Matrix reactivity of AlF and AlCl in the presence of HCl and HBr: Generation and characterization of the new Al(III) hydrides HAlFCl, HAlFBr, and HAlClBr and the monomeric mixed Al(III) halides AlX2Y (X, Y = F, Cl, or Br)

Article abstract of DOI:10.1021/ic020275a

The spontaneous and photolytically induced reactions of AlF and AlCl in the presence of HCl and HBr in solid argon matrices were followed and the products identified and characterized by means of IR spectroscopy. Quantum mechanical calculations allow for a further evaluation of the properties of the reaction products. These are the adducts AlF·HCl, AlF·HBr, and AlCl·HBr, representing the products of spontaneous reactions, and the trivalent Al(III) hydrides HAlFCl, HAlFBr, and HAlClBr, which were formed upon photoactivation of these complexes. All three hydrides are planar molecules (C_s symmetry) with bond angles in agreement with the predictions of the VSEPR theory. In addition, the mixed halides AlFCl₂, AlFBr₂, and AlClBr₂ were formed upon photolysis. The bisadducts AlF·(HCl)₂ and AlF·(HBr)₂ are likely to be the precursors to these species.

Full text of DOI:10.1021/ic020275a

Inorg. Chem. 2002, 41, 4952−4960
Matrix Reactivity of AlF and AlCl in the Presence of HCl and HBr:
Generation and Characterization of the New Al(III) Hydrides HAlFCl,
HAlFBr, and HAlClBr and the Monomeric Mixed Al(III) Halides AlX Y
2
(X, Y ) F, Cl, or Br)†
Hans-Jo1rg Himmel,* Jan Bahlo, Michael Haussmann, Fabian Kurth, Gregor Sto1sser, and
Hansgeorg Schno1ckel
Lehrstuhl fu¨r Analytische Chemie, Institut fu¨r Anorganische Chemie, UniVersita¨t Karlsruhe,
Engesserstrasse, Geb. 30-45, 76128 Karlsruhe, Germany
Received April 15, 2002
The spontaneous and photolytically induced reactions of AlF and AlCl in the presence of HCl and HBr in solid
argon matrices were followed and the products identified and characterized by means of IR spectroscopy. Quantum
mechanical calculations allow for a further evaluation of the properties of the reaction products. These are the
adducts AlF‚HCl, AlF‚HBr, and AlCl‚HBr, representing the products of spontaneous reactions, and the trivalent
Al(III) hydrides HAlFCl, HAlFBr, and HAlClBr, which were formed upon photoactivation of these complexes. All
three hydrides are planar molecules (C symmetry) with bond angles in agreement with the predictions of the
s
VSEPR theory. In addition, the mixed halides AlFCl2, AlFBr2, and AlClBr2 were formed upon photolysis. The bisadducts
AlF‚(HCl)2 and AlF‚(HBr)2 are likely to be the precursors to these species.
Introduction
group 13 atoms.5 Although Ga2H6 is stable in the gas phase
and has a sufficiently high vapor pressure for applications
in CVD processes, it is difficult to handle and decomposes
at temperatures higher than -20 °C. The monomers MH3
can be generated and characterized only in inert gas matrices,
e.g. by reaction of the metal atoms with molecular and/or
atomic hydrogen (see eq 1).6,7 Matrix isolation has also
The recent years witnessed the characterization of several
new hydrides of group 13 elements. These compounds are
of considerable interest because of their potential to act as
metal sources in chemical vapor deposition (CVD) pro-
cesses,1 and because of their structural richness.2 Hydrides
show a remarkable mutability, and the H atom can carry
either a formally negative or positive partial charge and the
element-H bonds can be either terminal or bridging. Stable
adducts of MH3 are now known not only for M ) B, Al, or
Ga but also, more recently, for In, e.g. (cyclohexyl)3P‚InH3.3
Without the stabilization by a suitable base, molecular
hydrides of the group 13 elements with the formula MH3
are known only in the form of dimers (MH3)2 for M ) B or
Ga to be accessible via synthetic routes.4 There are also
known species such as H2GaBH4 featuring two different
M + H2
9
hν8 MH2
9
hν8 MH
9
hν8 MH3
(1)
H2
proved to be extremely useful to study new monomer
compounds bearing the formula H2MX and HMX2, where
X represents a halogen atom. Thus H2GaCl8 and H2InCl9
(4) Downs, A. J.; Goode, M. J.; Pulham, C. R. J. Am. Chem. Soc. 1989,
111, 1936-1937. Pulham, C. R.; Downs, A. J.; Goode, M. J.; Rankin,
D. W. H.; Robertson, H. E. J. Am. Chem. Soc. 1991, 113, 5149-
5162.
(5) 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.; Greene, T. M.; Johnson, E.; Brain, P. T.; Morrison, C. A.;
Parsons, S.; Pulham, C. R.; Rankin, D. W. H.; Aarset, K.; Mills, I.
M.; Page, E. M.; Rice, D. A. Inorg. Chem. 2001, 40, 3484.
(6) Kurth, F. A.; Eberlein, R. A.; Schno¨ckel, H.; Downs, A. J.; Pulham,
C. R. J. Chem. Soc., Chem. Commun. 1993, 1302.
(7) Pullumbi, P.; Bouteiller, Y.; Manceron, L.; Mijoule, C. Chem. Phys.
1994, 185, 25.
(8) Ko¨ppe, R.; Schno¨ckel, H. J. Chem. Soc., Dalton Trans. 1992, 3393.
* Author to whom correspondense should be addressed. E-mail:
himmel@achpc9.chemie.uni-karlsruhe.de.
† Dedicated to Professor Ru¨diger Mews on the occasion of his 60th
birthday.
(1) See, for example: Downs, A. J., Ed. Chemistry of Aluminium, Gallium,
Indium and Thallium; Blackie: Glasgow, U.K., 1993.
(2) Aldridge, S.; Downs, A. J. Chem. ReV. 2001, 101, 3305.
(3) Hibbs, D. E.; Jones, C.; Smithies, N. A. J. Chem. Soc., Chem. Commun.
1999, 185.
4952 Inorganic Chemistry, Vol. 41, No. 19, 2002
10.1021/ic020275a CCC: $22.00 © 2002 American Chemical Society
Published on Web 08/23/2002

Matrix ReactiWity of AlF and AlCl
resolution of 0.5 and 1.0 cm-1 was used for measurements with
the MCT and DTGS detector, respectively.
