Preparation and properties of gallaborane, GaBH6: Structure of the gaseous molecule H2Ga(μ-H)2BH2 as determined by vibrational, electron diffraction, and ab initio studies, and structure of the crystalline solid at 110 K as determined by X-ray diffraction

Article abstract of DOI:10.1021/ic001338x

Gallaborane (GaBH₆, 1), synthesized by the metathesis of LiBH₄ with [H₂GaCl]_n at ca. 250 K, has been characterized by chemical analysis and by its IR and ¹H and ¹¹B NMR spectra. The IR spectrum of the vapor at low pressure implies the presence of only one species, viz. H₂Ga(μ-H)₂BH₂, with a diborane-like structure conforming to C_2ν symmetry. The structure of this molecule has been determined by gas-phase electron diffraction (GED) measurements afforced by the results of ab initio molecular orbital calculations. Hence the principal distances (rα in A) and angles (∠_α in deg) are as follows: r(Ga···B), 2.197(3); r(Ga-H_t), 1.555(6); r(Ga-H_b), 1.800(6); r(B-H_t), 1.189(7); r(B-H_b), 1.286(7); ∠H_b-Ga-H_b, 71.6(4); and ∠H_b-B-H_b, 110.0(5) (t = terminal, b = bridging). Aggregation of the molecules occurs in the condensed phases. X-ray crystallographic studies of a single crystal at 110 K reveal a polymeric network with helical chains made up of alternating pseudotetrahedral GaH₄ and BH₄ units linked through single hydrogen bridges; the average Ga···B distance is now 2.473(7) A. The compound decomposes in the condensed phases at temperatures exceeding ca. 240 K with the formation of elemental Ga and H₂ and B₂H₆. The reactions with NH₃, Me₃N, and Me₃P are also described.

Full text of DOI:10.1021/ic001338x

3
484
Inorg. Chem. 2001, 40, 3484-3497
Preparation and Properties of Gallaborane, GaBH6: Structure of the Gaseous Molecule
H2Ga(µ-H)2BH2 As Determined by Vibrational, Electron Diffraction, and ab Initio Studies,
and Structure of the Crystalline Solid at 110 K As Determined by X-ray Diffraction†
Anthony J. Downs,* Tim M. Greene, and Emma Johnsen
Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QR, UK
Paul T. Brain, Carole A. Morrison, Simon Parsons, Colin R. Pulham,* and
David W. H. Rankin
Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh EH9 3JJ, UK
Kirsten Aarset, Ian M. Mills, Elizabeth M. Page, and David A. Rice
Department of Chemistry, University of Reading, Whiteknights, PO Box 224, Reading RG6 6AD, UK
ReceiVed NoVember 28, 2000
Gallaborane (GaBH6, 1), synthesized by the metathesis of LiBH4 with [H2GaCl]n at ca. 250 K, has been
1
11
characterized by chemical analysis and by its IR and H and B NMR spectra. The IR spectrum of the vapor at
low pressure implies the presence of only one species, viz. H2Ga(µ-H)2BH2, with a diborane-like structure
conforming to C2V symmetry. The structure of this molecule has been determined by gas-phase electron diffraction
(GED) measurements afforced by the results of ab initio molecular orbital calculations. Hence the principal distances
(rR in Å) and angles (∠ R in deg) are as follows: r(Ga‚‚‚B), 2.197(3); r(Ga-Ht), 1.555(6); r(Ga-Hb), 1.800(6);
r(B-Ht), 1.189(7); r(B-Hb), 1.286(7); ∠ Hb-Ga-Hb, 71.6(4); and ∠ Hb-B-Hb, 110.0(5) (t ) terminal, b )
bridging). Aggregation of the molecules occurs in the condensed phases. X-ray crystallographic studies of a
single crystal at 110 K reveal a polymeric network with helical chains made up of alternating pseudotetrahedral
GaH4 and BH4 units linked through single hydrogen bridges; the average Ga‚‚‚B distance is now 2.473(7) Å. The
compound decomposes in the condensed phases at temperatures exceeding ca. 240 K with the formation of elemental
Ga and H2 and B2H6. The reactions with NH3, Me3N, and Me3P are also described.
t
Introduction
achieved (i) by making X very bulky (e.g. X ) 2,4,6-Bu 3-
4
,10
C6H2)
and/or imparting additional coordinating capacity to
it (e.g. X ) NR2 or 2,6-(Me2NCH2)2C6H3 ) or (ii) by external
coordination to a suitable Lewis base (e.g. Me3N, quinuclidine
or Cy3P). Several of these compounds bid fair as precursors
to epitaxial films or nanocrystals of gallium-containing semi-
The past decade has evinced a lively interest in gallium
hydrides.1 Synthetic advances have produced not only the
4
10
-4
2,5
binary hydride itself, [GaH3]n, but also mono- or disubstituted
4
,11
6
,7
8
derivatives, GaXnH3-n, where n ) 1 or 2 and X ) Cl, NH2,
NR2,4 N3, PR2,4 OR,4 or R (R ) an organic group).
9
4,10
4
,8,9,11
conductor materials such as GaN;
application in chemical synthesis.
others have found
Stabilization of the intrinsically labile GaHn unit may be
4
†
In terms of chemical vapor deposition (CVD), the most
suitable sources are volatile compounds in which all extraneous
bonds to the metal are easily and quantitatively cleaved. One
compound clearly answering to this description is gallaborane
(GaBH6, 1), the most volatile compound of gallium to be
reported to date, although its instability at ambient temperatures
is unlikely to command its widespread use for either CVD or
synthetic operations. Here we report the synthesis and some of
the physical and chemical properties of this compound. On the
evidence of its IR spectrum and electron-diffraction pattern, the
This paper is dedicated by A.J.D. to Professor Hansgeorg Schn o¨ ckel
of the Universit a¨ t Karlsruhe to mark his 60th birthday.
(
1) Taylor, M. J.; Brothers, P. J. In Chemistry of Aluminium, Gallium,
Indium and Thallium; Downs, A. J., Ed.; Blackie: Glasgow, UK, 1993;
pp 111-247.
(
(
(
(
2) Downs, A. J.; Pulham, C. R. AdV. Inorg. Chem. 1994, 41, 171.
3) Downs, A. J.; Pulham, C. R. Chem. Soc. ReV. 1994, 23, 175.
4) Downs, A. J. Coord. Chem. ReV. 1999, 189, 59.
5) Pulham, C. R.; Downs, A. J.; Goode, M. J.; Rankin, D. W. H.;
Robertson, H. E. J. Am. Chem. Soc. 1991, 113, 5149.
(
6) Goode, M. J.; Downs, A. J.: Pulham, C. R.; Rankin, D. W. H.;
Robertson, H. E. J. Chem. Soc., Chem. Commun. 1988, 768.
7) 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.
8) 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.
9) McMurran, J.; Kouvetakis, J.; Nesting, D. C.; Smith, D. J.; Hubbard,
J. L. J. Am. Chem. Soc. 1998, 120, 5233. McMurran, J.; Dai, D.;
Balasubramanian, K.; Steffek, C.; Kouvetakis, J.; Hubbard, J. L. Inorg.
Chem. 1998, 37, 6633.
(
(10) Cowley, A. H.; Gabba ¨ı , F. P.; Isom, H. S.; Decken, A. J. Organomet.
Chem. 1995, 500, 81.
(11) Jones, C.; Koutsantonis, G. A.; Raston, C. L. Polyhedron 1993, 12,
1829. Raston, C. L. J. Organomet. Chem. 1994, 475, 15. Gardiner,
M. G.; Raston, C. L. Coord. Chem. ReV. 1997, 166, 1. Luo, B.; Young,
V. G., Jr.; Gladfelter, W. L. J. Chem. Soc., Chem. Commun. 1999,
123.
(
(
1
0.1021/ic001338x CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/31/2001

Preparation and Properties of Gallaborane, GaBH6
Inorganic Chemistry, Vol. 40, No. 14, 2001 3485
vapor of 1 at pressures up to about 50 Torr consists exclusively
of the diborane-like molecules H2Ga(µ-H)2BH2, the structure
of which has been determined by a combination of experimental
and ab initio methods. Aggregation is the dominant feature of
the condensed phases, as demonstrated unequivocally by the
structure of a single crystal at 110 K; this has been shown by
X-ray diffraction to comprise helical chains of alternating BH4
and GaH4 units each linked to its neighbors by two single
hydrogen bridges. Chemical properties of 1 to be reported
include its thermal decomposition (which provides a means of
chemical analysis) and its reactions with the bases NH3, NMe3,
by small amounts of 1, from which it is not easily freed, and
there is evidence too that the decomposition of 2 proceeds
initially by the relatively facile reaction 2.
1
2
HGa(BH ) f H GaBH + B H
(2)
4
2
2
4
2
6
X-ray crystallographic studies have led to the determination of
the structure of a single crystal of 1 at 110 K, with the results
to be described in section d. The rate of vaporization of pure 1
is consistent with a vapor pressure of ca. 1 Torr at 195 K. In
the condensed phases, 1 decomposes rapidly at temperatures
exceeding 238 K; the half-life of the vapor at room temperature
is on the order of 2 min when the pressure is ca. 50 mmHg but
can be extended significantly by operating at lower pressures.
and PMe3. Preliminary accounts of some of these studies have
appeared previously,12,13 and the UV photoelectron spectrum
14
of the gaseous molecule has also been reported.
1
2
Results and Discussion
As reported briefly elsewhere, gallaborane has been au-
thenticated chemically by quantitative assay of its decomposition
products. Complete decomposition of a sample by warming to
343 K gives rise to elemental gallium and dihydrogen together
with diborane, in accordance with reaction 3.
a) Synthesis and Validation. As noted elsewhere,2,4
(
monochlorogallane, [H2GaCl]n, conveniently synthesized by the
6
,7
reaction of gallium(III) chloride with an excess of Me3SiH,
represents a potentially useful precursor to other monosubstituted
derivatives of gallane, e.g. H2GaB3H815 and H2GaN3. We have
found that monochlorogallane reacts with freshly recrystallized
9
2
GaBH f 2Ga + 3H + B H
(3)
6
2
2
6
2
LiBH4 in vacuo at ca. 250 K and in the absence of a solvent
The identity and likely structure of 1 have been endorsed by its
to yield, in addition to small amounts of H2 and B2H6, a highly
volatile product shown to be gallaborane ([H2GaBH4]n, 1),
formed in accordance with the metathesis reaction 1.
1
11
IR and H and B NMR spectra. The IR spectrum of the vapor
at pressures up to 50 Torr argues strongly for the presence of
the molecule H2Ga(µ-H)2BH2 with C2V symmetry as the pre-
dominant species. This view is supported by an analysis of the
electron diffraction pattern of the vapor at 240-260 K and by
the results of ab initio calculations. The vibrational and NMR
properties imply that aggregation of the GaBH6 molecules occurs
in solution and in the solid state, as borne out by the crystal
structure.
neat reagents
1
1
[
H GaCl] + LiBH
8 [H GaBH ] + LiCl (1)
n
2
n
4
ca. 250 K
n
2 4 n
Compound 1 can be isolated by fractional condensation in vacuo
of the volatile components of the reaction mixture, the vapor
passing through a trap at 210 K but condensing in, and being
retained by, a trap at 147 K. Fractionation gives a homogeneous
product judged to be essentially pure gallaborane on the evidence
of the spectroscopic properties to be described; the yields are
typically about 80% on the basis of eq 1 and the quantity of
monochlorogallane taken. The successful synthesis of 1, like
that of gallane, demands the use of scrupulously pure reagents
(b) Properties of the Vapor. (i) IR Spectrum. IR spectra
were recorded for isotopically normal and perdeuterated gall-
aborane vapor at pressures up to 50 Torr and near-ambient
temperatures and for the vapor species isolated in solid
dinitrogen matrixes at ca. 20 K. Representative spectra are
illustrated in Figure 1, and the details are itemized in Table 1.
Wavenumbers quoted in parentheses in the ensuing discussion
refer to the perdeuterated compound.
The similarity of the IR spectra exhibited by the vapor and
the solid N2 matrixes indicates their origin in a common
molecule. The pattern and frequencies of the absorptions, and
particularly the clearly defined rotational structure characterizing
many of the bands in the vapor spectrum, testify to a relatively
simple molecule with a momental ellipsoid in which at least
one of the principal moments of inertia is unusually small for
a gallium derivative. Three features appear to give clear notice
of the molecular identity.
and rigorous exclusion of moisture, oxygen, grease, or other
contaminants;2,5 accordingly, operations were carried out at
-
4
pressures <10 mmHg in all-glass apparatus, which had been
preconditioned by heating under continuous pumping, with short
distillation paths, and the maintenance of all glassware to which
the gallaborane had access at temperatures <263 K.
Gallaborane condenses at low temperatures as a white solid
that melts at ca. 228 K. Crystals of the solid compound were
formed adventitiously during attempts to grow single crystals
by controlled freezing of what proved to be an impure sample
of the bis(tetrahydroborate) ([HGa(BH4)2]n, 2), another thermally
1
6
(i) Many of the bands in the vapor spectrum display the
rotational envelopes typical of a symmetric-top molecule. For
labile gallium hydride with a melting point near 200 K. On
synthesis from gallium(III) chloride, 2 is invariably accompanied
-
1
example, the absorption at 2558 (1933.5) cm clearly shows
the P- and R-branches characteristic of a perpendicular-type
transition of a symmetric top, and under high resolution, it is
possible to discern the individual Q-branches corresponding to
the transitions between different K-levels of the molecule. At
(
(
(
(
12) Pulham, C. R.; Brain, P. T.; Downs, A. J.; Rankin, D. W. H.;
Robertson, H. E. J. Chem. Soc., Chem. Commun. 1990, 177.
13) Downs, A. J.; Parsons, S.; Pulham, C. R.; Souter, P. F. Angew. Chem.,
Int. Ed. Engl. 1997, 36, 890.
14) Dyke, J. M.; Haggerston, D.; Warschkow, O.; Andrews, L.; Downs,
A. J.; Souter, P. F. J. Phys. Chem. 1996, 100, 2998.
15) 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.
-
1
ca. 3.5 cm , the separation of these Q-branches points to a
molecule with a very small moment of inertia about the principal
axis, implying that only hydrogen atoms lie off this axis.
(ii) The strong doublet at 2558 (1933.5) and 2482 (1823)
-1
(
16) Downs, A. J.; Greene, T. M.; Harman, L. A.; Souter, P. F.; Brain, P.
T.; Pulham, C. R.; Rankin, D. W. H.; Robertson, H. E.; Hofmann,
M.; Schleyer, P. v. R. Inorg. Chem. 1995, 34, 1799. Downs, A. J.;
Harman, L. A.; Thomas, P. D. P.; Pulham, C. R. Polyhedron 1995,
cm , together with further prominent bands at ca. 2000 (1430),
-1
1
435 (1060), and 1130 (850) cm , are the clear hallmarks of
1
7,18
a bidentate BH4 group,
analogous to that found in the
molecule Me2Ga(µ-H)2BH2, for example.
1
9,20
14, 935.

