Dilead(II) chromium(III) hepta­fluoride

Single crystals of the title compound, Pb₂CrF₇, were obtained by solid-state reaction. The monoclinic structure is isotypic with Pb₂RhF₇ and is built up of CrF₆³⁻ octa­hedra isolated from each other, inserted in a fluorite-related matrix of PbF₆ distorted octa­hedra, and PbF₈ square anti­prisms sharing edges and corners. The seventh F atom is `independent', connected only to three Pb atoms within FPb₃ triangles, sharing an edge and building an almost planar Pb₄F₂ unit, so that the formula can alternatively be written as Pb₂F(CrF₆).

Related literature

For the Pb₂RhF₇ structure-type, see: Domesle & Hoppe (1983), and for isostructural Sr₂RhF₇, see: Grosse & Hoppe (1987). For the indexed powder pattern of Pb₂CrF₇, see: de Kozak et al. (1999). For other compounds containing `independent' fluorine atoms coordinated to three cations (Ca or Sr) in a plane, see: Ca₂AlF₇ (Domesle & Hoppe, 1980); Sr₅Zr₃F₂₂ (Le Bail, 1996); Sr₅(VOF₅)₃F(H₂O)₃ (Le Bail et al., 2009). For fluorite-related lead-based compounds, see: Pb₈MnFe₂F₂₄ (Le Bail & Mercier, 1992); Pb₂ZrF₈ (Le Bail & Laval, 1998). For the structure simulation of fluoride glasses containing lead by using crystalline models, see: Le Bail (1989, 2000). For details and parameters of the bond-valence model, see: Brown & Altermatt (1985); Brese & O'Keeffe (1991).

Experimental

Crystal data

• Pb₂CrF₇

• M_r = 599.40

• Monoclinic, P 2₁ /c

• a = 5.4626 (7) Å

• b = 11.2085 (15) Å

• c = 9.5738 (11) Å

• β = 91.197 (10)°

• V = 586.05 (13) ų

• Z = 4

• Mo Kα radiation

• μ = 59.61 mm⁻¹

• T = 293 K

• 0.12 × 0.07 × 0.01 mm

Data collection

• Siemens AED2 diffractometer

• Absorption correction: Gaussian (SHELX76; Sheldrick, 2008) T_min = 0.011, T_max = 0.321

• 6770 measured reflections

• 2506 independent reflections

• 2082 reflections with I > 2σ(I)

• R_int = 0.041

• 3 standard reflections every 120 min intensity decay: 15%

Refinement

• R[F² > 2σ(F²)] = 0.036

• wR(F²) = 0.084

• S = 1.64

• 2506 reflections

• 93 parameters

• Δρ_max = 3.17 e Å⁻³

• Δρ_min = −2.20 e Å⁻³

Table 1
Selected bond lengths (Å)

Pb1—F1	2.324 (5)
Pb1—F7^i	2.437 (5)
Pb1—F4^ii	2.439 (5)
Pb1—F5^iii	2.620 (6)
Pb1—F6^iv	2.640 (7)
Pb1—F2^v	2.899 (6)
Pb1—F6^v	3.067 (7)
Pb2—F1	2.412 (5)
Pb2—F1^ii	2.441 (4)
Pb2—F5^i	2.497 (6)
Pb2—F3^vi	2.512 (6)
Pb2—F2^iv	2.586 (5)
Pb2—F6	2.632 (6)
Pb2—F3^iv	2.653 (5)
Pb2—F7	2.743 (6)
Cr—F2	1.883 (5)
Cr—F3	1.900 (5)
Cr—F4	1.907 (5)
Cr—F5	1.909 (5)
Cr—F6	1.912 (6)
Cr—F7	1.931 (4)
Symmetry codes: (i) ; (ii) -x, -y+1, -z; (iii) x, y+1, z; (iv) ; (v) -x+1, -y+1, -z; (vi) .

Table 2
Valence-bond analysis according to the empirical expression from Brown & Altermatt (1985), using parameters for solids from Brese & O'Keeffe (1991), with two results for Pb1 and Pb2 according to their coordinations (respectively VI or XI, and VIII or IX)

	Cr	Pb1	Pb2	Σ	Σexpected
F1		0.45	0.36 + 0.33	1.14	1
F2	0.52	0.10 + 0.05	0.22	0.89	1
F3	0.50	0.04	0.27 + 0.19	1.00	1
F4	0.49	0.33 + 0.04	0.05	0.91	1
F5	0.48	0.20	0.28	0.96	1
F6	0.48	0.19 + 0.06	0.20	0.93	1
F7	0.46	0.33 + 0.03	0.15	0.97	1
Σ	2.93	1.60 (VI)	2.00 (VIII)
or		1.82 (XI)	2.05 (IX)
Σexpected	3	2	2

Table 3
Comparison of the cell parameters of the three isostructural hepta­fluorides showing a noticeable anomaly on the c parameter of the chromium compound

Formula	a	b	c	β	V
Pb2RhF7^a	5.569	11.854	8.832	91.00	582.96
Sr2RhF7^b	5.510	11.628	8.640	90.98	553.49
Pb2CrF7^c	5.463	11.208	9.574	91.20	586.05
Notes: (a) Domesle & Hoppe (1983); (b) Grosse & Hoppe (1987); (c) this work.

