η (eta or hapto)

https://doi.org/10.1351/goldbook.H01881
An affix giving a topological indication of the bonding between a π-electron ligand and the @C00930@ in a @C01330@. A right superscript numerical index indicates the number of ligating atoms in the π-electron system of the ligand which bind to the @C00930@. Examples:
  1. [Fe(CO)3(C4H6SO)], tricarbonyl(η2-dihydrothiophene 1-oxide-κO)iron.
  2. [Cr(CO)4(C4H6)], tetracarbonyl(η4-2-@M03881@-1,3-propanediyl)chromium.
Source:
Red Book, 3rd ed., p. 202 [Terms] [Book]
See also:
Blue Book, p. 465 [Terms] [Book]
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1120 [Terms] [Paper]