Polydentate

Polydentate is a term used in coordination chemistry to describe ligands that bind to a central metal atom or ion through multiple donor atoms. The name derives from poly- meaning many and dent- from Latin dens, “tooth,” an allusion to multiple binding sites that resemble teeth. The number of donor atoms that bind to the metal is called the denticity or dentate number. Ligands with two or more donor sites are described as multidentate or polydentate; those with a single donor are monodentate. Donor counts of three, four, five, or more lead to tridentate, tetradentate, pentadentate, hexadentate, etc.

When a single ligand binds through multiple donor atoms, it can form chelate rings with the metal. This chelation often stabilizes the complex relative to those formed with monodentate ligands, a phenomenon known as the chelate effect. Polydentate ligands can be cyclic (forming ring structures) or open chains with several donor atoms arranged to coordinate simultaneously.

Examples include classic polydentate ligands such as EDTA (ethylenediaminetetraacetic acid), a hexadentate chelating agent widely used to bind metal ions in medicine and chemistry. Other common polydentate ligands include bipyridine and phenanthroline (both bidentate), as well as larger polyaminocarboxylates used in catalysis, environmental chemistry, and bioinorganic contexts.

In summary, polydentate ligands bind metals at multiple sites, creating chelate complexes that are often more stable than those formed by single-donor ligands, and they play a central role in many chemical and biological systems.