sulfide

sulfide, any of three classes of chemical compounds containing the element sulfur. The three classes of sulfides include inorganic sulfides, organic sulfides (sometimes called thioethers), and phosphine sulfides. Inorganic sulfides are ionic compounds containing the negatively charged sulfide ion, S²⁻; these compounds may be regarded as salts of the very weak acid hydrogen sulfide. Organic sulfides are compounds in which a sulfur atom is covalently bonded to two organic groups. Phosphine sulfides are formed from the reaction of organic phosphines with sulfur, in which the sulfur atom is linked to the phosphorus by a bond that has both covalent and ionic properties.

Sulfides of many important metallic elements are naturally occurring minerals. For example, pyrite, which is also called fool’s gold owing to its brassy yellow colour, is a sulfide of iron with the formula FeS₂. Pyrite is a major source of iron and is one of the most abundant of the sulfur minerals. Zinc, cadmium, mercury, copper, silver, and many other elements occur in nature as sulfides.

Preparation of sulfides

Most metals react directly with sulfur to form metal sulfides—i.e., compounds that contain a metal atom and the sulfide ion, S²⁻. In addition to direct combination of the elements as a method of preparing sulfides, they can also be produced by reduction of a sulfate by carbon or by precipitation from acidic aqueous solution by hydrogen sulfide, H₂S, or from basic solution by ammonium sulfide, (NH₄)₂S. Another method, particularly suitable for water-soluble sulfides, involves bubbling H₂S into a basic solution of the metal to give the metal hydrogen sulfide, MHS. A further equivalent of metal hydroxide added will yield the metal sulfide. NaOH + H₂S → NaHS + H₂O
NaHS + NaOH → Na₂S + H₂O

Solubility of sulfides

The alkali metals and alkaline-earth metals are the only sulfides that have any appreciable water solubility and that appear to be primarily ionic. In contrast, the sulfides of the copper and zinc families are some of the least-soluble compounds known. When water-soluble metal sulfides are heated in aqueous solution with elemental sulfur, solutions of so-called polysulfides are formed. These solutions consist primarily of S₄²⁻ and S₃²⁻ anions. Sulfides are an important component of high-density power sources such as lithium and sodium sulfide batteries. The sulfides that have been utilized in these power sources are M₂S, M₂S₂, M₂S₄, and M₂S₅.

Structure of sulfides

Semimetals (metalloids) and some nonmetallic elements form sulfides that are molecular or that have sulfide bridges in a polymeric structure. For example, silicon disulfide, SiS₂, has a structure consisting of infinite chains of SiS₄ tetrahedrons that share edges. (Each SiS₄ tetrahedron consists of a central silicon atom surrounded by and bonded to four sulfur atoms.) Phosphorus forms a series of molecular sulfides that includes P₄S₃, P₄S₄ (two distinct forms), P₄S₅, P₄S₇, P₄S₉, and P₄S₁₀. The structures of all these compounds are derived from a P₄ tetrahedron in which P―P bonds are replaced by P―S―P units. All these phosphorus sulfides are stable in carbon disulfide (CS₂), and all react with water to produce phosphoric acid (H₃PO₄) or other phosphorus oxyacids. P₄S₁₀ + 16H₂O → 4H₃PO₄ + 10H₂S The tips of strike-anywhere matches contain P₄S₃, which ignites in air as a result of the friction produced when the match is rubbed against a rough surface. A flame is produced by reaction of the phosphorus sulfide with active chemicals in the match head. P₄S₁₀ is used in the preparation of industrial lubricant additives.