A sulfide is a compound formed when sulfur reacts directly with another, more electropositive element. Metals form ionic sulfides containing the S–2 ion; these are salts of hydrogen sulfide. Metal sulfides are mostly insoluble (except those of the alkali metals and alkaline earth metals), and are prepared by precipitation with hydrogen sulfide. Soluble sulfides are readily hydrolyzed to the soluble bisulfides (HS-), and are used as reducing agents and in making dyes. Compounds of sulfur with nonmetals, including hydrogen sulfide, carbon dusulfide, and several sulfides of nitrogen and phosphorus are covalent compounds formed by direct synthesis.
Sulfides can be important indicators of the presence of life since there is no way that they can be formed at normal temperatures except biologically.
Organic sulfides, called thioethers, contain the –S– group linked to two hydrocarbon groups. They are named after the linking groups, e.g., C2H5SCH3 is called ethyly methyl sulfate. Organic sulfates are analogs of ethers in which the oxygen is replaced by sulfur but are generally more reactive than ethers. They can be oxidized to form sulfoxides (containing the =S=O group linked to two other groups) and react with halogen compounds to form sulphonium compounds (containing the R3S+ ion, where R is any organic group).