[Mg(XeF2)n](AsF6)2 (n = 4, 2): First Compounds of Magnesium with XeF2

Article abstract of DOI:10.1021/ic034826o

The reaction between Mg(AsF₆)₂ and XeF₂ in anhydrous HF (aHF) at room temperature yields two compounds with XeF ₂ bonded directly to the Mg²⁺ cation: [Mg(XeF ₂)₄](AsF₆)₂; [Mg(XeF ₂)₂](AsF₆)₂. The 1:4 compound is obtained with excess XeF₂ while the 1:2 compound is prepared from stoichiometric amounts of Mg(AsF₆)₂ and XeF₂. [Mg(XeF₂)₄](AsF₆)₂ crystallizes in an orthorhombic crystal system, space group P2₁2₁2 ₁, with a = 8.698(15) A, b = 14.517(15) A, c = 15.344(16) A, V = 1937(4) A³, and Z = 4. The octahedral coordination sphere of Mg consists of one fluorine atom from each of the four XeF₂ molecules and two fluorine atoms from the two AsF₆ units. [Mg(XeF₂)₂](AsF₆)₂ crystallizes in the orthorhombic crystal system, space group Pbam, with a = 8.9767(10) A, b = 15.1687(18) A, c = 5.3202(6) A, V = 724.42(14) A, and Z = 2. The octahedral coordination sphere consists of two fluorine atoms, one from each of the two XeF₂ molecules and four fluorine atoms from the four bridging AsF₆ units.

Full text of DOI:10.1021/ic034826o

Inorg. Chem. 2004, 43, 699−703
[Mg(XeF2)n](AsF6)2 (n ) 4, 2): First Compounds of Magnesium with
XeF2
Melita Tramsˇek,* Primozˇ Benkicˇ,† and Boris Zˇemva*
Department of Inorganic Chemistry and Technology, Jozˇef Stefan Institute,
JamoVa 39, SI-1000 Ljubljana, SloVenia
Received July 15, 2003
The reaction between Mg(AsF ) and XeF in anhydrous HF (aHF) at room temperature yields two compounds
6 2
2
with XeF bonded directly to the Mg2+ cation: [Mg(XeF ) ](AsF ) ; [Mg(XeF ) ](AsF ) . The 1:4 compound is ob-
2
2 4
6 2
2 2
6 2
tained with excess XeF while the 1:2 compound is prepared from stoichiometric amounts of Mg(AsF6)2 and XeF .
2
2
[Mg(XeF ) ](AsF ) crystallizes in an orthorhombic crystal system, space group P212121, with a ) 8.698(15) Å, b
2 4
6 2
3
) 14.517(15) Å, c ) 15.344(16) Å, V ) 1937(4) Å , and Z ) 4. The octahedral coordination sphere of Mg
consists of one fluorine atom from each of the four XeF molecules and two fluorine atoms from the two AsF units.
2
6
[Mg(XeF ) ](AsF ) crystallizes in the orthorhombic crystal system, space group Pbam, with a ) 8.9767(10) Å, b
2 2
6 2
3
) 15.1687(18) Å, c ) 5.3202(6) Å, V ) 724.42(14) Å , and Z ) 2. The octahedral coordination sphere consists
of two fluorine atoms, one from each of the two XeF molecules and four fluorine atoms from the four bridging
2
AsF units.
6
very sensitive to traces of moisture were handled in an argon atmos-
phere in a drybox, which limited the maximum water vapor content
to 1 ppm (M Braun, Garching, Germany). PFA reaction vessels,
equipped with a Teflon valve and a Teflon-covered mixing bar,
were used for the syntheses. T-shaped PFA reaction vessels, con-
structed from large (16 mm i.d.) and smaller (4 mm i.d.) PFA tubes
joined at right angles and equipped with a Teflon valve, were used
for crystallization.
Reagents. MgF2 (Aldrich, 99.9%) was used as purchased. Its
purity was checked by elemental analysis (Anal. Calcd for MgF2:
Mg, 39.0; F, 61.0%. Found: Mg, 38.8; F, 59.7). Anhydrous HF
(Fluka, purum) was treated with K2NiF6 (Ozark-Mahoning, 99%)
for several days prior to use. AsF5 was prepared by high-pressure
fluorination of As2O3, as previously described for PF5.9 Its purity
was checked by IR spectroscopy. XeF2 was prepared by the
photochemical reaction between Xe and F2 at room temperature.10
Mg(AsF6)2 was prepared as previously described11 and characterized
by X-ray powder diffraction data and chemical analysis (Anal. Calcd
Introduction
1
[Ag(XeF2)2]AsF6 is the first example of a compound in
which XeF2 acts as a ligand coordinated to a metal ion.
