Hydrides H₂E (E = S, Se, Te)

  Selected physical data for hydrogen sulfide, selenide and telluride are listed in Table 1.1 and illustrated in Figures 9.6 and 9.7. Hydrogen sulfide is more toxic that HCN, but because H₂S has a very characteristic odour of rotten eggs, its presence is easily detected. It is a natural product of decaying S-containing matter, and is present in coal pits, gas wells and sulfur springs. Where it occurs in natural gas deposits, H₂S is removed by reversible absorption in a solution of an organic base and is converted to S by controlled oxidation. Figure 15.2 showed the increasing importance of sulfur recovery from natural gas as a source of commercial sulfur. In the laboratory, H₂S was historically prepared by reaction 1.1 in a Kipp’s apparatus. The hydrolysis of calcium or barium sulfides (e.g. equation 1.2) produces purer H₂S, but the gas is also commercially available in small cylinders.

                     (1.1)

                              (1.2)

Hydrogen selenide may be prepared by reaction 1.3, and a

similar reaction can be used to make H2Te.

                          (1.3)

    The enthalpies of formation of H₂S, H₂Se and H₂Ten (Table 1.1) indicate that the sulfide can be prepared by direct combination of H₂ and sulfur (boiling), and is more stable with respect to decomposition into its elements than H₂Se or H₂Te. Like H₂O, the hydrides of the later elements in group 16 have bent structures but the angles of ≈ 90^o (Table 1.1) are significantly less than that in H₂O (105⁰). This suggests that the E^_H bonds (E= S, Se or Te) involve p character from the central atom (i.e. little or no contribution from the valence s orbital). In aqueous solution, the hydrides behave as weak acids (Table 1.1 ).

Table 1.1 Selected data for H₂S, H₂Se and H₂Te.

The second ionization constant of H₂S is ≈ 10^-19 and, thus, metal sulfides are hydrolysed in aqueous solution. The only reason that many metal sulfides

can be isolated by the action of H₂S on solutions of their salts is that the sulfides are extremely insoluble. For example, a qualitative test for H₂S is its reaction with aqueous lead acetate (equation 1.4).

               (1.3)

    Sulfides such as CuS, PbS, HgS, CdS, Bi₂S₃, As₂S₃, Sb₂S₃ and SnS have solubility products less than ≈10^-30 and can be precipitated by H₂S in the presence of dilute HCl. The acid suppresses ionization of H₂S, lowering the concentration of S^2- in solution.  Sulfides such as ZnS, MnS, NiS and CoS with solubility products in the range ≈ 10^-15  to 10^-30 are precipitated only from neutral or alkaline solutions.  Protonation of H₂S to [H₃S]⁺‏ can be achieved using the superacid HF/SbF₅.   The salt [H₃S][SbF₆] is a white crystalline solid which reacts with quartz glass; vibrational spectroscopic data for [H₃S]⁺‏ are consistent with a trigonal pyramidal structure like that of [H₃O]⁺.  The addition of MeSCl to [H₃S][SbF₆] at 77K followed by warming of the mixture to 213K yields [Me₃S][SbF₆], which is stable below 263 K. Spectroscopic data (NMR, IR and Raman) are consistent with the presence of the trigonal pyramidal [Me₃S]‏ cation.