1) Organic chemicals that form two or more coordination bonds with a central metal ion. Heterocyclic rings are formed with the central metal atom as part of the ring. Some biological systems form metal chelates, e.g., the iron-binding porphyrin group of haemoglobin and the magnesium-binding chlorophyll of plants. They are used chemically to remove ions from solutions, medicinally against microorganisms, to treat metal poisoning, and in chemotherapy protocols.

2) Any of various compounds that combine with metals to form chelates and that include some used medically in the treatment of metal poisoning (as by lead).