Chelate, any of a class of coordination or complex compounds consisting of a central metal atom attached to a large molecule, called a ligand, in a cyclic or ring structure. An example of a chelate ring occurs in the ethylenediamine-cadmium complex:

Structural formula.

The ethylenediamine ligand has two points of attachment to the cadmium ion, thus forming a ring; it is known as a didentate ligand. (Three ethylenediamine ligands can attach to the Cd2+ ion, each one forming a ring as depicted above.) Ligands that can attach to the same metal ion at two or more points are known as polydentate ligands. All polydentate ligands are chelating agents.

Chelates are more stable than nonchelated compounds of comparable composition, and the more extensive the chelation—that is, the larger the number of ring closures to a metal atom—the more stable the compound. This phenomenon is called the chelate effect; it is generally attributed to an increase in the thermodynamic quantity called entropy that accompanies chelation. The stability of a chelate is also related to the number of atoms in the chelate ring. In general, chelates containing five- or six-membered rings are more stable than chelates with four-, seven-, or eight-membered rings.

In medical practice, chelating agents, particularly salts of EDTA, or edetic (ethylenediaminetetraacetic) acid, are widely used for direct treatment of metal poisoning because they bind the toxic metal ions more strongly than do the vulnerable components of the living organism. Chelating agents are also employed as extractants in industrial and laboratory separation of metals and as metal-ion buffers and indicators in analytical chemistry. Many commercial dyes and a number of biological substances, including chlorophyll and hemoglobin, are chelate compounds.

Learn More in these related articles:

Each molecule or ion of a coordination compound includes a number of ligands, and, in any given substance, the ligands may be all alike, or they may be different. The term ligand was proposed by the German chemist Alfred Stock in 1916. Attachment of the ligands to the metal atom may be through only one atom, or it may be through several atoms. When only one atom is involved, the ligand...
Bars of soap.
  • MLA
  • APA
  • Harvard
  • Chicago
You have successfully emailed this.
Error when sending the email. Try again later.
Edit Mode
Tips For Editing




Thank You for Your Contribution!



Uh Oh


Keep Exploring Britannica

iceberg illustration.
Nature: Tip of the Iceberg Quiz
Periodic table of the elements. Chemistry matter atom
Chemistry: Fact or Fiction?
Margaret Mead
Zeno’s paradox, illustrated by Achilles’ racing a tortoise.
foundations of mathematics
acid–base reaction
game theory
General Science: Fact or Fiction?
quantum mechanics
Email this page