Recently, an entire series of new compounds of the type
[Mx(XeF2)n](AF6)x (M ) Ca, Sr, Ba, Pb, Ag, La, Nd, A )
As, Sb, P) were synthesized and characterized structurally:
[Ln(XeF2)2.5](AsF6)3 (Ln ) La,2 Nd3); [Pb(XeF2)3](AsF6)2;4,5
[Sr(XeF2)3](AsF6)2;4 [Ba(XeF2)5](SbF6)2;6 [Ca(XeF2)n](AsF6)2,
n ) 4 and 2.5;7 [Ag(XeF2)2]PF6.8
In this paper the syntheses of the first two compounds in
which XeF2 is bonded directly to a magnesium ion are de-
scribed. The structures of the compounds and their Raman
spectra are also presented.
Experimental Section
General Experimental Procedure. Volatile materials (anhy-
drous HF (aHF), AsF5) were manipulated in an all-Teflon vacuum
line equipped with Teflon valves. Nonvolatile materials that were
(5) Tramsˇek, M.; Benkicˇ, P.; Turicˇnik, A.; Tavcˇar, G.; Zˇemva, B. J.
Fluorine. Chem. 2002, 114, 143-148.
(6) Turicˇnik, A.; Benkicˇ, P.; Zˇemva, B. Inorg. Chem. 2002, 41, 5521-
5524.
* To whom correspondence should be addressed. E-mail: melita.
tramsek@ijs.si (M.T.); boris.zemva@ijs.si (B.Zˇ.). Phone: +386 1 477 33
01. Fax: +386 1 423 21 25.
(7) Benkicˇ, P.; Tramsˇek, M.; Zˇemva, B. Solid State Sci. 2002, 4, 1425-
† E-mail: primoz.benkic@ijs.si.
1434.
(1) Hagiwara, R.; Hollander, F.; Maines, C.; Bartlett, N. Eur. J. Solid
State Inorg. Chem. 1991, 28, 855-866.
(8) Matsumoto, K.; Hagiwara, R.; Ito, Y.; Tamada, O. Solid State Sci.
2002, 4, 1465-1469.
(9) Jesih, A.; Zˇemva, B. Vestn. SloV. Kem. Drus. 1986, 33, 25-28.
(10) Sˇmalc, A.; Lutar, K. Inorganic Syntheses; Grimes R. N., Ed.); John
Wiley & Sons: New York, 1992; Vol. 29, pp 1-4.
(11) Frlec, B.; Gantar, D.; Holloway, J. H. J. Fluorine. Chem. 1982, 19,
485-500.
(2) Lutar, K.; Borrmann, H.; Mazej, Z.; Tramsˇek, M.; Benkicˇ, P.; Zˇemva,
B. J. Fluorine Chem. 2000, 101, 155-160.
(3) Tramsˇek, M.; Lork, E.; Mews, R.; Zˇemva, B. J. Solid State Chem.
2001, 162, 243-249.
(4) Tramsˇek, M.; Benkicˇ, P.; Zˇemva, B. Solid State Sci. 2002, 4, 9-14.
10.1021/ic034826o CCC: $27.50 © 2004 American Chemical Society
Published on Web 11/27/2003
Inorganic Chemistry, Vol. 43, No. 2, 2004 699

Tramsˇek et al.
for Mg(AsF6)2: Mg, 6.0; As, 37.3; Ft-, 56.7%. Found: Mg, 6.2;
As, 38.0; Ft-, 57.3).
Table 1. Crystal Data and Structure Refinement for
[Mg(XeF2)n](AsF6)2 (n ) 4, 2)a
Caution! Anhydrous HF (aHF), AsF5, and XeF2 must be handled
in a well-Ventilated hood, and protectiVe clothing must be worn at
all times.
param
MgXe4As2F20
MgXe2As2F16
fw
1079.35
740.75
20
temp (°C)
a (Å)
b (Å)
c (Å)
V (Å3)
Z
-173(2)
8.698(15)
14.517(15)
15.344(16)
1937(4)
8.9767(10)
15.1687(18)
5.3202(6)
724.42(14)
2
3.396
0.71069
9.417
Preparation of [Mg(XeF2)n](AsF6)2 (n ) 4, 2). Synthesis of
[Mg(XeF2)4](AsF6)2. Mg(AsF6)2 (1.056 g, 2.63 mmol) and excess
XeF2 (5.00 g, 29.5 mmol) were weighed into the reaction vessel
inside the drybox. Anhydrous HF was added and the reaction
allowed to proceed at room temperature for several days. The
product was isolated by pumping off the solvent and excess XeF2.
