Term: η (eta or hapto)
https://doi.org/10.1351/goldbook.H01881

Definition:
An affix giving a topological indication of the bonding between a π-electron ligand and the central atom in a coordination entity. A right superscript numerical index indicates the number of ligating atoms in the π-electron system of the ligand which bind to the central atom. Examples:  [Fe(CO)3(C4H6SO)], tricarbonyl(η2-dihydrothiophene 1-oxide-κO)iron.  [Cr(CO)4(C4H6)], tetracarbonyl(η4-2-methylene-1,3-propanediyl)chromium.  

Related Terms:
1) central atom (http://doi.org/10.1351/goldbook.C00930).
2) coordination entity (http://doi.org/10.1351/goldbook.C01330).
3) methylene (http://doi.org/10.1351/goldbook.M03881).

Source: Red Book, 3rd ed., p. 202 (https://doi.org/10.1515/pac-2014-0718)

Citation: 'η (eta or hapto)' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.H01881

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