Synthesis and structural investigation of the compounds containing HF2- anions: Ca(HF2)2, Ba4F4(HF2)(PF6)3 and Pb2F2(HF2)(PF6)

Article abstract of DOI:10.1016/j.jssc.2008.05.013

Three new compounds Ca(HF₂)₂, Ba₄F₄(HF₂)(PF₆)₃ and Pb₂F₂(HF₂)(PF₆) were obtained in the system metal(II) fluoride and anhydrous HF (aHF) acidified with excessive PF₅. The obtained polymeric solids are slightly soluble in aHF and they crystallize out of their aHF solutions. Ca(HF₂)₂ was prepared by simply dissolving CaF₂ in a neutral aHF. It represents the second known compound with homoleptic HF environment of the central atom besides Ba(H₃F₄)₂. The compounds Ba₄F₄(HF₂)(PF₆)₃ and Pb₂F₂(HF₂)(PF₆) represent two additional examples of the formation of a polymeric zigzag ladder or ribbon composed of metal cation and fluoride anion (MF⁺)_n besides PbF(AsF₆), the first isolated compound with such zigzag ladder. The obtained new compounds were characterized by X-ray single crystal diffraction method and partly by Raman spectroscopy. Ba₄F₄(HF₂)(PF₆)₃ crystallizes in a triclinic space group P1 with a=4.5870(2) A, b=8.8327(3) A, c=11.2489(3) A, α=67.758(9)°, β=84.722(12), γ=78.283(12)°, V=413.00(3) A³ at 200 K, Z=1 and R=0.0588. Pb₂F₂(HF₂)(PF₆) at 200 K: space group P1, a=4.5722(19) A, b=4.763(2) A, c=8.818(4) A, α=86.967(10)°, β=76.774(10)°, γ=83.230(12)°, V=185.55(14) A³, Z=1 and R=0.0937. Pb₂F₂(HF₂)(PF₆) at 293 K: space group P1, a=4.586(2) A, b=4.781(3) A, c=8.831(5) A, α=87.106(13)°, β=76.830(13)°, γ=83.531(11)°, V=187.27(18) A³, Z=1 and R=0.072. Ca(HF₂)₂ crystallizes in an orthorhombic Fddd space group with a=5.5709(6) A, b=10.1111(9) A, c=10.5945(10) A, V=596.77(10) A³ at 200 K, Z=8 and R=0.028.

Full text of DOI:10.1016/j.jssc.2008.05.013

ARTICLE IN PRESS
Journal of Solid State Chemistry 181 (2008) 2318– 2324
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Synthesis and structural investigation of the compounds containing HF2ꢀ
anions: Ca(HF2)2, Ba4F4(HF2)(PF6)3 and Pb2F2(HF2)(PF6)
Ã
ÃÃ
ˇ
ˇ
ˇ
Tina Bunic, Melita Tramsek, Evgeny Goreshnik , Boris Zemva
ˇ
Department of Inorganic Chemistry and Technology, Jozef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia
a r t i c l e i n f o
a b s t r a c t
Article history:
Three new compounds Ca(HF2)2, Ba4F4(HF2)(PF6)3 and Pb2F2(HF2)(PF6) were obtained in the system
metal(II) fluoride and anhydrous HF (aHF) acidified with excessive PF5. The obtained polymeric solids
are slightly soluble in aHF and they crystallize out of their aHF solutions. Ca(HF2)2 was prepared by
simply dissolving CaF2 in a neutral aHF. It represents the second known compound with homoleptic
HF environment of the central atom besides Ba(H3F4)2. The compounds Ba4F4(HF2)(PF6)3 and
Pb2F2(HF2)(PF6) represent two additional examples of the formation of a polymeric zigzag ladder or
ribbon composed of metal cation and fluoride anion (MF+)n besides PbF(AsF6), the first isolated
compound with such zigzag ladder. The obtained new compounds were characterized by X-ray single
crystal diffraction method and partly by Raman spectroscopy. Ba4F4(HF2)(PF6)3 crystallizes in a triclinic
Received 15 February 2008
Received in revised form
23 April 2008
Accepted 5 May 2008