The pumping was stopped when a 1:4 mole ratio between Mg and
XeF2 was reached (weight of the product: 2.833 g). The compound
still released XeF2 at room temperature in a dynamic vacuum
(approximately 8 mg of XeF2/(mmol of the product/h of pumping)).
The product was characterized by chemical analysis (Anal. Calcd
for [Mg(XeF2)4](AsF6)2: Mg, 2.2; As, 13.9; Ft-, 35.2; Ff-, 14.1;
AsF6-, 35.0%. Found: Mg, 2.1; As, 13.6; Ft-, 35.0; Ff-, 14.6;
AsF6-, 34.1), Raman spectroscopy, and the X-ray powder diffraction
pattern.
Synthesis of [Mg(XeF2)2](AsF6)2. Mg(AsF6)2 (0.835 g, 2.08
mmol) and XeF2 (0.705 g, 4.16 mmol) were weighed directly into
the reaction vessel inside the drybox. The reaction was allowed to
proceed for 24 h after the solvent (aHF) was added. A crystalline
white solid (1.531 g, 2.07 mmol) was isolated by pumping off aHF
at 0 °C for 4 h. The product was characterized by chemical analysis
(Anal. Calcd for [Mg(XeF2)2](AsF6)2: Mg, 3.3; As, 20.2; Ft-, 41.0;
Ff-, 10.3; AsF6-, 51.0%. Found: Mg, 3.5; As, 21.0; Ft-, 40.8; Ff-,
10.4; AsF6-, 50.9), Raman spectroscopy, and its X-ray powder
diffraction pattern.
4
D
calcd (g/cm3)
3.701
0.71069
10.529
P212121 (No. 19)
0.0508, 0.1098
λ (Å)
µ (mm-1
)
space group
R1, wR2
Pbam (No. 55)
0.0390, 0.0753
2
2
2
a R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) [∑w(Fo - Fc )2)/∑w(Fo )2]1/2
.
complexometric titration.15 AsF6- was determined gravimetrically
by precipitation with tetraphenylarsonium chloride.16 Arsenic was
determined by the ICP-AES method.17
Crystal Structure Determination. Both single-crystal data sets
were collected using a Mercury CCD area detector coupled to a
Rigaku AFC7 diffractometer with graphite-monochromated Mo KR
radiation. The data were corrected for Lorentz and polarization
effects. A multiscan absorption correction was applied to both data
sets. All calculations during the data processing were performed
using the CrystalClear software suite.18 Structures were solved using
direct methods19 and expanded using Fourier techniques. Full-matrix
least-squares refinement of F2 against all reflections was performed
using the SHELX 97 program.20 More details on the data collection
and structure determinations are given in Table 1.
Diffraction data for [Mg(XeF2)2](AsF6)2, collected for single
crystals at -73 °C, showed split reflections and systematic absence
violations for glide planes under the Pbam space group. Peaks
merged at higher temperature (about -15 °C). Systematic absences
under the Pbam space group were obeyed for diffraction data for
single crystals collected at room temperature. Data for the high-
temperature form are presented here, while the low-temperature
data will be analyzed in a further study.
Raman Spectroscopy. Raman spectra of powdered samples in
sealed quartz capillaries were recorded on a Renishaw Raman
Imaging Microscope System 1000, with the 632.8 nm exciting line
of a He-Ne laser.
X-ray Powder Diffraction. X-ray powder diffraction patterns
of samples in sealed quartz capillaries were obtained with a 114
mm diameter Debye-Scherrer camera with X-ray film, using Cu
KR radiation (λ ) 1.5418 Å) with a Ni filter. Intensities were
estimated visually.
