
(3) Goode, M. J.; Downs, A. J.; Pulham, C. R.; Rankin, D. W. H.;
Robertson, H. E. J. Chem. Soc., Chem. Commun. 1988, 768.
(4) Pulham, C. R.; Downs, A. J.; Goode, M. J.; Greene, T. M.; Mills, I.
M. Spectrochim. Acta, Part A 1995, 51, 769.
(8) Pulham, C. R.; Downs, A. J.; Rankin, D. W. H.; Robertson, H. E. J.
Chem. Soc., Dalton Trans. 1992, 1509. Morrison, C. A.; Smart, B.
A.; Brain, P. T.; Pulham, C. R.; Rankin, D. W. H.; Downs, A. J. J.
Chem. Soc., Dalton Trans. 1998, 2147.
10.1021/ic010128x CCC: $20.00 © 2001 American Chemical Society
Published on Web 07/28/2001

4756 Inorganic Chemistry, Vol. 40, No. 18, 2001
Johnsen et al.
“symmetrically” to give a molecular adduct, e.g., Me3N‚GaH2-
Cl, or “unsymmetrically” to give a saltlike product, e.g.,
[H2Ga(NH3)2]+Cl-; and (iv) insertion of ethene into the Ga-H
bonds in a stepwise manner to form first [EtGa(H)Cl]2 and then
[Et2GaCl]2 in reactions emulating that of ethene with dichlo-
rogallane.9
(i) Thermal Decomposition; Characterization of the In-
termediate Ga+[GaCl3H]-. As noted previously,2 vaporization
of monochlorogallane in vacuo invariably leads to slight
decomposition with the deposition of less volatile white solids.
During the synthesis of the gallane, moreover, it was noted that
the residue was inhomogeneous, typically comprising a clear
crystalline material and a white powder. Exactly how the
decomposition proceeds is hard to establish since the intermedi-
ate products varied significantly with temperature, phase, and
other conditions (e.g., according to whether the sample is in a
closed system or subject to continuous evacuation with removal
of any volatile materials as they are formed).
Figure 1. Crystal structure of Ga[HGaCl3] at 100 K showing the
presence of Ga+ and [HGaCl3]- units with 50% thermal ellipsoids.
Table 1. Single Crystal Data for Ga[GaCl3H]
Raising the temperature of a sample of the liquid in a sealed,
evacuated glass ampule to ca. 373 K caused complete decom-
position to occur in a matter of hours, with the evolution of
dihydrogen and the formation of an involatile, inhomogeneous
solid having an overall composition GaCl. The solid consisted
of a mixture of finely dispersed elemental gallium (giving a
gray coloration, sometimes with metallic flecks) and a white
powder identified by its Raman spectrum as Ga+[GaCl4]-.10
Quantitative chemical assay implied that decomposition had
occurred in accordance with eq 1, and thereby offered a means
of analysis of the monochlorogallane.
empirical formula
fw
crystal color; dimens, mm
cryst syst
space group
a, Å
b, Å
c, Å
HGa2Cl3
246.81
colorless, 0.30 × 0.30 × 0.35
monoclinic
P21/n
5.730(1)
6.787(1)
14.508(1)
97.902(5)
558.9(3)
4
â, deg
V, Å3
Z
DX, g cm-3
T, K
2.93
100
λ(Mo KR), Å
0.71069
109.01
27
µ, cm-1
θmax, deg
no. of reflns measd
5667
no. of unique reflns
1096
888
51
0.54
-0.95
0.029
0.035
1.067
no. of observations with I > 3σ(I)
Numerous experiments in which liquid or gaseous samples
of monochlorogallane were heated or left to stand at room
temperature invariably witnessed the release of dihydrogen, the
deposition of gallium, and the formation of gallium compounds
of varying volatility and stability. Only one such intermediate
was amenable to isolation and characterization, namely the
crystalline solid first noted during the synthesis of monochlo-
rogallane. The maximum yield of this solid was achieved by
leaving a sample of monochlorogallane in vacuo at room
temperature (283-288 K) for 14 days. A single crystal selected
from the reaction mixture was shown to be the new mixed-
valence compound gallium(I) trichlorogallate(III), Ga+[GaCl3H]-,
by X-ray diffraction measurements (see Tables 1 and 2 and
Figure 1).