Data collection: STADI4 (Stoe & Cie, 1998); cell refinement: STADI4; data reduction: X-RED (Stoe & Cie, 1998); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2005) and ORTEP-3 (Farrugia, 1997); software used to prepare material for publication: publCIF (Westrip, 2010).

Supporting information

S1. Comment

Continuing investigations of crystalline complex compounds of lead and 3d metal cation fluorides having chemical formulations similar to some fluoride glasses for structure modelling purposes (Le Bail, 2000), such as Pb₈MnFe₂F₂₄ (Le Bail & Mercier, 1992) or NaPbFe₂F₉ (Le Bail, 1989), the title compound, Pb₂CrF₇, was synthesized and characterized from single crystal X-ray data. The results confirm that it is isostructural with Pb₂RhF₇ (Domesle & Hoppe, 1983), as mentioned previously by de Kozak et al. (1999). The monoclinic structure is built up of CrF₆^3- octahedra (Fig. 1) isolated from each other, inserted in a fluorite-related matrix of PbF₆ distorted octahedra and PbF₈ square antiprisms sharing edges and corners. The seventh fluorine atom is "independent", connected only to 3 Pb atoms in FPb₃ triangles, sharing an edge and building an almost planar Pb₄F₂ unit (Fig. 2), so that the formula can be alternatively written as Pb₂F(CrF₆). There are some differences observed with the Pb₂RhF₇ structure-type. The Pb1 atom appears to be six-coordinated in Pb₂CrF₇ with Pb—F distances ranging from 2.324 (5) to 2.899 (6) Å, there are then 5 next F atoms being between 3.067 (7) and 3.364 (6) Å whereas in Pb₂RhF₇, Pb1 looks eightfold coordinated (Pb—F distances between 2.375 and 2.744 Å), with two next F atoms at 3.105 and 3.219 Å. The bond valence calculations show that the first 6 F atoms around Pb1 cannot really satisfy the Pb²⁺ charge and that the 5 next F atoms contribute to the overall bond valence (Table 2). The behaviour of the cell parameters is surprising in the series of the three isostructural compounds. If all cell parameters are logically smaller in Sr₂RhF₇ (Grosse & Hoppe, 1987) than in Pb₂RhF₇, due to the smaller Sr²⁺ size, in Pb₂CrF₇ one observes that only a and b are smaller but that c is much larger, in spite of the Cr³⁺ cation being smaller than Rh³⁺ (Table 3). Finally, the cell volume of Pb₂CrF₇ is even slightly larger than that of Pb₂RhF₇.

Usually, such fluorinated phases with high A^II/M ratio (A^II = Ca, Pb, Sr ; M = 3d element or In, Nb, Zr, etc), are found to be related to the fluorite structure adopted by PbF₂ or SrF₂. The relation is in general easy to establish due to the presence of "independent" F atoms (not bonded to M) which are found to form characteristic FPb₄ tetrahedra. But this is not obvious here (Fig. 3) since no such FPb₄ tetrahedron is observed. Other compounds containing "independent" fluorine atoms coordinated to three cations (Ca or Sr) in a plane are Ca₂AlF₇ (Domesle & Hoppe, 1980), Sr₅Zr₃F₂₂ (Le Bail, 1996) and Sr₅(VOF₅)₃F(H₂O)₃ (Le Bail et al., 2009), the last two being strongly related to the fluorite structure in which the FSr₃ triangles were found to interconnect the fluorite-related blocks. An examination of the Pb coordinates in Pb₂CrF₇ along the a axis (which is close to the PbF₂ fluorite cell parameter) shows that they alternate at values x~1/4 and 3/4, forming fluorite-related strips corrugating in the ac plane where highly distorted PbF₈ cubes as expected in the fluorite structure were obtained by small displacements of some of the F atoms as evidenced in Fig. 4 . Similar corrugated strips were observed for the Pb₂ZrF₈ structure (Le Bail & Laval, 1998), where the Zr⁴⁺ cations occupy bicapped trigonal prisms at positions similar to those of Cr³⁺ in the title compound. In Pb₂ZrF₈, the Pb²⁺ lone pair position was suggested by comparison of the Pb coordination with that of Ba in the isostructural α-Ba₂ZrF₈ structure, observing some strong distorsions, the stereochemichally active lone pair repelling clearly some F atoms. The same exercice is not so obvious here, even by comparing Sr₂RhF₇ with Pb₂RhF₇. However, there is a specific pattern of differences in the bond lengths (3 adjacent short Pb—F bonds and 3 adjacent long ones for Pb1; 4 short and 4 long for Pb2) which could be attributed to repulsion effects involving a somehow weak stereochemically active lone pair producing the longer Pb—F distances. The Pb1 lone pair is thus probably oriented towards the barycenter of the (F5—F6—F2) face of the Pb1F₆ octahedron, a similar reasoning applying to the Pb2 lone pair.