Preparation of Single Crystals. The product (approximately
200 mg) of the reaction between Mg(AsF6)2 and excess XeF2
(estimated molar ratio between Mg and XeF2 in this product was
1:3.5, calculated from the mass balance of the reaction) was
transferred to the wider tube of the reaction vessel and dissolved
in aHF. This solution was decanted into the narrower arm of the
reaction vessel, which was left at room temperature while the wider
arm was cooled with running water. In this way a small temperature
gradient of 6 °C was achieved. Crystals were obtained in the
narrower tube of the reaction vessel after several days. The mother
liquor was poured from them into the wider tube of the vessel, and
the aHF was pumped off from the reaction vessel. Crystals with
compositions [Mg(XeF2)4](AsF6)2 and [Mg(XeF2)2](AsF6)2 were
immersed in perfluorinated oil (ABCR, FO5960) in the drybox,
selected under the microscope, and transferred into the cold nitrogen
stream of the diffractometer. The quality of the data for crystals
having the composition [Mg(XeF2)2](AsF6)2 was not good enough
(see Crystal Structure Determination) so another crystallization pro-
cess was adopted. Mg(AsF6)2 (0.144 g, 0.36 mmol) and XeF2 (0.122
g, 0.72 mmol) were weighed at the exact 1:2 mole ratio (Mg:XeF2)
into the wider tube of the reaction vessel inside the drybox.
Anhydrous HF was added, and crystallization was carried out as
described above. Crystals were selected under the microscope inside
the drybox and mounted in 0.3 mm thin-walled quartz capillaries.
Elemental Analysis. The total fluoride content (Ft-) was
determined after total decomposition of the sample by fusion with
KNaCO3.12,13 The content of free fluoride (Ff-) was determined in
aqueous solution of the sample after it was hydrolyzed.14 Both
fluoride contents were determined by direct potentiometry using a
fluoride ion selective electrode.12 Magnesium was determined by
Results
Description of the Crystal Structure of [Mg(XeF2)4]-
(AsF6)2. Magnesium is octahedrally coordinated to six
(13) Ponikvar, M.; Zˇemva, B.; Liebman, J. F. J. Fluorine. Chem. 2003,
123, 217-220.
(14) Sedej, B. Talanta 1976, 23, 335-336.
(15) Pribil, R. Applied Complexometry; Pergamon Press: Oxford, U.K.,
1982; pp 200-201.
(16) Dess, H. M.; Parry, R. W.; Vidale, G. L. J. Am. Chem. Soc. 1956, 78,
5730-5734.
(17) Ingle, J. D.; Crouch, S. R. Spectrochemical analysis; Prentice Hall:
Englewood Cliffs, NJ, 1998; pp 225-256.
(18) CrystalClear; Rigaku Corp.: Woodlands, TX, 1999.
(19) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A. J. Appl.
Crystallogr. 1993, 26, 343-350.
(12) Ponikvar, M.; Sedej, B.; Pihlar, B.; Zˇemva, B. Anal. Chim. Acta 2000,
418, 113-118.
(20) Sheldrick, G. M. SHELX97-2; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
700 Inorganic Chemistry, Vol. 43, No. 2, 2004

[Mg(XeF2)n](AsF6)2
Figure 2. Coordination sphere of Mg in the structure of [Mg(XeF2)2]-
(AsF6)2 and connection of Mg atoms via AsF6 units (50% probability level).
Figure 1. Coordination sphere of Mg in the structure of [Mg(XeF2)4]-
(AsF6)2 (50% probability level).
Table 2. Selected Bond Lengths and Angles in [Mg(XeF2)4](AsF6)2
Distance (Å)
Mg1-F1
Angle (deg)
F1-Mg1-F2
1.966(10)
1.981(10)
2.003(10)
1.981(10)
1.984(10)
1.996(10)
2.059(9)
1.947(9)
2.079(9)
1.963(10)
2.087(8)
1.940(8)
2.083(8)
1.945(10)
1.773(9)
1.707(12)
1.703(11)
1.701(12)
1.705(12)
1.726(10)
1.786(9)
1.720(10)
1.707(9)
1.702(9)
1.710(9)
1.713(10)
95.9(5)
170.8(5)
178.4(5)
92.2(4)
87.5(4)
90.2(4)
133.9(5)
178.0(4)
156.3(6)
177.9(4)
124.0(5)
178.9(4)
130.2(5)
178.0(4)
149.7(6)
91.1(6)
88.5(5)
177.5(6)
89.3(7)
178.2(5)
148.7(5)
87.8(4)
175.8(5)
176.6(5)
88.5(4)
Mg1-F2
Mg1-F3
Mg1-F4
Mg1-F11
Mg1-F21
Xe1-F1
Xe1-F5
Xe2-F2
Xe2-F6
Xe3-F3
Xe3-F7
Xe4-F4
Xe4-F8
As1-F11
As1-F12
As1-F13
As1-F14
As1-F15
As1-F16
As2-F21
As2-F22
As2-F23
As2-F24
As2-F25
As2-F26
F2-Mg1-F11
F3-Mg1-F1
F4-Mg1-F11
F11-Mg1-F21
F21-Mg1-F3
Mg1-F1-Xe1
F5-Xe1-F1
Mg1-F2-Xe2
F6-Xe2F2
Mg1-F3-Xe3
F7-Xe3-F3
Mg1-F4-Xe4
F8-Xe4-F4
As1-F11-Mg1
F12-As1-F16
F13-As1-F11
F14-As1-F12
F15-As1-F12
F16-As1-F11
As2-F21-Mg1
F22-As2-F21
F23-As2-F25
F24-As2-F22
F25-As2-F21
F26-As2-F21
Figure 3. Arrangement of chains in the structure of [Mg(XeF2)2](AsF6)2
viewed down the c axis.