As in the familiar parent compound Ga+[GaCl4]-,11 there are
two distinct gallium centers, with the four-coordinated Ga(III)
separated from Ga(I) that is at least eight-coordinated by an
average distance of 4.13 Å. The novel [GaCl3H]- anions closely
approximate C3ν symmetry, with a mean Ga-Cl bond length
of 2.210 Å (cf. 2.172 Å for [GaCl4]- in Ga2Cl411) and Cl-
Ga-Cl angles of 104.0°. The coordination environment of the
Ga(I) center is not only more diffuse, but is also distinctly less
symmetrical, with eight Ga‚‚‚Cl contacts at separations ranging
no. of variables
max peak in final diff map, e Å-3
min peak in final diff map, e Å-3
R
Rw
S
Table 2. Selected Distances (Å) and Angles (deg) in
Ga+[GaCl3H]-
Ga(1)-Cl(1)
2.1985(9)
Ga(1)-Cl(2)
2.212(1)
2.2199(8)
1.32(6)
Ga(1)-Cl(3)
Ga(1)-H(1)
Cl(1)-Ga(1)-Cl(2)
Cl(1)-Ga(1)-Cl(3)
Cl(2)-Ga(1)-Cl(3)
Cl(1)-Ga(1)-H(1)
Cl(2)-Ga(1)-H(1)
Cl(3)-Ga(1)-H(1)
104.24(4)
103.39(4)
104.35(4)
117.3(24)
113.4(25)
112.8(23)
from 3.069 to 3.552 Å, a ninth at 3.814 Å, and a Ga‚‚‚H contact
measuring 2.79 Å.
It is of interest to see how the structures of Ga[GaCl4] and
Ga[GaCl3H] fit into the context of families having the general
formula A[BX4], where BX4 is tetrahedral and A is eight-
coordinated. A starting point is provided by zircon, Zr[SiO4]
(Figure 2a),12 in which the Zr atoms are eight-coordinated by
oxygen atoms, defining a triangulated dodecahedron; this can
be considered to be two intersecting tetrahedra which have been
(9) Schmidbaur, H.; Findeiss, W.; Gast, E. Angew. Chem., Int. Ed. Engl.
1965, 4, 152. Schmidbaur, H.; Klein, H.-F. Chem. Ber. 1967, 100,
1129.
(10) Chemouni, E. J. Inorg. Nucl. Chem. 1971, 33, 2325.
(11) Schmidbaur, H.; Nowak, R.; Bublak, W.; Burkert, P.; Huber, B.;
Mu¨ller, G. Z. Naturforsch. B: Chem. Sci. 1987, 42, 553. Wilkinson,
A. P.; Cheetham, A. K.; Cox, D. E. Acta Crystallogr. 1991, B47, 155.
(12) See, for example, Wells, A. F. Structural Inorganic Chemistry, 5th
ed.; Clarendon Press: Oxford, U.K., 1984.

Chemical Properties of Monochlorogallane
Inorganic Chemistry, Vol. 40, No. 18, 2001 4757
Figure 3. Coordination geometry of GaI in Ga[HGaCl3].
Figure 2. Coordination geometries of the metal atom A in solids with
the general formula A[BX4] where BX4 is tetrahedral: (a) Zr in zircon,
Zr[SiO4]; (b) Ca in scheelite, Ca[WO4]; (c) GaI in Ga[GaCl4]; (d) GaI
in Ga[AlCl4]; and (e) Ba in barytes, Ba[SO4]. (f) For comparison the
coordination geometry of GaI in Ga[HGaCl3]. The coordination
geometries in (a)-(d) can each be considered, in varying degrees, in
terms of two intersecting tetrahedra which have been squashed (open
circles) or elongated (filled circles) about one diad; the coordination
geometries in (e) and (f) can each be related to a trigonal prism capped
(filled circles) on two square faces, as well as having longer contacts
with additional atoms (open circles).
Figure 4. Raman spectrum of a crystal of Ga[HGaCl3] at room
temperature.
squashed (open circles) or elongated (filled circles) about one
diad (spheroids). Replacement of the spheroids with tetrahedra
would yield a cubic array about the central metal atom. A step
in this direction is observed in scheelite, Ca[WO4] (Figure 2b),
the coordination polyhedron now being described as a bisdis-
phenoid. Scheelite is really quite similar to Ga[GaCl4] (Figure
2c);11 in both structures the Ca‚‚‚O or Ga‚‚‚Cl distances fall
into two sets of four. While the Ga‚‚‚Cl contacts in Ga[GaCl4]
thus occupy a narrow range (3.17-3.19 Å), those in the related
salt Ga[AlCl4] are much more irregular.13 Here we find six
contacts in the range 3.10-3.25 Å forming a trigonal prism
that is capped on two “square” faces by Cl atoms. making Ga‚
‚‚Cl contacts of 3.52 Å (Figure 2d). As noted previously,13 the
structure of Ga[AlCl4] is related to that of barytes, Ba[SO4],12
where the large Ba2+ is 12-coordinated; eight Ba‚‚‚O contacts
of 2.76-2.86 Å form a bicapped trigonal prism about the Ba,
and then four longer contacts (3.07-3.32 Å) are made through
the edges between the triangular and capped square faces of
the prism (Figure 2e). In Ga[AlCl4], the Ga+ is not large enough
to be 12-coordinated and so the four long contacts are lost,
leaving the bicapped prismatic array. The coordination sphere
of the Ga+ center in crystalline Ga[GaCl3H], like that in Ga-
[AlCl4], accommodates a rather broad spread of Ga‚‚‚Cl contacts
within the sum of the van der Waals radii (3.07-3.55 Å) and
one Ga‚‚‚H contact of 2.79 Å. The coordination sphere can again
be derived from the barytes structure with a bicapped trigonal
prismatic array similar to that observed in Ga[AlCl4] but, instead
of being formed by eight Cl atoms, it comprises seven Cl and
one H atom (Figure 3). It appears that the small size of the H
enables the Ga to take up a higher coordination number than in
the aluminum compound, and a ninth (longest) contact is formed
through one of the edges between a triangular face and one of
the capped square faces, i.e., in the same direction as one of
the long contacts in the Ba[SO4] structure.