3486 Inorganic Chemistry, Vol. 40, No. 14, 2001
Downs et al.
Figure 1. IR spectra of gallaborane and gallaborane-d
6 6 2
in the vapor or matrix-isolated states: (a) vapor of GaBH isolated in a solid N matrix at
ca. 20 K; (b) vapor of GaBH ; (c) vapor of GaBD ; (d and e) ν14 and ν
6
6
1
bands of GaBH
6
vapor (pressure 0.15 Torr, path length 3.9 m, spectral
-
1
resolution 0.06 cm ).
-1
(
iii) Absorptions near 2000 (1430) and at 729 (527) cm-1
of about 20 cm , type-B bands by a doublet structure also with
-
1
suggest by their frequencies, contours, and H/D shifts that they
originate in the stretching and scissoring vibrations, respectively,
of a GaH2 group [cf. H2Ga(µ-X)2GaH2, where X ) H (ca. 1980
and 671 cm ) or X ) Cl (ca. 2000 and 710 cm ) ].
The IR spectrum is then most easily interpreted in terms of
the diborane-like molecule H2Ga(µ-H)2BH2 with C2V symmetry
a spacing of about 20 cm , and type-C bands by a prominent
Q-branch and relatively diffuse P- and R-branches.22 With one
very small moment of inertia (IA) about the axis passing through
the two heavy atoms, gallaborane may be expected in many
respects to behave like B2H6 so that, for example, Coriolis
interactions due to rotation about the A-axis are likely to produce
relatively large perturbations in the observed spectra.
-
1 5
-1 7
2
3
(
Table 2). The 18 vibrational fundamentals are accommodated
by the representation 7a1 + 2a2 + 4b1 + 5b2, all but the 2a2
modes being IR-active. Earlier quantum chemical studies have
predicted such a structure,21 and the moments of inertia
estimated from the optimum dimensions determined in this way
imply that the molecule, although formally an asymmetric top,
approximates to a prolate symmetric top, with IA ) 8.394 amu,
Our interpretation leads to the vibrational assignments given
in Tables 3 and 4. The analysis has been based on five main
criteria: (a) the selection rules expected to govern the activity
of modes in IR absorption; (b) the effect of deuteration on the
energy of a given spectroscopic transition; (c) the envelope of
the IR absorption displayed by the vapor (vide infra); (d)
analogies with the vibrational properties of related molecules,
2
IB ) 65.449 amu, and IC ) 69.485 amu Å (1 amu ) 1.6605 ×
-
27
19
16
24
1
2
0
kg). With the choice of coordinate axes shown in Figure
2 4
notably Me GaBH , HGa(BH4)2, and Al(BH4)3; and (e)
, a1 modes are expected to give rise in the IR spectrum of the
comparisons with the frequencies and relative intensities of the
absorptions forecast by ab initio calculations carried out
vapor to type-A bands, b1 modes to type-C bands, and b2 modes
to type-B bands. Overall, the infrared bands are expected, and
observed, to be those characteristic of a prolate asymmetric top;
they are either type-A parallel bands, or type-B or -C perpen-
dicular bands. Type-A bands should be recognizable by P-, Q-,
and R-branches of comparable intensity and a P-R separation
2
1
previously or by us. We attach particular weight to the
evidence of the rovibrational features, the isotopic shifts, and
the theoretical predictions, which has led to the unambiguous
identification of most of the vibrational fundamentals.
There are some complications, however. First, the presence
1
0
11
of two naturally occurring isotopes of boron, B and B,
(
17) Marks, T. J.; Kolb, J. R. Chem. ReV. 1977, 77, 263.
(18) Marks, T. J.; Kennelly, W. J.; Kolb, J. R.; Shimp, L. A. Inorg. Chem.
1
972, 11, 2540.
19) Downs, A. J.; Thomas, P. D. P. J. Chem. Soc., Dalton Trans. 1978,
09.
(22) Seth Paul, W. A.; Dijkstra, G. Spectrochim. Acta 1967, 23A, 2861.
Seth Paul, W. A. J. Mol. Struct. 1969, 3, 403.
(23) See for example: Smith, W. L.; Mills, I. M. J. Chem. Phys. 1964, 41,
1479. Duncan, J. L.; McKean, D. C.; Torto, I. J. Mol. Spectrosc. 1981,
85, 16. Duncan, J. L.; Harper, J.; Hamilton, E.; Nivellini, G. D. J.
Mol. Spectrosc. 1983, 102, 416.
(
(
(
8
20) Barlow, M. T.; Downs, A. J.; Thomas, P. D. P.; Rankin, D. W. H. J.
Chem. Soc., Dalton Trans. 1979, 1793.
21) van der Woerd, M. J.; Lammertsma, K.; Duke, B. J.; Schaefer, H. F.,
III J. Chem. Phys. 1991, 95, 1160.
(24) Coe, D. A.; Nibler, J. W. Spectrochim. Acta 1973, 29A, 1789.

Preparation and Properties of Gallaborane, GaBH6
Inorganic Chemistry, Vol. 40, No. 14, 2001 3487
Table 1. IR Spectra of Gaseous and Matrix-Isolated Gallaborane and Gaseous Gallaborane-d
6
(Wavenumbers in cm-1)
vapor
GaBH
6
GaBD
6
GaBH
6
in N
2
matrix
intensb
ν˜
intensb
vw
ν˜
intensb
ν˜
H
/ν˜
D
ν˜
assignmentb
+ ν14 (B
3
2
2
2
2
2
2
2
2
2
2
1
1
1
1
1
1
1
1
1
1
1
1
670
565
549
492
482
472
255
217
156
007
005
982
950
937
925
870
575
532
446
435
426
350
315
ν
5
2
)
c
R
1940
1927
1828
1823
1816
R
P
R
2558
m
}
P
m, B
}
m, B
m, A
1.3230:1
1.3615:1
}
}
ν(B-H
t
), ν14 (b
2
)
2536
2482
2465
m
m
m
c
R
ν(B-H
t
), ν (a
1
1
)
Q
P
m, A
Q
P
}
}
vw
2ν
5
(A
1
)
vw, A
vw, A
m, C
m, B
w, A
2220
vw
s
s, br
m
ν
1
15 + ν18 (A )
ν
4
+ ν
6
(A )
1
1450
1447
1428
m, C
m, B
w, A
1.3841:1
1.3856:1
1.3880:1
2018
1992
1965
ν(B-H
b
), ν10 (b )
1
ν(Ga-H
ν(Ga-H
t
), ν15 (b
2
)
t
), ν (a )
2
1
R
Q
P
m, A
1383
m, A
1.4006:1
1925
1870
m
w
ν(B-H
b
), ν
3 1
(a )
}
mw, C
vw, A
vw, A
ν
5
+ ν12 (B )
1
2ν12 (A
1
)
ν16 + ν17 (A )
1
R
Q
P
1066
1060
1053
R
Q
P
c
s, A
s, A
1.3538:1
1432
1422
s
s
}
}
ν(Ga-H
b
), ν
4
(a
1
)
}
vw, B
w, C
ν16 + ν17 (B
2
)
c
980
850
w, C
w, A
1.3418:1
1.3294:1
1330
w
m
m
ν(Ga-H
b 1
), ν11 (b )
}
1
290
c
1
130
m, A?
1125
1
1
120
105
m
}
s
δ[B(H
t
)
2
], ν
5
(a
1
)
9
7
7
7
7
6
30
82
41
29
18
20
vw, B
vw, C
670
w, B?
1.3881:1
F[B(H
F[B(H
t
)
)
2
], ν16 (b
], ν12 (b
2
1
)
)
770
730
630
12
570
60
460
52
w
s
t
2
R
Q
P
537
527
515
442
R
Q
P
m, A
w, B?
m, C?
m, A
m, A
m, B?
s, C?
s, A
1.3833:1
1.4027:1
1.2922:1
1.1359:1
δ[Ga(H
t
2 6 1
) ], ν (a )
}
}
c
w
w
m
m
F[Ga(H
t
)
2
2
], ν17 (b
], ν13 (b
2
)
)
}
6
c
5
4
44
68
421
412
F[Ga(H
t
)
1
}
5
c
s
7 1
ν(Ga‚‚‚B), ν (a )
}
4
m
a
Key: s, strong; m, medium; w, weak; v, very; br, broad. P, Q, and R refer to rotational branches, A, B, and C to band types (see the text).
b
2 2
Ga(µ-H) BH
with C2V symmetry. c Matrix splitting.
Based on the structure H
2
Table 2. Fundamental Modes of the GaBH
6
Molecule in C2V Symmetry
H
2
Ga(µ-H)
2 2
BH with C2V symmetry
expected wavenumber
approx description of modea
range /cm
b
-1
irred rep
no of IR-active modes
no of IR bands observed
ν(B-H
ν(B-H
t
)
2400-2600
1900-2100
1900-2050
1200-1500
1000-1200
900-1000
700-800
650-800
600-700
500-600
350-500
<500
a
a
a
a
a
b
b
a
b
b
a
b
a
1
1
1
1
1
2
1
1
2
1
1
2
2
+ b
+ b
+ b
+ b
2
1
2
1
2
2
2
2
1
1
1
1
1
1
1
1
0
2
2
2
2
1
1
1
1
1
1
1
b
) + ring def
ν(Ga-H
t
)
ν(Ga-H
b
) + ring def
B(H
t
)
2
scissoring and twisting
+ a
2
in-plane F[B(H
out-of-plane B(H
Ga(H scissoring
in-plane F[Ga(H
out-of-plane Ga(H
t 2
) ]
t
)
2
wagging
t 2
)
t
)
2
]
t
)
2
wagging
b
ν(Ga‚‚‚B) ring def + ν(Ga-H )
Ga(µ-H)
Ga(µ-H)
2
2
BH bending
2
B bridge twist
<500
a
,19 HGa(BH
,16 Ga
,5 Al(BH ,24 and [H
.7
Description of modes is based on those suggested for the analogous molecules Me
2
GaBH
4
)
4 2
2
H
6
4 3 2 2
) GaCl]
Key: t ) terminal, b ) bridging. b See, for example, refs 5, 7, 16-19, and 24.
produces overlapping absorptions; at high resolution the features
due to the different isotopomers may be resolvable, but more
often they blend. Contamination is another source of confusion
with a compound as reactive and thermally labile as gallaborane.
amounts of incompletely deuterated isotopomers such as
GaBD5H. While sharpening the absorptions facilitated the
identification of some near-degenerate transitions, matrix isola-
tion tended also to introduce its own complications. Thus,
several of the bands were split, presumably because the
gallaborane molecules occupied different matrix sites, and
although the spectrum underwent no marked changes when the
Small concentrations of HGa(BH4)2,16 B2H6, chlorogallanes,
23
7
and other impurities are difficult to exclude, and the sample of
the deuterated compound was certainly contaminated with small

