New Coordination Compounds of Cd(AsF6)2 with HF and XeF2

Article abstract of DOI:10.1021/ic035166q

Two new coordination compounds of cadmium with HF and XeF₂ as ligands have been synthesized. Solid white [Cd(HF)](AsF₆) ₂ is obtained from an anhydrous HF (aHF) solution of Cd(AsF ₆)₂. It crystallizes in a monoclinic P2₁/c space group with a = 9.4687(14) A, b = 9.2724(11) A, c = 10.5503(18) A, β = 104.887(7)°, and Z = 4. The coordination sphere of Cd consists of 7 + 2 fluorine atoms, which are in a capped trigonal-prismatic arrangement. The reaction between Cd(AsF₆)₂ and XeF ₂ in aHF yields a solid white product at room temperature having the composition [Cd(XeF₂)₄](AsF₆)₂ after the excess XeF₂ and solvent have been removed under dynamic vacuum. [Cd(XeF₂)₄](AsF₆)₂ crystallizes in the orthorhombic space group P2₁2₁2 ₁, with a = 8.6482(6) A, b = 13.5555(11) A, c = 16.6312(14) A, and Z = 4. The coordination sphere of Cd consists of eight fluorine atoms, which are at the corners of a trigonal prism with two capped side faces.

Full text of DOI:10.1021/ic035166q

Inorg. Chem. 2004, 43, 1452−1457
New Coordination Compounds of Cd(AsF6)2 with HF and XeF2
Gasˇper Tavcˇar, Primozˇ Benkicˇ, and Boris Zˇemva*
Department of Inorganic Chemistry and Technology, Jozˇef Stefan Institute,
JamoVa 39, SI-1000 Ljubljana, SloVenia
Received October 8, 2003
Two new coordination compounds of cadmium with HF and XeF2 as ligands have been synthesized. Solid white
[Cd(HF)](AsF ) is obtained from an anhydrous HF (aHF) solution of Cd(AsF6)2. It crystallizes in a monoclinic P21/c
6 2
space group with a ) 9.4687(14) Å, b ) 9.2724(11) Å, c ) 10.5503(18) Å, â ) 104.887(7)°, and Z ) 4. The
coordination sphere of Cd consists of 7 + 2 fluorine atoms, which are in a capped trigonal-prismatic arrangement.
The reaction between Cd(AsF ) and XeF in aHF yields a solid white product at room temperature having the
6 2
2
composition [Cd(XeF ) ](AsF ) after the excess XeF and solvent have been removed under dynamic vacuum.
2 4
6 2
2
[Cd(XeF ) ](AsF ) crystallizes in the orthorhombic space group P212121, with a ) 8.6482(6) Å, b ) 13.5555(11)
2 4
6 2
Å, c ) 16.6312(14) Å, and Z ) 4. The coordination sphere of Cd consists of eight fluorine atoms, which are at
the corners of a trigonal prism with two capped side faces.
Introduction
under a dynamic vacuum at room temperature and slowly
loses XeF2, yielding AgAsF6 as a final product. A whole
series of new compounds of the type [Mx(XeF2)n](AF6)x (M
is a metal in the oxidation state +x, x ) 2, 3; A ) As, Sb,
Bi), stable at room temperature, were isolated recently,
and the majority of their crystal structures were determined:
[Ln(XeF2)3](AF6)3 (Ln ) La, Nd, Sm, Eu, Gd; A ) As, Bi),3
[Nd(XeF2)2.5](AsF6)3,8 [Pb(XeF2)3](AsF6)2, [Sr(XeF2)3](AsF6)2,9
[Ca(XeF2)n](AsF6)2 (n ) 2.5, 4),10 and [Ba(XeF2)5](SbF6)2.11
One of the interesting subjects of research in fluorine
chemistry is the synthesis of coordination compounds of
various metal cations with weakly coordinating anions
-
-
-
(AsF6 , SbF6 , BiF6 ,...) and different ligands. With the right
choice of metal cation, anion, and solvent, coordination
compounds can be obtained in which even weaker ligands
and solvents are coordinated to the metal ion. The first
coordination compound to be made with HF bonded to the
metal ion was [La(HF)2](AsF6)3.1 Similar compounds with
HF as the ligand are [Pb(HF)](AsF6)2,2 [Ca(HF)](AsF6)2,3
[Ca(HF)2](SbF6)2,4 [Mg(HF)2](SbF6)2,4 ([OsO3F)(HF)2]-
(AsF6))2,5 [(OsO3F)(HF)](SbF6),5 and [Au(HF)2](SbF6)2‚
2HF.6
The syntheses, crystal structures, and vibrational spectra
of [Cd(XeF2)4](AsF6)2 and [Cd(HF)](AsF6)2 are described
in the present paper.