To what extent the unsymmetrical hemidirected14 coordination
geometry of the Ga(I) center reflects the role of the 4s2 lone
pair of electrons, as opposed to the purely steric demands of
the Ga+ and GaCl3H- units, is an open question. There are at
present insufficient crystallographic data on gallium(I) com-
pounds15 to warrant the kind of detailed analysis that has recently
been undertaken by Glusker et al.14 for lead(II) compounds.
The air-sensitive intermediate Ga+[GaCl3H]- melts at ca. 303
K and decomposes on further heating with cleavage of the
Ga-H bond to form dihydrogen, gallium, and Ga+[GaCl4]-.
The Raman spectrum of the solid displayed the emissions
illustrated in Figure 4 and with the wavenumbers listed in Table
3. It can be satisfactorily interpreted on the basis of the
vibrational transitions expected for an [HGaCl3]- anion having
C3V symmetry with the aid of a quantum chemical model; density
functional theory (DFT) calculations employing a 6-311G* basis
set were found to give the best account of the observed
frequency and intensity pattern (see Table 3). The proposed
assignments are also supported by comparisons with the
vibrational properties reported not only for the [GaCl4]- anion
but also for the isoelectronic molecule HGeCl3.16
The most prominent feature of the spectrum, occurring at
1960.4 cm-1, is plainly associated with the ν(Ga-H) funda-
(14) Shimoni-Livny, L.; Glusker, J. P.; Bock, C. W. Inorg. Chem. 1998,
37, 1853.
(15) Taylor, M. J.; Brothers, P. J. In Chemistry of Aluminium, Gallium,
Indium and Thallium; Downs, A. J., Ed.; Blackie: Glasgow, U.K.,
1993; pp 111-247.
(16) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coor-
dination Compounds, Part A, 5th ed.; Wiley: New York, 1997.
(13) Staffel, T.; Meyer, G. Z. Anorg. Allg. Chem. 1987, 552, 108.

4758 Inorganic Chemistry, Vol. 40, No. 18, 2001
Johnsen et al.
a sample gave a 1H spectrum similar to that of the decomposed
sample of monochlorogallane. At ambient temperatures it
showed but one broad resonance at δH 5.98 (W1/2 ) 190 Hz).
Decreasing the temperature to 243 K brought about resolution
into two main resonances, one at δH 6.45 with a shoulder at ca.
δH 7.2 and the other at δH 5.63; further cooling caused the first
resonance to develop a broad triplet pattern, with components
at δH 7.29, 6.67, and 6.23, and the second to split into two
sharper lines, at δH 5.77 and 5.14. Hence, it was evident that
all the solutions of decomposed samples of monochlorogallane
contained a mixture of products including some, like the parent
compound itself,2 in different degrees of aggregation. The most
plausible interpretation of the results is that resonances at δH <
6 arise from uncharged species of the type [HmGaCl3-m]n (m )
Table 3. Observed and Calculated Raman Frequencies and
Intensities for the Vibrational Fundamentals of Solid Ga+[HGaCl3]-
and Comparison with HGeCl3
Ga[HGaCl3]
obs
calcb
HGeCl3
description
of modec
mode ν/cm-1 intensa ν/cm-1 intensa ν/cm-1
ν1 (a1) 1960.4
ν2 (a1) 349.5
ν3 (a1) 167.9
ν4 (e) 609.3
ν5 (e) 320.6
100.0 1996.4
100.0 2155.7 ν(M-H)
85.8
0.6
20.0
11.6
344.9
163.0
653.6
341.8
1.7
1.1
14.0
2.8
418.4
181.8
708.6
454
sym ν(M-Cl)
sym δ(MCl3)
δ(M-H)
antisym
ν(M-Cl)
F(MCl3)
ν6 (e)
d
d
124.4
this work
2.8
145.0
ref 16
this work
a Relative intensities with the strongest band set to 100 arbitrary units.
b Calculated by DFT methods (B3LYP) with a 6-311G* basis set. c M
) Ga for [HGaCl3]- or M ) Ge for HGeCl3. d Not observed; outside
range of detection.
1 or 2), whereas resonances at δH > 6 arise from anions of the
-
type [HmGaCl4-m
]
(m ) 1-4) which are subject to rapid
scrambling.