UV photolysis (λmax ) 254 nm) of the matrices was achieved
by the aid of a low-pressure Hg lamp (Graentzel, Karlsruhe)
operating at 200 W, the radiation being transmitted through a quartz
window.
Density Functional Theory (DFT) and ab initio quantum chemical
calculations were performed with the TURBOMOLE program
package,15 applying the BP86 and the MP2 methods in combination
with an SVP type basis set. Normal coordinate analysis relied on
the program ASYM 40.16
are among the species with the formula H2MX which were
generated and characterized in matrix isolation experiments
by reaction of MX with dihydrogen. HAlCl2,10 HGaCl2,11
and HInCl2,9 the products of the photoinduced reactions of
the subvalent MX species with HX (see eq 2), are known
representatives of compounds with the formula HMX2.12
Herein we will report on the spontaneous and photolyti-
cally induced matrix reactions of AlCl with HBr and of AlF
with HCl and HBr. The spontaneous reactions lead to the
adducts AlCl‚HBr, AlF‚HCl, and AlF‚HBr. Further reactions
can be set in train by the action of photolysis. It has been
shown previously that the effect of photolysis is to excite
the subhalide from its singlet electronic ground state into its
triplet excited state.13 These reactions lead to representatives
of the general formula HAlXY, where X and Y are two
different halogen atoms, namely, HAlClBr, HAlFCl, and
HAlFBr. Additionally, AlClBr2, AlFCl2, and AlFBr2 are
formed. The results of this study shed light not only on the
properties of all these compounds, but also on the reaction
pathways leading to them and to the Al(III) trihalides. The
knowledge of these pathways is most likely of relevance for
a better understanding of the mechanism of Al oxidation by
HX (X ) F, Cl, or Br) leading in several steps finally to
AlX3.
Results
AlCl and HBr. Experiments in which AlCl was co-
deposited together with HBr in an excess of argon gave
evidence for a sharp and strong IR band at 454 cm-1
immediately upon deposition due to AlCl monomers. In
addition to this band, the spectrum contained a somewhat
broader absorption at 425 cm-1 and a weaker one at 2420
cm-1 belonging to a first product 1a of the reaction of AlCl
with HBr (see Figures 1 and 2). There followed a period of
UV photolysis of the matrix. In the IR spectrum taken after
this photolysis treatment (see Figure 2), the absorption due
to product 1a was observed to decrease. At the same time,
several new absorptions appeared. One group of bands,
located at 1959.6, 644.2, 552.4/547.7, 462.1, and 404 cm-1,
can be assigned to a second distinct reaction product 2a.
There also were two new bands at 588 and 511 cm-1
belonging to a third product 3a. Finally, two weak features
appeared at 616.1 and 560.1 cm-1 due to a fourth product
4a.
Experimental Section
The experiment was repeated, but now with DBr in place
of HBr. The IR spectrum taken upon co-deposition of AlCl
and DBr in an excess of solid argon gave evidence of one
absorption attributable to the D-version of product 1a. At
425 cm-1, the wavenumber was not significantly affected
by H/D substitution. By analogy with the experiments
conducted with HBr, UV photolysis brought about the
decrease of the absorption due to 1a. At the same time,
new absorptions were seen to grow in. Four of these, at 1425,
560/566, 410, and 400 cm-1, are attributable to the D-version
of product 2a. The wavenumbers of all observed vibrational
modes and their response to photolysis are included in Table
1.
Finally, experiments were conducted with different con-
centrations of HBr in the matrix. The same absorptions were
detected in these experiments, but the relative intensities of
the absorptions belonging to different products 1a-4a varied.
For low concentrations of HBr in the matrix (2%), the
intensities of all absorptions were reduced. However, the
In the matrix experiments argon gas containing up to 5% of HCl
or HBr was sprayed slowly and typically over a period of 1 h at
relatively low pressures (5 × 10-5 mbar) onto a polished copper
block kept at 12 K by means of a closed cycle refrigerator (Leybold
LB 510). The flow of the gas was controlled by a needle valve.
AlF and AlCl were generated by passing CHF3 or HCl, respectively,
over aluminum (Merck, 99.999%) in a Knudsen-type graphite cell,
heated resistively to 900 °C. Hence the AlF or AlCl vapor emitted
from the cell was co-deposited together with HCl or HBr in an
excess of Ar onto the copper block. Other details of the relevant
procedures have been described elsewhere.14
HCl (Messer, 99.995%), HBr (Messer, 99.98%), and CHF3
(Messer, 99.995%) were used as supplied and with the quoted
purities. DCl and DBr were prepared by reaction of D2O with
SiMe3Cl and PBr3, respectively, followed by trap-to-trap distillation.
The purity was checked by gas-phase IR measurements.
Infrared spectra of the matrix samples were recorded on a Bruker
113v FTIR instrument equipped with a liquid N2-cooled MCT
detector, covering the spectral range 4000-400 cm-1, and a DTGS
detector for measurements in the spectral range 700-200 cm-1. A
(9) Himmel, H.-J.; Downs, A. J.; Greene, T. M. J. Am. Chem. Soc. 2000,
122, 922.
(10) Schno¨ckel, H. J. Mol. Struct. 1978, 50, 275.
(11) Ko¨ppe, R.; Tacke, M.; Schno¨ckel, H. Z. Anorg. Allg. Chem. 1991,
605, 35.
(12) HAlBr2 was also sighted in matrix isolation experiments, but generated
via a different route. See: Mu¨ller, J.; Wittig, B. Eur. J. Inorg. Chem.
1998, 1807.
(15) Ahlrichs, R.; Ba¨r, M.; Ha¨ser, M.; Horn, H.; Ko¨lmel, C. Chem. Phys.
Lett. 1989, 162, 165. Eichkorn, K.; Treutler, O.; O¨ hm, H.; Ha¨ser, M.;
Ahlrichs, R. Chem. Phys. Lett. 1995, 240, 283. Eichkorn, K.; Treutler,
O.; O¨ hm, H.; Ha¨ser, M.; Ahlrichs, R. Chem. Phys. Lett. 1995, 242,
652. Eichkorn, K.; Weigend, F.; Treutler, O.; Ahlrichs, R. Theor.
Chem. Acc. 1997, 97, 119. Weigend, F.; Ha¨ser, M. Theor. Chem. Acc.