Available online 16 May 2008
Keywords:
Poly-(hydrogen fluorides)
Hydrogen bonding
Crystal structure
Raman spectrum
˚
˚
˚
¯
space group P1 with a ¼ 4.5870(2) A, b ¼ 8.8327(3) A, c ¼ 11.2489(3) A, a ¼ 67.758(9)1, b ¼ 84.722(12),
g ¼ 78.283(12)1, V ¼ 413.00(3) A3 at 200 K, Z ¼ 1 and R ¼ 0.0588. Pb2F2(HF2)(PF6) at 200 K: space group
˚
+
(MF)n ribbon
˚
˚
˚
¯
P1, a ¼ 4.5722(19) A, b ¼ 4.763(2) A, c ¼ 8.818(4) A, a ¼ 86.967(10)1, b ¼ 76.774(10)1, g ¼ 83.230(12)1,
Homoleptic HF environment
3
˚
˚
¯
V ¼ 185.55(14) A , Z ¼ 1 and R ¼ 0.0937. Pb2F2(HF2)(PF6) at 293 K: space group P1, a ¼ 4.586(2) A,
3
˚
˚
˚
b ¼ 4.781(3) A, c ¼ 8.831(5) A, a ¼ 87.106(13)1, b ¼ 76.830(13)1, g ¼ 83.531(11)1, V ¼ 187.27(18) A ,
˚
Z ¼ 1 and R ¼ 0.072. Ca(HF2)2 crystallizes in an orthorhombic Fddd space group with a ¼ 5.5709(6) A,
3
˚
˚
˚
b ¼ 10.1111(9) A, c ¼ 10.5945(10) A, V ¼ 596.77(10) A at 200 K, Z ¼ 8 and R ¼ 0.028.
& 2008 Elsevier Inc. All rights reserved.
1. Introduction
metal centers forming close packed three-dimensional networks.
Various compounds with HF acting as a ligand bonded directly
to the metal center of the general formula [Mn+(HF)x](AF6)n are
also known. The first such compound was [La(HF)2](AsF6)3 [6].
Recently, the following compounds were isolated and character-
ized: [Pb(HF)](AsF6)2 [7], [Cd(HF)](AsF6)2 [8], [M(HF)2](SbF6)2
(M ¼ Mg, Ca) [9], [Ca(HF)6](AsF6)2, [Ca(HF)](AsF6)2 [10], [Au(HF)2]
(SbF6)2 ꢁ 2HF [11], [(OsO3F)(HF)2](AsF6)2 [12], [Hg2(OH2)2] (SbF6)2 ꢁ
4HF [13] and [Ir(CO)6] (SbF6)3 ꢁ 4HF [14].
It has been known for a long time that the metal fluorides form
complex salts with HF [1,2]. These salts are important inorganic
compounds that have found extensive use at the inorganic
synthesis in the laboratory and in the engineering, e.g., in the
production of elemental fluorine and hydrogen fluoride [3]. They
contain HFꢀ2 bifluoride anions, which are characterized by the
shortest hydrogen bond known to date. The investigation of the
crystal structures of the bifluorides and other salts with HnFn+1
anions is of particular importance in the elucidation of the nature
of a strong hydrogen bonding. The structures of the acidic
fluorides of the general formula MHF2 (M ¼ Li, Na, K, Rb, Cs)
and Rb5F4(HF2) ꢁ 2H2O have been determined [4].
2. Experimental
2.1. General experimental procedures
In the recent years, we have synthesized compounds with
poly-(hydrogen fluoride) anions bonded as ligands to the metal
cation: M2(H2F3)(HF2)2(AF6) (M ¼ Ca, A ¼ As; M ¼ Sr, A ¼ As, P)
[5]. In these polymeric compounds, H2Fꢀ3 and HF2ꢀ anions bridge
A nickel vacuum line and Teflon vacuum system were used as
previously described [15]. Volatile materials (aHF, PF5) were
manipulated in an all-Teflon vacuum line equipped with Teflon
valves. Non-volatile materials sensitive to traces of moisture were
handled in the dry argon atmosphere in a glove box with
maximum content of 0.1 ppm of water vapor (MBraun, Garching,
Germany). A FEP reaction vessel equipped with a Teflon valve and
a Teflon-covered mixing bar was used for the syntheses. The
Ã
ÃÃ
Corresponding author. Fax: +38614773155.
Also to be corresponded to.
E-mail addresses: evgeny.goreshnik@ijs.si (E. Goreshnik),
ˇ
boris.zemva@ijs.si (B. Zemva).
