The disulfide anion is , or S−S. In disulfide, sulfur exists in the reduced state with oxidation number −1. Its electron configuration then resembles that of a chlorine atom.  It thus tends to form a covalent bond with another S center to form  group, similar to elemental chlorine existing as the diatomic Cl. Oxygen may also behave similarly, e.g. in peroxides such as HO. Examples:
*Hydrogen disulfide (SH), the simplest inorganic disulfide
*Disulfur dichloride (SCl), a distillable liquid.
*Iron disulfide (FeS), or pyrite.