chelate (rus. хелаты otherwise хелатные соединения; клешневидные соединения) — coordination compounds composed of atoms (or ions) and polydentate ligands.

Description

The term derives from Latin chelate meaning a claw. To form the chelate, a ligand is needed with at least two donor sites bound with the central atom. The number of such sites is called the ligand denticity. The ligands forming chelate rings are called chelating agents (chelants, chelators). The examples of polydentate ligands include: ethylenediamine H2NCH2CH2NH2, glycerol HOCH2CH(OH)CH2OH, ethylenediaminetetraacetic acid (EDTA) (Fig.). The formation of chemical bonds between the polydentate ligand and the central atom is called chelation. Chelation is one of the pathways to form supramolecular guest-host complexes.

The most extensive and important class of chelates is chelated metal complexes (metallochelates). The ability to coordinate ligands is inherent in metals of all oxidation states.

Chelates are considerably more stable than complexes of a similar nature formed by monodentate ligands. This phenomenon is called the chelate effect. Largely due to the chelate rings, chelates have unique physical, chemical and biological properties. They are widely used in analytical chemistry to determine metals. Chelate-coated nanoparticles are capable of forming chelate complexes with gadolinium (Gd), which enables their use as a contrasting agent in nuclear magnetic tomography. Chelates play an important role in life processes: hemoglobin, chlorophyll, vitamin B12 are chelates.

Illustrations

Chelate formed by EDTA anion.
Chelate formed by EDTA anion.

Authors

  • Eremin Vadim V.
  • Streletskiy Alexey V.

Sources

  1. Chelation // Chemical encyclopedia (in Russian) V. 5. — Мoscow: Bol'shaja Rossijjskaja ehnciklopedija, 1998. 224–225 pp.
  2. Steed J. W., Atwood J. L. Supramolecular Chemistry. 2nd Ed. — J. Wiley & Sons: Chichester, 2009. — 745 pp.

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