1997, 97, 331. Weigend, F.; M. Ha¨ser, M.; Patzelt, H.; Ahlrichs, R.
Chem. Phys. Lett. 1998, 294, 143.
(13) Himmel, H.-J. J. Chem. Soc., Dalton Trans. 2002, 2678.
(14) Schno¨ckel, H.; Schunck, S. Chem. Unserer Zeit 1987, 21, 73.
(16) ASYM 40, version 3.0, upgrade of program ASYM 20; Hedberg, L.;
Mills, I. M. J. Mol. Spectrosc. 1993, 160, 117.
Inorganic Chemistry, Vol. 41, No. 19, 2002 4953

Himmel et al.
Table 1. Observed Wavenumbers (cm-1) for a Matrix Containing AlCl
and HBr/DBr in an Excess of Ar before and after UV Photolysis (λmax
) 254 nm)
HBr
2420
DBr
depositiona
photolysisa
absorber
b
v
V
v
1a
2a
1959.6
644.2
616.1
588.3
560.1
552.4/547.7
511.0
462.1
454
1425
566/560
616.1
588.3
560.1
566/560
511.0
b
454
425
410
272
v
v
v
v
v
v
V
V
v
V
4a
3a
4a
2a
3a
2a
AlCl
1a
2a
v
v
425
404
272
v
(AlCl)2
a v ) increase in intensity. V ) decrease in intensity. b Too weak to be
detected, obscured by more intense absorptions, or outside the range of
detection.
Figure 1. IR spectrum taken for an Ar matrix containing AlCl and HBr
(a) before photolysis and (b) following UV photolysis (λmax ) 254 nm).
Figure 3. IR difference spectra taken for an Ar matrix containing AlF
and HCl after UV photolysis (λmax ) 254 nm) minus before photolysis.
Figure 4. IR difference spectra taken for an Ar matrix containing AlF
and HBr after UV photolysis (λmax ) 254 nm) minus before photolysis.
Figure 2. IR difference spectra taken for an Ar matrix containing AlCl
and HBr after UV photolysis (λmax ) 254 nm) minus before photolysis.
bands due to species 2a and 4a gained intensity relative to
the ones due to species 3a. This might imply that 2a and 4a
are the products of reactions with one HBr moiety, while
3a is formed by reaction with two moieties of HBr or, more
likely, with the dimer [HBr]2.
AlF and HCl. The spectra taken upon co-deposition of
AlF together with 3% HCl in an excess of argon are
displayed in Figure 3. They are dominated by sharp absorp-
tions due to HCl, AlF, and (AlF)217 near 2854, 776.3/772.4,
and 431.0 cm-1. In addition, the spectrum contained new
4954 Inorganic Chemistry, Vol. 41, No. 19, 2002

Matrix ReactiWity of AlF and AlCl
Table 2. Observed Wavenumbers (cm-1) for a Matrix Containing AlF
and HCl/DCl in an Excess of Ar before and after UV Photolysis (λmax
) 254 nm)
exposure of the matrix to UV photolysis. All bands assigned
to product 2b were observed to shift toward lower wave-
numbers. The largest effect was observed for the band at
2005 cm-1, which shifted to 1437.9 cm-1, giving a ν(H)/
ν(D) ratio of 1.3944:1. The band at 857.8/852.5 cm-1 in the
experiments with HCl experienced only a small shift to
856.4/850.9 cm-1. The band at 655.1 cm-1 was now observed
at 538.3 cm-1 [ν(H)/ν(D) ) 1.2170:1]. Unfortunately, the
spectra failed to give any sign of the D-counterparts of the
bands at 524.6 and 496.2/490.1 cm-1, which were observed
in the experiments with HCl and belong to species 2a. The
failure to detect these absorptions is caused either by lack
of intensity or by the absorptions being outside the range of
detection of our experiments. The two bands due to species
3a showed no shifts, and this possibly implies that they do
not contain hydrogen. The two weak features at 887.1/884.7
and 831.3 cm-1 were also not affected by substitution of
HCl by DCl.
AlF and HBr. The reaction of AlF with HBr was observed
to follow the same pattern as the reaction with HCl. Thus, a
new absorption red-shifted from that of AlF at 736.2 cm-1
was present in the IR spectrum taken immediately upon co-
deposition and can be assigned to a first product 1c, the Br
analogue of 1b. Following 10 min of UV photolysis (λmax
) 254 nm), this band decreased and gave way to several
new absorptions. These were located near 1990 cm-1 and at
858.1/853.8, 650.5, and 486.8 cm-1, all belonging to a
common absorber 2c. Additional absorptions due to a third
species 3c were observed at 871.1 and 522.2 cm-1. Finally,
the spectrum witnessed the appearance of two weaker
features at 873.6 and 831.1 cm-1 due to a fourth species 4c.
In the experiments with DBr in place of HBr, the IR
spectra again showed an absorption immediately following
deposition due to 1c in its D-version. At 735.9 cm-1, the
wavenumber was only slightly shifted with respect to that
detected in the experiment with HBr. Upon UV photolysis,
the bands due to 1c again decreased and several new bands
were seen to grow in, belonging to the D-equivalents of
products 2c, 3c, and 4c. The bands due to product 2c were
all red-shifted. Thus the band at 1990 cm-1 in the experiment
with HBr appeared at 1455 cm-1 in the experiments with
DBr, giving a ν(H)/ν(D) ratio of 1.3677:1. The band at 650.5
cm-1 was now observed at 488.1 cm-1 [ν(H)/ν(D) ) 1.3327:
1]. The smallest shift was monitored for the absorption at
858.5/853.8 cm-1, which now occurred at 851.9/ 846.3 cm-1.
The D-counterpart of the absorption at 486.8 cm-1 escaped
detection, most likely because of lack of intensity. Species
3c again gave rise to two absorptions, with wavenumbers
similar to those observed in the experiment with HBr (871.1
and 522.2 cm-1). As with 3c, the absorptions due to 4c
exhibited no significant changes and were observed at 873.6
and 831.1 cm-1. The obvious inference is that 3c and 4c,
like 3a,b and 4a,b, do not contain hydrogen.