There are several close intra- and intermolecular electro-
static interactions between the positively charged Xe atoms
and fluorine atoms of the XeF2 and AsF6 units (for the
arrangement of [Mg(XeF2)4](AsF6)2 molecules, see Support-
ing Information). The [Mg(XeF2)4](AsF6)2 molecules in the
crystal packing are connected by electrostatic forces only,
with Xe‚‚‚F distances ranging from 3.180 to 3.698 Å (the
sum of the respective van der Waals radii is 3.63 Å 21). As
a consequence of these long-range interactions and of the
steric activity of the electron lone pairs of Xe atoms,
F-Xe-F angles deviate from the ideal linear arrangement
and range from 177.9(4) to 178.9(4)°. Similarly, due to
178.5(5)
fluorine atoms. Four fluorine atoms originate from the four
XeF2 molecules, and two cis fluorine atoms, from the two
monodentate AsF6 units. Each of the four XeF2 molecules
and the two AsF6 units are crystallographically different. The
structure is illustrated in Figure 1, and bond lengths and
angles are given in Table 2.
-
packing interactions and intramolecular contacts, the AsF6
The Mg-F distances range from 1.966(10) to 2.003(10)
Å. The octahedron of fluorine atoms around Mg is distorted,
which is apparent from the cis and trans F-Mg-F angles.
Xe-F bridging distances range from 2.059(9) to 2.087(8)
Å while the Xe-F terminal distances range from 1.940(8)
to 1.963(10) Å. The bridging As-F distances are 1.773(9)
and 1.786(9) Å while terminal As-F distances range from
1.701(12) to 1.726(10) Å.
octahedra are deformed (see Table 2).
Description of the Crystal Structure of [Mg(XeF2)2]-
(AsF6)2. Magnesium is octahedrally coordinated to six
fluorine atoms. Two fluorine atoms originate from two XeF2
molecules in the axial position and four fluorine atoms from
four AsF6 units in the equatorial plane. The two XeF2
(21) Bondi, A. J. Phys. Chem. 1964, 68, 441-451.
Inorganic Chemistry, Vol. 43, No. 2, 2004 701

Tramsˇek et al.
a
Table 3. Selected Bond Lengths and Angles in [Mg(XeF2)2](AsF6)2
Distance (Å) Angle (deg)
Mg1-F1, Mg1-F12
1.917(4)
F1-Mg1-F12
180.0(2)
180.0(2)
Mg1-F11, Mg1-F112
Mg1-F115, Mg1-F116
Xe1-F1
F11-Mg1-F115
2.019(3)
2.051(4)
1.913(5)
1.747(3)
1.708(7)
1.677(4)
1.669(8)
Mg1-F1-Xe1
F2-Xe1-F1
As1-F11-Mg1
F12-As1-F11
F13-As1-F11
F14-As1-F12
177.5(3)
179.5(3)
151.0(2)
87.9(2)
176.6(2)
174.2(4)
Xe1-F2
As1-F11, As1-F116
As1-F12
As1-F13
As1-F14
a Symmetry operations used for generating equivalent atoms: (2) -x,
-y, z; (3) x + 1/2, -y + 1/2, -z; (4) -x + 1/2, y + 1/2, -z; (5) -x, -y,
-z; (6) x, y, -z; (7) -x + 1/2, y + 1/2, z; (8) x + 1/2, -y + 1/2, z.
molecules and all four AsF6 units are crystallographically
identical. The structure is illustrated in Figures 2 and 3, and
selected bond lengths and angles are given in Table 3.
