metalhalide

Metal halides are binary compounds consisting of a metal cation and a halide anion, with halogens such as fluorine, chlorine, bromine, or iodine functioning as ligands. Common stoichiometries include MX, MX2, and M2X3, reflecting the metal’s oxidation state. Many metal halides are ionic solids with high lattice energies, but several transition-metal halides exhibit substantial covalent character and a range of oxidation states, leading to diverse structures and colors.

Solid metal halides typically have high melting points; alkali and alkaline-earth halides are often colorless and highly soluble in water, whereas transition-metal halides can be colored and less soluble. Some, like titanium tetrachloride (TiCl4) and aluminum trichloride (AlCl3), are volatile or easily hydrolyze in air, reflecting their polynuclear and molecular tendencies.

Preparation methods include direct reaction of a metal with a halogen (M + X2), halogenation of metal oxides or hydroxides in the presence of a hydrogen halide, or metathesis reactions in solution to exchange halide ligands.

Applications include catalysts, particularly Lewis acids such as AlCl3 and TiCl4; precursors for chemical vapor deposition and synthesis of organometallic reagents; and, in lighting technology, metal halide lamps that use mixtures of metal halides to produce high-intensity broad-spectrum light. In materials science, metal halides serve as precursors to metal oxides and as components in perovskites and other halide-based materials.

Safety considerations are important: many metal halides are toxic and corrosive; some are moisture sensitive and can release hydrogen halide or halogen gases upon hydrolysis or combustion. They are typically handled with appropriate ventilation and protective equipment.