Related literature

For the Pb₂RhF₇ structure-type, see: Domesle & Hoppe (1983), and for the isostructural Sr₂RhF₇, see: Grosse & Hoppe (1987). For the indexed powder pattern of Pb₂CrF₇, see: de Kozak et al. (1999). For other sompounds containing `independent' fluorine atoms coordinated to three cations (Ca or Sr) in a plane, see: Ca₂AlF₇ (Domesle & Hoppe, 1980); Sr₅Zr₃F₂₂ (Le Bail, 1996); Sr₅(VOF₅)₃F(H₂O)₃ (Le Bail et al., 2009). For fluorite-related lead-based compounds, see: Pb₈MnFe₂F₂₄ (Le Bail & Mercier, 1992); Pb₂ZrF₈ (Le Bail & Laval, 1998). For the structure simulation of fluoride glasses containing lead by using crystalline models, see: Le Bail (1989, 2000). For details and parameters of the bond-valence model, see: Brown & Altermatt (1985); Brese & O'Keeffe (1991).

S2. Experimental

Solid state reaction between 2PbF₂ and CrF₃ at 773 K for 96 hours in a platinum tube sealed under argon yieded single crystal of the title compound.

S3. Refinement

The highest residual peak and deepest hole in the final difference map were located respectively 0.67 Å and 0.83 Å from the Pb2 atom.

Computing details

Data collection: STADI4 (Stoe & Cie, 1998); cell refinement: STADI4 (Stoe & Cie, 1998); data reduction: X-RED (Stoe & Cie, 1998); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2005) and ORTEP-3 (Farrugia, 1997); software used to prepare material for publication: publCIF (Westrip, 2010).

Figures

	Fig. 1. ORTEP-3 view (Farrugia, 1997) of the regular [Cr^IIIF6]^3- octahedron and of the "independent" fluoride ion F1 connected to Pb1 and Pb2 completing the Pb2F(CrF6) formula (ellipsoids at the 50% probability level).
	Fig. 2. Diamond (Brandenburg, 2005) view of the two F1Pb3 triangles sharing an edge in order to form the planar F2Pb4 unit with Pb1F6 distorted octahedra and Pb2F8 square antiprisms.
	Fig. 3. Diamond (Brandenburg, 2005) view of the crystal packing along [100] showing the isolated [CrF6] octahedra,
	Fig. 4. Idealized (with small displacement of some F atoms) view of the Pb atoms alternating positions close to x = 1/4 and 3/4, building corrugated strips with the PbF2 fluorite structure. Similar kinked blocks were observed for Pb2ZrF8 (Le Bail & Laval, 1998).

Dilead(II) chromium(III) heptafluoride

Crystal data

Pb2CrF7	F(000) = 1004
Mr = 599.40	Dx = 6.793 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71069 Å
Hall symbol: -P 2ybc	Cell parameters from 40 reflections
a = 5.4626 (7) Å	θ = 2.8–35°
b = 11.2085 (15) Å	µ = 59.61 mm^−1
c = 9.5738 (11) Å	T = 293 K
β = 91.197 (10)°	Platelet, green
V = 586.05 (13) Å^3	0.12 × 0.07 × 0.01 mm
Z = 4

Data collection

Siemens AED2 diffractometer	2082 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.041
Graphite monochromator	θmax = 35.0°, θmin = 2.8°
2θ/ω scans	h = −8→8
Absorption correction: gaussian (SHELX76; Sheldrick, 2008)	k = 0→18
Tmin = 0.011, Tmax = 0.321	l = 0→15
6770 measured reflections	3 standard reflections every 120 min
2506 independent reflections	intensity decay: 15%

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.036	w = [exp(2.00(sinθ/λ)^2)]/[σ^2(Fo^2) + (0.0528P)^2] where P = 0.33333Fo^2 + 0.66667Fc^2
wR(F^2) = 0.084	(Δ/σ)max < 0.001
S = 1.64	Δρmax = 3.17 e Å^−3
2506 reflections	Δρmin = −2.20 e Å^−3
93 parameters	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.0171 (7)
0 constraints