3488 Inorganic Chemistry, Vol. 40, No. 14, 2001
Downs et al.
Table 3. Measured and Calculated Wavenumbers (in cm-1), Assignments, and Product Rule Calculations for the Vibrational Fundamentals of
a
the Molecules GaBH
6
and GaBD
6
in C2V Symmetry
c
GaBH
6
GaBD
6
Π(ν˜
calc
H
D i
/ ν˜ )
number of
mode
irred rep
descriptionb
ν˜
H
obs ν˜
H
calc
ν˜
D
obs ν˜
D
calc
ν˜
H
/ν˜
D
(obs)
obs
a
1
1
2
3
4
5
6
7
8
9
sym ν(B-H
sym ν(Ga-H
sym ν(B-H ) + ring def
sym ν(Ga-H ) + ring def
B(H scissor
Ga(H scissor
ν(Ga‚‚‚B) ring def + ν(Ga-H
B(H twist
Ga(µ-H) B bridge twist
ν(B-H ) + ring def
ν(Ga-H ) + ring def
out-of-plane B(H wag
out-of-plane Ga(H wag
asym ν(B-H
asym ν(Ga-H
in-plane F[B(H
in-plane F[Ga(H
Ga(µ-H)
t
)
t
2482
1982
1937
1435
1130
729
2551.9
2030.3
1996.0
1473.3
1143.6
712.4
461.9
1105.7
464.3
1823
1428
1383
1060
850
527
412
1877.6
1472.4
1434.3
1081.4
870.3
531.9
439.4
807.1
319.0
1509.3
1013.3
593.2
422.0
2012.2
1455.7
740.9
401.1
140.7
1.3615
1.3880
1.4006
1.3538
1.3294
1.3833
1.1359
7.7318
7.4849
)
b
b
t 2
)
t
)
2
b
)
468
a
2
t
)
2
1.9922
3.4107
2
b
1
10
b
2007
1315
782
1950.3
1379.5
794.7
1450
980
1.3841
1.3418
11
12
13
b
t 2
)
t
)
2
544
550.5
421
1933.5
1447
670
1.2922
1.3230
1.3856
1.3881
1.4027
b
2
14
t
)
t
)
2558
2005
930
2639.9
2046.9
957.8
556.8
214.5
4.6892
15
16
17
18
t 2
) ]
t
)
2
]
620 d
442
2
B bending
(212)
a
The observed frequencies are taken from the IR spectra of the vapors and the calculated frequencies (unscaled) from MP2/6-311G* calculations.
b
c
t b
Key: H , terminal H atom; H , bridging H atom. The use of observed (anharmonic) vibration frequency data is expected to give an observed
product several percent smaller than the calculated one (see ref 23, for example). Value deduced from a possible combination mode.
d
antisymmetric vibration, ν14 (b2), is identifiable with the type-B
-
1
band centered at 2558 (1933.5) cm , and the symmetric
-
1
vibration, ν1 (a1), with the type-A band at 2482 (1823) cm ,
in compliance with the corresponding modes of the molecules
1
6
19
HGa(BH4)2 and Me2GaBH4. This part of the spectrum is
also strongly reminiscent of the corresponding region in the
-
1
spectrum of B2H6 that features a type-B band at 2609 cm
and a type-A band at 2520 cm , and the resemblance extends
to the isotopic shifts. Thus, the H/D ratios and B- B shifts
are as follows: (a) for gallaborane, ν14 1.3230:1 and 13.7 cm ;
-
1 23
1
0
11
-1
2 2 2
Figure 2. View of the H Ga(µ-H) BH molecule in the optimum
-
1
ν1 1.3615:1 and 6.0 cm ; for diborane, ν8 (b2u) 1.3197:1 and
refinement showing the coordinate axes and the axes of the momental
ellipsoid.
-
1
-1 23
13.9 cm ; ν16 (b3u) 1.3714:1 and 5.3 cm . On this evidence
it is reasonable to conclude that the dimensions of the BH2 unit
in GaBH6 are close to those in B2H6, where the best experi-
mental estimates of r0(B-Ht) and ∠ 0Ht-B-Ht are 1.201(2) Å
Table 4. Geometrical Parameters Obtained in the New Analysis of
the GED Data Collected for GaBH
6
(r
R
/Å, ∠
R
/deg)
parametera
GEDb
ab initioa
25
and 121.0(4)°, respectively; such a conclusion finds support
independent
too in the results of ab initio calculations (q.v.).
p
1
p
2
p
3
p
4
p
5
p
6
p
7
r(Ga‚‚‚B)
2.197(3)
1.458(6)
0.245(6)
0.440(5)
0.096(1)
120.8(9)
130.1(9)
2.192
1.452
0.228
The two ν(Ga-Ht) [ν2 (a1) and ν15 (b2)] and one of the ν-
r(B/Ga-H)av
(
B-Hb) modes [ν10 (b1)] (Hb ) bridging H atom) must together
∆r(Ga-Hb-t
)
be responsible for the broad, intense absorption centered near
∆r(B/Ga-H)av
0.422
-
1
2
000 (1440) cm . This accounts for the complicated nature of
∆r(B-Hb-t
)
0.0962(15)
121.1(10)
130.1(10)
∠ H
∠ H
t
-B-H
t
the spectrum hereabouts and the difficulty in deciding on the
precise assignments. The spectrum of the vapor reveals an
obvious pseudo-perpendicular band centered at 2005 (1447)
t
-Ga-H
t
dependent
r(Ga-H
r(Ga-H
b
)
)
1.800(6)
1.777
1.549
1.289
1.193
72.0
-
1
cm , which is likely therefore to arise from ν10 and/or ν15.
t
1.555(6)
1.286(7)
1.189(7)
71.6(4)
-
1
Closer inspection under high resolution (0.06 cm ) shows that
the spacing of the Q-branches is irregular and more complicated
r(B-H
b
)
r(B-H
t
)
-
1
∠
∠
H
H
b
-Ga-H
b
than in the absorption at 2558 cm (see Figure 1). We believe
that the feature actually comprises two overlapping pseudo-
perpendicular vibrational transitions with near-coincident band
origins. The overlapping results in four sets of Q-branches,
namely two sets of very closely spaced lines associated with
b
-B-H
b
110.0(5)
108.3
a
Values derived from B3PW91/6-311++G(2d,2p) calculation.
Values quoted with uncertainties represent restraints applied in ac-
cordance with the SARACEN method; see the text for details. b Values
in parentheses represent one standard deviation.
69
71
the Ga/ Ga components of the antisymmetric ν(Ga-Ht) mode,
ν15, and two sets of rather more widely spaced lines associated
matrix was annealed at temperatures up to 33 K, there was also
the risk that some of the weaker features arose not from the
monomer but from aggregates, [GaBH6]n. There are limitations,
too, imposed by the restricted wavenumber range of the current
1
0
11
with the B/ B components of the antisymmetric ν(B-Hb)
mode, ν10. Support for this assignment comes from the band at
-
1
1447 cm in the spectrum of GaBD6 vapor, where the low-
energy flank (∆K ) -1) comprises two sets of regularly spaced
Q-branches, the spacing of which identifies their origin in a
-
1
measurements (400-4000 cm ).
The clear doublet pattern of the absorptions near 2500 (1900)
cm- must be associated with the ν(B-Ht) fundamentals (Ht
1
1
6-19
)
terminal H atom) of a bidentate BH4 group.
The
(25) Duncan, J. L.; Harper, J. Mol. Phys. 1984, 51, 371.