71Ga NMR spectra of these solutions typically revealed a
single signal at δGa -677 (W1/2 ) 120 Hz) at ambient
temperatures. By virtue of its position,20 this may be identified
with the presence of a Ga(I) species. Decreasing the temperature
caused the resonance to shift to lower frequency and to broaden
(δGa -742 and W1/2 ) 560 Hz at 230 K) and an additional,
substantially weaker feature to appear at a slightly higher
frequency (δGa -713 at 203 K). A possible explanation is that
the more intense signal originates in the ion pair Ga+[HGaCl3]-,
the weaker one in Ga+[GaCl4]-. There was no sign of any
feature attributable to a Ga(III) component, which would be
expected to resonate near δGa +200.20 This is due in all
probability to the pronounced broadening effect of the electric
field gradient created by the unsymmetrical environment of the
metal atom in a species such as [HGaCl3]-; as no 71Ga signal
has been detected for monochlorogallane itself, the result comes
as no particular surprise.
mental, ν1 (a1). The frequency is, as expected, somewhat lower
than that of the equivalent modes in the neutral molecules
HGeCl3 (2155.7 cm-1 16 and HGaCl2 (2015 cm-1),17 in keeping
)
with the increased polarity of the metal-hydrogen bond induced
by the negative charge. On the other hand, replacement of three
of the hydrogens of [GaH4]- (with a weighted average ν(Ga-
H) frequency of 1807 cm-1 18
) by more electronegative chlorine
substituents clearly results in a strengthening of the surviving
Ga-H bond. The symmetric and antisymmetric ν(Ga-Cl)
fundamentals, ν2 (a1) and ν5 (e), are identified with emissions
at 349.5 and 320.6 cm-1, respectively. It is clear from Table 3
that the small differences in the calculated values, both for the
positions and intensities of these two features, are of little
assistance in deciding their relative ordering. We have been
guided, instead, by the observed intensities of the corresponding
bands in the Raman spectrum of HGeCl3, where the symmetric
M-Cl stretching mode is observed to have approximately 10
times the intensity of the antisymmetric one. There are no such
problems in identifying the feature associated with the δ(GaCl3)
fundamental, ν3(a1), since here theory anticipates well both the
frequency and Raman intensity of this mode, which is assigned
to the very weak scattering at 167.9 cm-1. The feature at 609.3
cm-1 may be identified with the Ga-H wagging mode, ν4 (e).
This leaves only the F(GaCl3) vibration, ν6 (e), which is expected
to appear near 120 cm-1, and so it would fall outside the range
of detection in the present experiments.
The results of these and other experiments2 imply that
disproportionation precedes the elimination of H2, and the
identification of Ga+[HGaCl3]- as a long-lived intermediate
suggests that monochlorogallane decomposes in the liquid phase
via stages such as those represented in eq 2. Disproportionation
need not be confined to the intermediates given here, but is
Attempts have also been made to investigate the decomposi-
tion of monochlorogallane by monitoring the 1H and 71Ga NMR
spectra of toluene-d8 solutions over a range of temperatures.
After such a solution had been left to stand for a period of days
likely to give rise also to H(Cl)Ga(µ-Cl)2Ga(Cl)H9 and highly
labile Ga2H6;6 although this makes no difference to the ultimate
decomposition products, the vapor may well be host to these
intermediates, as suggested in our analysis of its electron-
diffraction pattern.2
(ii) Metathesis Reactions: Preparation of other Gallanes
of the Type [H2GaX]n. Much of the significance of mono-
chlorogallane derives from its potential as a source of other
gallanes of the type [H2GaX]n, where X ) H, BH4, or B3H8,
through metathesis reactions involving the relevant anion X-
[eq 3]. The reactions, which have been described elsewhere,5-8,21
1
at room temperature, the broad H resonance associated with
gallium-bound hydrogen atoms2 was observed to have shifted
from δH 5.46 to ca. 6.0; decreasing the temperature to 193 K
resolved the signal into two components, the first centered near
δH 6.4 (W1/2 ) 48 Hz) and the second near δH 5.5 (W1/2 ) 13
Hz), the latter being attributable to unchanged monochlorogal-
lane and/or other species of the type [HmGaCl3-m]n (m ) 1 or
2).19 Continued decomposition with deposition of gallium metal
was seen to cause the first resonance to grow at the expense of
the second. It proved impossible to prepare a sample of Ga-
[HGaCl3] that was entirely free from monochlorogallane and
other decomposition products, but a toluene-d8 solution of such
1
n
1
n
[H2GaCl]n + M+X- f [H2GaX]n + M+Cl-
(3)
(17) Ko¨ppe, R.; Tacke, M.; Schno¨ckel, H. Z. Anorg. Allg. Chem. 1991,
605, 35.
(18) Shirk, A. E.; Shriver, D. F. J. Am. Chem. Soc. 1973, 95, 5904.