Experimental Section
Xenon difluoride bonded directly to a metal ion was first
General Experimental Procedures. A Teflon and nickel
vacuum line and system were used as described previously.12
Moisture-sensitive materials were handled in a dry argon atmo-
sphere in a drybox having a maximum water content of 0.1 ppm
of water vapor (M. Braun, Garching, Germany). Reaction vessels
made of tetrafluoroethylene-hexafluoropropylene copolymer (FEP)
and equipped with Teflon valves and Teflon-coated stirring bars
observed in [Ag(XeF2)2]AsF6.7 This compound is not stable
* Author to whom correspondence should be adressed. E-mail:
boris.zemva@ijs.si. Phone: +386 1 477 35 40. Fax: +386 1 423 21 25.
(1) Mazej, Z.; Borrmann, H.; Lutar, K.; Zˇemva, B. Inorg. Chem. 1998,
37, 5912-5914.
(2) Tramsˇek, M.; Lork, E.; Mews, R.; Zˇemva, B. J. Fluorine Chem. 2001,
110, 123-131.
(3) Tramsˇek, M.; Benkicˇ, P.; Turicˇnik, A.; Tavcˇar, G.; Zˇemva, B. J.
Fluorine Chem. 2002, 114, 143-148.
(8) Tramsˇek, M.; Lork, E.; Mews, R.; Zˇemva, B. J. Solid State Chem.
(4) Turicˇnik, A.; Benkicˇ, P.; Zˇemva, B. J. Fluorine Chem. 2003, 121,
245-251.
2001, 162, 243-249.
(9) Tramsˇek, M.; Benkicˇ, P.; Zˇemva, B. Solid State Sci. 2002, 4, 9-14.
(5) Gerken, M.; Dixon, D. A.; Schrobilgen, G. J. Inorg. Chem. 2002, 41,
259-277.
(10) Benkicˇ, P.; Tramsˇek, M.; Zˇemva, B. Solid State Sci. 2002, 4, 1425-
1434.
(6) Hwang, I. C.; Seppelt, K. Z. Anorg. Allg. Chem. 2002, 628, 765-
769.
(11) Turicˇnik, A.; Benkicˇ, P.; Zˇemva, B. Inorg. Chem. 2002, 41, 5521-
5524.
(7) Hagiwara, R.; Hollander, F.; Maines, C.; Bartlett, N. Eur. J. Solid
State Inorg. Chem. 1991, 28, 855-866.
(12) Mazej, Z.; Benkicˇ, P.; Lutar, K.; Zˇemva, B. J. Fluorine Chem. 2001,
112, 173-183.
1452 Inorganic Chemistry, Vol. 43, No. 4, 2004
10.1021/ic035166q CCC: $27.50 © 2004 American Chemical Society
Published on Web 01/21/2004

Cd(AsF6)2 with HF and XeF2
9.6; As, 12.8; AsF6-, 32.4; Ftotal, 32.6; Ffree, 13.0. Found: Cd, 9.6;
As, 13.0; AsF6-, 32.4; Ftotal, 32.0; Ffree, 13.0), and X-ray powder
diffraction pattern.
The reaction was subsequently repeated with all of the reagents
added in stoichiometric amounts [Cd(AsF6)2 (0.340 g, 0.694 mmol);
XeF2 (0.471 g, 2.78 mmol); mole ratio of 1:4 Cd/Xe]. The Raman
and IR spectra and X-ray powder diffraction pattern of the product
were identical to those obtained in the experiment described above.
Preparation of Single Crystals of [Cd(HF)](AsF6)2. Cd(AsF6)2
(0.250 g) was placed in the wider tube of a crystallization vessel
and dissolved in aHF. This solution was decanted into the narrower
part of the T-shaped crystallization vessel. The crystals were grown
with a temperature gradient of 28 °C, isolated from the mother
liquid, and immersed in perfluorinated oil (ABCR, FO5960) in a
drybox. A suitable crystal was selected under the microscope and
transferred into the cold nitrogen stream of the X-ray diffractometer.
Preparation of Single Crystals of [Cd(XeF2)4](AsF6)2. Ap-
proximately 0.3 g of [Cd(XeF2)4](AsF6)2 was placed in the wider
part of a crystallization vessel and dissolved in aHF. The remainder
of the procedure was the same as that described above.