0022-4596/$ - see front matter & 2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2008.05.013

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T. Bunic et al. / Journal of Solid State Chemistry 181 (2008) 2318–2324
2319
crystals were grown in a crystallization vessel made from a
2.4. Preparation of single crystals
T-shaped FEP reaction vessel, constructed from one 16 mm i.d. and
one 4 mm i.d. FEP tubes joint at a right angle and equipped with a
Teflon valve.
The 100–200 mg of the compounds Ca(HF2)2, Pb2F2(HF2)(PF6)
and Ba4F4(HF2)(PF6)3 obtained during the syntheses (see
Section 2.3) were transferred in the dry box into the FEP
crystallization vessel. Anhydrous HF was added and a saturated
solution was prepared. This solution was decanted into a narrower
part of the reaction vessel, which was left at room temperature
while the wider part was cooled with tap water. In this way, a
small temperature gradient was achieved. The crystallization
proceeded for several days. Crystals were isolated by pumping off
aHF on the vacuum line at room temperature. Inside the dry box,
the crystals were put in a perfluorinated oil (ABCR, FO5960),
selected under the microscope outside of the dry box and
transferred into the cold nitrogen stream at the X-ray diffract-
ometer. One of Pb2F2(HF2)(PF6) crystals was placed in the capillary
and its structure determination has been performed also at room
temperature.
2.2. Reagents
PbF2, CaF2 (Merck, Suprapur), BaF2 (Alfa Aesar, 99.99%) and
fluorine (Solvay, 99.98%) were used as purchased. PF5 was
prepared by fluorination of P2O5 powder under high-pressure as
previously described [16]. Its purity was checked by IR spectro-
scopy. Anhydrous HF (Fluka, purum) was treated with K2NiF6
(Ozark-Mahoning, 99%) for several days prior to use. Caution:
anhydrous hydrogen fluoride and PF5 must be handled in a well-
ventilated hood and protective clothing must be worn all the
times! The experimentalist must become familiar with these
reagents and the hazards associated with them. Fresh tubes of
calcium gluconate gel should always be at hand for the fast
treatment of skin exposed to these reagents. For treatment of HF
injuries, see Ref. [17].
Crystals of Ba4F4(HF2)(PF6)3 were grown also directly from the
reaction mixture. BaF2 (0.088 g, 0.502 mmol) was added into the
crystallization vessel which was cooled with liquid nitrogen and
2.3. Syntheses
Table 2
˚
Selected bond distances (A) in Ba4F4(HF2)(PF6)3
The compounds Ca(HF2)2, Pb2F2(HF2)(PF6) and Ba4F4(HF2)(PF6)3
were synthesized in the system MF2–PF5–aHF. The metal(II)
fluoride (M ¼ Ca, Ba, Pb) was weighed into the reaction vessel
inside the dry box. The reaction vessel was evacuated and then
cooled down to ꢀ196 1C. Anhydrous HF was added by sublimation.
Then the reaction vessel was warmed up to room temperature and
weighed. Suspension was cooled again in liquid nitrogen and excess
of PF5 was added. The reaction proceeded at room temperature.
After the reaction aHF and excessive PF5 were carefully removed at
temperature around ꢀ301C. The obtained products in the case
of Ba and Pb are loosing PF5 in a dynamic vacuum at room
temperature. Details of the syntheses are CaF2 (0.166 g, 2.13 mmol),
PF5 (0.321 g, 2.55 mmol), product (0.385 g, 3.2 mmol); PbF2
(0.448 g, 1.83 mmol), PF5 (0.243 g, 1.93mmol), product (0.636 g,
1.00 mmol); BaF2 (0.259g, 1.47 mmol), PF5 (0.107 g, 0.848mmol),
product (0.557 g, 0.51 mmol).