(19) Goode, M. J. D.Phil. Thesis, University of Oxford, U.K., 1987.
are normally carried out at low temperatures (e.g., 243-253
K) and in the absence of a solvent, despite any reduction in the

Chemical Properties of Monochlorogallane
Inorganic Chemistry, Vol. 40, No. 18, 2001 4759
efficiency of the exchange, mainly in order to facilitate the
isolation of pure samples of the reactive, thermally frail products
[H2GaX]n.
reaction does occur, however, when a benzene solution of the
gallane is maintained at room temperature under ethene at an
overpressure of ca. 2 atm. After 24 h, 1 mol of [H2GaCl]2 was
found to have consumed 2.4 mol of ethene. Evaporation of the
material volatile at 250 K left a colorless, viscous liquid which
was a mixture of two components. Warming to 303 K in an
all-glass apparatus resulted in the vaporization of a more volatile
fraction identified by its IR and H NMR spectra as [Et(H)-
GaCl]2. Thus, the IR spectrum of the solid condensate formed
at 77 K showed, in addition to the absorptions characteristic of
(iii) Reactions with Nitrogen Bases. Monochlorogallane
reacts with an excess of trimethylamine at 178 K. With the
mixture warmed to 250 K and the excess of the base recovered,
there is but a single product, a white solid identified by mass
balance and by its IR and Raman spectra as the known molecular
adduct Me3N‚GaH2Cl.22 When the vapor over the adduct is
cocondensed with additional NMe3 at 77 K, the IR spectrum of
the deposit shows, besides the bands characteristic of Me3N‚GaH2-
Cl, extra bands attributable to the 2:1 adduct (Me3N)2GaH2Cl.
Evidence of this second adduct is provided, for example, by
the appearance in the IR spectrum of a broad ν(Ga-H)
absorption at 1831 cm-1 accompanying the absorption at 1909
cm-1 due to Me3N‚GaH2Cl; the shift of ν(Ga-H) to lower
energy with the coordination of a second molecule of NMe3 is
consistent with the formation of a molecule in which NMe3
ligands are sited above and below a planar GaH2Cl substrate,
1, in line with the behaviors of the adducts Me3N‚MH3 and
(Me3N)2MH3 (M ) Al23 or Ga6). Annealing the deposit to 250
1
Et-Ga moieties,25 prominent bands at 1961 and 708 cm-1
,
implying the presence of terminal Ga-H bonds.5,6,22 The H
NMR spectrum of a toluene-d8 solution at room temperature
pointed to the presence of both trans- (2a) and cis-isomers (2b)
of the [Et(H)GaCl]2 molecule. Raising the temperature of the
residue to 333 K led to the vaporization of a second fraction,
also a liquid at room temperature but with a Raman spectrum
which identified it as [Et2GaCl]2.25
1
Experimental Section
(a) Synthesis and Manipulation of Monochlorogallane; Chemical
Studies and Reagents. The vacuum-line methods used for the
preparation and manipulation of monochlorogallane and related com-
pounds have been described elsewhere.2,5,6 The thermal instability and
reactivity of monochlorogallane made it difficult to determine the mass
of a sample prior to any chemical reaction. Hence, it was necessary
typically to treat the sample with a measured quantity of the reagent,
judged to be in excess, under the appropriate conditions, then to separate
and identify the components of the reaction mixture (usually on the
basis of their vibrational and/or NMR spectra), and, where appropriate,
to assay one or more of these components (e.g., by manometric
measurements or elemental analysis). The reaction itself was carried
out in a sealed, preconditioned all-glass ampule equipped with a break-
seal.
The following reagents, from the commercial sources indicated, were
purified before use by fractionation in vacuo: NH3 (B.O.C.) and C2H4
(B.O.C.). Toluene-d8 and benzene, both supplied by Aldrich, were dried
and fractionated in vacuo prior to use as solvents. Trimethylamine was
prepared by the action of alkali on [NMe3H]+Cl- (Aldrich); fractional
condensation in vacuo gave a sample judged to be pure by the criteria
of tensimetric and IR measurements.
K under pumping results in the decay and ultimate disappearance
of the bands associated with (Me3N)2GaH2Cl, which evidently
develops an appreciable dissociation pressure under these
conditions.
With ammonia at 195 K, monochlorogallane forms a white
solid with the composition H2GaCl‚2NH3, which is long-lived
at room temperature. The IR spectrum of the product is
consistent with the presence of the cation [H2Ga(NH3)2]+,
showing a distinct resemblance in the pattern and frequencies
of its bands to the spectra reported previously for the compound
[H2Ga(NH3)2]+[GaH4]-‚2NH3 and also for the isoelectronic
6
species H2Ge(CH3)2.24 Hence, it emerges that the Ga(µ-Cl)2Ga
skeleton of the gallane is cleaved unsymmetrically in accordance
with eq 4:
(b) Spectroscopic Measurements and Analysis. IR Spectra were
recorded using one of three spectrometers, a Perkin-Elmer Model 580
dispersive (4000-200 cm-1), a Mattson “Polaris” FT-IR (4000-400
cm-1), or a Mattson “Galaxy” FT-IR instrument (4000-400 cm-1).
Raman spectra were excited either at λ ) 514.5 nm with a Spectra-
Physics Model 165 Ar+ laser or at 632.8 nm with a He-Ne laser and
measured with a Spex Ramalog 5 spectrophotometer operated with a
computerized data-handling center or with a Dilor “Labram” system,
respectively; the resolution was normally 2 cm-1. Solid deposits of
volatile materials were presented for spectroscopic analysis by allowing
the vapor to condense on a CsI window (for IR measurements) or a
copper block (for Raman measurements) contained in an evacuated
glass shroud and maintained at 77 K.