Preparation and Properties of Gallaborane, GaBH6
Inorganic Chemistry, Vol. 40, No. 14, 2001 3489
ν(B-Db) mode; the high-energy flank is more complicated,
presumably because of overlap with the antisymmetric ν(Ga-
Dt) feature. There is an obvious pseudo-parallel or type-A band
Calculation offers the only access we have to the IR-silent a2
fundamentals, ν8 and ν9, which involve twisting motions of the
B(Ht)2 and Ga(Ht)2 fragments; frequencies near 1200 and 500
-
1
10 11
-1
-1
at 1937 (1383) cm ; a B/ B isotopic shift of 3 (4) cm
cm are thus estimated. The remaining features in the observed
identifies it as the symmetric ν(B-Hb) mode [ν3 (a1)]. The
remaining a1 mode [the symmetric ν(Ga-Ht) mode, ν2] must
then be responsible for a partially obscured absorption at 1982
spectrum of gallaborane, all of low intensity, can be interpreted
satisfactorily as overtones or combinations of fundamentals. A
-
1
very weak absorption near 2220 cm may be associated with
-1
-1
cm in the spectrum of GaBH6 and for a more easily discerned
the combination ν15 + ν18, implying that ν18 ) 212 cm . The
-
1
-1
band at 1428 cm in the spectrum of GaBD6. The detailed
assignments proposed in Tables 3 and 4 are fortified by the
results of the quantum chemical calculations and by the
medium-weak type-C band at 1870 cm can realistically be
attributed only to the combination ν5 + ν12 (B1), for which the
-
1
frequency should be ca. 1910 cm . Hence significant Fermi
resonance must occur between this combination and the adjacent
b1 fundamental ν10. For those symmetry classes offering a
complete or virtually complete set of observed frequencies, the
proposed assignments are consistent with the operation of the
product rule, as indicated by the results included in Table 3.
16
frequencies of analogous modes in the molecules HGa(BH4)2,
Me2GaBH4,19 Ga2H6, and [H2GaCl]2.
5
7
For each of the two ν(B-Hb) fundamentals there is an
analogous mode approximating to ν(Ga-Hb) (while having
significant Ga(µ-H)2B ring deformation character), which is of
the same symmetry. Analogies with related molecules, e.g.
-1
At high resolution (0.06 cm ) the pseudo-perpendicular band
5
16
19
26
Ga2H6, HGa(BH4)2, Me2GaBH4, and Me2Ga(µ-H)2GaMe2,
imply that these two modes are likely to occur in the region
-1
of gaseous GaBH6 (GaBD6) at 2558 (1933.5) cm clearly
exhibits a series of well-defined Q-branches with a separation
-
1
1
150-1450 (850-1100) cm . The prominent type-A band at
435 (1060) cm is then the obvious choice for the a1 mode,
-1
-1
of ca. 3.5 (1.8) cm . A preliminary analysis of the 2558 cm
band by combination differences then gives values of 1.7579-
-
1
1
ν4. The b1 mode, ν11, is harder to identify positively, being only
(
(
10) and 1.7675(10) for A′ - Bh ′ and A′′ - Bh ′′, respectively
where the rotational constants A′ and Bh ′ relate to the V ) 1
weak in IR absorption, but it can reasonably be linked to the
-
1
weak absorption at 1320 (980) cm .
state and A′′ and Bh ′′ to the V ) 0 state, Bh being the average of
the constants B and C). To evaluate Bh ′ and Bh ′′ it would be
necessary to carry out a similar analysis of the rotational
Next in order of decreasing frequency comes the B(Ht)2
scissoring mode, ν5 (a1), for which the band at 1130 (850) cm-
is the logical choice. Such an assignment is in keeping with
the frequencies of the corresponding fundamentals in Me2-
1
structure associated with a pseudo-parallel band, e.g. that at 2482
-1
(
(
1823) cm . Further studies requiring still higher resolution
1
9
16
24
GaBH4, HGa(BH4)2, and Al(BH4)3. Although a type-A
contour is clearly in evidence for the band of GaBD6 at 850
-1
<0.01 cm ) are still under way to resolve the closely spaced
-1
rotational lines forming the P- and R-branches of the 2482 cm
-
1
-1
cm , it is far from obvious in the case of the 1130 cm band
of GaBH6, which gives the appearance of considerable pseudo-
perpendicular character. A possible explanation of this is that
the type-A absorption at 1130 cm is coincident with a
combination transition with the B1 or B2 representation. Such a
combination might be ν9 (a2) + ν17 (b2) (setting the IR-silent
fundamental ν9 at ca. 510 cm ). Somewhat lower in frequency
and predicted to give a weak type-B absorption is the B(Ht)2
in-plane wagging vibration, ν16 (b2), which we associate with a
27
-1
band, although the separation of ca. 0.51 cm clearly signifies
-1
21
that Bh is on the order of 0.25 cm . The earlier ab initio studies
furnish estimates for Bh ′′ and A′′ - Bh ′′ of 0.2501 and 1.7581
-
1
-1
cm , respectively, in excellent agreement with these experi-
mental values. Accordingly, there can be little doubt about the
essential structural features of the GaBH6 molecule.
-1
As expected, the frequencies deduced from HF/6-311G(d)
calculations are mostly higher than the experimental ones, but
the overall level of agreement is highly satisfactory. Discrep-
ancies are well within the range set by the frequency differences
between harmonic (theoretical) and anharmonic (experimental)
vibrations, by the various approximations inherent in the
theoretical method, and by the uncertainties associated with
some of the experimental frequencies that may be subject, for
example, to appreciable perturbation under the action of Fermi
-
1
weak feature at 930 (670) cm . Another weak band, this time
-1
at 782 cm and appearing to have a type-C contour (but without
any obvious counterpart in the spectrum of the deuterated
compound), is most probably due to the corresponding rocking
vibration, ν12 (b1).
The frequency, well-developed type-A contour, and precedent
all support the attribution of the band of medium intensity at
23
resonance (cf. B2H6 ). Hence we conclude that the assignments
-1
7
29 (527) cm to the Ga(Ht)2 scissoring mode, ν6 (a1). Wagging
proposed here are unlikely to be far wide of the mark, with all
the evidence vouching for a molecule with the structure H2Ga-
and rocking motions of the Ga(Ht)2 unit [ν17 (b2) and ν13 (b1)],
expected to occur at successively lower frequency, are then
(µ-H)2BH2 conforming to C2V symmetry. We have not in the
-
1
identified with the features at 620 (442) and 544 (421) cm ,
respectively. The measured spectrum shows only one other band
of appreciable intensity. With a type-A contour and medium-
circumstances attempted a detailed refinement of the vibrational
force field, but have drawn on the theoretically based field, with
appropriate scaling of the constants, to estimate vibrational
amplitudes. This supplementary information then becomes input
to our analysis of the electron diffraction pattern of the gaseous
molecule (q.v.) and so serves an important function in fixing
the most reliable structure that can be determined experimentally
from the data currently available to us.
-1
to-strong intensity, this occurs at 468 cm and is characterized
-
1
by a relatively modest shift on deuteration (to 412 cm ),
credentials leading us to associate it with ν7 (a1), a complex
motion incorporating deformation of the Ga(µ-H)2B ring with
Ga‚‚‚B stretching as a primary feature.
All but one of the 16 IR-active fundamentals of the H2Ga-
(ii) Structure of Gaseous Gallaborane: Electron Diffrac-
(µ-H)2BH2 molecule have thus been located with some confi-
tion Measurements. Earlier studies have repeatedly emphasized
dence. The one absentee is the skeletal puckering vibration, ν18
the technical problems of carrying out gas electron diffraction
-
1
(
b2), which is anticipated to lie below 250 cm , i.e. beyond
1
2
(
GED) measurements on such labile molecules as gallaborane
the low-frequency threshold of the current measurements.
5
,6,15,16,26
5
and other gallium hydrides.
As with digallane, it has
(
26) Baxter, P. L.; Downs, A. J.; Goode, M. J.; Rankin, D. W. H.;
Robertson, H. E. J. Chem. Soc., Dalton Trans. 1990, 2873.
(27) Pulham, C. R.; Law, M. M.; Newnham, D. A. Unpublished results.

3490 Inorganic Chemistry, Vol. 40, No. 14, 2001
Downs et al.
been necessary to use an all-glass inlet system which can be
adequately preconditioned and then cooled to minimize thermal
decomposition of the compound. The first measurements were
made with the Edinburgh apparatus,12 but despite repeated
attempts, it proved impossible to secure satisfactory, reproduc-
ible results except at one, relatively short camera distance (ca.
defined by the experimental GED data (vide infra), which varied
by as much as 0.04 Å. Introducing higher order excitations to
Hartree-Fock theory (MP2, MP3, and MP4) consistently
returned parameter values longer than the experimental value
by 0.02 Å. Interestingly, incorporating all electrons into the
correlation treatment at the MP2 and MP3 level shortened this
bond, in line with experiment, but unfortunately at this level
rather erratic values were returned for the parameters r(Ga-
Hb) and r(Ga-Ht), as indicated above. On balance, the most
reliable geometries were determined at the B3PW91 DFT level,
where the majority of parameter values are within two esds of
the experimental geometry.
2
00 mm); otherwise, the experiments failed through extreme
problems associated either with fogging of the photographic
plates or with extraneous scattering due to decomposition pro-
ducts of the gallaborane. The problem of fogging was partially
alleviated by extending the cold trap of the Reading apparatus
7
so as to envelop the nozzle of the inlet. Hence, a measure of
success was achieved at a long camera distance (ca. 500 mm)
but not at a shorter distance, where a series of experiments with
different sample and nozzle temperatures and developing
conditions failed to elicit measurable scattering at the sort of
vapor pressures it was necessary to use. It was therefore decided
to combine the results from Edinburgh and Reading to produce
Modeling the geometric and dynamic properties of the central
Ga(µ-H)2B ring of gallaborane must account for significant
shrinkage effects. There may not be any vibrational modes with
-1
7,16,32
frequencies less than ca. 250 cm , but precedent
suggests
that the Ga(µ-H)2B ring-puckering mode, ν18, is likely to have
a large amplitude. Such large amplitude motion is better
approximated by curvilinear motion, rather than the conventional
rectilinear system. Thus, estimates of vibrational amplitudes (u
and k) required in the GED analysis were obtained from the
construction of a harmonic force field [MP2/6-311G(d) force
-
1
a data set extending to a maximum value of 20.0 Å in the
-
1
variable s () 4πλ sin θ, where 2θ is the scattering angle).
Theoretical Calculations. Taken alone, the GED pattern of
gallaborane gives only limited guidance to the structural and
dynamic properties of the molecule. To arrive at the best
estimate of the molecular structure it has therefore been vital
to supplement the GED findings not only with all possible
information about the vibrational properties (as noted above),
but also with the results of ab initio calculations carried out
2
9
constants] using the program SHRINK, which was scaled
against the experimental vibrational frequencies (vide supra)
to correct for the systematic overestimation of force constants
by the ab initio calculation.
Structural Model Used in the Analysis of the GED Data.
28
21
with the GAUSSIAN 94 suite of programs. In keeping with
the findings of earlier studies,21 these calculations confirm that
the global minimum in the potential energy surface corresponds
to the C2V equilibrium structure H2Ga(µ-H)2BH2. Particular
attention has been paid to the response of the calculated structure
parameters to the level of theory and the basis set employed.
Hartree-Fock (HF), Møller-Plesset (MP2, MP3, and MP4),
and density functional theory (DFT) treatments were deployed
with the 6-311G basis set, incorporating varying numbers of
diffuse and polarization functions. The calculation which most
closely reproduces the experimental geometry is given in Table
In the light of the IR spectrum and of the present and earlier
quantum chemical calculations, the gaseous gallaborane mol-
ecule was assumed to resemble B2H6 with an equilibrium
structure H2Ga(µ-H)2BH2 possessing C2V symmetry (see Figure
2). The model adopted for simulation of the molecular scattering
intensities was then defined by seven independent structural
parameters. To reduce correlation effects between similar bond
lengths, it should be noted that a number of parameters must
be defined as average and difference pairs; this is standard
practice in gas-phase electron diffraction work. The structural
parameters were thus defined as (i) r(Ga‚‚‚B), (ii) r(B/Ga-H)av
) [r(B-Ht) + r(B-Hb) + r(Ga-Ht) + r(Ga-Hb)], (iii)
∆r(Ga-Hb-t), (iv) ∆r(B/Ga-H)av ) [r(Ga-H)av - r(B-H)av],
(v) ∆r(B-Hb-t), (vi) ∠ Ht-B-Ht, and (vii) ∠ Ht-Ga-Ht. A
theoretical sIm(s) curve was adjusted simultaneously to the two
intensity curves (derived from the different camera distances)
using a unit weight matrix (Figure 4). Least-squares analysis30
of the molecular scattering intensities has then been performed
to refine the structure of the gallaborane molecule.
4
; a summary of the geometric parameters obtained from all
calculations performed is available in the Supporting Informa-
tion.
From this graded series of calculations it was observed that
distances relating to the gallium atom [r(Ga‚‚‚B), r(Ga-Hb),
and r(Ga-Ht)] were most sensitive to the quality of calculation.
Supplementing the 6-311G basis set with further polarization
and diffuse functions gave rise to only minor structural changes
overall, with bond distances shortening on average by no more
than 0.005 Å as the basis set improved with any given level of
theory. The only major exceptions to this rule were the Ga-Hb
and Ga-Ht distances, which could shorten by as much as 0.015
Å when polarization functions were added to the hydrogen atoms
at levels above MP2(FC).
Refinement of the Structural Model. The presence of a
large number of similar interatomic distances and of some
parameters involving hydrogen (a poor scatterer of electrons)
prevented a complete structure solution using only experimental
data. Information obtained from theory has therefore been
included to allow a complete structural determination using the
3
1
SARACEN method. The essential feature of SARACEN is
that information calculated ab initio is introduced into the
refinement procedure as additional observations (or restraints),
While the choice of basis set expressed minimal influence
on the values of geometric parameters, larger structural changes
were observed with changing the level of theory. The most
obvious example is the Ga‚‚‚B distance, the parameter best
(29) (a) Sipachev, V. A. J. Mol. Struct. (THEOCHEM) 1985, 121, 143.
(
b) Sipachev, V. A. In AdVances in Molecular Structure Research;
(
28) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson,
B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G. A.;
Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski,
V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.;
Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen,
W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.;
Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.;
Stewart, J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A. GAUSSIAN
Hargittai, I., Hargittai, M., Eds.; JAI Press: Greenwich, 1999; Vol.
5, p 263.
(30) Hedberg, K.; Iwasaki, M. Acta Crystallogr. 1964, 17, 529.
(31) (a) Blake, A. J.; Brain, P. T.; McNab, H.; Miller, J.; Morrison, C. A.;
Parsons, S.; Rankin, D. W. H.; Robertson, H. E.; Smart, B. A. J. Phys.
Chem. 1996, 100, 12280. (b) Brain, P. T.; Morrison, C. A.; Parsons,
S.; Rankin, D. W. H. J. Chem. Soc., Dalton Trans. 1996, 4589.
(32) Morrison, C. A.; Smart, B. A.; Brain, P. T.; Rankin D. W. H.; Downs,
A. J. J. Chem. Soc., Dalton Trans. 1998, 2155.
94, Revision D.4; Gaussian, Inc.: Pittsburgh, PA, 1995.

