The formation or presence of
bonds (or other attractive interactions) between two or more separate
binding sites within the same ligand and a single
central atom.
A
molecular entity in which there is chelation (and the corresponding
chemical species ) is called a '
chelate'. The terms bidentate (or didentate), tridentate, tetradentate, ... multidentate
are used to indicate the number of potential binding sites of the ligand, at least
two of which must be used by the ligand in forming a '
chelate'. For example, the bidentate ethylenediamine forms a
chelate with
CuI
in which both nitrogen atoms of ethylenediamine are bonded to copper. (The use of
the term is often restricted to metallic central atoms.) The phrase '
separate binding sites' is intended to exclude cases such as
[PtCl3(CH2=CH2)]−, ferrocene and (benzene)tricarbonylchromium in which ethene, the cyclopentadienyl
group and benzene, respectively, are considered to present single binding sites to
the respective metal atom, and which are not normally thought of as chelates.
Source:
PAC, 1994, 66, 1077
(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
on page 1094