Figure 1. Raman spectrum of [Cd(XeF2)4](AsF6)2.
were used for the syntheses. Crystals were grown in a crystallization
vessel made from a T-shaped FEP reaction vessel, constructed from
one 16 mm i.d. FEP tube and a smaller 4 mm i.d. FEP tube
connected by a Teflon valve.
Elemental Analysis. The total fluoride content (Ftotal-) was
determined after complete decomposition of the sample by fusion
with KNaCO3.16,17 The content of free fluoride (Ffree-) was
determined in an aqueous solution of the sample after hydrolysis.18
Both fluoride contents were determined by direct potentiometry
using a fluoride ion selective electrode.16 Total arsenic was
determined potentiometrically by titration with Na2S2O319 after prior
Reagents. Cadmium difluoride was prepared by direct fluorina-
tion of CdCl2‚H2O (2.805 g) (Zorka Sˇabac, 99%) with F2 in
anhydrous HF (aHF) as a solvent at 298 K. Fluorine was partially
condensed three times at a pressure of 700 kPa from a vessel of
known volume into a reaction vessel cooled in liquid nitrogen.
Cadmium difluoride was characterized by the X-ray powder
diffraction pattern and by chemical analyses (Calcd: Cd, 74.7; F,
25.3. Found: Cd, 74.6; F, 25.2). aHF (Fluka, purum) was treated
with K2NiF6 (Ozark-Mahoning, 99%) for several days prior to use.
Arsenic pentafluoride was prepared by high-pressure fluorination
of As2O3, using the procedure described for PF5.13 Xenon difluoride
was prepared by the photochemical reaction between Xe and F2 at
room temperature.14 (Caution: aHF and AsF5 must be handled in
a well-ventilated hood, and protective clothing must be worn at all
times.)
-
fusion of the sample with KNaCO3. The content of AsF6 was
determined gravimetrically by precipitation with tetraphenyl-
arsonium chloride,20 and the cadmium content was determined by
complexometric titration with EDTA.21
X-ray Powder Diffraction Patterns. Diffraction data of the
samples were taken in sealed quartz capillaries on a 143 mm
Debye-Scherer camera with X-ray film, using Cu KR radiation.
Intensities were estimated visually.
X-ray Structure Determination. Both single-crystal data sets
were collected at -73 °C using a Mercury CCD area detector
coupled with a Rigaku AFC7 diffractometer with graphite-mono-
chromatized Mo KR radiation. The data were corrected for Lorentz
and polarization effects. Multiscan absorption corrections were
applied to each data set. All calculations during the data processing
were performed using the CrystalClear software suite.22 Structures
were solved by direct methods23 and expanded using Fourier
techniques. Full-matrix least-squares refinement of F2 against all
reflections was performed using the SHELX97 program.24 More
details on the data collection and structure determination are given
in Table 1.
Synthesis of Cd(AsF6)2. Cd(AsF6)2 was prepared from CdF2
(0.518 g, 3.44 mmol) and AsF5 (1.341 g, 7.89 mmol) in an aHF
solvent as previously described.15 The product was dried under
dynamic vacuum for 3 days to remove all traces of aHF and
characterized by the X-ray powder diffraction pattern and chemical
-
analysis (Calcd for Cd(AsF6)2: Cd, 22.9; As, 30.6; Ftotal, 46.5; Ffree
0; AsF6-, 77.1. Found: Cd, 22.7; As, 31.4; Ftotal-, 46.3; Ffree-, 0.5;
AsF6-, 77.0).
Synthesis of [Cd(HF)](AsF6)2. All attempts to synthesize this
compound in bulk were unsuccessful. [Cd(HF)](AsF6)2 is not stable
under dynamic vacuum even at -20 °C, losing HF to get Cd(AsF6)2
as the final product.
,
Raman Spectroscopy. Raman spectra of the powdered samples
in sealed quartz capillaries were recorded on a Renishaw Raman
Synthesis of [Cd(XeF2)4](AsF6)2. Cd(AsF6)2 (1.032 g, 2.11
mmol) was dissolved in aHF. Xenon difluoride (2.387 g, 14.10
mmol) was sublimated into the reaction vessel at -196 °C followed
by warming of the reaction vessel to room temperature. The product
of the reaction was not completely dissolved in aHF. The excess
of XeF2 and aHF were pumped off at room temperature for 6 h,
yielding a white product (1.4131 g) with a mole ratio of 1:3.97
Cd/XeF2. The compound was characterized by Raman spectroscopy
(Figure 1), chemical analyses (Calcd for [Cd(XeF2)4](AsF6)2: Cd,
(16) Ponikvar, M.; Sedej, B.; Pihlar, B.; Zˇemva, B. Anal. Chim. Acta 2000,
418, 113-118.