Preparation and Properties of Gallaborane, GaBH6
Inorganic Chemistry, Vol. 40, No. 14, 2001 3491
Table 5. Bond Distances (r
a
/Å) and Amplitudes of Vibration (Å)
for Gallaborane Obtained in the GED Refinement
atom pair
distance
amplitude
0.108(5)
0.086(13)
0.047(17)
0.308(22)
0.082(11)
0.094 (tied to 5)
0.158a
1
2
3
4
5
6
7
8
9
1
1
1
1
B(2)‚‚‚Ga(1)
H(5)-Ga(1)
H(3)-Ga(1)
H(7)‚‚‚Ga(1)
H(7)-B(2)
H(3)-B(2)
H(5)‚‚‚B(2)
H(7)‚‚‚H(3)
H(5)‚‚‚H(3)
H(7)‚‚‚H(5)
H(8)‚‚‚H(7)
H(4)‚‚‚H(3)
H(7)‚‚‚H(6)
H(6)‚‚‚H(5)
2.173(4)
1.551(6)
1.798(6)
2.904(9)
1.185(7)
1.280(7)
3.147(11)
1.969(11)
2.732(11)
3.393(8)
2.054(15)
2.077(14)
4.160(13)
2.804(15)
a
0.146
0.182a
a
0
1
2
3
0.334
0.131a
0.133a
a
0.155
0.146a
Figure 3. Observed and final difference radial distribution curves for
14
H
2
Ga(µ-H)
2 2
BH . Before Fourier inversion the data were multiplied by
a
2
Fixed vibrational amplitudes originating in the Shrink force field
derived from the MP2/6-311G(d) set of force constants.
s[exp(-0.000 02s )/(ZGa - fGa)(Z
B
B
- f )].
assigned a restraint, in accordance with the SARACEN method.31
The two remaining parameters, p6 (∠ Ht-B-Ht) and p7 (∠ Ht-
Ga-Ht), were also poorly expressed by the GED data, so they
too were assigned restraints. For the three parameters so
restrained, all values returned in the least-squares analysis were
in very close agreement with the imposed values and the
uncertainties slightly smaller. This demonstrates an important
principle behind the SARACEN method: if a parameter refines
to give a value and standard deviation in exact agreement with
its restraint, then clearly no information regarding this parameter
is contained within the GED data. If, however, the same value
and a lower standard deviation are returned, then some
experimental information must be present, since the same
information is effectively delivered twice (i.e. once from the
restraint and once from the experimental data).
Figure 4. Observed and final difference combined molecular-scattering
curves for H Ga(µ-H) BH
2
2
2
.
the weight of the observation being assigned according to the
level of convergence achieved in a series of graded ab initio
calculations. Using this approach it has been possible to refine
the values of all structural parameters and all significant
amplitudes of vibration simultaneously for the first time.
The radial distribution curve, P(r)/r vs r, derived from the
experiments after scaling, combination, and Fourier transforma-
tion, is depicted in Figure 3. In essence it comprises just four
main peaks centered near r ) 1.25, 1.70, 2.20, and 3.00 Å.
The first two peaks correspond to scattering from the bonded
atom pairs B-H and Ga-H, whereas peaks three and four are
due primarily to scattering from the nonbonded atom pairs Ga‚
Bond distances (r ) and amplitudes of vibration deduced or
fixed in the final refinement of the model are given in Table 5.
Additional tables documenting the refinement (Cartesian coor-
dinates and correlation matrix) are available in the Supporting
R
Information. At 0.180 the final R factor (R ) for the combined
G
data sets was rather higher than usual but is a reflection of the
far from ideal signal-to-noise ratio of the measured scattering.
The molecular parameters evaluated on the basis of the
present analysis are similar in magnitude to those determined
1
2
previously. The Ga‚‚‚B distance does not vary appreciably as
between H Ga(µ-H) BH [2.197(3) Å], HGa[(µ-Η) BH ]
2
2
2
2
2 2
16
20
[2.185(2) Å], and Me Ga(µ-H) BH [2.163(8) Å]. Although
2
2
2
‚
‚B and Ga‚‚‚H, respectively. The best defined feature is r(Ga‚
‚B), which at 2.197 Å (cf. 2.192 Å from theory) is wholly
the Ga-H bond appears at 1.555(6) Å to be longer than in any
t
‚
other gaseous gallium(III) hydride molecule investigated so
5
,7,15,16,33
consistent with a molecule containing a bis(hydrogen-bridged)
far,
the difference is not significant within the limits
Ga(µ-H)2BH2 unit, as in HGa(BH4)2 16 and Me2GaBH4, rather
20
of experimental uncertainty. The average ν(Ga-H ) stretching
t
1
5
-1
than a mono- or tris(hydrogen-bridged) unit (cf. H2GaB3H8
frequency of gallaborane is actually some 80 cm higher than
32
and Me2GaB3H8 ). There being observed no scattering corre-
sponding to atom pairs with separations of more than 3.50 Å,
we have every reason to suppose that gallaborane vapor, at
pressures up to ca. 10 Torr and temperatures of 240-260 K, is
essentially composed of monomeric molecules GaBH6, with no
significant contribution from any higher oligomer.
that of the gallane complex Me N·GaH [for which r (Ga-H )
3
3
R
t
34
-1
has been estimated to be 1.511(13) Å] and only about 20 cm
lower than that of H Ga(µ-Cl) GaH [r (Ga-H ) ) 1.523(20)
2
2
2
R
t
7
Å]. The Ga-H distance [1.800(6) Å] is in keeping with the
b
corresponding parameters in other molecules featuring Ga-
H -B bridges, as attested by the following values (given as ra
b
Defining all four B-H and Ga-H bond distances by
parameter averages and differences (p2 - p5) greatly reduced
the degree of correlation between these refining parameters,
resulting in three of the four parameters refining to values
exactly within the range predicted by theory (see Table 4). The
parameter which failed to refine [p5, ∆r(B-Hb-t)] corresponds
to a feature which is too weakly defined on the radial distribution
curve to be determined by GED data alone. It was therefore
in Å, for comparison): H Ga(µ-H) BH , 1.798(6); HGa[(µ-H) -
2
2
2
2
1
6
20
BH ] , 1.788(7); Me Ga(µ-H) BH , 1.79(3); H Ga(µ-
2
2
2
2
2
2
(
33) Downs, A. J.; Greene, T. M.; Collin, S. E.; Whitehurst, L. A.; Brain,
P. T.; Morrison, C. A.; Pulham, C. R.; Smart, B. A.; Rankin, D. W.
H.; Keys, A.; Barron, A. R. Organometallics 2000, 19, 527.
34) Brain, P. T.; Brown, H. E.; Downs, A. J.; Greene, T. M.; Johnsen, E.;
Parsons, S.; Rankin, D. W. H.; Smart, B. A.; Tang, C. Y. J. Chem.
Soc., Dalton Trans. 1998, 3685.
(

3492 Inorganic Chemistry, Vol. 40, No. 14, 2001
Downs et al.
the reversible collapse of the quartet with a simultaneous
sharpening of the higher frequency resonance. Warming the
sample to temperatures above 243 K resulted not only in the
decay of both resonances but also in the growth of a new 1:1:
1
1
[
:1 quartet [ J(B-H) ) 90 Hz] centered at δH 3.85 and a septet
1
J(B-H) ) 48 Hz] centered at δH -0.85 attributable to
3
6
diborane, as well as a sharp singlet at δH 4.5 attributable to
37
dihydrogen; at room temperature the walls of the sample tube
were observed to be coated with a metallic film, presumed to
be elemental gallium. Of the resonances due to gallaborane,
the broad singlet at δH 5.0 is characteristic of hydrogen atoms
5,7,15,16,38
forming terminal bonds to gallium,
whereas the quartet
at lower frequency is clearly identifiable with the magnetically
equivalent hydrogen atoms of a tetrahydroborate group, by
analogy with the behavior of other tetrahydroborate derivatives.17
Changes in the line shape reflect varying degrees of thermal
decoupling of the protons from the quadrupolar Ga and B
3
9
nuclei to which they are bound.
1
1
The B NMR spectrum of the gallaborane solution at 238 K
revealed not one but two partially resolved 1:4:6:4:1 quintets
1
at δB -33.7 and -24.0, with J(B-H) ) 90 Hz in each case
(Figure 5b). Increasing the temperature or reducing the con-
centration of the solution caused the high-frequency resonance
to grow at the expense of the low-frequency one. Hence it
appears that gallaborane exists in solution in different states of
1
1
aggregation, with the high-frequency B feature probably
arising from the monomer H2GaBH4 and the low-frequency
feature from a dimer, [H2GaBH4]2. The latter may well adopt
the cyclic structure I made up of alternating GaH4 and BH4
units linked to each other through single Ga-H-B bridges; it
would thus resemble the hypothetical molecules B4H12 and
Ga4H12, the structures of which have been calculated ab initio.40
Figure 5. 1H and 11B NMR spectra of a toluene-d
gallaborane at different temperatures.
solution of
8
H)2B3H6, 1.81(4);15 and Me2Ga(µ-H)2B3H6, 1.86(6).31 Rather
shorter distances (also ra in Å) seem to be the rule in
symmetrical Ga-Hb-Ga bridges, where the charge separation
between the Ga and Hb centers is likely to be smaller, e.g. H2-
Ga(µ-H)2GaH2, 1.71(4);5 Me2Ga(µ-H)2GaMe2, 1.708(14).
Clearly the problem of placing Ga-H bond lengths on a reliable
quantitative scale has yet to be resolved. The dimensions of
26
2
the η -H2BH2 moiety do not differ significantly from those in
analogous molecules, cf. ra(B-Ht) (in Å), ra(B-Hb) (in Å), and
∠
Hb-B-Hb (in deg) in the following molecules: H2Ga(µ-
H)2BH2, 1.185(7), 1.280(7), 110.0(5); HGa[(µ-H)2BH2]2, 1.183-
(
1
16
4), 1.269(4), 109.5(5); Me2Ga(µ-H)2BH2, 1.19(2), 1.22(2),
20
35
12(4); and Al[(µ-H)2BH2]3, 1.196(12), 1.283(12), 114.0(2).
A more precise structural specification of gallaborane may yet
be gained from a microwave study, but the presence of Ga and
B nuclei with I ) /2 will cause substantial splitting of lines
Broad-band 11B irradiation has no effect on the H resonance
1
3
at δ 5.0 but causes the quartet at δ 1.05 to collapse not to a
singlet but to a sharp doublet at δH 1.02, with a separation of
.5 Hz, and to a partially overlapping singlet at δH 1.09.
Irradiation at the resonant frequency corresponding to the
signal at δB -33.7, attributed to the dimer, showed that it
correlates with the doublet H resonance, whereas the B signal
at δ -24.0, believed to be due to the monomer, was shown
similarly to correlate with the singlet H resonance. The doublet
H
H
and is likely therefore to hamper both detection and analysis.
On balance, the best prospect for access to accurate estimates
of the relevant rotational constants seems to be offered by the
rovibrational spectrum of the molecule measured under high
8
11
B
1
11
-
1
resolution (<0.01 cm ), and work is proceeding along these
lines.27
B
1
c) Properties of the Liquid Phase: 1H and 11B NMR
(
1
11
pattern of the H{ B} resonance at δH 1.02 may correspond to
an AB-pattern arising from the exo and endo Ht atoms bound
to boron in structure I, but we lack proof on this point.
Spectra. Liquid gallaborane has only a narrow range of stability,
from its melting point near 228 K to its decomposition point
near 238 K. Attempts to measure the Raman spectrum of the
liquid at 233 K were frustrated by decomposition under the
1
(36) Leach, J. B.; Ungermann, C. B.; Onak, T. P. J. Magn. Resonance
action of the exciting radiation. The H NMR spectrum of a
1972, 6, 74.
2
[
H8]toluene solution of gallaborane at 233 K comprised a broad
(
37) Dayan, E.; Widenlocher, G.; Chaigneau, M. C. R. Acad. Sci. Paris
1963, 257, 2455.
38) Greenwood, N. N.; Ross, E. J. F.; Storr, A. J. Chem. Soc. A 1966,
1
1:1:1:1 quartet [ J(B-H) ) 89 Hz] centered at δΗ 1.05 and a
(
broad singlet at δH 5.0 with intensities roughly in the proportions
7
7
06. Beachley, O. T., Jr.; Simmons, R. G. Inorg. Chem. 1980, 19,
2
:1 (see Figure 5a). Lowering the temperature brought about
83.
(
39) Beall, H.; Bushweller, C. H. Chem. ReV. 1973, 73, 465. Marks, T. J.;
(
35) Almenningen, A.; Gundersen, G.; Haaland, A. Acta Chem. Scand.
968, 22, 328.
Shimp, L. A. J. Am. Chem. Soc. 1972, 94, 1542.
1
(40) Shen, M.; Liang, C.; Schaefer, H. F., III Chem. Phys. 1993, 171, 325.