Special details

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Pb1	0.21926 (4)	0.81248 (3)	0.05527 (3)	0.01669 (9)
Pb2	0.23992 (4)	0.43908 (2)	0.13531 (3)	0.01410 (9)
Cr	0.30002 (17)	0.13284 (9)	0.20773 (11)	0.01105 (17)
F1	0.1204 (9)	0.6150 (4)	0.0059 (6)	0.0212 (9)
F2	0.5511 (9)	0.0321 (5)	0.1456 (6)	0.0240 (10)
F3	0.3978 (9)	0.0941 (5)	0.3938 (6)	0.0233 (10)
F4	0.2089 (10)	0.1752 (6)	0.0208 (6)	0.0253 (10)
F5	0.0826 (9)	−0.0003 (5)	0.1978 (7)	0.0251 (11)
F6	0.5281 (10)	0.2622 (6)	0.2141 (8)	0.0307 (13)
F7	0.0430 (8)	0.2415 (5)	0.2594 (5)	0.0190 (8)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Pb1	0.01615 (12)	0.01664 (12)	0.01736 (14)	−0.00212 (8)	0.00190 (8)	−0.00028 (8)
Pb2	0.01326 (11)	0.01521 (11)	0.01376 (12)	−0.00018 (7)	−0.00137 (7)	0.00039 (8)
Cr	0.0087 (3)	0.0131 (4)	0.0113 (4)	0.0010 (3)	0.0002 (3)	0.0013 (3)
F1	0.0202 (18)	0.0153 (17)	0.028 (3)	−0.0008 (16)	−0.0086 (17)	0.0004 (17)
F2	0.023 (2)	0.032 (2)	0.018 (2)	0.0174 (19)	0.0007 (17)	−0.0013 (18)
F3	0.023 (2)	0.037 (3)	0.010 (2)	0.0123 (19)	−0.0022 (16)	0.0025 (17)
F4	0.026 (2)	0.039 (3)	0.011 (2)	0.010 (2)	0.0022 (17)	0.0067 (19)
F5	0.023 (2)	0.0195 (19)	0.033 (3)	−0.0058 (18)	0.001 (2)	−0.0061 (19)
F6	0.019 (2)	0.024 (2)	0.049 (4)	−0.0056 (19)	0.000 (2)	0.002 (2)
F7	0.0140 (16)	0.022 (2)	0.021 (2)	0.0084 (16)	0.0022 (15)	−0.0009 (16)

Geometric parameters (Å, º)

Pb1—F1	2.324 (5)	Cr—F4	1.907 (5)
Pb1—F7^i	2.437 (5)	Cr—F5	1.909 (5)
Pb1—F4^ii	2.439 (5)	Cr—F6	1.912 (6)
Pb1—F5^iii	2.620 (6)	Cr—F7	1.931 (4)
Pb1—F6^iv	2.640 (7)	F2—F3	2.630 (8)
Pb1—F2^v	2.899 (6)	F2—F5	2.643 (8)
Pb1—F6^v	3.067 (7)	F2—F6	2.665 (9)
Pb2—F1	2.412 (5)	F2—F4	2.721 (7)
Pb2—F1^ii	2.441 (4)	F3—F6	2.659 (9)
Pb2—F5^i	2.497 (6)	F3—F5	2.734 (8)
Pb2—F3^vi	2.512 (6)	F3—F7	2.835 (6)
Pb2—F2^iv	2.586 (5)	F4—F7	2.584 (8)
Pb2—F6	2.632 (6)	F4—F5	2.696 (8)
Pb2—F3^iv	2.653 (5)	F4—F6	2.698 (9)
Pb2—F7	2.743 (6)	F5—F7	2.784 (8)
Cr—F2	1.883 (5)	F6—F7	2.704 (7)
Cr—F3	1.900 (5)
F2—Cr—F3	88.1 (2)	F4—Cr—F6	89.9 (3)
F2—Cr—F4	91.8 (2)	F5—Cr—F6	177.6 (3)
F3—Cr—F4	178.3 (3)	F2—Cr—F7	176.1 (2)
F2—Cr—F5	88.4 (3)	F3—Cr—F7	95.5 (2)
F3—Cr—F5	91.7 (3)	F4—Cr—F7	84.6 (2)
F4—Cr—F5	89.9 (3)	F5—Cr—F7	92.9 (2)
F2—Cr—F6	89.2 (3)	F6—Cr—F7	89.4 (3)
F3—Cr—F6	88.5 (3)

Symmetry codes: (i) −x, y+1/2, −z+1/2; (ii) −x, −y+1, −z; (iii) x, y+1, z; (iv) −x+1, y+1/2, −z+1/2; (v) −x+1, −y+1, −z; (vi) x, −y+1/2, z−1/2.