1
n
[H2GaCl]n + 2NH3 f [H2Ga(NH3)2]+ Cl-
(4)
(iv) Insertion of Ethene into the Ga-H Bonds. Ethene does
not react at a measurable rate with neat monochlorogallane at
178 K or with the gallane in isopentane solution at 250 K. A
(20) Hinton, J. F.; Briggs, R. W. In NMR and the Periodic Table; Harris,
R. K., Mann, B. E., Eds.; Academic Press: London, 1978; p 279.
Akitt, J. W. In Multinuclear NMR; Mason, J., Ed.; Plenum Press: New
York and London, 1987; p 259.
(21) McMurran, J.; Kouvetakis, J.; Nesting, D. C.; Smith, D. J.; Hubbard,
J. L. J. Am. Chem. Soc. 1998, 120, 5233.
(22) Greenwood, N. N. In New Pathways in Inorganic Chemistry; Ebsworth,
E. A. V., Maddock, A. G., Sharpe, A. G., Eds.; Cambridge University
Press: Cambridge, U.K., 1968; pp 37-64. Greenwood, N. N.; Storr,
A. J. Chem. Soc. 1965, 3426.
(23) Mu¨ller, J. J. Am. Chem. Soc. 1996, 118, 6370, and references therein.
Ehrlich, R.; Young, A. R., II.; Lichstein, B. M.; Perry, D. D. Inorg.
Chem. 1963, 2, 650.
1H NMR measurements on toluene-d8 solutions at temperatures
ranging between 193 and 293 K were made at 250, 300, or 500 MHz
using a Bruker Model AM250, a Bruker AM300, or a Varian UNITY-
plus 500 instrument, respectively. 71Ga NMR measurements over the
(25) Kurbakova, A. P.; Leites, L. A.; Aleksanyan, V. T.; Golubinskaya, L.
M.; Zorina, E. N.; Bregadze, V. I. Zh. Strukt. Khim. 1974, 15, 1083.
Weidlein, J. J. Organomet. Chem. 1969, 17, 213.
(24) van de Vondel, D. F.; Van der Kelen, G. P. Bull. Soc. Chim. Belg.
1965, 74, 467.

4760 Inorganic Chemistry, Vol. 40, No. 18, 2001
Johnsen et al.
Table 4. Vibrational Frequencies (cm-1) for the Products Formed from the Reaction between Monochlorogallane and Trimethylaminea
Raman shift of
IR of solid film
at 77 K
crystalline solid at
room temp.
IR in benzene
solutionb
species
assignment
2980 m
2935 m
2913 m
2894 m
2986 w
2944 w
2920 w
2970 sh m
2936 m
2914 m
2883 m
2864 m
2840 w
2818 w
2804 w
2770 m
2729 w
Me3N‚GaH2Cl
ν(C-H)
}
}
2866 w
overtones and
combinations of
δ(CH3) modes
Me3N‚GaH2Cl
Me N‚GaH Cl
2803 w
1909 vs
1911 vs
1472 m
1905 vs
1899 sh vs
ν(Ga-H)
ν(Ga-H)
3
2
1831 s
1478 s
1467 s
1448 sh
1407 m
(Me3N)2‚GaH2Cl
Me3N‚GaH2Cl
1447 m
δ
δ
asym(CH3)
sym(CH3)
}
}
1450 m
1411 w
1438 sh w
1409 m
1395 sh w
1250 m
1235 w
1105 m
Me N‚GaH Cl
3
2
1252 m
1230 sh
1105 w
1205 w
1020 m
1000 s
828 m
1241 w
F(CH3)
F(CH3)
Me3N‚GaH2Cl
}
1110 vw
(Me N) ‚GaH Cl
}
3
2
2
998 s
826 s
996 m
824 m
819 sh m
730 vs
710 sh s
603 m
Me3N‚GaH2Cl
Me N‚GaH Cl
ν
ν
asym(C-N)
sym(C-N)
}
}
3
2
731 vs
677 vs
610 vw
515 sh
492 m
335 s
734 m
682 m
607 w
520 m
494 m
340 s
Me N‚GaH Cl
δ(GaH2)
δ(GaH2)
ν(Ga-N)
ν(Ga-Cl)
3
2
Me3N‚GaH2Cl
Me N‚GaH Cl
}
3
2
491 s
345 m
Me3N‚GaH2Cl
a vs very strong; s strong; m medium; w weak; vw very weak; sh shoulder; br broad. b Values taken from ref 22.
same temperature range were made at 152.335 MHz using the Varian
system; chemical shifts were referenced to [Ga(OH2)6]3+ 20
Elemental analyses were performed by the Analytische Laboratorien,
Engelskirchen, Germany, or by the analytical services of the Inorganic
Chemistry Laboratory at Oxford.
