MetalHalogen

Metalhalogen is a broad term used to describe chemical compounds in which a metal is bonded to one or more halogen elements (fluorine, chlorine, bromine, iodine, and sometimes astatine). In common usage these are referred to as metal halides, and they span a wide range of structures from ionic salts to molecular species.

Bonding in metalhalogens varies with the metal and the halogen. Ionic halides, such as alkali and alkaline earth halides, form extended crystal lattices held by electrostatic attraction. Many transition and post‑transition metal halides display smaller, sometimes covalent, bonding and may exist as discrete molecules or polymeric networks. Volatile metal halides (for example, titanium tetrachloride) exist as molecular liquids or vapors. The degree of covalency and the structural motif influence properties such as melting point, solubility, and reactivity.

Classification commonly includes alkali and alkaline earth halides (e.g., NaCl, CaCl2), transition metal halides (e.g., FeCl3, CuCl), and post‑transition metal halides (e.g., PbCl2). Some metalhalogens are highly soluble in water, while many form insoluble salts or undergo hydrolysis. Others are volatile or form layered or networked solids. Solubility, color, and conductivity reflect the balance between ionic and covalent character.

Preparation methods include direct halogenation of the metal, metathesis reactions between metal salts and halide sources, and industrial routes such as halogenation of oxides or carbonates. Metalhalogens are central to several technologies, notably lead halide perovskites in photovoltaics, various catalysts, and specialty optical materials. Safety considerations are important, as many halides are corrosive, toxic, or reactive with moisture.

See also metal halides, halogen chemistry.