trihalogens

Trihalogens are chemical species that contain three halogen atoms within a single molecule. The term covers both inorganic compounds in which a central atom is bonded to three halogen ligands (tri-, or trihalides) and organic or mixed compounds that bear three halogen substituents on a framework. In practice, the class includes both trihalide molecules such as MX3 (for example, ClF3) and trihalogenated compounds where three halogens are attached to a nonmetal or carbon.

Inorganic trihalides form the most explicit examples. Boron trihalides, BX3, include BF3, BCl3, BBr3, and BI3. These species typically feature a central boron atom with three halogen ligands and an empty p orbital, making many of them strong Lewis acids. Phosphorus trihalides, PCl3, PBr3, and PI3, have a similar three-halogen framework but differ in geometry due to the presence of lone pairs on phosphorus, giving them a pyramidal shape. Nitrogen trihalides such as NCl3 and NI3 are more labile and frequently explosive, reflecting the instability of three halogen substituents around nitrogen. Some tri-halides, such as chlorine trifluoride ClF3, are notable for extreme reactivity and oxidizing power.

Organic and mixed tri-halogen compounds include tri-halogenated hydrocarbons, where three halogen atoms substitute hydrogen atoms on a carbon skeleton. Classic examples are chloroform (CHCl3), bromoform (CHBr3), and iodoform (CHI3). These compounds have widespread historical use as solvents, reagents, or precursors, but many pose health or environmental hazards and must be handled with care.

Safety and reactivity vary widely within tri-halogen compounds. While some BX3 species are valuable catalysts or reagents, others are highly reactive or unstable, and organic tri-halogen compounds may be toxic or carcinogenic.