Halides of germanium, tin and lead

There are many similarities between the tetrahalides of Ge and Si, and GeX₄ (X= F, Cl, Br or I) is prepared by direct combination of the elements. At 298 K, GeF₄ is a colourless gas, GeCl₄, a colourless liquid, and GeI₄ a red-orange solid (mp 417 K); GeBr₄ melts at 299 K. Each hydrolyses, liberating HX. Unlike SiCl₄, GeCl₄ accepts Cl^- .

   The Si(II) halides SiF₂ and SiCl₂ can be obtained only as unstable species (by action of SiF₄ or SiCl₄ on Si at ≈1500 K) which polymerize to cyclic products. In contrast, Ge forms stable dihalides; GeF₂, GeCl₂ and GeBr₂ are produced when Ge is heated with GeX4, but the products disproportionate on heating (equation in below).

Reaction between GeF₂ and F^- gives [GeF₃]^-. Several compounds of type MGeCl₃ exist where M‏ may be an alkali metal ion or a quaternary ammonium or phosphonium ion. Crystal structure determinations for [BzEt₃N][GeCl₃] (Bz=benzyl) and [Ph₄P][GeCl₃] confirm the presence of well-separated trigonal pyramidal [GeCl₃]^- ions. In contrast, CsGeCl₃ adopts a perovskite-type structure  which is distorted at 298K and non-distorted above 328 K. CsGeCl₃ belongs to a group of semiconducting compounds CsEX₃ (E=Ge, Sn, Pb; X=Cl, Br, I).

  The preference for the ‏2 over ‏4 oxidation state increases down the group, the change being due to the thermodynamic 6s inert pair effect. Whereas members of the GeX₄ family are more stable than GeX₂, PbX₂ halides are more stable than PbX₄. Tin tetrafluoride is prepared from SnCl₄ and HF. At 298 K, SnF₄ is a white solid and has a sheet structure with octahedral Sn atoms. At 978 K, SnF₄ sublimes to give a vapour containing tetrahedral molecules. SnF₄ is thermally stable, but PbF₄ (which has the same solid state structure as SnF₄) decomposes into PbF₂ and F₂ when heated, and must be prepared by the action of F₂ or halogen fluorides on Pb compounds.

Tin(II) fluoride is water-soluble and can be prepared in aqueous media. In contrast, PbF₂ is only sparingly soluble.

One form of PbF₂ adopts a CaF₂ lattice  while the solid state structure of SnF₂ consists of puckered Sn₄F₈ rings  with each Sn being trigonal pyramidal consistent with the presence of a lone pair. In above structures the Sn^_F bridge bonds are longer than the terminal bonds, a feature that is common in this type of structure. Many tin fluoride compounds show a tendency to form F^_Sn^_F bridges in the solid state, as we illustrate later. Tin(IV) chloride, bromide and iodide are made by combining the respective elements and resemble their Si and Ge analogues. The compounds hydrolyse, liberating HX, but hydrates such as SnCl₄.4H₂O can also be isolated.

   The reaction of Sn and HCl gives SnCl₂, a white solid which is partially hydrolysed by water. The hydrate SnCl₂.2H₂O is commercially available and is used as a reducing agent. In the solid state, SnCl₂ has a puckeredlayer structure, but discrete, bent molecules are present in the gas phase.  The Sn(IV) halides are Lewis acids, their ability to accept halide ions  following the order SnF₄ > SnCl₄ > SnBr₄ > SnI₄.

   Similarly, SnCl₂ accepts Cl^- to give trigonal pyramidal [SnCl₃]^_, but the existence of discrete anions in the solid0 state is cation-dependent (see earlier discussion of CsGeCl₃). The [SnF₅]^- ion can be formed from SnF₄, but in the solid state, it is polymeric with bridging F atoms and octahedral Sn centres. The bridging F atoms are mutually cis to one another. Bridge formation is similarly observed in Na‏ salts of [Sn₂F₅]^_ and [Sn₃F₁₀]^4-, formed by reacting NaF and SnF₂ in aqueous solution. Figure 1.1 shows the structures of the [SnCl₂F]^- and [Sn₂F₅]^- ions.  Lead tetrachloride is obtained as an oily liquid by the reaction of cold concentrated H₂SO₄ on [NH₄]₂[PbCl₆]; the latter is made by passing Cl₂ through a saturated solution of PbCl₂ in aqueous NH₄Cl. The ease with which [PbCl₆]^2_ is obtained is a striking example of stabilization of a higher oxidation state by complexation in contrast, PbCl₄ is hydrolysed by water and decomposes to PbCl₂ and Cl₂ when gently heated. The Pb(II) halides are considerably more stable than their Pb(IV) analogues and are crystalline solids at 298 K; they can be precipitated by mixing aqueous solutions of soluble halide and soluble Pb(II) salts . Note that few Pb(II) salts are very soluble in water.

  Lead(II) chloride is much more soluble in hydrochloric acid than in water owing to the formation of [PbCl₄]^2-. In the solid state, PbCl₂ has a complicated structure with 9-coordinate Pb centres, but PbF₂ has the fluorite structure (Figure 5.18a). The yellow diiodide adopts the CdI₂ lattice. Discrete iodoplumbate anions such as [Pb₃I₁₀]^4- (Figure 1.2a), [Pb₇I₂₂]^{8- ,} [Pb₁₀I₂₈]^8- and [Pb₅I₁₆]^6- (Figure 1.2b) as well as related polymeric iodoplumbates† can be formed by reacting PbI₂ and NaI in the presence of large cations such as [R₃N(CH₂)₄NR₃]²⁺‏ (R=Me, nBu) or [P(CH₂Ph)₄]‏. The reactions can be driven towards a particular product by varying the reactant stoichiometry, reaction conditions and counter-ion. In these iodoplumbates, the Pb(II) centres are in either octahedral or square-based pyramidal environments (Figure 1.2).