Ba1–F3
2.276(11)
2.450(12)
2.475(12)
2.478(12)
2.775(11)
2.984(14)
3.050(13)
3.050(12)
3.160(14)
3.163(12)
3.237(13)
3.364(18)
Ba2–F2
2.325(12)
2.410(12)
2.516(12)
2.525(12)
2.587(11)
2.878(13)
2.914(13)
2.927(12)
3.125(15)
3.221(14)
3.245(13)
3.502(23)
Ba1–F2i
Ba1–F1
Ba2–F3
Ba2–F1iv
Ba2–F3v
Ba1–F2
Ba1–F11
Ba1–F22
Ba1–F16
Ba1–F13ii
Ba1–F21
Ba1–F13iii
Ba1–F16ii
Ba1–F14ii
Ba2–F15
Ba2–F23vi
Ba2–F14vii
Ba2–F12
Ba2–F22iv
Ba2–F21viii
Ba2–F23iv
Ba2–F11v
P1–F11v
P1–F12ix
P1–F13
P1–F14
P1–F15
P1–F16
1.594(12)
1.596(13)
1.600(13)
1.615(13)
1.622(13)
1.634(13)
P2–F21x
P2–F21i
P2–F22xi
P2–F22
P2–F23
P2–F23xi
1.587(13)
1.587(13)
1.595(14)
1.595(14)
1.603(13)
1.603(13)
Ca(HF2)2 could also be prepared by direct reaction between
CaF2 (0.0943 g, 1.21 mmol) and HF (3.5 mL), product (0.150 g,
1.27 mmol). Ca(HF2)2 was also found as by-product of the partial
decomposition of Ca(RuF6)2 ꢁ nXeF2 in aHF solution.
Symmetry codes: (i) xꢀ1, y, z; (ii) ꢀx+1, ꢀy+1, ꢀz+1; (iii) ꢀx+2, ꢀy+1, ꢀz+1; (iv)
ꢀx+1, ꢀy+1, ꢀz+2; (v) x+1, y, z; (vi) ꢀx, ꢀy+1, ꢀz+2; (vii) ꢀx+1, ꢀy+2, ꢀz+1; (viii) x,
y+1, z; (ix) ꢀx+2, ꢀy+2, ꢀz+1; (x) ꢀx, ꢀy, ꢀz+2; (xi) x, yꢀ1, z.
Table 1
Crystal data and structure refinement of Ba4F4(HF2)(PF6)3, Pb2F2(HF2)(PF6) and Ca(HF2)2
Formula
Ba4F24HP3
Pb2F10HP
Pb2F10HP
CaF4H2
Fw
1099.24
200
636.362
200
636.362
293
118.088
200
T (K)
¯
P1
¯
P1
¯
P1
Space group
Fddd
˚
a (A)
4.5870(2)
8.8327(3)
11.2489(3)
67.758(9)
84.722(12)
78.283(12)
413.00(3)
1
4.5722(19)
4.763(2)
8.818(4)
86.967(10)
76.774(10)
83.230(12)
185.55(14)
1
4.586(2)
4.781(3)
8.831(5)
87.106(13)
76.830(13)
83.531(11)
187.27(18)
1
5.5709(6)
10.1111(9)
10.5945(10)
90
˚
b (A)
˚
c (A)
a (deg)
b (deg)
g (deg)
90
90
3
˚
V (A )
596.77(10)
8
Z
Dcalculated (g/cm3)
4.42
5.695
5.643
2.629
˚
l (A)
0.71069
9.943
0.71069
45.661
0.71069
45.242
0.71069
2.007
m (mmꢀ1
)
R1a; wR2
0.0588; 0.1761
1.145
0.0937; 0.2466
1.073
0.072; 0.192
1.08
0.028; 0.0679
1.205
GOF
P
P
P
P
R1 ¼ ||Fo|–|Fc||/ |Fo|, wR2 ¼ ½wðF2o ꢀ F2c Þ Þ= wðFo2Þ ꢂ1=2, GOF ¼ ½ wðFo2 ꢀ Fc2Þ Þ=No ꢀ NpÞꢂ1=2, where No is no. of reflns and Np is no. of refined parameters.
a
2
2
2

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2320
Table 3
aHF was added by sublimation. Then the crystallization vessel was
˚
Selected bond distances (A) in Pb2F2(HF2)(PF6)
warmed up to room temperature and weighed. Solution was
cooled again in liquid nitrogen and excess of PF5 (0.332 g,
2.63 mmol) was added. The saturated solution was decanted into
the narrower part of the reaction vessel, which was left at room
temperature while the wider part was cooled with tap water.
Crystals were isolated by pumping off aHF and PF5 at ꢀ35 1C.