(17) Ponikvar, M.; Zˇemva, B.; Liebman, J. F. J. Fluorine Chem. 2003,
123, 217-220.
(18) Sedej, B. Talanta 1976, 23, 335 and 336.
(19) Ponikvar, M.; Pihlar, B.; Zˇemva, B. J. Fluorine Chem. 2003, 122,
215-217.
(20) Dess, H. M.; Parry, R. W.; Vidale, G. L. J. Am. Chem. Soc. 1956, 78,
5730-5734.
(21) Pribil, R. Applied Complexometry; Pergamon Press: Oxford, 1982;
pp 169-171.
(22) CrystalClear; Rigaku Corporation: The Woodlands, TX, 1999.
(23) SIR92: Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.
J. Appl. Crystallogr. 1993, 26, 343.
(13) Jesih, A.; Zˇemva, B. Vestn. SloV. Kem. Drus. 1986, 33, 25.
(14) Sˇmalc, A.; Lutar, K. In Inorganic Syntheses; Grimes, R. N., Ed.;
Wiley: New York, 1992; Vol. 29, p 1.
(24) Sheldrick, G. M. SHELX97-2: Programs for Crystal Structure
Analysis, release 97-2; University of Go¨ttingen: Go¨ttingen, Germany,
1998.
(15) Frlec, B.; Gantar, D.; Holloway, J. J. Fluorine Chem. 1982, 19, 485-
500.
Inorganic Chemistry, Vol. 43, No. 4, 2004 1453

Tavcˇar et al.
Table 1. Crystal Data and Structure Refinement for
a
[Cd(XeF2)4](AsF6)2 and [Cd(HF)](AsF6)2
[Cd(XeF2)4](AsF6)2
[Cd(HF)](AsF6)2
a (Å)
b (Å)
c (Å)
â (deg)
V (Å)
Z
8.6482(6)
13.5555(11)
16.6312(14)
90
1949.7(3)
4
9.4687(14)
9.2724(11)
10.5503(18)
104.887(7)
895.2(2)
4
formula weight
space group
T (°C)
λ (Å)
1167.4
P212121
-73(1)
0.710 69
3.977
510.2
P21/c
-73(1)
0.710 69
3.786
D
calcd (g/cm3)
µ (mm-1
R1
)
11.491
0.037
9.960
0.050
wR2
0.076
0.127
Figure 3. Connected ladders in the [Cd(HF)](AsF6)2 structure, viewed
along the [1, 1, 1] direction.
2
2
2
a R1 ) ∑||Fo| - |Fc||/∑|Fo|, wR2 ) [∑(w(Fo - Fc )2)/∑w(Fo )2]1/2
.
a
Table 2. Selected Bond Lengths and Angles in [Cd(HF)](AsF6)2
bond
distance (Å)
angle
degrees
Cd1-F1
2.376(5)
2.299(3)
2.326(3)
2.921(4)
2.263(3)
2.285(3)
2.309(3)
2.681(4)
2.287(4)
1.752(3)
1.742(3)
1.717(3)
1.760(3)
1.689(4)
1.681(4)
1.749(3)
1.755(3)
1.719(3)
1.764(4)
1.665(4)
1.689(4)
F11b-Cd1-F12c
F11b-Cd1-F1
F12-Cd1-F1
74.3(1)
78.2(1)
145.3(1)
114.0(1)
74.7(1)
81.1(1)
80.4(1)
89.1(1)
142.9(1)
71.5(2)
72.9(1)
83.2(1)
74.9(1)
115.7(1)
73.8(1)
94.4(1)
80.8(1)
146.9(2)
124.8(2)
155.2(2)
117.0(2)
Cd1-F11b
Cd1-F12c
Cd1-F13
Cd1-F14
Cd1-F21d
Cd1-F22
Cd1-F23c
Cd1-F24c
As1-F11
As1-F12
As1-F13
As1-F14
As1-F15
As1-F16
As2-F21
As2-F22
As2-F23
As2-F24
As2-F25
As2-F26
F13-Cd1-F1
F14-Cd1-F1
F14-Cd1-F11b
F14-Cd1-F12c
F14-Cd1-F22
F21d-Cd1-F1
F21d-Cd1-F12c
F21d-Cd1-F22
F21d-Cd1-F24c
F22-Cd1-F1
F23c-Cd1-F1
F24c-Cd1-F1
F24c-Cd1-F11b
F24c-Cd1-F22
As1b-F11b-Cd1
As1-F14-Cd1
As2-F22-Cd1
As2c-F24c-Cd1
Figure 2. Coordination sphere of Cd in the structure of [Cd(HF)](AsF6)2.
