Chelation (from Greek χηλη, chelè, meaning claw) is the process of reversible binding of an organic ligand, the chelator or chelating agent, to a metal ion, forming a metal complex, the chelate. In contrast to the simple monodentate ligands like H₂O or NH₃, which are easily broken apart by other chemical processes, the polydentate chelators form multiple bonds with the metal ion, resulting in more stable complexes. A typical chelator is EDTA.

The term chelate was first applied in 1920 by Sir Gilbert T. Morgan and H. D. K. Drew in J. Chem. Soc., 1920, 117, 1456, who stated: "The adjective chelate, derived from the great claw or chele (Greek) of the lobster or other crustaceans, is suggested for the caliperlike groups which function as two associating units and fasten to the central atom so as to produce heterocyclic rings."

Metal complexes are of widespread interest and studied by inorganic chemists, physical and organic chemists, biochemists, pharmacologists, molecular biologists, and environmentalists.

Chelators are used in chemical analysis, as water softeners, as preservatives, and in medicine (chelation therapy), where they are employed to safely bind with poisonous metal agents such as mercury or arsenic in order to stabilize them and allow them to be excreted without further interaction with the body. Natural chelators include the porphyrin rings in hemoglobin or chlorophyll and the Fe³⁺ chelating siderophores secreted by microorganisms.

Antibiotic drugs of the tetracycline family are also chelators of Ca²⁺ and Mg²⁺ ions and are incorporated into bones and teeth.

See also

• Chelation therapy
• Electron counting
• Organometallic chemistry