Pb1–F2
2.307(15)
2.312(12)
2.460(17)
2.459(14)
2.473(16)
2.469(15)
2.48(2)
Pb1–F12iii
Pb1–F11iii
Pb1–F13iii
P1–F13iv
P1–F13
3.22(3)
3.26(3)
Pb1–F2i
Pb1–F2ii
Pb1–F1
3.30(2)
3.30(2)
3.37(3)
3.41(3)
1.566(16)
1.619(13)
1.566(16)
1.619(13)
1.587(18)
1.602(17)
1.587(18)
1.602(17)
1.597(18)
1.579(17)
1.597(18)
1.579(17)
2.474(16)
2.55(2)
2.5. Crystal structure determination
Pb1–F1iii
Pb1–F13vi
Pb1–F11vii
Pb1–F12
2.582(16)
2.92(3)
P1–F12
Data were collected on Rigaku AFC7S diffractometer equipped
by Mercury CCD area detector using graphite monochromated
MoKa radiation. The treatment of all data was performed using
CrystalClear program package [18]. The data were corrected for
Lorentz and polarization effects. A multiscan absorption correc-
tion was applied to all datasets. All structures were solved by
direct methods using SIR-92 [19] program implemented in
program package TeXsan [20] and refined with SHELXL-97 [21]
software (program packages TeXsan and WinGX [22]). In the case
of Pb2F2(HF2)(PF6) compound, majority of checked crystals
2.85(3)
2.99(3)
P1–F12iv
P1–F11iv
P1–F11
3.01(3)
2.99(3)
2.97(3)
Symmetry codes: (i) ꢀx+1, ꢀy, ꢀz+1; (ii) ꢀx, ꢀy, ꢀz+1; (iii) x, y+1, z; (iv) ꢀx, ꢀy–1,
ꢀz; (v) x, yꢀ1, z; (vi) 1+x, 1+y, z; (vii) 1ꢀx, ꢀ1ꢀy, ꢀz.
Regular font: data at 200 K; italic font: data at room temperature.
Fig. 1. Coordination of barium atoms and bridging function of PFꢀ6 anions in Ba4F4(HF2)(PF6)3.
Fig. 2. Packing in the structure of Ba4F4(HF2)(PF6)3.

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2321
demonstrate a reproducible affinity to twinning with the rotating
of sub-domains on 1801 around a-axis. The data were treated
using Twinsolve [23] software, and hklf5 reflections file was
generated. The final refinement has led to 0.23:0.77 ratio of twin
domains. In this compound, as well as in the compound
Ba4F4(HF2)(PF6)3, the positions of hydrogen atoms were found
on the basis of the geometrical reasons. In the case of the
compound Ca(HF2)2, the positions of the hydrogen atoms were
found on a difference Fourier map. The figures were prepared
using DIAMOND 3.1 software [24]. The crystal data and the details
of structure refinement for these compounds are given in Table 1,
selected bond distances for compounds Pb2F2(HF2)(PF6) and
Ba4F4(HF2)(PF6)3 are placed in Tables 2 and 3.
System 1000 with the exiting line at 632.8 nm of a He–Ne laser.
Geometry for all Raman experiments was 1801 back scattering
with a laser power of 25 mW.
3. Results and discussion
3.1. Syntheses
In the case of the compounds Ba4F4(HF2)(PF6)3 and
Pb2F2(HF2)(PF6), the corresponding metal(II) fluoride reacted with
PF5 and HF in aHF solution yielding first M(PF6)2 or M(PF6)2(HF)n
derivatives, which are stable in the presence of excessive PF5. After
the excess of PF5 was pumped away, these compounds start to
decompose in now neutral aHF. PFꢀ6 anions (RuF6ꢀ in the case of Ca
derivative) could be a source of Fꢀ anions in the solution. Because
of the competition between metal centers and hydrogen atoms for
bonding with fluoride anions, the metal coordination sphere
containing Fꢀ, HF2ꢀ and, possibly, HF and PFꢀ6 moieties forms. The
crystallization process completely removed the solvent from
the metal environment with a formation of the title compounds.
