Sulfides of The group 14 elements

  The disulfides of C, Si, Ge and Sn show the gradation in properties that might be expected to accompany the increasingly metallic character of the elements. Pertinent properties of these sulfides are given in Table 1.1. Lead(IV) is too powerful an oxidizing agent to coexist with S^2- , and PbS₂ is not known. Carbon disulfide is made by heating charcoal with sulfur at 1200K, or by passing CH₄ and sulfur vapour over Al₂O₃ at 950 K. It is highly toxic (by inhalation and absorption through the skin) and extremely flammable, but is an excellent solvent which is used in the production of rayon and cellophane. Carbon disulfide is insoluble in water, but is, by a narrow margin, thermodynamically unstable with respect to hydrolysis to CO₂ and H₂S. However, this reaction has a high kinetic barrier and is very slow. Unlike CO₂, CS₂ polymerizes under high pressure to give a black solid. When shaken with solutions of group 1 metal sulfides, CS₂ dissolves readily to give trithiocarbonates, M₂CS₃, which contain the [CS₃]^2- ion the sulfur analogue of [CO₃]^2-. Salts are readily isolated, e.g. Na₂CS₃ forms yellow needles (mp 353K). The free acid H₂CS₃ separates as an oil when salts are treated with hydrochloric acid  and behaves as a weak acid in aqueous solution: pKa(1( = 2.68, pKa(2) = 8.18.

   The action of an electric discharge on CS₂ results in the formation of C₃S₂, a red liquid which decomposes at room temperature, producing ablack polymer (C₃S₂)x. When heated, C₃S₂ explodes. Incontrast to CO, CS is a short-lived radical species whichdecomposes at 113 K; it has, however, been observed in theupper atmosphere.

Several salts of the [C₂S₄]^2- anion are known  although the free acid (an analogue of oxalic acid) has not been isolated.

   In [Et₄N]₂[C₂S₄], the anion has D₂d symmetry, i.e. the dihedral angle between the planes containing the two CS₂-units is 908  whereas in [Ph₄P]₂[C₂S₄].6H₂O, this angle is 79.58. It is interesting to compare these structural data with those for salts of the related oxalate ion, [C₂O₄]^2-. The solid state structures of anhydrous alkali metal oxalates respond to an increase in the size of the metal ion. In Li₂C₂O₄, Na₂C₂O₄, K₂C₂O₄ and in one polymorph of Rb₂C₂O₄, the [C₂O₄]^2- ion is planar. In the second polymorph of Rb₂C₂O₄ and in Cs₂C₂O₄, the [C₂O₄]^2- ion adopts a staggered conformation.

  Oxalate salts in general tend to exhibit planar anions in the solid state. The C^_C bond length (157 pm) is consistent with a single bond and indicates that the planar structure is not a consequence of  π-delocalization but is, instead, a result of intermolecular interactions in the crystal lattice.

Silicon disulfide is prepared by heating Si in sulfur vapour. Both the structure of this compound (Table 1.1) and the chemistry of SiS₂ show no parallels with SiO₂; SiS₂ is instantly hydrolysed.

The disulfides of Ge and Sn (Table 1.1) are precipitated when H₂S is passed into acidic solutions of Ge(IV) and Sn(IV) compounds. Some sulfides have cluster structures.

   Tin(IV) forms a number of thiostannates containing discrete anions, e.g. Na₄SnS₄ contains the tetrahedral [SnS₄]^4- ion, and Na₄Sn₂S₆ and Na₆Sn₂S₇ . The monosulfides of Ge, Sn and Pb are all obtained by precipitation from aqueous media. Both GeS and SnS crystallize with layer structures similar to that of black phosphorus. Lead(II) sulfide occurs naturally as galena and adopts an NaCl lattice. Its formation as a black precipitate (K_sp ≈ 10^-30) is observed in the qualitative test for H₂S. The colour and very low solubility of PbS suggest that it is not a purely ionic compound.

    Pure PbS is a p-type semiconductor when S-rich, and an n-type when Pb-rich. It exhibits photoconductivity and has applications in photoconductive cells, transistors and photographic exposure meters.  If a material is a photoconductor, it absorbs light with the result that electrons from the valence band are excited into the conducting band; thus, the electrical conductivity increases on exposure to light.

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