HCl
2721.4
2005
887.1/884.7
881.7
857.8/852.5
831.3
776.3/772.0
743.1/736.2
714.8
655.1
627.1/624.3
524.6
DCl
1903.9
1437.9
887.1/884.7
882.1
856.4/850.9
832.5
776.3/772.0
733.3
709.7
538.3
627.1/624.3
b
b
depositiona
photolysisa
absorber
v
V
v
v
v
v
v
V
V
V
v
v
v
v
V
1b
2b
4b
3b
2b
4b
AlF
1b
1b
2b
3b
2b
2b
v
v
v
496.2/490.1
430.1
430.1
v
(AlF)2
a v ) increase in intensity. V ) decrease in intensity. b Too weak to be
detected, obscured by more intense absorptions, or outside the range of
detection.
Table 3. Observed Wavenumbers (cm-1) for a Matrix Containing AlF
and HBr/DBr in an Excess of Ar before and after UV Photolysis (λmax
) 254 nm)
HBr
2419.3
1990
873.6
871.1
858.5/853.8
831.1
776.3/772.0
736.2
650.5
DBr
1742.9
1455
874.0
871.1
851.9/846.3
831.1
776.3/772.0
735.9
488.1
depositiona
photolysisa
absorber
v
V
v
v
v
v
v
V
V
v
v
v
V
1c
2c
4c
3c
2c
4c
AlF
1c
2c
v
v
522.2
486.8
430.1
522.2
b
430.1
3c
2c
(AlF)2
v
a v ) increase in intensity. V ) decrease in intensity. b Too weak to be
detected, obscured by more intense absorptions, or outside the range of
detection.
absorptions at 743.1/736.2 and 714.8 cm-1, which can be
assigned to a first, spontaneously formed product 1b of the
reaction of AlF with HCl. Following 10 min of UV
photolysis (λmax ) 254 nm) of the matrix, the IR spectra
witnessed the decrease of the bands associated with product
1b and the simultaneous appearance of several new absorp-
tions. One group of absorptions consisted of not less than
five bands, located at 2005, 857.8/852.5, 655.1, 524.6, and
496.2/490.1 cm-1, all belonging to the same absorber 2b,
representing a second product of the reaction of AlF with
HCl. Two new absorptions at 881.7 and 627.1/624.3 cm-1
can be assigned to a third product 3b. Finally, two broad
and weak bands appeared at 887.1/884.7 and 831.3 cm-1
and were assigned to a fourth species 4b.
An additional experiment was carried out with DCl in
place of HCl. The spectrum taken upon deposition gave again
evidence for absorptions due to a first, spontaneously formed
product 1b of the reaction of AlCl with DCl, now in its
deuterated version. These were located at 733.3 and 709.7
cm-1. In the same way as already reported for the reaction
of AlCl with HBr, additional products can be generated upon
Discussion
On the basis of the IR spectra, comparison with related
species, and the results of quantum mechanical calculations
applying ab initio (MP2) as well as DFT (BP86) methods,
(17) Ahlrichs, R.; Zhengyan, L.; Schno¨ckel, H. Z. Anorg. Allg. Chem. 1984,
519, 155.
Inorganic Chemistry, Vol. 41, No. 19, 2002 4955

Himmel et al.
Table 4. Comparison between the IR Absorptions Observed and
HAlClBr (2a), HAlFCl (2b), and HAlFBr (3c) will be shown
to be the primary products of the photolytically induced
reactions of AlCl with HBr and of AlF with HCl or HBr,
respectively. The experiments give also evidence for the
presence of the complexes AlF‚HCl (1a), AlF‚HBr (1b), and
AlCl‚HBr (1c), formed immediately upon deposition, and
which most likely act as precursors to HAlFCl, HAlFBr, and
HAlClBr, respectively. The photoinduced reactions result in
the additional formation of AlFCl2, AlFBr2, and AlClBr2
(3a-c). One possible route to these species starts from the
bisadducts AlF‚(HCl)2, AlF‚(HBr)2, and AlCl‚(HBr)2, which,
however, escaped detection in our experiments. Finally, 4a-c
will be shown to be the trivalent Al-halides AlX2Y, which
were formed in small quantities by reaction with traces of
HF and HCl.18 The adducts AlF‚HF‚HCl, AlF‚HF‚HBr, and
AlCl‚HCl‚HBr are possible precursors to these species. All
compounds will be discussed in turn, first the adducts AlX‚
HY (X ) F or Cl or Y ) Cl or Br), then the insertion
products HAlXY, and finally the trihalides AlXY2 and
AlYX2.
Calculated for HAlClBr and DAlClBr (2a), HAlFCl and DAlFCl (2b),
and HAlFBr and DAlFBr (2c) [wavenumbers in cm-1, with calculated
IR intensities (in km mol-1) in parentheses]
HAlClBr
DAlClBr
calcda
exptl
1959.6
calcda
exptl
1425
assignment
1959 (111)
651 (243)
552/547 (49)
391 (34)
1423 (73)
570 (196)
451 (43)
379 (23)
133 (5)
ν1(a′)
ν2(a′)
ν3(a′)
ν4(a′)
ν5(a′)
ν6(a′′)
644.2
552.4/547.7
404
560/566
b
400
b
b
134 (5)
462.1
438 (91)
410
334 (59)
HAlFCl
DAlFCl
exptl
calcda
exptl
calcda
assignment
2005
857.8
655.1
524.6
b
1973.4 (93)
859.9 (129)
665.3 (123)
524.9 (52)
222.6 (19)
478.8 (137)
1437.9
856.4
538.3
b
b
b
1427.3 (63)
856.8 (113)
541.6 (116)
465.3 (20)
221.3 (19)
376.4 (95)
ν1(a′)
ν2(a′)
ν3(a′)
ν4(a′)
ν5(a′)
ν6(a′′)
496.2
HAlFBr
DAlFBr
exptl
calcda
exptl
calcda
assignment
AlCl‚HBr, AlF‚HCl, and AlF‚HBr, 1a-c. The absorp-
tions due to 1a-c are close to the absorptions of AlCl (in
the case of 1a) and AlF (in the case of 1b and 1c), but red-
shifted by 29.6, 33.2/28.9, and 40.1/35.8 cm-1. We anticipate
the species responsible for these absorptions to be the adducts
AlCl‚HBr, AlF‚HCl, and AlF‚HBr, respectively, formed
spontaneously upon deposition. Support for such an assign-
ment comes from comparison with related species, namely
the complexes AlCl‚HCl,10 GaCl‚HCl,11 and InCl‚HCl,9
which all were found to be formed immediately upon co-
deposition of the M(I) halides (M ) Al, Ga, or In,
respectively) with HCl in solid argon matrices. A possible
structure for such an adduct was recently suggested by Tacke,
