Lead(IV) chloride at 150 K

Lead(IV) chloride, PbCl₄, is a volatile air-sensitive liquid under ambient conditions. The crystal structure consists of tetrahedral mol­ecules residing on crystallographic twofold axes, with no intermolecular interactions within the sum of van der Waals radii. Unlike the lighter Group 14 analogues, which are based on hexagonal close packing of Cl atoms, the Cl atoms in PbCl₄ form a cubic close-packed array.

Key indicators

• Single-crystal X-ray study
• T = 150 K
• Mean [sigma](Pb-Cl) = 0.003 Å
• R factor = 0.019
• wR factor = 0.057
• Data-to-parameter ratio = 21.4

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry


 Alert Level A:



ABSTM_02  Alert A Crystal and compound unsuitable for non-numerical
          corrections. Product of mu and tmid > 3.0
           Value of mu given =    29.350 tmid =     0.200


 Alert Level C:



CRYSR_01  Alert C _exptl_crystal_size_rad not in the CIF when expected.

REFLE_01  Alert C The _reflns_threshold_multiplier given is >= 4
          Premultiplier =    4.00


 1 Alert Level A = Potentially serious problem

 0 Alert Level B = Potential problem

 2 Alert Level C = Please check

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S1. Comment

Lead(IV) chloride exists under ambient conditions as a volatile air-sensitive liquid with a melting point of 258 K (Biltz & Meinecke, 1923). The compound is thermally fragile and decomposes readily under ambient conditions to give lead(II) chloride and chlorine. A gas electron-diffraction study of the vapour has shown the presence of tetrahedral molecules with a Pb—Cl bond length of 2.373 (3) Å (Haaland et al., 1992). Vibrational spectra of the liquid and solid also suggest a structure composed of essentially isolated tetrahedral molecules (Clark & Willis, 1971; Clark & Hunter, 1971), in contrast to the pseudo-octahedral geometries found for lead in PbF₄ (Bork & Hoppe, 1996).

The crystal structures of the Group 14 tetrachlorides, MCl₄, have been reported for M = C (Piermarini, 1973), Si (Zakharov et al., 1986), Ge (Merz & Driess, 2002) and Sn (Reuter & Pawlak, 2000). They form an isostructural series, being monoclinic, P2₁/c, with cell dimensions in the ranges a = 9.07–9.86, b = 5.76–6.68 and c = 9.20–9.94 Å, which all increase as the group is descended from carbon to tin. All structures have values of β in the range 103–105°. The structures consist of isolated tetrahedral MCl₄ molecules.

The structure of lead(IV) chloride also consists of essentially isolated tetrahedral molecules of PbCl₄ (Fig. 1). However, though the crystal system is monoclinic, the lattice type is centred (I-centred in the setting given here) rather than primitive, and it is clear that this compound does not follow the trend established by the lighter members of Group 14. The molecule resides on a crystallographic twofold axis at (3/4, y, 0), where y is very close to 1/4. This introduces pseudo-C-centring operation into the lead positions, with the result that though the mean I/σ for the data set as a whole is 27.1, that for data with h+k ≠ 2n is only 5.4. There is also peudosymmetry in the Cl positions, and this is discussed in more detail below. The Pb—Cl bond lengths are 2.360 (2) and 2.363 (2) Å, which correspond to exactly one bond-valence unit (Brown, 2002).

The shortest Cl···Cl contact observed in this structure is 3.661 (5) Å, which is slightly larger than twice the van der Waals radius of chlorine (3.5 Å; Bondi, 1964). The Pb.·Cl contacts are well outside sum of the van der Waals radii of Pb and Cl (4.09 Å). There are concentric shells of Cl atoms around the lead atom, the first of them consisting of 12 Cl atoms at a distance of 4.340–4.455 (2) Å. The shortest Pb···Pb distance is 5.359 Å. As in the structures of the lighter analogues, therefore, there are no intermolecular contacts of any significance.

The structures of carbon, silicon, germanium and tin tetrachlorides can all be considered to be based on hexagonal close packing (h.c.p.) of Cl atoms (Reuter & Pawlak, 2000). Fig. 2 is taken from the structure of SnCl₄ and shows the ABA layers characteristic of h.c.p. arrays. By contrast the Cl atoms in PbCl₄ form a cubic close-packed (CCP) array; Fig. 3 shows the characteristic ABC layering sequence. Deviations from ideal symmetry may be gauged using the continuous symmetry measure (CSM) parameter described by Pinsky & Avnir (1998). The collection of the 13 atoms shown in Fig. 3 has a CSM of 0.044% relative to perfect CCP; the corresponding figure for the geometry about Cl2 is 0.037%. For comparison, the CSM values for the four independent Cl atoms in SnCl₄ versus perfect h.c.p. are 0.103, 0.070, 0.073 and 0.081% for Cl1 to Cl4, respectively. The agreement with perfect close packing is excellent in both cases; the slightly larger deviations for perfect h.c.p. are to be expected because the layer separation in h.c.p. is not restricted by symmetry.

S2. Experimental

The sample of PbCl₄ was prepared by the reaction between pyridinium hexachloroplumbate(IV) and sulfuric acid according to the method of Kauffman et al. (1983).