1.067. Despite being surrounded by heavy atoms, the unique hydrogen
atom could be reliably located through the combination of good crystal
quality, low crystal temperature, and the use of image plate technology
for data collection.30 All calculations made use of the CRYSTALS
program package,31 neutral atom scattering factors being taken from
ref 32. The CAMERON program33 was used to produce Figure 1.
(d) Theoretical Calculations. Density functional theory (DFT)
calculations were performed by using the GAUSSIAN98 program
package34 and applying the B3LYP method, which has been shown to
give satisfactory results for small group 13 metal compounds.35
(e) Chemical Properties of Monochlorogallane. (i) Thermal
Decomposition. When heated to 373 K for 4 days to ensure complete
.
(c) X-ray Crystallography. A crystal of Ga+[GaCl3H]- was
immersed in highly viscous perfluoropolyether under argon, very
quickly mounted on a glass fiber, and placed in the cold stream of an
Oxford Cryosystems low-temperature device26 attached to an Enraf-
Nonius DIP2020 image-plate diffractometer. Measurements were thus
made with the crystal at 100 K using graphite-monochromated Mo KR
radiation. Other details are given in Table 1.
(i) Data Collection and Processing. Of the 5667 reflections
measured (1 < θ < 27°; -6 e h e 6, -8 e k e 8, -18 e l e 18)
1096 were unique, giving 888 reflections with I > 3σ(I). The images
were processed with the DENZO and SCALEPACK programs.27
Corrections for Lorentz and polarization effects and for absorption
(“multiscan”) were made.
(ii) Structure Solution and Refinement. The crystal structure was
solved by direct methods28 and refined by full-matrix least squares with
anisotropic displacement parameters for all non-hydrogen and isotropic
displacement parameters for the hydrogen atoms. Fifty-one refined
parameters from 888 observations gave an observation: refined
parameter ratio of 17:1. Corrections for secondary extinction were
applied, and the refinement was completed using a three-term Cheby-
shev weighting scheme29 with the coefficients 1.53, 0.641, and 1.16.
Refinement on F converged at R ) 0.029, Rw ) 0.035, and GOF )
(30) Mu¨ller, M.; Williams, V. C.; Doerrer, L. H.; Leech, M. A.; Mason, S.
A.; Green, M. L. H.; Prout, K. Inorg. Chem. 1998, 37, 1315.
(31) Watkin, D. J.; Prout, C. K.; Carruthers, J. R.; Betteridge, P. W.
CRYSTALS; Chemical Crystallography Laboratory, University of
Oxford: Oxford, U.K., 1996; Issue 10.
(32) Wilson, A. J. C., Ed. International Tables for Crystallography; Kluwer
Academic Publishers: Dordrecht, The Netherlands, 1995; Vol. C.
(33) Pearce, L. J.; Prout, C. K.; Watkin, D. J. CAMERON; Chemical
Crystallography Laboratory, University of Oxford: Oxford, U.K.,
1996.
(34) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels,
A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone,
V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.;
Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R.
L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara,
A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.;
Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle,
E. S.; Pople, J. A. Gaussian 98, revision A.3; Gaussian Inc.:
Pittsburgh, PA, 1998.
(26) Cosier, J.; Glazer, A. M. J. Appl. Crystallogr. 1986, 19, 105.
(27) Gewirth, D. The HKL Manual (written with the cooperation of the
program authors Z. Otwinowski and W. Minor); Yale University: New
Haven, CT, 1995.
(28) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Burla,
M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435.
(29) Carruthers, J. R.; Watkin, D. J. Acta Crystallogr. 1979, A35, 698.
(35) Jursic, B. S. J. Mol. Struct.: THEOCHEM 1998, 428, 61.

Chemical Properties of Monochlorogallane
Inorganic Chemistry, Vol. 40, No. 18, 2001 4761
decomposition, a sample of monochlorogallane (48.5 mg, 0.452 mmol
GaH2Cl) gave 0.41 mmol of H2 and 47.7 mg of a slightly grayish solid
shown by elemental analysis to have the composition GaCl (Found:
Ga, 66.0; Cl, 34.0; GaCl requires Ga, 66.3; Cl, 33.7%).
C2H4 (450 mmHg) at 250 K. However, a benzene solution of the gallane
(257 mg, 2.40 mmol GaH2Cl) reacted with C2H4 (overpressure 2 atm)
at room temperature. After 24 h, evaporation of the material that was
volatile at 250 K (C2H4 and C6H6) revealed that 2.89 mmol of C2H4
had been consumed, corresponding to a stoichiometry C2H4:GaH2Cl
of 1.20:1. The residue consisted of a colorless, viscous liquid which
gave two fractions on volatilization in vacuo, first at 303 and then at
333 K.
(ii) Reactions with Anionic Hydride Derivatives. Details of these
experiments are given elsewhere.5-8 Essential features are the use of
evacuated, rigorously preconditioned, all-glass apparatus; freshly
prepared, finely divided solid reagents; and low temperature for all
parts of the apparatus, including those accessible by the vapors of the
products. The volatile products could be removed under continuous
pumping and fractionated; the relevant gallane [GaH3]n, [GaBH6]n, or
GaB3H10 could thus be isolated in yields up to 80% based on eq 3 and
the amount of monochlorogallane taken. Subsequent characterization
included chemical and spectroscopic analysis, physical and chemical
trapping, and structure determination of the gaseous molecules by
electron diffraction.