Imaging Microscope System 1000 by use of the 632.8 nm exciting
line of a He-Ne laser. The geometry for all of the Raman
experiments was 180° backscattering with a laser power of 25 mW.
a Symmetry operations used for the generation of equivalent atoms. b -x,
1
1
1
1
y + /2, -z + /2. c -x, -y, -z. d x, -y + /2, z + /2.
Results
Description of the Crystal Structure of [Cd(HF)]-
(AsF6)2. The cadmium cation is surrounded by six bulky
AsF6 units and one HF molecule (Figure 2). The AsF6 units
interact with cadmium via six fluorine atoms arranged at the
apexes of distorted trigonal prisms, with the Cd-F(As)
distances in the range of 2.263-2.326 Å. The HF molecule
caps a trigonal-prismatic face at a distance of 2.376(5) Å.
Additionally, the AsF6 units interact with cadmium at slightly
longer distances, Cd-F23(As2) ) 2.681(4) Å and Cd-
F13(As1) ) 2.921(4) Å. The sum of the respective van der
Waals radii25 is 3.05 Å. The cadmium cation is thus located
in an environment of nine fluorine atoms, forming a distorted,
tricapped trigonal prism. Two longer Cd-F contacts that
bring the edges of the AsF6 units into the coordination sphere
of cadmium are considered as being mainly electrostatic.
There are two crystallographically different AsF6 units in
the structure, but both bridge cadmium cations in a similar
manner. The two Cd atoms are connected by fac-F ligands
from two As1F6/As2F6 units, thus forming a square ring of
10 atoms. The square rings [Cd2(As1F6)2]2+/[Cd2(As2F6)2]2+
are alternately connected by trans bridges of As1F6 and
As2F6 units, forming zigzag ladders running in the [1, 1, 1]
direction (Figure 3). Each Cd atom from the [1, 1, 1] ladder
is part of an identical ladder running in the [-1, -1, -1]
direction. Cadmium atoms connect both types of ladders in
a 3D network, where channels are formed along the a axis.
Space in the channels is occupied by HF molecules coor-
dinated through fluorine directly to Cd atoms. According to
the position of the hydrogen atoms found in the Fourier
difference map, the HF molecule alternately forms a hydro-
gen bond to the F26 atoms from the neighboring ladder
running in the opposite direction and in addition joins the
ladders.
Selected bond distances and angles are given in Table 2.
Description of the Crystal Structure of [Cd(XeF2)4]-
(AsF6)2. The coordination sphere around cadmium consists
of eight fluorine atoms forming a trigonal prism with two
capped side faces. The fluorine atoms of one trigonal face
are contributed by the monodentate As2F6 unit, the non-
bridging Xe4F2 molecule, and the bridging Xe1F2 molecule,
while those of the other trigonal face are from the nonbridg-
(25) Bondi, A. J. Chem. Phys. 1964, 68, 441-451.
1454 Inorganic Chemistry, Vol. 43, No. 4, 2004

Cd(AsF6)2 with HF and XeF2
a
Table 3. Selected Bond Lengths and Angles in [Cd(XeF2)4](AsF6)2
bond
distance (Å)
angle
degrees
Xe1-F1
Xe1-F5
Xe2-F2
Xe2-F6
Xe3-F3
Xe3-F7
Xe4-F4
Xe4-F8
Cd1-F1
Cd1-F2
Cd1-F3
Cd1-F4
Cd1-F5b
Cd1-F11
Cd1-F12
Cd1-F21
As1-F11
As1-F12
As1-F13
As1-F14
As1-F15
As1-F16
As2-F21
As2-F22
As2-F23
As2-F24
As2-F25
As2-F26
2.017(5)
1.995(5)
2.045(6)
1.952(8)
2.043(6)
1.968(6)
2.079(6)
1.953(6)
2.328(6)
2.220(6)
2.271(6)
2.211(6)
2.426(5)
2.399(6)
2.418(6)
2.309(6)
1.750(6)
1.762(6)
1.675(7)
1.711(6)
1.699(7)
1.689(7)
1.774(6)
1.712(7)
1.685(8)
1.720(7)
1.690(7)
1.694(6)
F5-Xe1-F1
F6-Xe2-F2
F7-Xe3-F3
F8-Xe4-F4
F1-Cd1-F11
F1-Cd1-F12
F2-Cd1-F1
F2-Cd1-F3
F2-Cd1-F5b
F2-Cd1-F11
F2-Cd1-F12
F2-Cd1-F21
F3-Cd1-F1
F3-Cd1-F5b
F3-Cd1-F11
F3-Cd1-F12
F3-Cd1-F21
F4-Cd1-F1
F4-Cd1-F2
F4-Cd1-F3
F4-Cd1-F5b
F4-Cd1-F11
F4-Cd1-F12
F4-Cd1-F21
F11-Cd1-F12
F21-Cd1-F1
F21-Cd1-F11
F21-Cd1-F12
177.3(2)
179.2(4)
177.7(3)
179.9(3)
75.4(2)
110.3(2)
140.3(2)
76.6(2)
74.0(2)
80.4(3)
82.6(2)
148.5(2)
141.9(2)
138.1(2)
133.2(2)
77.4(2)
77.7(2)
87.9(2)
93.1(3)
79.0(2)
73.5(2)
142.8(2)
156.4(2)
99.6(2)
59.5(2)
69.4(2)
104.9(2)
74.3(2)
Figure 4. Coordination sphere of Cd in the structure of [Cd(XeF2)4]-
(AsF6)2.