In the direction of the c axis, the Mg atoms are connected
by two cis-AsF6 units, with arsenic atoms in the mirror plane
perpendicular to the c axis, forming eight-membered rings
with the composition [Mg2(AsF6)2]2+. Because the Mg atoms
are at inversion centers, these rings are further connected,
forming infinite chains along the c axis (Figure 3). The
Mg-F1 distance of 1.917(4) Å is shorter than the Mg-F11
distance of 2.019(3) Å, resulting in compression of the
octahedron of fluorine atoms around the Mg atoms. The
Xe-F terminal distance is 1.913(5) Å while the Xe-F
bridging distance is 2.051(4) Å. The As-F bridging distance
is 1.747(3) Å, and the As-F terminal distances range from
1.669(8) to 1.708(7) Å. Positively charged Xe atoms interact
with negatively charged fluorine atoms arising from AsF6
units and XeF2 molecules of neighboring chains at distances
ranging from 3.245 to 3.440 Å. These electrostatic forces
apparently hold the neighboring chains together.
Figure 4. Raman spectra of [Mg(XeF2)n](AsF6)2 (n ) 4, 2).
When Mg(AsF6)2 was reacted with excess XeF2, using aHF
as a solvent, and Raman spectra were recorded during
isolation of the product, it was found that there was no free
XeF2 present when the mole ratio Mg:XeF2 was e1:6. The
compound [Mg(XeF2)6](AsF6)2 was not stable as a solid but
kept losing XeF2 in dynamic vacuum at room temperature
until a composition 1:4 was reached. Characterization of this
product by X-ray powder diffraction, Raman spectroscopy,
chemical analysis, and a single-crystal X-ray structure
determination established that the product is [Mg(XeF2)4]-
(AsF6)2. This compound is not stable but slowly loses XeF2
under dynamic vacuum at room temperature. The preparation
of pure 1:2 compound was possible only by reacting
stoichiometric amounts of Mg(AsF6)2 and XeF2 in aHF and
subsequently removing the solvent under dynamic vacuum
at 0 °C. The 1:2 compound was also characterized by X-ray
powder diffraction, Raman spectroscopy, chemical analysis
and a single-crystal X-ray structure determination.
Raman Spectroscopy. Raman spectra of [Mg(XeF2)n]
(AsF6)2, n ) 4 and 2, are shown in Figure 4.
Discussion
Synthesis. The lattice energy of Mg(AsF6)2 is rather low
-
as a consequence of the relatively large volume of AsF6
(110 Å3 22). The AsF6- anion is a weak Lewis base so even
poor Lewis base solvents, such as aHF, can provide suffi-
cient solvation energy to dissolve the Mg(AsF6)2 salt to form
-
[Mg(HF)n]2+ cations and AsF6 anions. XeF2 is a medium
Crystal Structures. The crystal structure of [Mg(XeF2)4]-
(AsF6)2 is the first molecular structure found in the system
Mx(AF6)x/XeF2/aHF (M ) Mg, Ca, Sr, Ba, La, Nd, Pb, Ag;
A ) P, As, Sb). In all other known coordination compounds
containing XeF2 as a fluorine-donating ligand, the metal ions
strong Lewis base, stronger than HF, and, on addition to such
a solution, competes not only with HF but also with AsF6
-
in providing Coulomb energy, because the charge on the F
ligands in free XeF2 is about -0.5e.23 The Mg2+ ion is, like
other alkaline earth cations, a relatively weak Lewis acid
and cannot withdraw F- from XeF2 to form an XeF+ salt.
The XeF2 molecule acts as a fluoro ligand, coordinating to
the magnesium ion as a consequence of the relatively high
charge on the fluorine atoms, the polarizability of the XeF2
molecule, and the partially covalent character of the Mg-F
bonds.
-
are connected, either by XeF2 molecules and AF6 (A )
As, Sb, P) anions1-8 forming polymeric compounds (chains,
layers, three-dimensional networks) or just by XeF2 mol-
ecules, as in the case of [Ca(XeF2)4](AsF6)2.7 The Mg-F(Xe)
and Mg-F(As) distances in the 1:4 compound are practically
the same. This implies that F ligands from monodentate
-
AsF6 can, in terms of relative Lewis basicity, compete
(22) Jenkins, H. D. B.; Roobottom, H. K.; Passmore, J.; Glasser, L. Inorg.