Dunne, El-Gamati, Fox, Rous, and Cuffe on the basis of
quantum chemical (DFT) calculations.19
HAlClBr, HAlFCl, and HAlFBr, 2a-c. Product 2a of
the photolytically induced reaction of AlCl with HBr is
characterized by IR absorptions at 1959.6, 644.2, 552.4/
547.7, 462.1, and 404 cm-1. Of these, the absorption at
1959.6 cm-1 occurs in a region characteristic of the ν(Al-
H) stretching vibration in Al(III) compounds (cf. HAlCl2
1967.6 cm-1,10 H2AlNH2 1899.3/1891.0 cm-1,20 H2AlPH
1874.1/1866.1 cm-1 21). The absorptions at 552.4/547.7 and
404 cm-1 appear in regions where ν(Al-Cl) and ν(Al-Br)
stretching modes are expected to show (cf. HAlCl2 νas 578.9,
νs 481.310 and HAlBr2 νas 476.0, νs 366.022). We thus
anticipate 2a to be the Al(III) hydride HAlClBr. Our
calculations using DFT methods (BP86) resulted in a planar
geometry with Cs symmetry for this molecule, which is
characterized by Al-H, Al-Cl, and Al-Br distances of
1990
858.5
650.5
b
1964.6 (105)
851.5 (121)
654.3 (141)
415.4 (51)
203.0 (11)
468.5 (120)
1455
851.9
488.1
b
b
b
1420.5 (68)
848.9 (109)
493.6 (124)
394.3 (20)
200.6 (11)
365.8 (82)
ν1(a′)
ν2(a′)
ν3(a′)
ν4(a′)
ν5(a′)
ν6(a′′)
b
486.8
a Cs symmetry. b Too weak to be detected, obscured by more intense
absorptions, or outside the range of detection.
1.5844, 2.1091, and 2.2610 Å, respectively, and H-Al-Cl,
H-Al-Br, and Cl-Al-Br bond angles of 120.1°, 121.1°,
and 118.9°. The trend of the bond angles (Br-Al-Cl <
H-Al-Cl < H-Al-Br) is thus in full agreement with what
is anticipated on the basis of simple VSEPR theory.23 In
Table 4 the wavenumbers calculated for the molecule in its
optimum geometry and the two relevant isotopic forms are
compared with the experimentally observed values. The
comparison lends support to our assignment. The modes with
high ν(Al-H), ν(Al-Cl), and ν(Al-Br) stretching mode
character [ν1(a′), ν3(a′), and ν4(a′)] were calculated to have
wavenumbers of 1959, 552, and 438 cm-1, respectively, in
excellent agreement with the experimental values.
In its H-version, product 2b gave rise to five absorptions
located at 2005, 857.8, 655.1, 524.6, and 496.2/490.1 cm-1.
The broad absorption near 2005 cm-1 again occurs in a
region characteristic of the stretching vibration of terminal
Al(III)-H bonds. We thus anticipate 2a to be the Al(III)
hydride HAlFCl, formed by the photolytically induced
reaction of AlF with HCl. Calculations with DFT methods
(BP86) again predict a planar geometry with Cs symmetry
for such a molecule. The Al-H, Al-F, and Al-Cl distances
were calculated to be 1.5748, 1.6724, and 2.1022 Å, and
the H-Al-F, H-Al-Cl, and F-Al-Cl angles 119.9°,
122.6°, and 117.5°, respectively. Again, the bond angles are
in full agreement with the predictions of the VSEPR theory.
The results of the frequency calculations are included in
(18) These traces are caused by incomplete reaction of liquid Al metal
with an excess of HF or HCl, which passed the Knudsen cell (see
Experimental Section).
(19) Tacke, M.; Dunne, J. P.; El-Gamati; Fox, S.; Rous, A.; Cuffe, L. P. J.
Mol. Struct. 1999, 447, 221.
(20) Himmel, H.-J.; Downs, A. J.; Greene, T. M. J. Am. Chem. Soc. 2000,
122, 9793.
(21) Himmel, H.-J.; Downs, A. J.; Greene, T. M. Inorg. Chem. 2001, 40,
396.
(22) Mu¨ller, J.; Wittig, B. Eur. J. Inorg. Chem. 1998, 1807.
(23) Gillespie, R. J.; Hargittai, I. The VSEPR Model of Molecular Geometry;
Allyn and Bacon: Boston, MA, 1991. Gillespie, R. J. Chem. Soc.
ReV. 1992, 21, 59.
4956 Inorganic Chemistry, Vol. 41, No. 19, 2002

Matrix ReactiWity of AlF and AlCl
Table 4, which compares the theoretical with the experi-
mental values for the two isotopomers HAlFCl and DAlFCl.
Our experiments hit on not less than five of the six vibrational
fundamentals of HAlFCl and four of the fundamentals of
DAlFCl. According to our calculations, the ν(Al-H) stretch-
ing vibration should occur at 1973.4 cm-1, in satisfactory
agreement with the observed wavenumber (2005 cm-1). The
ν(H)/ν(D) ratios calculated and observed (1.3826:1 and
1.3686:1, respectively) give support to this assignment. Next
comes the mode with a high degree of ν(Al-F) character,
located at 857.8 cm-1 in our experiments. Again the
calculated wavenumber (859.9 cm-1) is in excellent agree-
ment with the experimental one and compares with a value
of 892.4 cm-1 for the similar mode of FAl(O2)2.24 The band
at 655.1 cm-1 [ν(H)/ν(D) ) 1.2170:1] in our experiments
can then be assigned to a motion with a high contribution
from the in-plane Al-H rocking fundamental, ν3(a′). At
665.3 cm-1 [ν(H)/ν(D) ) 1.2284:1] the calculated value is
close to the experimental one. The observed wavenumber is
also close to the wavenumber of the similar mode in HAlCl2
(654.5 cm-1). The ν4(a′) mode, which can roughly be
described as an in-plane deformation mode of HAlClF, is
observed at 524.6 cm-1 and calculated to occur at 524.9
cm-1. Additionally, the spectra gave evidence for an absorp-
tion at 496.2 cm-1 attributable to a mode with a high
contribution from the out-of-plane Al-H bending mode,
ν6(a′′), of HAlClF. The calculations predict a value of 478.8
cm-1 for this mode. The only vibration of HAlFCl that
escaped detection was the ν5(a′) mode, which is calculated
to appear at 222.6 cm-1, i.e., near the limit of detection in
our experiments, and to carry very little intensity.