Preparation and Properties of Gallaborane, GaBH6
Inorganic Chemistry, Vol. 40, No. 14, 2001 3493
Table 6. Vibrational Spectra of a Solid Gallaborane Film at 77 K (Wavenumbers in cm-1
)
Raman scattering
GaBH
IR absorption
6
GaBH
6
GaBD
6
ν˜
intensa
ν˜
intensa
ν˜
intensa
ν˜
H
/ν˜
D
assignment
b
1
930
mw
m
sh
m
2532
2450
2381
2193
2056
w
2529
2450
m
1910
820
1.3241
ν
t
asym(B-H )
b
1
m
m
sh
w, br
w, br
w
1798
1767
1.3626
1.3758
ν
t
sym(B-H )
}
}
2
431
2381
270
sh
m, br
m
2
1659
w
s
1.3683
2187
ν(B-H ) +
combinations or
overtone bands
b
1
563
1548
540
w
2089
s, br
vs
s
s
s
1.3495
}
1
2032
2009
1989
s
m
vs
2023
2008
1989
s
s
s
1464
1453
1426
1.3818
1.3820
1.3948
ν
asym(Ga-H
t
)
m
w
ν
sym(Ga-H )
t
b
1
396
1
1
1
1
472
449
387
363
vw
w
1146 + 338
920 + 520
1450
1400
vw
m
sh
s
}
1028
mw
1.3619
ν(Ga-H
679 + 625
ν(Ga-H
b
)
1
311
1239
211
vw
m
1
236
vw
9
57
m
w
1.2800
1.3023
b
)
}
1
m
1
1
146
126
m
w
1146
1111
w
m
m
m
880
t 2
δ[B(H ) ]
1080
838
m
1.2888
}
1059
920
702
690
625
520
500
w, br
vw, br
m
1061
652
510
445
vw
m
1.4110
1.3314
1.3708
F[B(H
δ[Ga(H
F[Ga(H
t 2
) ]
679
s
}
}
t 2
) ]
}
sh
m, br
w
610
517
w
vw
m
t 2
) ]
vw
274 + 230
ν(Ga‚‚‚B)
4
78
462
52
m, sh
m
m, sh
4
67
m
411
vs
1.1241
}
4
c
c
401
338
274
230
181
w, br
vw
w
w
s
skeletal
deformation
modes
}
a
s, strong; m, medium; w, weak; v, very; br, broad; sh, shoulder. b Band probably due to H-containing isotopomer. c Region not explored.
In its behavior in hydrocarbon solution, gallaborane thus
resembles closely the bis(tetrahydroborate), [HGa(BH4)2]n,
which appears also to exist as a mixture of the monomer and
an oligomer of indeterminate nature.
(
IR and Raman spectra of annealed solid films of gallaborane-
h6 and -d6 at 77 K give the frequencies listed in Table 6.
Comparison with the IR spectrum of the vapor reveals signifi-
cant changes of pattern and frequency, implying association of
the discrete gallaborane molecules in the solid phase. Some
indication of the form of association is provided by the following
features.
The spectra differ in a number of respects from those of the
1
6
41
42
solid tetrahydroborates Be(BH4)2, MeZnBH4, and Me2-
4
3
AlBH4, where the metal atoms are linked together through
BH4 groups each acting as bis(bidentate) bridges. Whether the
BH2 and GaH2 units in solid gallaborane are bound through
single hydrogen bridges to form an oligomer such as I or to
give a polymeric array, however, it is impossible to judge on
the evidence of the vibrational spectra alone.
(ii) Crystal Structure. The implications of the spectra are
confirmed by the structure determined by X-ray diffraction of
a single crystal at 110 K, illustrated in Figure 6. The compound
crystallizes under these conditions in the monoclinic space group
I2/a with 12 GaBH6 molecules per unit cell. Details of the data
collection, structure solution, and refinement are presented in
Table 7, and selected bond distances and angles are in Table 8.
A preliminary account of the results has already been given.13
d) Properties of the Solid. (i) Vibrational Spectra. The
(
1) The doublet pattern of bands at 2529 (1910) and 2450
-
1
(
1798) cm implies the retention of terminal BH2 moieties in
the solid.
2) Prominent absorptions near 2000 (1460) and 679 (510)
(
-
1
cm give grounds for believing that terminal GaH2 moieties
also survive the transition from the vapor to the solid state.
(
(
(
41) Marynick, D. S.; Lipscomb, W. N. J. Am. Chem. Soc. 1971, 93, 2322;
Inorg. Chem. 1972, 11, 820. Nibler, J. W.; Shriver, D. F.; Cook, T.
H. J. Chem. Phys. 1971, 54, 5257.
42) Nibler, J. W.; Cook, T. H. J. Chem. Phys. 1973, 58, 1596. Aldridge,
S.; Blake, A. J.; Downs, A. J.; Parsons, S.; Pulham, C. R. J. Chem.
Soc., Dalton Trans. 1996, 853.
(
3) The appearance of IR and Raman bands in the region
-1
2
130-2390 cm attributable to ν(B-Hb) vibrations means that
the frequency separation between the ν(B-Ht) and ν(B-Hb)
fundamentals is significantly reduced in the solid. Such a
situation is consistent with the presence of relatively ionic BH4
43) Oddy, P. R.; Wallbridge, M. G. H. J. Chem. Soc., Dalton Trans. 1976,
8
69. Aldridge, S.; Blake, A. J.; Downs, A. J.; Gould, R. O.; Parsons,
S.; Pulham, C. R. J. Chem. Soc., Dalton Trans. 1997, 1007.
4
1-43
units.

3494 Inorganic Chemistry, Vol. 40, No. 14, 2001
Downs et al.
Table 8. Interatomic Distances (in Å) and Angles (in deg) in
Crystalline Gallaborane at 110 K
Ga(1)-B(2)
Ga(1)-B(1)
Ga(1)-H(1A)
Ga(1)-H(1B)
Ga(1)-H(13)
Ga(1)-H(22)
Ga(2)-B(1)
Ga(2)-B(1)#1
Ga(2)-H(2)
2.463(3) Ga(2)-H(14)
2.499(7) B(1)-H(11)
1.86(7)
1.09(7)
1.06(7)
1.14(7)
1.13(7)
2.463(3)
0.98(7)
1.17(7)
1.40(4)
1.41(4)
2.04(7)
1.87(7)
B(1)-H(12)
B(1)-H(13)
B(1)-H(14)
B(2)-Ga(1)#2
2.457(7) B(2)-H(21)
2.457(7) B(2)-H(22)
1.41(4)
B(2)-Ga(1)-B(1)
B(2)-Ga(1)-H(1A)
B(1)-Ga(1)-H(1A)
B(2)-Ga(1)-H(1B)
B(1)-Ga(1)-H(1B)
125.5(3) H(2)-Ga(2)-H(14) 104(4)
105(3)
99(3)
Ga(2)-B(1)-Ga(1) 160.3(3)
Ga(2)-B(1)-H(11) 75(3)
Ga(1)-B(1)-H(11) 119(4)
Ga(2)-B(1)-H(12) 128(4)
Ga(1)-B(1)-H(12) 63(4)
H(11)-B(1)-H(12) 112(5)
Ga(2)-B(1)-H(13) 113(3)
Ga(1)-B(1)-H(13) 54(3)
H(11)-B(1)-H(13) 98(5)
H(12)-B(1)-H(13) 117(5)
Ga(2)-B(1)-H(14) 46(4)
Ga(1)-B(1)-H(14) 117(4)
H(11)-B(1)-H(14) 120(5)
H(12)-B(1)-H(14) 111(5)
H(13)-B(1)-H(14) 98(5)
Ga(1)-B(2)-H(21) 64(4)
Ga(1)-B(2)-H(22) 47(3)
H(21)-B(2)-H(22) 111(5)
99(3)
100(3)
H(1A)-Ga(1)-H(1B) 133(4)
B(2)-Ga(1)-H(13)
B(1)-Ga(1)-H(13)
99(2)
27(2)
H(1A)-Ga(1)-H(13) 113(4)
H(1B)-Ga(1)-H(13) 103(3)
B(2)-Ga(1)-H(22)
B(1)-Ga(1)-H(22)
27(2)
98(2)
H(1A)-Ga(1)-H(22) 114(3)
H(1B)-Ga(1)-H(22) 106(3)
H(13)-Ga(1)-H(22)
B(1)-Ga(2)-H(2)
B(1)-Ga(2)-B(1)#1
B(1)-Ga(2)-H(14)
72(3)
96(3)
99(3)
26(2)
Figure 6. Structure of crystalline gallaborane at 110 K viewed (a)
perpendicular to, and (b) along the helical axis. Primed atoms are
symmetry-related to nonprimed atoms.
B(1)#1-Ga(2)-H(14) 102(2)
tetrahydroborates, with almost linear M‚‚‚B‚‚‚M fragments,
depend on the angularity of the B‚‚‚M‚‚‚B fragments to provide
the winding of the spirals. (d) To a first approximation, the
polymeric structure affords the Ga atoms in 1 the tetracoordi-
Table 7. Crystal Data and Structure Refinement for Gallaborane at
1
10(2) K
empirical formula
formula weight
wavelength
crystal system
space group
unit cell dimensions
GaBH
86.58
6
1
nated environment they so often favor, whereas hexa- or
0.710 73 Å
monoclinic
I2/a
pentacoordinated metal atoms are otherwise the rule. There are
in fact two distinct types of BH4 unit in the structure of 1. As
shown in Figure 6, the unit based on B(1) forms primary
interactions through H(13) to Ga(1) [2.04(7) Å] and through
H(14) to Ga(2) [1.86(7) Å], and a weaker, secondary contact
through H(11) to Ga(2) [2.41(6) Å], so that it could be said to
function as a semi-tridentate bridge. On the other hand, the BH4
unit based on B(2), which, like Ga(2), lies on a 2-fold axis,
incontrovertibly links the two adjacent Ga centers through single
hydrogen bridges [via H(22) and its symmetry equivalent, with
Ga(1)-H(22) ) 1.87(7) Å]. The only other metal tetrahydrobo-
a ) 10.036(3) Å, R ) 90°
b ) 8.932(3) Å, â ) 100.57(2)°
c ) 11.294(4) Å, γ ) 90°
3
volume
Z
density (calc)
995.2(6) Å
12
3
1.733 g/cm
-
1
absorption coefficient
F(000)
7.970 mm
504
crystal size
0.39 × 0.19 × 0.19 mm
θ range for data collection
2.93-24.99°
index ranges
-14 e h e 0, 0 e k e 12, -15 e l e 15
1015
reflections collected
independent reflections
conventional R [F > 4 σ(F)]
R indices (all data)
largest diff peak and hole
5
864 [R(int) ) 0.0216]
rates known to display a similar mode of coordination are [(η -
R
R
1
) 0.0344 [744 data]
44a
5
44b
C5Me5)Ir]2H3(BH4) and [(η -C5H5)Mo]2(µ-SMe)3(BH4), in
which a metal-metal bond is bridged on one side by the BH4
group.
1
) 0.0472, wR ) 0.0990
2
-3
0.604 and -0.604 e A
Structural comparisons with the gaseous H2Ga(µ-H)2BH2
monomer are complicated by differences of temperature and
technique, X-ray diffraction, unlike electron diffraction, measur-
ing distances between centers of maximum electron density.
There is little doubt, though, that crystallization and the attendant
polymerization have the effects (i) of attenuating the Ga‚‚‚B
distance (from 2.18 to 2.47 Å) and (ii) of accentuating the greater
length of the bridging compared with the terminal Ga-H
distances [∆r(Ga-H) ) r(Ga-Hb) - r(Ga-Ht) increasing from
Hence we find that the crystal structure consists of helical
chains made up of alternating pseudotetrahedral GaH4 and BH4
units; between the chains there are no contacts short enough to
imply significant secondary interactions. 1 thus resembles the
41
42
crystalline tetrahydroborates Be(BH4)2, MeZnBH4, and Me2-
AlBH443 in forming helical chains of metal atoms linked by
BH4 bridges. However, it differs from these compounds in
several respects. (a) The polymer chains repeat along a glide
plane and not around a screw axis. (b) The BH4 groups in 1
function primarily as bis(monodentate) bridges, M(µ-H)BH2-
0
.25 to 0.52 Å]. While the BH4 groups of the solid, with
H-B-H angles spanning the range 98-120°, do not deviate
far from tetrahedral geometry, the GaH4 groups are far from
tetrahedral, with ∠ Ht-Ga-Ht ) 133(3)° and 145(3)° and ∠ Hb-
Ga-Hb ) 72(3)° and 76(3)° at Ga(1) and Ga(2), respectively.
(
µ-H)M (M ) metal atom), and not as bis(bidentate) bridges,
M(µ-H)2B(µ-H)2M, as in the other tetrahydroborate helices,
which feature no terminal B-H bonds.
41-43
(c) The structure
of 1 is based around two molecules in an asymmetric unit with
angular Ga‚‚‚B‚‚‚Ga linkages (∠ Ga‚‚‚B‚‚‚Ga ) ca. 159°), as
well as angular B‚‚‚Ga‚‚‚B linkages (∠ B‚‚‚Ga‚‚‚B ) ca. 127°),
contributing to the winding of the spirals; by contrast, the other
(
44) (a) Gilbert, T. M.; Hollander, F. J.; Bergman, R. G. J. Am. Chem.
Soc. 1985, 107, 3508. (b) Schollhammer, P.; Cabon, N.; P e´ tillon, F.
Y.; Talarmin, J.; Muir, K. W. Chem. Commun. 2000, 2137.