a Symmetry operations used for the generation of equivalent atoms.
1
1
b x + /2, -y + /2, -z.
Figure 5. Chain of molecules in the structure of [Cd(XeF2)4](AsF6)2.
Lewis base solvents, such as aHF, can supply enough
solvation energy to dissolve Mx(AF6)x salts. Cd(AsF6)2 readily
ing Xe2F2 molecule and the edge-sharing bidentate As1F6
unit. The cadmium coordination prism is additionally capped
by the nonbridging Xe3F2 molecule and by the symmetry-
generated bridging Xe1F2 molecule, both placed at the square
faces opposite to the edge shared by the As1F6 unit (Figure
4). Thus, each Cd atom is only connected to the neighboring
cadmium atoms via two bridging Xe1F2, forming a spiral
chain running along the a axis (Figure 5). There are long-
range electrostatic interactions between positively charged
Xe atoms and negatively charged F atoms from XeF2 and
AsF6 units inside each chain and also between the chains.
The latter are responsible for keeping the chains together.
The consequences of these interactions and of the steric
activity of the electron lone pairs at the XeF2 molecules are
the slightly deformed angles F7-Xe3-F3 ) 177.7° and F1-
Xe1-F5 ) 177.3°. Sterically reasonable electrostatic contacts
range from 3.143 to 3.639 Å (see the Supporting Informa-
tion).
-
dissolves in aHF, forming [Cd(HF)y]2+ cations and AsF6
anions. During the process of crystallization from a saturated
aHF solution, crystals of the compound [Cd(HF)](AsF6)2
were obtained. They could be separated from the solvent by
rapid removal of the solvent under dynamic vacuum at
20 °C. The material in noncrystalline form is unstable under
dynamic vacuum, even at -20 °C. The final product was
always only pure Cd(AsF6)2, which was used for the reaction
with XeF2 (eq 1).
298 K
Cd(AsF6)2 + nXeF2
8
aHF
[Cd(XeF2)4](AsF6)2 + (n - 4)XeF2
n g 4 (1)
Addition of XeF2 to a solution of Cd(AsF6)2 in aHF replaces
the latter with the stronger Lewis base, XeF2, to yield
[Cd(XeF2)4](AsF6)2. On the other hand, Cd2+ is not a
sufficiently strong Lewis acid to withdraw F- from XeF2 to
The Cd-F(Xe) distances range from 2.211 to 2.271 Å for
nonbridging XeF2 molecules, shorter than the Cd-F distance
in the structure of CdF2 (2.333 Å26) and in the bridging XeF2
molecules (2.328(6) and 2.426(5) Å). The Cd-F(As) dis-
tances range from 2.309 to 2.418 Å.
+
-
give CdF2 and Xe2F3 AsF6 .
The compound [Cd(XeF2)4](AsF6)2 is a white solid having
a negligible vapor pressure at room temperature and does
not release significant quantities of XeF2 during prolonged
pumping at room temperature.
Selected bond distances and angles are given in Table 3.
Crystal Structures. Although the Cd2+ and Ca2+ cations
have very similar ionic radii (Cd2+, CN ) 8, r ) 1.24 Å;
Ca2+, CN ) 8, r ) 1.26 Å), the structures of their analogous
coordination compounds are not always the same. [Cd(HF)]-
Discussion
-
Syntheses. Metal salts containing the large anions AF6 ,
where A is As, Sb, or Bi, have rather low lattice energies.