Chem. 1999, 38, 3609-3620.
(23) Jortner, J.; Wilson, E. G.; Rice, C. A. J. Am. Chem. Soc. 1963, 85,
814-815.
effectively with F ligands of XeF2 molecules in coordinating
to the Mg2+ ion. Significant elongation of the bridging As-F
bonds of each monodentate AsF6 unit (1.773(9) and 1.786-
702 Inorganic Chemistry, Vol. 43, No. 2, 2004

[Mg(XeF2)n](AsF6)2
(9) Å) also indicates accumulation of negative charge at those
F ligands relative to the terminal ones. A similar situation
was observed in the Ca compounds [Ca(XeF2)n](AsF6)2, n
) 4 and 2.5.7 In the compounds [Mg(XeF2)n](AsF6)2, n ) 4
and 2, all XeF2 molecules are nonbridging (i.e. they do not
connect two metal centers), which is again unique among
the compounds of the type [Mx(XeF2)n](AF6)x (M ) Mg,
Ca, Sr, Ba, Pb, Ag, La, Nd; A ) As, Sb, P). This indicates
that the electron charge of the XeF2 molecule is delocalized
toward the magnesium cation, which renders the XeF2
molecule less capable of bridging two magnesium cations.
In addition to bridging XeF2 molecules, nonbridging mol-
ecules of XeF2 were observed in [Ca(XeF2)4](AsF6)2 and in
[Ln(XeF2)2.5](AsF6)3 (Ln ) La,2 Nd3) while crystal structures
of [Ca(XeF2)2.5](AsF6)2, [M(XeF2)3](AsF6)2 (M ) Pb, Sr4,5),
[Ba(XeF2)5](SbF6)2,6 and [Ag(XeF2)2]AF6 (A ) As,1 P8)
contain no nonbridging XeF2 molecules. This is the conse-
quence of the decreasing Lewis acidity of the cation and
charge transfer from the XeF2 molecule to the cation due
the covalent character of the M-F bond. It appears that
compounds with magnesium cations prefer to crystallize in
molecular ([Mg(XeF2)4](AsF6)2) or chain ([Mg(XeF2)2]-
(AsF6)2) arrangements, while [Ln(XeF2)2.5](AsF6)3 (Ln ) La,2
Nd3) contains double chains, [Ca(XeF2)4](AsF6)27 is a layer
structure, [M(XeF2)3](AsF6)2 (M ) Pb, Sr)4 contains strongly
interconnected double layers, and [Ca(XeF2)2.5](AsF6)2,7
[Ba(XeF2)5](SbF6)2,6 and [Ag(XeF2)2]AF6 (A ) As,1 P8) form
a 3D network.
Raman Spectroscopy. The Raman spectra of [Mg(XeF2)n]-
(AsF6)2 with n ) 4 and 2 are shown in Figure 4. The high
polarizability of xenon usually results in intense Raman bands
for the symmetric Xe-F stretching modes. Modes involving
Mg-F and As-F vibrations are usually far less intense and
broader.
The totally symmetric (a1g) stretching mode for XeF2
occurs at 497 cm-1.25 When XeF2 is distorted by bridging
through one F atom to Mg2+ ion, the band at 497 cm-1 is
replaced by two bands: that at higher frequency is labeled
as the shorter bond Xe-F stretching (ν(Xe-F)), and the band
at lower frequency is labeled as the longer bond Xe-F
stretching (ν(Xe‚‚‚F)). In the 1:4 compound ν(Xe-F) is 565
cm-1, and in the 1:2 compound, it is 578 cm-1. The higher
frequency in the case of the latter is a consequence of a
shorter and therefore stronger Xe-F(terminal) bond. The
vibration of the longer Xe-F bond (ν(Xe‚‚‚F)) in the 1:4
compound is probably hidden under the broad band at 469
cm-1. In the case of the 1:2 compound, ν(Xe‚‚‚F) is at 412
cm-1. The origin of the band at 469 cm-1 is not clear but
could be a consequence of the vibrational coupling of four
nonbridging XeF2 molecules. A similar band was found in
the compounds with nonbridging XeF2 molecules, e.g.