Preparation and Properties of Gallaborane, GaBH6
Inorganic Chemistry, Vol. 40, No. 14, 2001 3495
+
-
Scheme 1. Formation and Some Chemical Properties of
cally”, to give a single salt-like product [H2GaL2] [BH4] (e.g.
Gallaborane
L ) NH3).
(
i) Decomposition. As noted previously, the complete thermal
decomposition of 1 in accordance with eq 3 has afforded a
means of chemical analysis. The decomposition process was
investigated more closely at temperatures ranging from 233 to
2
73 K to discover whether partial decomposition might yield
gallaboranes of higher nuclearity or gallane itself. In this we
were guided by the precedents set by diborane, pyrolysis of
49
which yields initially tetraborane(10), and also by the proper-
ties of 2-galla-arachno-tetraborane(10), H2GaB3H8,15 which is
rather more robust than either gallane or gallaborane. The
decomposition products of gallaborane, as the neat compound
or in hydrocarbon solution, were variously fractionated and
1
analyzed by mass balance and by their H NMR and IR spectra,
but no isolable products other than elemental gallium, dihydro-
gen, and diborane (and always in the proportions required by
eq 3) were found, irrespective of the precise conditions.
(ii) Reactions with Nitrogen and Phosphorus Bases. With
an excess of ammonia at 195 K gallaborane reacts to form a
white solid with the composition GaBH6·2NH3, which is long-
lived at ambient temperatures. The IR spectrum of this solid
provides persuasive evidence that it is most aptly formulated
In keeping with the vibrational spectra of the solid (q.v.), these
properties imply a significant degree of ionic character in the
+
-
sense [H2Ga] [BH4] . The behavior follows the trend set by
the crystal structures not only of various metal hydroborates,
+
-
as [H2Ga(NH3)2] [BH4] . Thus, the features characteristic of
50
coordinated NH3 molecules are clearly present in the form of
e.g. MeZnBH4,42 Me2AlBH4, and Me2InB3H8, but also of
43
45
-1
bands at 3250, 1620, ca. 1300, and 790 cm , whereas bands
other derivatives of the heavier main group elements, e.g. TeCl4,
-1
at 1927, 750, and 640 cm are identifiable with the stretching
+
- 46
which tends toward TeCl3 Cl . It is curious then that the
dimethyl derivative of gallaborane should adopt a more or less
discrete molecular structure Me2Ga(µ-H)2BH2 in the solid state
with r(Ga‚‚‚B) ) 2.269(5) Å and shortest intermolecular Ga‚
+
and bending modes of the GaH2 unit in the [H2Ga(NH3)2]
cation. There are, in addition, prominent bands at 2259 and 1099
-1
cm attributable to the IR-active t2 fundamentals ν3 and ν4,
-
17,18
respectively, of a more or less discrete BH4 anion.
Indeed,
‚
‚H contacts of 2.74(5) and 2.82(5) Å;47 presumably this reflects
the spectrum can be well simulated by a superposition of those
both the partiality of gallium for 4-fold coordination (cf. Me2-
AlBH4 where 6-fold coordination of the Al center prevails) and
the inadequacy of methyl groups as ligands able to bridge Ga
centers.
+
- 51
17,18
due to [H2Ga(NH3)2] Cl
and NaBH4.
There is a distinct
resemblance also to the spectra reported previously for the
+
-
5
52
compounds [H2Ga(NH3)2] [GaH4] ·2NH3 and H2Ge(CH3)2
+
(
isoelectronic with [H2Ga(NH3)2] ). Hence, the reaction evi-
Several factors must determine the way hydride molecules
such as GaBH6 aggregate and the structures they adopt in the
condensed phases, namely the electronic properties and size of
the metal atom, as well as the nature and number of any other
substituents. It is possible that solid gallaborane provides a clue
to the structure of solid gallane, [GaH3]n, which, unlike its
dently proceeds with asymmetric cleavage of the Ga(µ-H)B
linkages of gallaborane, in accordance with eq 4.
+
-
GaBH + 2NH f [H Ga(NH ) ] [BH ]
(4)
6
3
2
3 2
4
Gallaborane also reacts with an excess of trimethylamine at
78 K. Warming the mixture to room temperature and recovery
48
aluminum analogue, features both terminal and bridging Ga-H
1
2,3,5
bonds but has so far eluded definitive characterization.
More
of the unchanged base left a white sublimable solid product
that could be identified by its IR spectrum as an equimolar
crystallographic data are needed, ideally including the results
of at least some neutron diffraction studies, to develop a fuller
picture of the patterns and principles that are at work.
Unfortunately, however, several of the compounds inviting
structural investigation, e.g. HGa(BH4)2, H2GaCl, Me2AlH, and
MeM(BH4)2 (M ) Al or Ga), have withstood so far all attempts
at crystallization.
33,53
mixture of the two known molecular adducts Me3N·GaH3
54
and Me3N·BH3. When the reagents, condensed together on a
CsI window initially at 77 K, were allowed to warm to 193 K
and the excess of trimethylamine evaporated, the IR spectrum
of the resulting solid film implied the formation of an adduct
GaBH6·nNMe3, in which the polymeric framework of the
gallaborane was at least partially retained, but with red shifts
(e) Chemical Properties of Gallaborane. We have inves-
tigated some of the chemical properties of gallaborane, with
the results summarized in Scheme 1. Two aspects merit
discussion: (i) the decomposition of the compound, and (ii)
cleavage of the Ga(µ-H)B units by a nitrogen or phosphorus
base either “symmetrically”, to give the molecular adducts L·
GaH3 and L·BH3 (e.g. L ) Me3N or Me3P), or “unsymmetri-
-1
averaging 55 and 73 cm for the ν(B-Ht) and ν(Ga-Ht)
vibrations, respectively. In this and other respects, the spectrum
resembled quite closely those reported previously for the
(49) See for example: Parry, R. W.; Walter, M. K. Prepr. Inorg. Reactions
1
968, 5, 45.
(
(
50) Schmidt, K. H.; M u¨ ller, A. Coord. Chem. ReV. 1976, 19, 41.
51) Johnsen, E.; Downs, A. J.; Goode, M. J.; Greene, T. M.; Himmel,
H.-J.; M u¨ ller, M.; Parsons, S.; Pulham, C. R. Inorg. Chem., in press.
(
(
(
(
45) Aldridge, S.; Downs, A. J.; Parsons, S. J. Chem. Soc., Chem. Commun.
1996, 2055.
(52) van de Vondel, D. F.; Van der Kelen, G. P. Bull. Soc. Chim. Belges
1965, 74, 467.
(53) Durig, J. R.; Chatterjee, K. K.; Li, Y. S.; Jalilian, M.; Zozulin, A. D.;
Odom, J. D. J. Chem. Phys. 1980, 73, 21.
(54) Odom, J. D.; Barnes, J. A.; Hudgens, B. A.; Durig, J. R. J. Phys.
Chem. 1974, 78, 1503.
46) See for example: Wells, A. F. Structural Inorganic Chemistry, 5th
ed.; Clarendon Press: Oxford, UK, 1984; p 709.
47) Konietzny, S.; Fleischer, H.; Parsons, S.; Pulham, C. R. J. Chem. Soc.,
Dalton Trans. 2001, 304.
48) Turley, J. W.; Rinn, H. W. Inorg. Chem. 1969, 8, 18.

3496 Inorganic Chemistry, Vol. 40, No. 14, 2001
Downs et al.
Table 9. GED Data Analysis Parameters
weighting functions/Å-
sw sw
11.0
18.75
1
camera
distance/mm
∆s
s
min
1
2
s
max
correlation parameter
scale factor, ka
electron wavelength /pm
b
4
2
95.04
01.12
0.25
0.25
2.5
7.0
4.5
9.0
13.0
20.0
0.1348
0.4462
1.36(5)
1.18(10)
0.058 561
0.056 720
a
Figures in parentheses are the estimated standard deviations. b Determined by reference to the scattering pattern of benzene vapor.
complexes Me3N·AlH2(BH4)55 and Me3N·GaH(BH4)2.16 Warm-
ing the window to 243 K resulted in decomposition of the
gallaborane derivative with the formation of gallium metal and
the release of H2 as well as Me3N.
There is a smooth reaction between gallaborane and an excess
of trimethylphosphine at 195 K, also to give a sublimable white
solid. This was shown by its IR spectrum (at 77 K) and by the
solid condensate at 77 K, and/or the H and 11B NMR spectra of a
1
2
[ H
8
]toluene solution at subambient temperature. Such a sample was
stored at 77 K in a sealed, evacuated, all-glass ampule equipped with
a break-seal, until required.
(
c) Spectroscopic Measurements. IR spectra were recorded using
one of two spectrometers, viz. a Mattson Galaxy (4000-400 cm ) or
-
1
-1
a Nicolet 7199 FT-IR instrument (4000-700 cm ), the former having
a DTGS detector and the latter an MCT-A detector. Gaseous samples
were contained either in a Pyrex-bodied cell fitted with CsI windows
and having a path length of 10 cm (pressure ca. 50 Torr, near-ambient
1
11
31
2
H, B, and P NMR spectra of a [ H8]toluene solution (at
43 K) to be an equimolar mixture of the molecular adducts
2
5
6
57
Me3P·GaH3 and Me3P·BH3. It follows, therefore, that the
reactions with both Me3N and Me3P parallel those of diborane
with the same reagents, resulting ultimately in symmetrical
cleavage of the Ga(µ-H)B linkages of gallaborane, as in eq 5.
-
1
temperatures, spectral resolution 2 cm ) or in a multipass gas cell
equipped with aluminum mirrors giving a path length of 3.9 m (pressure
.15 Torr, ambient temperatures, spectral resolution 0.06 cm ). Solid
-1
0
nitrogen matrixes, typically at dilutions estimated to be ca. 1:200, were
prepared by continuous codeposition of the gallaborane vapor (con-
ducted via an appropriately cooled, preconditioned glass inlet) with an
excess of the matrix gas on a CsI window cooled to ca. 20 K by means
of a Displex closed-cycle refrigerator (APD CS 202).57 Raman spectra
were excited at λ ) 514.5 nm with the output of a Spectra-Physics
GaBH + 2L f L·GaH + H B·L (L ) Me N or Me P)
6
3
3
3
3
(5)
Experimental Section
+
(
a) Synthesis of Gallaborane. Gallaborane was prepared by the
Model 165 Ar laser and measured with a Spex Ramalog 5 spectro-
photometer operated with a computerized data-handling center; the
reaction between LiBH
4
(Aldrich, 95% pure, freshly recrystallized from
GaCl] , itself made by the
-1
dry Et
2
O) and monochlorogallane, [H
3
with an excess of Me SiH. This took place at 250
2
n
resolution was 2 cm . Solid films of the gallaborane and other 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.
1H, 11B, and 31P NMR spectra of [ H
rane, its decomposition products, and other derivatives were measured
at temperatures between 190 and 300 K using a Bruker AM300
instrument, the operating frequency being 300, 96.25, and 121.5 MHz
6
reduction of GaCl
3
K (eq 1) under rigorously solvent- and grease-free conditions in all-
glass apparatus; all the exposed sections to which the gallaborane had
2
,5
access were cooled by a stream of cold nitrogen gas. In a typical
experiment, finely powdered LiBH (100 mg, 4.59 mmol) reacted with
monochlorogallane (ca. 200 mg, 1.9 mmol H GaCl) in the space of 1
4
2
8
]toluene solutions of gallabo-
2
h. The volatile products were separated by fractional condensation in
vacuo through a train of traps held at 210, 147, and 77 K. Gallaborane
(
ca. 130 mg, 1.5 mmol GaBH
solid. Minor products included dihydrogen and diborane. Perdeuterated
gallaborane was synthesized similarly from LiBD and [D GaCl]
b) Chemical Analysis and Authentication. A sample of gallabo-
6
) collected in the 147 K trap as a white
1
11
31
for H, B, and P, respectively.
(
d) Electron Diffraction Measurements. Electron diffraction
4
2
n
.
measurements were made with an all-glass inlet system which could
be cooled to minimize decomposition of the vapor en route to the
diffraction chamber. The assembly used with the Edinburgh apparatus
A somewhat different design was
adopted for the inlet system to the Reading apparatus,60 and to alleviate
the problem of fogging of the photographic plates, the cold trap of this
(
rane was allowed to decompose at room temperature with the deposition
of gallium metal on the walls of the ampule; to ensure complete
decomposition, the temperature was held latterly at 343 K for 12 h.
The volatile products, consisting of diborane and dihydrogen, were
withdrawn quantitatively by means of a Toepler pump and separated
by condensation of the diborane in a spiral trap cooled to 77 K. Assay
of the metallic residue by atomic absorption and of the gaseous products
by tensimetric means gave the following results:
5,12,58
has been described previously.
7
apparatus was extended so as to envelope the nozzle. The gallaborane
sample was held at a temperature between 231 and 243 K, while the
nozzle temperature was held between 244 and 258 K. Before any
measurements were made on the gallaborane vapor, the inlet system
was preconditioned by exposing it to a slow stream of the vapor for
ca. 2 min. Various measures were adopted in an attempt to produce
plates with a uniform background, e.g. leaving the exposed plates under
continuous pumping for 24 h before developing them and/or leaving
the washed plates exposed to air for 24 h. Several sets of measurements
were made at both Edinburgh and Reading with varying degrees of
success. Careful analysis to determine which of the results offered the
best signal-to-noise ratio led to the selection of one set of measurements
at a camera distance of ca. 50 cm (made at Reading) and one set of
measurements at a camera distance of ca. 20 cm (made at Edinburgh),
the relevant conditions being specified in Table 9. The precise nozzle-
to-plate distances and electron wavelengths were determined from the
scattering patterns measured for benzene vapor61 immediately prior to
GaBH
taken
6
Ga
residue
H
2
2 6
B H
evolved
evolved
expt 1 found/mmol
proportions
expt 2 found/mmol
proportions
2.34
1.00
1.10
1.00
2.28
0.97
1.10
1.00
3.44
1.47
1.74
1.58
1.32
0.56
0.60
0.55
7
Within the limits of experimental uncertainty, these results confirm
the empirical formula GaBH
6
.
The authenticity of a sample of the compound was checked by
reference to its vaporization properties, the IR spectrum of the annealed
(55) Ruff, J. K. Inorg. Chem. 1963, 2, 515.
(
56) Greenwood, N. N.; Ross, E. J. F.; Storr, A. J. Chem. Soc. 1965, 1400.
Odom, J. D.; Chatterjee, K. K.; Durig, J. R. J. Phys. Chem. 1980, 84,
(58) See for example: Hawkins, M.; Downs, A. J. J. Phys. Chem. 1984,
88, 1527, 3042.
1843.
(
57) Cowley, A. H.; Damasso, M. C. J. Am. Chem. Soc. 1971, 93, 6815.
(59) Pulham, C. R. D. Philos. Thesis, University of Oxford, UK, 1991.
(60) Holwill, C. J. Ph.D. Thesis, University of Reading, UK, 1987.
(61) Tamagawa, K.; Iijima, T.; Kimura, M. J. Mol. Struct. 1976, 30, 243.
Odom, J. D.; Hudgens, B. A.; Durig, J. R. J. Phys. Chem. 1973, 77,
1972.