S3. Refinement

A stable solid-liquid equilibrium was established at 262 K in a sample of lead(IV) chloride held in a capillary mounted on a Stoe Stadi-4 diffractometer equipped with an Oxford Cryosystems low-temperature device. A crystal was then obtained by cooling the sample at a rate of 10 K h⁻¹. Data were collected at 150 K, a previous experiment having shown that the sample decomposed in the X-ray beam during data collection at 220 K.

The length of a crystal is difficult to control when growing a crystal of a low-melting compound in a capillary. It was almost certainly larger than the 0.8 mm collimator used. The value of µ is high, and a numerical absorption correction would normally be required. However, the cylindrical symmetry of the axially mounted sample meant that absorption anisotropy was low. A cylindrical absorption correction was attempted, and the results were satisfactory [R(F) = 2.3%, Δρ_max = +1.8 e Å⁻³]. However, slightly better results were obtained by applying ψ scans, and these are the results presented here.

The structure was solved by Patterson methods (DIRDIF; Beurskens et al., 1996) and refined by full-matrix least squares against F² using all data (CRYSTALS; Watkin et al., 2001). The highest difference Fourier peak was 0.935 Å from Pb1.

Continuous symmetry measures were calculated using a locally written program based on the procedure given by Pinsky & Avnir (1998).

The data for calculating bond valences were taken from https://www.CCP14.ac.uk/CCP/web-mirrors/i_d_brown/bond_valence_parm/.

Computing details

Data collection: DIF4 (Stoe & Cie, 1990); cell refinement: DIF4; data reduction: REDU4 (Stoe & Cie, 1990); program(s) used to solve structure: DIRDIF96 (Beurskens et al., 1996); program(s) used to refine structure: CRYSTALS (Watkin et al., 2001); molecular graphics: CAMERON (Watkin et al., 1996) and XP (Sheldrick, 2001); software used to prepare material for publication: CRYSTALS.

Figures

	Fig. 1. The structure of PbCl4 in the crystal. The view is along the b axis, and the ellipsoids enclose 50% probability surfaces.
	Fig. 2. The hexagonal close packing of the Cl atoms in SnCl4. The geometry around atom Cl1 is shown.
	Fig. 3. The cubic close packing of the Cl atoms in PbCl4. The geometry around atom Cl1 is shown.

Lead(IV) chloride

Crystal data

PbCl4	F(000) = 588.601
Mr = 349.01	Dx = 3.812 Mg m^−3
Monoclinic, I2/a	Melting point: 262 K
Hall symbol: -I 2ya	Mo Kα radiation, λ = 0.71073 Å
a = 10.542 (8) Å	Cell parameters from 88 reflections
b = 5.359 (3) Å	θ = 13–21°
c = 11.958 (5) Å	µ = 29.35 mm^−1
β = 115.83 (2)°	T = 150 K
V = 608.0 (6) Å^3	Cylinder, colourless
Z = 4	1.00 × 0.20 × 0.20 mm

Data collection

Stoe Stadi-4 diffractometer	Rint = 0.07
Graphite monochromator	θmax = 25.0°, θmin = 3.8°
ω/θ scans	h = −11→12
Absorption correction: empirical (using intensity measurements) via ψ scans [XPREP (Sheldrick (2001) based on method of North et al. (1968)]	k = −6→6
Tmin = 0.002, Tmax = 0.002	l = −14→2
2322 measured reflections	3 standard reflections every 0 reflections
536 independent reflections	intensity decay: 0.0%
363 reflections with I > 4σ(I)

Refinement

Refinement on F^2	Primary atom site location: Patterson
Least-squares matrix: full	Method: Tukey & Prince (Carruthers & Watkin, 1979) W = [weight][1-(ΔF/6σF)^2]2 where [weight] is given by a four-term Chebychev polynomial with coefficients 43.7, 56.2, 32.6 and 13.2.
R[F^2 > 2σ(F^2)] = 0.019	(Δ/σ)max = 0.0002
wR(F^2) = 0.057	Δρmax = 1.64 e Å^−3
S = 0.94	Δρmin = −0.71 e Å^−3
534 reflections	Extinction correction: Larson (1970) eq. 22
25 parameters	Extinction coefficient: 50.6 (27)

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

	x	y	z	Uiso*/Ueq
Pb1	0.7500	0.25755 (7)	0.0000	0.0184
Cl1	0.5587 (3)	0.0050 (4)	−0.1292 (2)	0.0263
Cl2	0.8146 (3)	0.5092 (4)	−0.1292 (2)	0.0275

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Pb1	0.0187 (4)	0.0184 (4)	0.0159 (4)	0.0000	0.0055 (2)	0.0000
Cl1	0.0219 (11)	0.028 (1)	0.0239 (12)	−0.0045 (9)	0.0051 (9)	−0.0033 (9)
Cl2	0.0303 (12)	0.027 (1)	0.0258 (12)	−0.0015 (9)	0.013 (1)	0.0044 (9)

Geometric parameters (Å, º)

Pb1—Cl1	2.360 (2)	Pb1—Cl2	2.363 (2)
Cl1—Pb1—Cl1^i	110.01 (12)	Cl1—Pb1—Cl2^i	110.35 (8)
Cl1—Pb1—Cl2	107.86 (8)	Cl2—Pb1—Cl2^i	110.41 (12)

Symmetry code: (i) −x+3/2, y, −z.