[Ca(XeF2)4](AsF6)2 at 463 cm-1 and [Nd(XeF2)2.5](AsF6)3 at
461 cm-1.3,7
-
In both compounds, the Oh symmetry of the AsF6 octa-
hedra is reduced, due to interactions with Mg2+ ions. Thus,
instead of three Raman bands (ν1 685, ν2 576, and ν5 372
cm-1),26 more bands appear. The bands at 686, 704, 599,
and 380 cm-1 in the case of the 1:4 compound and 687,
739, and the 374 cm-1 in the 1:2 compound can be assigned
[Mg(XeF2)4](AsF6)2 is not stable in a dynamic vacuum at
room temperature, slowly losing XeF2 to yield [Mg(XeF2)2]-
(AsF6)2. The lost XeF2 ligands around the Mg2+ cation are
-
-
to As-F vibrations. The symmetry reduction of AsF6
replaced by F ligands from AsF6 units that transform
+
octahedra in the XeF+, XeF3 , and KrF+ salts has been
from monodentate to cis bridging units (compare Figures 1
and 2). The Mg-F(Xe) distances are significantly shorter
(1.917(4) Å) than the Mg-F(As) distances (2.019(3) Å),
reported by several authors in the past.27-29
As reported,30 the ν1 vibrations of matrix-isolated MgF2
were found at frequencies between 514 and 544 cm-1,
depending on the matrix used. The band at 510 cm-1 in the
Raman spectra of [Mg(XeF2)n](AsF6)2 can tentatively be
assigned to the Mg-F vibrations.
-
indicating that four cis-bridging AsF6 units in the Mg
coordination sphere are electrostatically and sterically less
favorable than the two F ligands of the XeF2 molecules for
coordinating a magnesium cation (Figure 2). In addition, the
Raman stretching frequency of the nonbridging XeF2 mol-
ecule (ν(Xe-F)) in the compound [Mg(XeF2)2](AsF6)2 is
higher (578 cm-1) than that in [Mg(XeF2)4](AsF6)2 (565
cm-1; see Raman spectroscopy), which can be attributed
to a higher positive charge on the magnesium cation in
Acknowledgment. The authors gratefully acknowledge
the financial support of the Ministry of Education, Science,
and Sport of the Republic of Slovenia. We thank Dr. M.
Ponikvar for the chemical analyses of these compounds.
Supporting Information Available: An X-ray crystallo-
graphic file in CIF format, a figure showing the arrangement of
[Mg(XeF2)4](AsF6)2 molecules, and tables of X-ray powder dif-
fraction data for [Mg(XeF2)n](AsF6)2 (n ) 4, 2). This material is
available free of charge via the Internet at http://pubs.asc.org.
-
[Mg(XeF2)2](AsF6)2. This shows that the four AsF6 units
accompanied by two XeF2 molecules in the magnesium
coordination sphere of [Mg(XeF2)2](AsF6)2 are weaker
-
electron donors that the two AsF6 units accompanied by
four XeF2 molecules in the magnesium coordination sphere
of [Mg(XeF2)4](AsF6)2. [Mg(XeF2)2](AsF6)2 has a structure
similar to that of [Mg(SO2)2](AsF6)2.24 The Mg-O distances
are 2.077(12) and 2.121(13) Å, showing that SO2 is a
weaker electron donor than the cis-bridged AsF6 unit. The
Mg-F(As) distances range from 1.953(11) to 1.987(10) Å.24
IC034826O
(25) Agron, P. A.; Begun, G. M.; Levy, H. A.; Mason, A. A.; Jones, C.
G.; Smith, D. F. Science 1963, 139, 842-844.
(26) Begun, G. M.; Rutenberg, A. C. Inorg. Chem. 1967, 6, 2212-2216.
(27) Gillespie, R. J.; Landa, B. Inorg. Chem. 1973, 12, 1383-1388.
(28) Gillespie, R. J.; Landa, B.; Schrobilgen, G. J. Inorg. Chem. 1976, 15,
1256-1263.
(29) Lehmann, J. F.; Dixon, D. A.; Schrobilgen, G. J. Inorg. Chem. 2001,
40, 3002-3017.
(30) Lisiecki, M. L.; Nibler, J. W. J. Chem. Phys. 1976, 64, 871-883.
(24) Hoppenheit, R.; Isenberg, W.; Mews, R. Z. Naturforsch. 1982, 37b,
1116-1121.
Inorganic Chemistry, Vol. 43, No. 2, 2004 703