[ν(H)/ν(D) ) 1.3650:1] according to the calculations can
again be assigned to the ν(Al-H) stretching fundamental,
ν1(a′), which was observed near 1990 cm-1 [ν(H)/ν(D) )
1.3650:1] in the experiments. Next comes the mode at 851.5
cm-1 [ν(H)/ν(D) ) 1.0031:1] according to the calculations
and 858.1 cm-1 [ν(H)/ν(D) ) 1.0077:1] according to the
experiments, exhibiting a high contribution from the ν(Al-
F) stretching fundamental, ν2(a′). The absorption at 654.3
cm-1 [ν(H)/ν(D) ) 1.3256:1] in the calculations can again
be assigned to a mode with in-plane Al-H rocking funda-
mental character, ν3(a′), and the experimental wavenumber
(650.5 cm-1) and ν(H)/ν(D) ratio (1.3327:1) are in excellent
agreement with the calculated values. The experiments
succeeded also in detecting an absorption at 486.8 cm-1,
which is a strong candidate for the out-of-plane Al-H
bending mode, ν6(a′′), calculated to appear at 468.5 cm-1.
Normal Coordinate Analysis. On the basis of the
experimentally observed wavenumbers, a normal coordinate
analysis was carried out for each of the three hydrides
HAlClBr, HAlFCl, and HAlFBr. The following six (not
normalized) symmetry coordinates were used in this analy-
sis: S1 ) r(Al-H), S2 ) r(Al-F), S3 ) δ(H-Al-Y) -
δ(H-Al-F), S4 ) r(Al-Y), S5 ) δ(X-Al-F), S6 ) γ(H-
Al-F-Y). For HAlClBr, a force constant f(Al-H) of 222
N m-1 was derived. This value is in good agreement with
that derived previously for HAlCl2 (219 N m-1).10 The force
constant f(Al-Cl) of HAlClBr was determined to be 266 N
m-1 and compares with values of 277, 274, and 292 N m-1
derived for HAlCl2,10 AlCl3,25 and ClAlO.26 Finally, the
normal coordinate analysis yielded a value of 223 N m-1
for the force constant f(Al-Br). Again, the agreement with
the force constants in other Al(III) compounds featuring
terminal Al-Br bonds is pleasing (cf. AlBr3 223 N m-1 27
and BrAlO2 255 N m-1 28). For HAlFCl, force constants
f(Al-H), f(Al-F), and f(Al-Cl) of 204, 476, and 327 N
m-1 were derived. The value for f(Al-F) is in good
agreement with those derived for f(Al-F) of FAlO (501 N
m-1),26 AlF3 (487 N m-1),29 and FAl(O2)2 (493 N m-1).24
The increase of the force constant f(Al-Cl) with respect to
that of HAlClBr reflects the higher electronegativity of the
fluorine ligand. Finally, HAlFBr is characterized by force
constants f(Al-H), f(Al-F), and f(Al-Br) of 230, 423, and
248 N m-1, respectively. The force constant f(Al-Br) is
larger for HAlFBr than for HAlClBr, showing again the
effect of the fluorine ligand.
The circumstances of its formation and the wavenumbers
of its vibrational modes suggest that 2c is the Br equivalent
AlClBr2, AlFCl2, and AlFBr2, 3a-c. The spectra taken
upon photolysis contained additional features at 588 and 511
cm-1 for AlCl and HBr, at 881.7 and 627.1 cm-1 for AlF
and HCl, and at 871.1 and 522.2 cm-1 for AlF and HBr,
which belong to a third reaction product 3a-c. These
absorptions exhibited no shift when HCl was replaced by
DCl. Thus the product responsible for these absorptions is
not likely to contain hydrogen. There is also no sign of any
of 2b, namely the Al(III) species HAlFBr. DFT calculations
resulted in a planar energy minimum structure with Al-H,
Al-F, and Al-Br bond lengths of 1.5762, 1.6741, and
2.2551 Å and H-Al-F, H-Al-Br, and F-Al-Br angles
of 119.3°, 122.7°, and 118.0°. Again the observed IR
spectrum shows characteristic vibrational modes which lend
support to our assignment. Table 4 compares the calculated
and experimental wavenumbers for this species. The general
level of agreement is pleasing. The mode at 1964.6 cm-1
(25) Schno¨ckel, H. Z. Anorg. Allg. Chem. 1976, 424, 203.
(26) Schno¨ckel, H. J. Mol. Struct. 1978, 50, 267.
(27) Beattie, I. R.; Horder, J. R. J. Chem. Soc. A 1969, 2655.
(28) Bahlo, J.; Himmel, H.-J.; Schno¨ckel, H. Inorg. Chem. 2002, 41, 2678.
(29) McCory, L. D.; Paul, R. C.; Margrave, J. L. J. Phys. Chem. 1963, 67,
1986. Snelson, A. J. Phys. Chem. 1967, 71, 3201.
(24) Bahlo, J.; Himmel, H.-J.; Schno¨ckel, H. Angew. Chem. 2001, 113,
4820; Angew. Chem., Int. Ed. 2001, 40, 4696.
Inorganic Chemistry, Vol. 41, No. 19, 2002 4957

Himmel et al.
