Lead(IV) chloride at 150 K

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.