(iii) Reaction with NMe3. Monochlorogallane (72 mg, 0.67 mmol
GaH2Cl) reacted with an excess of NMe3 at 178 K. Evaporation of the
material that was volatile at 250 K (unchanged NMe3) showed that
0.68 mmol of NMe3 had been consumed, corresponding to a stoichi-
ometry Me3N:GaH2Cl of 1.01:1. The white, solid product was
characterized by its Raman spectrum (see Table 4). On the other hand,
the IR spectrum of the annealed solid film formed by condensing the
vapor of this adduct with a small excess of NMe3 on a CsI window at
77 K showed, in addition to absorptions associated with Me3N‚GaH2-
Cl,18 extra features attributable to the formation of a second adduct
(Me3N)2GaH2Cl at low temperatures (see also Table 4). Raising the
temperature of the deposit to 250 K under continuous pumping resulted
in the decay of these features.
(iv) Reaction with NH3. Monochlorogallane (188 mg, 1.75 mmol
GaH2Cl) reacted with an excess of NH3 at 195 K. After 2 h, the material
that was volatile at this temperature (NH3) was evaporated to leave a
white solid found to be stable at room temperature. Hence, it was found
that 3.66 mmol of NH3 had been consumed, corresponding to a
stoichiometry NH3:GaH2Cl of 2.09:1. Elemental analysis of the solid
confirmed the composition H2GaCl‚2NH3 (Found: Ga, 48.8; Cl, 24.8;
H2GaCl‚2NH3 requires Ga, 49.4; Cl, 25.1%). The IR spectrum of the
solid showed the following bands in the region 4000-400 cm-1
(wavenumbers in cm-1; s strong, m medium, w weak, br broad, sh
shoulder): 3280 s, 3166 s, br [ν(N-H)]; 1972 sh, 1915 m [ν(Ga-H)];
1618 w, br [δas(NH3)]; 1407 w [impurity]; 1327/1303 m [δs(NH3)];
789 m, br [F(NH3)]; 734 m [δ(GaH2)]; 648 w [F(GaH2)]; 475/451 w
[ν(Ga-N)].
The solid formed by condensing the vapor of the more volatile
fraction at 77 K gave an IR spectrum with the following absorptions
in the region 4000-400 cm-1: 2959 s, 2930 sh, 2901 s, 2871 s, 2805
w, 2733 w, 1961 s, br [ν(Ga-H)], 1461 m, 1414 m, 1377 w, 1239/
1205 w, 1013 m, 962/945 m, 708 m, 672 s, 602 s, br, 551/521 m. The
1H NMR spectrum of a toluene-d8 solution at 223 K exhibited two
sets of resonances, A and B, with relative intensities ca. 3:1, the more
intense features (A) being appreciably broader than the weaker ones
(B). The resonances comprised (a) triplets at δH 1.25 (A) and 1.14 (B)
[GaCH2CH3], (b) quartets at δH 0.66 (A) and 0.46 (B) [GaCH2CH3],
and (c) broad singlets at δH 6.08 (A) and 5.53 (B) [terminal GaH]. The
results are consistent with the presence of trans- and cis-isomers of the
dimeric compound [Et(H)GaCl]2 (2a and 2b, respectively), although it
is not possible to identify with any confidence which isomer is which.
Derivatives containing fewer or more ethyl groups, Et(H)Ga(µ-Cl)2GaH2
and Et2Ga(µ-Cl)2Ga(H)Et, are likely also to be formed at some stage,
but the comparative simplicity of both the ν(Ga-H) region of the IR
1
spectrum and the H NMR spectrum argues against their presence as
more than minor constituents, at least under the conditions of these
experiments.
The less volatile component, vaporizing at ca. 333 K in vacuo, was
also a liquid at room temperature. This displayed a Raman spectrum
identical in all essential respects with that reported previously for [Et2-
GaCl]2,25 the absence of significant scattering in the region 1600-
2000 cm-1 appearing to rule out the retention of any Ga-H bonds.
Acknowledgment. We thank (i) the EPSRC (formerly
SERC) for research studentships (to M.J.G., E.J., and C.R.P.),
research fellowships (to T.M.G. and C.R.P.), and other financial
support of the Oxford group; and (ii) Drs. P. T. Brain and W.
M. Laidlaw for practical assistance and advice.
Supporting Information Available: Full crystallographic data are
available in CIF format. This material is available free of charge via
the Internet at http://pubs.acs.org.
(v) Reaction with C2H4. Monochlorogallane reacted neither with
neat C2H4 at 178 K nor in isopentane solution with an overpressure of
IC010128X