trivalency

Trivalency is the quality or condition of having valence three, meaning the ability to form three chemical bonds or to adopt a +3 oxidation state. The term is used across chemistry, materials science, and related fields to describe centers or species that can engage in three substituent interactions or coordinate to three ligands. In many contexts, a trivalent element can complete three covalent bonds, sometimes with a lone pair still present (as in ammonia, NH3) or in a higher coordination environment.

In chemistry, several elements are described as trivalent in common compounds. Boron and aluminum frequently occur in three-coordinate species such as BF3 and AlCl3. Phosphorus often forms three bonds in compounds like PH3, PCl3, or various organophosphorus compounds. Nitrogen can be trivalent in amines (three N–R bonds with a lone pair) or in other three-coordinate species, while arsenic and antimony can exhibit +3 oxidation states in appropriate compounds. In coordination and organometallic chemistry, metal centers may also display three-coordinate or +3 configurations, depending on ligands and geometry.

In vaccinology, trivalency refers to formulations that incorporate antigens from three strains or serotypes. Historically, trivalent influenza vaccines contained three strains, though many modern products are quadrivalent, covering four strains.

In broader terms, trivalency is related to the concept of multivalency, where molecules bearing multiple binding sites can interact with targets more strongly through avidity than single-site (monovalent) interactions. Trivalent ligands and polymers are studied in supramolecular chemistry and bioconjugate design for their potential to enhance binding and assembly.