polyhalides

Polyhalides are chemical species that contain more than one halogen atom within a single unit. The term encompasses interhalogen compounds—neutral molecules comprising two or more different halogen elements, such as ICl and IBr, and related species—as well as polyhalide ions, for example I3−, Br3−, and Cl3− that form in solution or in salts. The common feature is the presence of halogen–halogen bonds and multiple halogen atoms in the same species.

Structural and bonding aspects: Interhalogen compounds exhibit varied geometries depending on the halogens involved; polyhalide ions like I3− are typically linear or near-linear, formed by halogen molecules associating with halide ions. The bonding involves covalent halogen–halogen interactions and, in salts, electrostatic stabilization by countercations.

Formation and occurrence: Polyhalides form when a halogen X2 is dissolved in a medium containing an excess of halide X− or when halogen molecules react with other halogen atoms to yield anions or neutral interhalogen compounds. In aqueous solutions or molten salts, equilibria exist between different polyhalide species (for example I2, I3−, I5−), depending on concentration, solvent, and temperature.

Properties and significance: Polyhalides typically show distinctive optical properties and redox behavior associated with halogen–halogen interactions. They are encountered in analytical chemistry (iodometric and bromometric titrations), redox chemistry, and materials science, where they serve as reagents, charge-transfer partners, or stabilizing components in larger halogen-bonded assemblies.