Preparation and Properties of Gallaborane, GaBH6
Inorganic Chemistry, Vol. 40, No. 14, 2001 3497
each experiment. Tracing of the plates was carried out with the aid
either of a computer-controlled Joyce-Loebl MDM6 microdensitometer
details of the crystal structure investigations may be obtained from the
Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopold-
shafen, Germany on quoting the depository number CSD-406271.
62
63
at Daresbury or of an AGFA Arcus II scanner at Reading. The results
-
1
spanning the ranges 2.5 e s e 13.0 and 7.0 e s e 20.0 Å were
(
g) Chemical Properties. (i) Reaction with NH
3
. Gallaborane (57
(BOC, purified
6
4
processed by the methods described elsewhere using the scattering
factors taken from ref 65.
mg, 66 mmol GaBH ) reacted with an excess of NH
6
3
by condensing on Na and fractionating in vacuo) at 195 K; after 30
min the mixture was allowed to warm to 238 K over a period of 1 h
to ensure completion of the reaction. The material volatile at 195 K
(
e) Theoretical Calculations. Ab initio calculations were carried
2
8
out with the program suite GAUSSIAN 94. How the method, level
of theory, and basis set (number of diffuse and polarization functions,
etc.) influenced the computed structural parameters was assessed by
scrutinizing the results of various calculations. Hartree-Fock (HF),
Møller-Plesset (MP2, MP3, and MP4), and DFT (B3PW91) calcula-
tions were performed with a 6-311G basis set, to which differing
numbers of diffuse and polarization functions were added. In addition,
a set of calculations was undertaken to assess the effects of the Gaussian
frozen-core (FC) approximation, which divides electrons into two
categories, core and valence, with only the valence electrons considered
in the electron-correlation treatment. The approximation is known to
be critical for the group 13 elements Ga, In, and Tl, as the corresponding
(unchanged NH ) was evaporated to leave a white solid stable at room
3
temperature as the sole product. Hence, it was found that 23 mg (1.35
mmol) of NH had been consumed, corresponding to a stoichiometry
3
of NH :GaBH ) 2.05:1. The IR spectrum of the solid showed the
3
6
-
1
-1
following bands in the region 4000-400 cm (wavenumbers in cm
;
s strong, m medium, w weak, v very, br broad, sh shoulder): 3250 vs,
br [ν(N-H)]; 2259 vs, br [ν (t ) BH ]; 1968 sh, 1927 m [ν(Ga-
H )]; 1620 w [δ (NH )]; 1401 m (impurity); 1331/1280 m [δ (NH )];
1186 w, 1099 m [ν (t ) BH ]; 790 m [F(NH )]; 750 sh [δ(GaH )];
640 w [F(GaH )]; 520 w, br, 480 w [ν(Ga-N)].
-
3
2
4
t
as
3
s
3
-
4
2
4
3
2
2
(
ii) Reaction with NMe
NMe (prepared from [Me
tionation in vacuo) for 1 h at 178 K. The mixture was then warmed to
43 K before evaporation of the unchanged base to leave a white solid.
This was identified as a mixture of the known molecular complexes
Me and Me after sublimation and condensation of a
3
. Gallaborane reacted with an excess of
NH]C1 and NaOH and purified by frac-
3
d, 4d, or 5d orbitals are often incorrectly placed in the core region,
3
3
when a close examination of the orbital energies shows these orbitals
to lie closer in energy to the outer valence orbitals than to the inner
core orbitals. Consequently, sets of MP2(FULL) and MP3(FULL)
calculations were implemented with all electrons (core and valence)
included in the electron correlation treatment.
2
3
N‚GaH
3
3
N‚BH
3
portion of the vapor on a CsI window held at 77 K; the IR spectrum
The vibrational force field associated with the optimized structure
of the resulting deposit was entirely consistent with a superposition of
of GaBH
6
at the MP2/6-311G(d) level served as the starting point for
53,54
the spectra previously reported for each of these two adducts.
other product was detected.
No
computing the vibrational parameters of the molecule. Sets of u and k
vibrational corrections based on curvilinear motion were derived using
In a separate experiment, small amounts of gallaborane and NMe
3
29
the program SHRINK, with scaling against the experimental frequen-
cies being carried out to eliminate systematic errors in the ab initio
frequency calculations.
were condensed together on a CsI window cooled to 77 K. The window
was allowed to warm to 193 K before evaporation of any material
volatile at this temperature. The IR spectrum of the solid film remaining
on the window showed the following bands in the region 4000-400
(f) X-ray Crystallography. Hydridogallium bis(tetrahydroborate)
[
HGa(BH , 2] was prepared from GaCl and LiBH and purified by
4
)
2
3
4
-1
-1
cm (wavenumbers in cm ; s strong, m medium, w weak, br broad,
sh shoulder): 2979 m, 2945 w, br [ν(C-H)]; 2448/2410 s [ν(B-H )];
353 m, 2313 m, 2228 m, 2198 w, 2040 m, br [ν(B-H )]; 1939/1912
s [ν(Ga-H )]; 1468 s [δas(CH )]; 1403 w [δ (CH )]; 1253 m [F(CH
as(C-N)]; 1169 sh, 1165 m [F(CH )]; 1114 s, 1100 m [δ(BH
001 s [F(BH )]; 956 w, 858 w [ν(C-N)]; 824 w, 713 s [δ(GaH
13 m [F(GaH
1
6
the procedures described elsewhere. A single crystal of what proved
to be gallaborane, present as a contaminant, was grown by careful
cooling of the sample that was contained in a preconditioned Pyrex
glass capillary mounted on the diffractometer. A stable solid/liquid
phase boundary was established at about 213 K, and crystal growth
effected from this boundary by slow cooling to 190 K at a rate of
t
2
b
t
3
s
3
3
)
+
1
5
ν
3
2
)];
)];
2
2
2
)]; 497 m [ν(Ga-N)]. The solid decomposed at 243 K,
and H
. Gallaborane reacted with an excess of
(purified by fractionation in vacuo) at 195 K. Evaporation of the
-
1
approximately 90 K h . Crystal data, data collection details, metrical
parameters, and solution refinement procedures are collected in Tables
turning gray with the simultaneous release of NMe
iii) Reaction with PMe
PMe
3
2
.
(
3
7
and 8. The data were collected on a Stoe Stadi 4 four-circle
6
6
3
diffractometer, an Oxford Cryosystems low-temperature device
unchanged phosphine left a white sublimable solid. Vacuum sublimation
on to a CsI window at 77 K gave a deposit whose IR spectrum displayed
maintaining a crystal temperature of 110 K. Following corrections for
Lp effects and absorption (ψ scan, Tmin ) 0.457, Tmax ) 0.740), the
structure was solved by Patterson methods for the Ga positions, and
5
6
all the absorptions characteristic of the two known adducts Me
3
P‚GaH
3
57
Confirmation was provided by the H, B, and 31P
solution of the solid product. Thus, the
P‚GaH and Me P‚BH was
P‚GaH 0.725
-39.2 singlet. (b)
0.695 doublet, J(P-H) 58 Hz (PMe ), 1.09
-36.9 doublet of 1:3:
1
11
and Me
NMR spectra of a toluene-d
presence of equimolar quantities of Me
verified by the following features. (a) For Me
doublet (PMe ), 4.13 broad singlet (GaH
); 31P δ
For Me P‚BH
3
P‚BH
3
.
67
the B and H atoms were located in subsequent F maps (SHELXTL ).
2
8
The structure was refined by full-matrix least squares against F . The
3
3
3
3
positions of H atoms bound to B centers were refined freely, and those
of H atoms bound to Ga centers made subject to H-Ga similarity
1 2
restraints. At convergence, R stood at 0.0344 and wR at 0.0990 (for
1
3
3
:
H δ
H
56
3
3
P
1
2
3
3
:
H δ
H
3
all data) for 60 parameters. The final ∆F-synthesis maximum and
1
11
1
3
:1:1:1 quartet, J(B-H) 95 Hz (BH
3
); B δ
B
-
3
minimum were +0.604 and -0.604 e Å , respectively.
3
1
57
:1 quartets (BH
3
); P δ
P
-4.4 1:1:1:1 quartet (PMe ).
3
Even with the relatively low precision of the H-atom parameters,
the H atoms showed up very clearly in the electron density maps. Since
Acknowledgment. We thank (i) the EPSRC for a research
1
1
the presence of boron (at least B) impairs the potential of neutron
diffraction measurements, our results probably represent the best
structural data available for a simple mixed hydride of this type. Further
studentship (to E.J.), an Advanced Fellowship (to T.M.G.), and
other financial support of the Oxford group, and also for
financial support of the Edinburgh Electron Diffraction Service
(
(
Grant GR/K44411), (ii) the Norwegian Research Council
NFR) for a postdoctoral grant (to K.A.), and (iii) the Norwegian
(
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