The equations for such a process could be written as follows:
2.6. Raman spectroscopy
Raman spectra of the powdered samples in the sealed quartz
capillaries were taken on a Renishaw Raman Imaging Microscope
4BaðPF6Þ2 þ HF ! Ba4F4ðHF2ÞðPF6Þ3 þ 5PF5m
2PbðPF6Þ2 þ HF ! Pb2F2ðHF2ÞðPF6Þ þ 3PF5m:
The crystals of the compound Ca(HF2)2 were found in three
different reaction systems: CaF2–HF, CaF2–PF5–HF and CaF2–
RuF5–XeF2–HF. The main reason for obtaining the same com-
pound in all three systems is rather low solubility of Ca(HF2)2 and
its relatively high stability.
3.2. Crystal structures
3.2.1. Description of crystal structure of Ba4F4(HF2)(PF6)3
The crystal structure of the compound Ba4F4(HF2)(PF6)3
contains two crystallographically independent barium atoms.
The coordination sphere of Ba1 atom includes 12 fluorine atoms:
one F center belongs to HFꢀ2 moiety, three other are Fꢀ anions, five
fluorine atoms are from five mono-dentate PFꢀ6 anions and the last
three F atoms are from tri-dentate PFꢀ6 unit. Ba1–F distances vary
from 2.276(11) to 2.478ꢀ(12) A, Ba1–F(HFꢀ2 ) bond length is
˚
˚
2.475(12) A. The Ba1–F(PF6 ) distances appear to be much longer
˚
Fig. 3. Coordination sphere of the lead atom in Pb2F2(HF2)(PF6).
(2.775(11) to 3.36(2) A). The environment of Ba2 also consists of
Fig. 4. Double layers in Pb2F2(HF2)(PF6).

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2322
12 F centers: one from HFꢀ2 unit, three Fꢀ anions, three fluorine
atoms from three mono-dentate PFꢀ6 anions, two from bi-dentate
PFꢀ6 moiety and three more from tri-dentate PF6ꢀ unit (Fig. 1) with
the bond distances of 2.325(12)–2.525(12), 2.516(12) and
these puckered layers (Fig. 5). Each F center act as m2-bridge,
and PFꢀ6 unit is connected to six lead atoms. Similarly to
Ba4F4(PF6)3HF2 structure symmetrical HF2 unit with F?F distance
of 2.32(4) A is bound to four metal centers located roughly on the
common plane.
˚
2.587(11)–3.50(2) A for the bonds Ba2–F, Ba2–F(HFꢀ2 ) and
˚
Ba2–F(PFꢀ6 ), respectively (Table 2). The last value coincides
practically with the sum oꢀf Van der Waals radii of Ba and F atoms
3.2.3. Description of crystal structure of Ca(HF2)2
˚
(3.47 A). Ba cations and F anions form infinite ribbons. Similar
Ca(HF2)2 compound represents a unique case of homoleptic HF
environment of Ca atom. Each cation is surrounded by eight F
atoms from eight HFꢀ2 moieties (Fig. 6). Because of the location of
the metal atom at 8a Wyck off position with 222 symmetry, Ca–F
infinite metal-fluoride fragments were found earlier in PbF(AsF6)
[7]. These ribbons, in turn, are interconnected by HFꢀ2 anions into
infinite slabs oriented along a-axis. Each HFꢀ2 unit is linked to four
barium atoms located roughly on the common plane. The HF2ꢀ
anion demonstrates symmetrical geometry with hydrogen atom
˚
distances form two sets of 4 ꢃ 2.3492(11) and 4 ꢃ 2.3678(11) A.
˚
located on inversion center and F?F distance of 2.28(2) A. The
crystallografically independent PFꢀ6 anions differ in the coordina-
tion behavior. In P(1)F6 anion, connected to seven Ba cations, four
F centers demonstrate m3-bridging functions, and two other are
m2-bridges. In the case of P2 anion, all fluorine atoms act as
m3-bridges, and the P(2)F6 anion is bound to eight metal centers.
Because of the difference in the shortest Ba–F(PFꢀ6 ) bond lengths,
P(1)Fꢀ6 anion will be described as strongly bonded (Ba–Fmin
2.59(1) A, shown as balls-and-sticks), and P(2)Fꢀ6 moiety as weakly
˚
˚
bonded (Ba–Fmin 2.88(2) A, drawn in polyhedra style) to the metal
centers. Mentioned above slabs are ‘‘capped’’ by strongly bonded
PFꢀ6 anions, and in between are located weakly bonded PFꢀ6 units
(Fig. 2).