Table 5. Comparison between the IR Absorptions Observed and
Calculated for AlClBr2 (3a), AlFCl2 (3b), and AlFBr2 (3c)
[wavenumbers in cm-1, with calculated IR Intensities (in km mol-1) in
parentheses]
Table 6. Comparison between the IR Absorptions Observed and
Calculated for AlCl2Br (4a), AlF2Cl (4b), and AlF2Br (4c)
[wavenumbers in cm-1, with calculated IR intensities (in km mol-1) in
parentheses]
AlClBr2 (3a)
exptl
calcda
511 498 (145) 881.7 880.5 (107) 871.1 865.0 (101)
AlFCl2 (3b)
AlFBr2 (3c)
AlCl2Br (4a)
exptl
calcda
560.1 556 (148) 831.3 826.4 (144) 831.1 808.2 (143)
AlF2Cl (4b)
AlF2Br (4c)
exptl
calcda
exptl
calcda
assignment
exptl
calcda
exptl
calcda
assignment
ν1(a1)
ν2(a1)
ν3(a1)
ν4(b1)
ν5(b1)
ν6(b2)
ν1(a1)
ν2(a1)
ν3(a1)
ν4(b1)
ν5(b1)
ν6(b2)
b
b
265 (2)
98 (2)
b
b
426.1 (7)
147.8 (6)
b
b
300.4 (6)
102.5 (2)
b
b
314 (6)
131 (5)
b
b
496.4 (34)
221.9 (29)
b
b
396.9 (52)
202.5 (21)
588 583 (132) 627.1 629.8 (181) 522.2 517.2 (184)
616.1 610 (137) 887.1 945.6 (110) 873.6 940.3 (102)
b
b
116 (4)
176 (20)
b
b
201.9 (20)
226.5 (54)
b
b
210.1 (37)
173.6 (14)
b
b
187 (26)
119 (4)
c
b
193.7 (13)
259.6 (83)
c
b
172.2 (8)
260.3 (71)
b
a C2V symmetry. b Too weak to be detected, obscured by more intense
a C2V symmetry. Too weak to be detected, obscured by more intense
absorptions, or outside the range of detection.
absorptions, or outside the range of detection.
observed at 588 and 511 cm-1. The mode at 588 cm-1 can
be assigned to the antisymmetric Al-Br stretching mode,
ν4(b1), which is calculated to appear at 581 cm-1. The
absorption at 511 cm-1 belongs to the Al-Cl stretching
fundamental, ν1(a1); the calculations predict a wavenumber
of 507 cm-1 for this mode. The absorptions due to AlFCl2
(3b) at 881.7 and 627.1 cm-1 can then be assigned to the
symmetric Al-F stretching fundamental, ν1(a1), and the
antisymmetric Al-Cl stretching mode, ν4(b1), for which the
calculations yielded wavenumbers of 880.5 and 629.8 cm-1,
respectively. These two modes carry most of the IR intensity.
Finally, bands at 871.1 and 522.2 cm-1 were assigned to
product 3c. Of these, the band at 871.1 cm-1 can again be
assigned to the ν(Al-F) stretching mode ν1(a1), which is,
as anticipated, only slightly affected by replacement of the
two Cl atoms by Br atoms. The experimental value is in
good agreement with the calculated one (865.0 cm-1). The
band that carries most of the intensity can be attributed to
the antisymmetric Al-Br stretching mode, ν4(b1). Our
calculations resulted in a wavenumber of 517.2 cm-1 for this
mode.
AlCl2Br, AlF2Cl, and AlF2Br, 4a-c. The IR spectra gave
evidence for a fourth reaction product. The absorptions at
616.1 and 560.1 cm-1 due to species 4a occur in a region
characteristic of ν(Al-Cl) stretching modes. The absorptions
at 887.1 and 831.3 cm-1 (4b) and at 873.6 and 831.1 cm-1
(4c) most likely belong to ν(Al-F) stretching fundamentals.
The detection of two Al-Cl modes of 4a and two Al-F
modes of 4a and 4b (asymmetric and symmetric stretching
fundamentals) indicates the presence of two Cl atoms in 4a
and two F atoms in 4b and 4c. Therefore 4a-c are likely to
be the Al(III) halides AlCl2Br, AlF2Cl, and AlF2Br.
Calculations were again performed to simulate the IR
properties. Table 6 includes a comparison between the
wavenumbers observed and calculated for these species. In
pleasing agreement with the experimental results, the cal-
culations predict the symmetric and antisymmetric Al-Cl
(AlCl2Br) and Al-F (AlF2Cl and AlF2Br) stretching modes
to give rise to the most intense absorptions in the IR
spectrum. With wavenumbers of 610 and 556 (AlCl2Br),
945.6 and 826.4 (AlF2Cl), and 940.3 and 808.2 cm-1, the
calculated wavenumbers are in fair agreement with the
observed ones.
ν(Al-H) stretching vibrations associated with 3a-c. The
absorptions are, however, close to two of the absorptions
due to HAlClBr, HAlFCl, and HAlFBr, which may indicate
some structural similarities. The bands at 881.7 (3b) and
871.1 (3c) cm-1 occur in a region where ν(Al-F) stretching
fundamentals of Al(III) species are expected to show. The
absorptions at 588 (3a) and 627.1 (3b) cm-1 are strong
candidates for ν(Al-Cl) stretching fundamentals. Finally,
the absorptions at 511 (3a) and 522.2 (3c) cm-1 occur in a
region characteristic of ν(Al-Br) stretching modes. The
obvious inference is that 3a-c are the mixed Al(III) halides
AlClBr2, AlFCl2, and AlFBr2. Of these, only AlClBr2 has to
our knowledge been studied previously in matrix isolation
experiments,30 and the reported vibrational properties tally
nicely with those observed here.
DFT calculations were employed to characterize further
these species. Like HAlClBr, HAlFCl, or HAlFBr, the
molecules exhibit a planar global energy minimum structure,
this time resulting in C2V symmetry. Because of the increased
electronegativities of its ligands, the Al-F distances of
AlFCl2 and AlFBr2 are slightly shorter than those of HAlFCl
and HAlFBr, respectively. For the same reason, the Al-Cl
distance in AlFCl2 is shorter than those in HAlFCl, HAlClBr,
or AlClBr2 (see below and Table 10 for a more detailed
evaluation of the structures).
Table 5 compares the experimental IR properties with
those calculated for the molecules in their global energy
minimum structure. Two of the six vibrational modes of the
molecules were detected. For AlClBr2 (3a), absorptions were
Comparison between the Structures of the Character-
ized Al(III) Species. Table 7 compares the dimensions for
(30) Pong, R. G. S.; Shirk, A. E.; Shirk, J. S. J. Chem. Phys. 1979, 70,
525.