These anions are also poor Lewis bases, so that even poor
3
(AsF6)2 and [Ca(HF)](AsF6)2 are isostructural, while the
10
structures of [Cd(XeF2)4](AsF6)2 and [Ca(XeF2)4](AsF6)2
(26) Haendler, H. M. J. Am. Chem. Soc. 1951, 73, 5218 and 5219.
differ. This implies that the XeF2 molecule, as a ligand
Inorganic Chemistry, Vol. 43, No. 4, 2004 1455

Tavcˇar et al.
Table 4. Frequencies and Intensities of the [Cd(XeF2)4](AsF6)2 Raman
Spectra
coordinated to a metal cation, is more affected by the cation
properties than the HF molecule.
wavenumber
wavenumber
The XeF2 molecule can bridge two metal cations, thus
effectively substituting the AsF6 units in forming the crystal
network. In the structure of [Cd(XeF2)4](AsF6)2, the Cd2+
ions are surrounded by one monodentate AsF6 unit, one
bidentate AsF6 unit, three nonbridging XeF2 molecules, and
only two bridging XeF2 molecules, to yield the chain-
structure arrangement. In the structure of [Ca(XeF2)4]-
(AsF6)2,10 the Ca2+ ions are surrounded by two nonbridging
XeF2 molecules, two monodentate AsF6 units, and four
bridging XeF2 molecules, resulting in a layer-structure
arrangement. In accordance with the much higher electron
affinity of Cd2+ (16.91 eV27) than that of Ca2+ (11.87 eV27),
a higher charge transfer from the XeF2 molecule to the metal
ion, and therefore a higher degree of covalency in the M-F
bond, is expected in the case of the Cd compound. This
makes bridging interactions in the cadmium structure less
favorable, resulting in the chain arrangement of the crystal
packing. Although the bridging XeF2 molecules in the
cadmium and calcium compounds are not readily distin-
guishable, they lie in different electric fields, as is evident
from their Raman stretching frequencies (Ca, 533 cm-1; Cd,
521 cm-1; Table 4). The difference can be attributed to the
lower positive charge at the Cd2+ cations. The average
M-F(Xe) distance for the calcium compound is 2.346(11)
Å, and that for the cadmium compound is 2.377(6) Å.
[Cd(XeF2)4](AsF6)2 and [Ca(XeF2)4](AsF6)2 are the only
known compounds in which metal cations are only bridged
by XeF2 molecules.
(cm-1
)
intensity
(cm-1
)
intensity
751
715
707
678
587
547
0.1
0.3
0.3
1.9
0.7
10
ν(As-F)
ν(As-F)
ν(As-F)
ν(As-F)
ν(As-F)
ν(Xe-F)
521
463
448
408
375
5.1
1.4
0.9
0.6
0.7
ν(XeF2)
ν(Xe‚‚‚F)
ν(Xe‚‚‚F)
ν(As-F)
ν(As-F)
bear a lower positive charge than Ca2+ cations, which is
probably a consequence of the higher covalency of the Cd-F
bonds, which compensates for the lower Coulombic contri-
bution to the lattice energy of the cadmium compound.
Additionally, the narrower range of the M-F(As) distances
in the calcium compound when compared with the cadmium
compound shows a lower tendency toward multidentate and
bridging AsF6 moieties in the cadmium compound, as already
discussed in the case of the compounds with XeF2 as a ligand.
Vibrational Spectra. The high polarizability of xenon
usually results in intense Raman bands for the symmetric
Xe-F stretching modes. Ordinarily, As-F and Cd-F vibra-
tions are far less intense and are broad in their Raman spectra.
The intense Raman band of the compound [Cd(XeF2)4]-
(AsF6)2 at 521 and 547 cm-1 (Figure 1 and Table 4) can be
confidently assigned to the symmetric XeF2 stretching modes.
