MCl3

MCl3 refers to a class of chemical compounds composed of a trivalent metal M and three chloride ligands. The formula encompasses a range of metals, including aluminum (AlCl3), chromium (CrCl3), iron (FeCl3), scandium (ScCl3), and bismuth (BiCl3). The best known member is aluminum trichloride, widely used as a Lewis acid catalyst in numerous organic reactions, with others serving as precursors in coordination chemistry, catalysts, or electronic materials.

In the solid state, many MCl3 compounds form polymeric networks or bridged structures, rather than discrete molecules. AlCl3 is particularly notable for existing as or forming dimers such as Al2Cl6 in the gas phase or melted chloride media, reflecting a strong covalent character. In solution, the coordination environment and stoichiometry of MCl3 species depend on the solvent and chloride concentration, with some systems generating higher-coordination anions such as MCl4− or MCl6− under appropriate conditions.

Synthesis of MCl3 compounds typically involves direct chlorination of the metal: M + 3 Cl2 → MCl3, or chlorination of lower chlorides or oxides followed by adjustment of oxidation state. Many MCl3 are moisture sensitive and hydrolyze in water to form M(OH)3 and HCl.

Key properties include high reactivity with water, tendency to act as Lewis acids, and variable volatility and color across the series. Applications span catalysis (for example, AlCl3 in Friedel–Crafts alkylation and acylation), etching and material synthesis (FeCl3, CrCl3), and as precursors to more complex metal complexes. Safety considerations center on their corrosivity and hydrolysis to produce hydrogen chloride.