The formation or presence of @[email protected] (or other attractive interactions) between two or more separate @[email protected] within the same ligand and a single @[email protected] A @[email protected] in which there is chelation (and the corresponding @[email protected] ) is called a '@[email protected]'. 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 '@[email protected]'. For example, the bidentate ethylenediamine forms a @[email protected] 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.
See also:
η (eta or hapto) (in inorganic nomenclature)
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1094 [Terms] [Paper]
Red Book, 3rd ed., p. 147 [Terms] [Book]