4958 Inorganic Chemistry, Vol. 41, No. 19, 2002

Matrix ReactiWity of AlF and AlCl
Table 7. Calculated Distances (in Å) and Bond Angles (in deg) of the Al(III) Species Studied in This Work
HAlClBr
1.5844
HAlFCl
HAlFBr
AlClBr2
AlFCl2
AlFBr2
AlCl2Br
AlF2Cl
AlF2Br
Al-H
1.5748
1.6724
2.1022
1.5762
1.6741
Al-F
1.6651
2.0890
1.6680
2.2437
1.6608
2.0827
1.6621
2.2333
Al-Cl
Al-Br
2.1091
2.2610
2.1000
2.2495
2.0974
2.2464
2.2551
119.3
H-Al-F
H-Al-Cl
H-Al-Br
F-Al-F
F-Al-Cl
F-Al-Br
Cl-Al-Cl
Cl-Al-Br
Br-Al-Br
119.9
122.6
120.1
121.1
122.7
118.0
118.1
120.7
118.6
120.9
117.5
119.1
121.8
118.8
122.5
119.2
120.4
118.9
119.7
120.6
all Al(III) species studied in this work. From this comparison
it can be seen how the different electronegativities of the
ligands influence the bond distances and the bond angles.
The bond lengths are short if the overall electronegativity
of the ligands is high. Thus, e.g., the Al-F bond length is
shortest in AlF2Cl, which exhibits the highest overall
electronegativity of its ligands and increases in the order
AlF2Cl < AlF2Br < AlFCl2 < AlFBr2 < HAlFCl < HAlFBr.
The force constants f(Al-F) follow the opposite trend,
decreasing in the row AlF2Cl > AlF2Br > AlFCl2 > AlFBr2
> HAlFCl > HAlFBr. In HAlFCl, for example, f(Al-F)
amounts to 501 N m-1, but in HAlFBr it is reduced to 423
N m-1. The Al-H, Al-Cl, and Al-Br bond lengths follow
the same pattern. Additionally, the bond angles are small if
ligands with high electronegativities are involved, in agree-
ment with the predictions made by the VSEPR model.23 Thus
in HAlFCl, for example, the bond angles follow the order
F-Al-Cl < H-Al-F < H-Al-Cl, reflecting the increas-
ing electronegativity in the direction H < Cl < F. The largest
bond angle is adopted for H-Al-Br in HAlFBr (122.7°),
which features the combination of atoms with the smallest
total electronegativity.
Reaction Pathways. AlCl and AlF have each been shown
to form an adduct of the form AlX‚HY (X ) F or Cl, Y )
Cl or Br) upon co-deposition together with HCl or HBr in a
solid Ar matrix. These adducts are photolabile and undergo
conversion to the Al(III) hydrides HAlXY upon UV pho-
tolysis (λ ) ca. 254 nm). The overall photoinduced reactions
of AlF with HCl or HBr to give the Al(III) species HAlFCl
and HAlFBr, respectively, and of AlCl with HBr to give
HAlClBr (see eq 3) proceed exothermally, the calculated
reaction energies being -204.7, -197.4, and -214 kJ mol-1,
respectively.
activation (see eq 4). Experiments with Al atoms and NH3
molecules gave evidence for the Al(II) bisamide Al(NH2)2,
which is, in the same way, likely to be formed by photo-
induced hydrogen elimination from the bisadduct Al(NH3)2.31
The reactions yielding AlClBr2, AlFCl2, and AlFBr2 and
dihydrogen were calculated to be exothermic by -347,
-330.8, and -317.6 kJ mol-1.
Under the conditions of the experiments, there also
appeared weak IR absorptions which can be assigned to the
mixed halides AlCl2Br, AlF2Cl, and AlF2Br (4a-c). AlCl2-
Br is most likely formed as a product of the reaction of AlCl
with both HBr and HCl, the latter being present in traces
due to incomplete reaction with liquid Al in the Knudsen
cell. In the same way, AlF2Cl and AlF2Br are products of
the reactions of AlF and HF with HCl and HBr, respectively.
No absorption attributable to a photoproduct of (AlF)2
appears in the IR spectra. Most likely, photolysis achieves
decomposition of (AlF)2 to give two AlF molecules, which
subsequently react with HCl or HBr in the ways described
herein.
Conclusions
In this work the reactions of AlCl with HBr and of AlF
with HCl and HBr were studied experimentally in a solid
Ar matrix by IR spectroscopy, including the effects of
isotopic substitution, and theoretically by quantum chemical
calculations. Twelve species were characterized, most of
them for the first time. These were the adducts AlCl‚HBr,
AlF‚HCl, and AlF‚HBr and the monomeric Al(III) species
HAlClBr, HAlFCl, HAlFBr, AlClBr2, AlFCl2, AlFBr2, AlCl2-
Br, AlF2Cl, and AlF2Br. The adducts AlCl‚HBr, AlF‚HCl,
and AlF‚HBr were spontaneously formed upon co-deposition
of the reactants. They can be converted by UV photolysis
into the new Al(III) hydrides HAlClBr, HAlFCl, and
HAlFBr, respectively. In addition, the experiments gave
evidence for the presence of AlClBr2 as the product of the
In addition to HAlClBr, HAlFCl, and HAlFBr, the
experiments gave evidence for the photoinduced formation
of AlClBr2, AlFCl2, and AlFBr2 (3a-c). These species are
most likely the products of the reaction of AlCl and AlF
with two molecules of HBr or HCl. A possible route to these
species starts with the bisadducts AlCl(HBr)2, AlF(HCl)2,
and AlCl(HBr)2, which eliminate dihydrogen upon photo-
(31) Gaertner, B.; Himmel, H.-J. Inorg. Chem. 2002, 41, 2496. Kasai, P.;
Himmel, H.-J. J. Phys. Chem. A 2002, 106, 6765.
Inorganic Chemistry, Vol. 41, No. 19, 2002 4959

Himmel et al.
photoinduced reaction of AlCl with two molecules of HBr,
and of AlFCl2 and AlFBr2 as products of the reaction of AlF
with two molecules of HCl and HBr, respectively. The trends
in the calculated bond lengths and angles were established
for all the species addressed in this work. In addition to the
characterization of several new monomeric aluminum com-
pounds, this work should shed light on fundamental processes
such as the oxidation of aluminum metal by hydrogen
halides, e.g. HCl, leading in several steps to AlCl3.
Acknowledgment. The authors thank the Deutsche For-
schungsgemeinschaft for financial support and the award of
a Habilitationsstipendium to H.-J. H.
IC020275A
4960 Inorganic Chemistry, Vol. 41, No. 19, 2002