Trichlorides

Trichlorides are chemical compounds with the formula MCl3, in which the central atom is typically trivalent and bonded to three chlorine atoms. The class includes metal trichlorides such as aluminum trichloride (AlCl3), ferric chloride (FeCl3), chromium(III) chloride (CrCl3), vanadium(III) chloride (VCl3), and titanium(III) chloride (TiCl3), as well as nonmetal boron trichloride (BCl3). Many trichlorides are important reagents in inorganic and organometallic chemistry and often serve as Lewis acids or as precursors to other chlorides.

Structurally, MCl3 compounds exhibit a range of motifs. Some form discrete molecules (for example BCl3 is trigonal planar), while others polymerize in the solid state or form extended networks. In the case of aluminum trichloride, dimerization to Al2Cl6 occurs in the gas phase and under certain conditions. In solutions, many trichlorides behave as Lewis acids, accepting electron pairs from donors such as alkenes or ethers.

Reactivity and properties vary with the metal. They are often moisture sensitive and hydrolyze upon contact with water, producing hydrochloric acid and metal oxychlorides or hydroxides. They tend to be corrosive and require careful handling and storage. Many trichlorides are used as catalysts in organic synthesis and as sources of chloride ligands for the preparation of other metal chlorides.

Applications span catalysis, materials science, and synthesis. Aluminum trichloride is a classical Friedel–Crafts catalyst; ferric and chromium(III) chlorides serve as Lewis acids in various reactions and as catalysts in polymerization and oxidation processes. Boron trichloride is a reactive Lewis acid used in polymerization and in chemical vapor deposition precursors. Safety considerations include their moisture sensitivity and corrosive, often toxic, fumes; they are typically handled under inert atmosphere or in fume hoods.