The totally symmetric (ν1) stretching mode for XeF2 is close
to 497 cm-1.29 By donating one of its F ligands, XeF2
becomes distorted, and the band at 497 cm-1 is replaced by
two bands. The band at the higher frequency is labeled as
the shorter bond Xe-F stretch [ν(Xe-F)] and that at the
lower frequency as the Xe-F stretch of the longer bond
[ν(Xe‚‚‚F)]. In (Xe-F)+ salts, one F ligand is pulled away
from the XeF2 molecule. The (Xe-F)+ stretching frequency
is normally higher than 600 cm-1.30 In the case of the present
compound, the peaks at 547 and 448 cm-1 can be assigned
to the terminal XeF2 molecules, while the peak at 521 cm-1
is assigned to the Xe-F stretching mode of the bridging XeF2
molecules. The symmetric stretching mode of the bridging
XeF2 might have been expected at ∼497 cm-1. Because
bridging XeF2 molecules in the moiety Cd‚‚‚F-Xe-F‚‚‚Cd
are anchored between two Cd2+ ions, a higher energy is
required for the symmetric stretching mode, which results
in an increase in the stretching frequency. The band at 463
cm-1 is most probably arisen from the vibrational coupling
of the nonbridging XeF2 vibrations. This band was also
observed in other [Mx+(XeF2)n](AsF6)x (M ) Nd,8 Ca10)
coordination compounds.
The HF molecule is a monodentate ligand and is therefore
less effective in influencing crystal packing because of the
charge transfer from the ligand to the metal ion as an XeF2
molecule. Thus, [Cd(HF)](AsF6)2 and [Ca(HF)](AsF6)2 are
isostructural. The structures are stabilized mostly by similar
interactions between the metal cations, small HF molecules,
and bulky AsF6 units.
In the structure [Cd(HF)](AsF6)2, all of the Cd-F distances
to the fluorines at the apexes of the coordination prism are
shorter than those in the structure of CdF2 (Cd-F ) 2.333
Å26), indicating that the Cd2+ cations in [Cd(HF)](AsF6)2 bear
a larger positive charge than those in the structure of CdF2.
This is a consequence of the weaker electron donor capabili-
ties of fluorine atoms in the Cd-F-As bridges compared
with those in the Cd-F-Cd bridges in the structure of CdF2.
Because Cd has a higher electron affinity than Ca, the
M-F bond is expected to be less ionic in the Cd compound.
It should be mentioned that, in the [Cd(HF)](AsF6)2 com-
pound, fluorine atoms from the AsF6 units capping the side
faces of the basic trigonal-prismatic polyhedra are at longer
distances from the metal (Cd-F13 and Cd-F23 are 2.921(4)
and 2.681(4) Å, respectively) than those in the Ca compound
(2.773 and 2.679 Å28), although the other Cd-F(As)
distances are shorter (average 2.295 Å) than those in Ca-
F(As) (average 2.376 Å). This indicates that Cd2+ cations
The AsF6- octahedra in the compound are deformed. The
ν1 mode of AsF6 is at 678 cm-1 (ν1 of Oh AsF6 is at 689
-
cm-1).31 The Oh symmetry of AsF6- is reduced to the lower
symmetry resulting in more Raman-active modes.32-34
(29) Agron, P. A.; Begun, G. M.; Levy, H. A.; Mason, A. A.; Jones, C.
G.; Smith, D. E. Science 1963, 139, 842-844.
(30) Sladky, F. O.; Bulliner, P. A.; Bartlett, N. J. Chem. Soc. A 1969, 2179.
(31) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coor-
dination Compounds, 4th ed.; Wiley: New York, 1986; p 110.
(32) Gillespie, R. J.; Landa, B. Inorg. Chem. 1973, 12, 1383-1388.
(27) Pearson, R. G. Inorg. Chem. 1988, 27, 734-740.
(28) Tramsˇek, M.; Benkicˇ, P. Unpublished results.
1456 Inorganic Chemistry, Vol. 43, No. 4, 2004

Cd(AsF6)2 with HF and XeF2
Supporting Information Available: An X-ray crystallographic
file in CIF format, atomic coordinates and equivalent displacement
parameters Ueq of [Cd(XeF2)4](AsF6)2 and [Cd(HF)](AsF6)2, a figure
illustrating the [Cd(HF)](AsF6)2 basic building unit, a figure
showing the [Cd(XeF2)4](AsF6)2 packing, and a table of the Xe-F
interactions in the [Cd(XeF2)4](AsF6)2 structure. This information
is available free of charge via the Internet at http://pubs.asc.org.
Acknowledgment. The authors gratefully acknowledge
the financial support of the Ministry of Education, Science
and Sport of the Republic of Slovenia. We thank Dr. Maja
Ponikvar for the chemical analyses.
(33) Gillespie, R. J.; Landa, B.; Schrobilgen, G. J. Inorg. Chem. 1976, 15,
1256-1263.
(34) Lehmann, J. F.; Dixon, D. A.; Schrobilgen, G. J. Inorg. Chem. 2001,
40, 3002-3017.
IC035166Q
Inorganic Chemistry, Vol. 43, No. 4, 2004 1457