3.2.2. Description of crystal structure of Pb2F2(HF2)(PF6)
In the structure of Pb2F2(HF2)(PF6), lead atom possess
coordination number 8+3: two F atoms belong to two HF2ꢀ
˚
moieties with the Pb–F distances of 2.48(2) and 2.55(2) A, three
other are Fꢀ anions (Pb–F 2.307(15)–2.473(16) A), three fluorine
˚
atoms are from three mono-dentate PFꢀ6 anions (Pb–F 2.92(2)–
2.99(2) A) and three more from tri-dentate PFꢀ6 unit (Pb–F
˚
˚
3.27(3)–3.36(3) A) (Fig. 3; Table 3). Metal and fluorine atoms also
form infinite double ribbons. Contrary to previously discussed
structure, HF2 moieties link such ribbons into infinite double
layers (Fig. 4). Weakly bonded PFꢀ6 anions are located between
Fig. 6. Homoleptic environment of Ca atoms and bridging role of HF2 anions in
Ca(HF2)2.
Fig. 5. Packing diagram of Pb2F2(HF2)(PF6).

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2323
Fig. 7. 3D network in Ca(HF2)2.
[26]. In the Raman spectrum of Ba4F4(HF2)(PF6)3, the band at
754 cmꢀ1 can be confidentially assigned to n1 of PFꢀ6 while the
vibrations at 570 and 473 cmꢀ1 could be assigned to n2 and n5 of
PFꢀ6 anion, respectively. The weak band at 592 cmꢀ1 could be
assigned to n1(Sg+)nsym) of the anion HF2ꢀ (Fig. 8). In [N(CH3)4]HF2
the peak at 576 cmꢀ1 [25], in NaHF2 the peak at 630 cmꢀ1 [27] and
in Sr2(H2F3)(HF2)2(AsF6) the peak at 620 cmꢀ1 [5] were assigned
to the symmetric HFꢀ2 anion.
In the Raman spectrum of Pb2F2(HF2)(PF6), the peaks at 751,
567 and 469 cmꢀ1 could be assigned to n1, n2 and n5 of PFꢀ6 anion,
respectively. The peak which could be assigned to n1 of the anion
HFꢀ2 is either too weak to be seen or the compound decomposed in
the laser beam.
In the case of Ca(HF2)2, we were not able to obtain a reasonable
Raman spectrum.
4. Supplementary material
Further details of the crystal structure investigation(s) can be
obtained from the Fachinformationszentrum Karlsruhe, 76344
Eggenstein-Leopoldshafen, Germany (fax: (49) 7247-808-666;
e-mail: crysdata@fiz.karlsruhe.de) on quoting the depository
numbers CSD-419141 for Pb2F2(HF2)(PF6) at 293 K, CSD-419142
for Pb2F2(HF2)(PF6) at 200 K, CSD-419143 for Ba4F4(HF2)(PF6)3,
CSD-419144 for Ca(HF2)2.
Fig. 8. Raman spectra of Pb2F2(HF2)(PF6) (top) and Ba4F4(HF2)(PF6)3 (bottom).
In turn, each anion is bonded to four metal centers. Fluorine atoms
of the HFꢀ2 anion are related through a crystallographic two-fold
axis and the hydrogen atom was refined on the special position on
axes 2. The F?F distance of 2.283(2) A) in HFꢀ2 moiety is practically
˚
the same as in Ba4F4 ꢁ HF2(PF6)3 and Pb2F2(HF2)(PF6) is very similar
˚
to that in KHF2 (2.277(6)A), and is somewhat longer than that in
˚
the crystal structure of [N(CH3)4][HF2] (2.213(4)A) [25]. Contrary to
Acknowledgment
earlier discussed structures of Ba4F4 ꢁ HF2(PF6)3 and Pb2F2(HF2)(PF6)
compounds, metal atoms bonded to one HFꢀ2 unit, are mutually
The authors gratefully acknowledge to the Slovenian Research
Agency (ARRS) for the financial support of the Research Program
P1-0045 (Inorganic Chemistry and Technology).
oriented in tetrahedral manner, resulting in
complicated 3D structure (Fig. 7).
a formation of
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In the case of Oh symmetry of the PFꢀ6 anion n1 is found at
756 cmꢀ1, n2 at 570 and 580 cmꢀ1 and n5 at 468, 475 and 480 cmꢀ1

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