The η-notation is encountered in many coordination compounds:
*Side-on bonding of molecules containing σ-bonds like H:
**W(CO)(PPr)(η-H)
*Side-on bonded ligands containing multiple bonded atoms, e.g. ethylene in Zeise's salt or with fullerene, which is bonded through donation of the π-bonding electrons:
**K[PtCl(η-CH)]HO
*Related complexes containing bridging π-ligands:
**(μ-η:η-CH)Co(CO) and (Cp*Sm)(μ-η:η-N)
**Dioxygen in bis{(trispyrazolylborato)copper(II)}(μ-η:η-O),
::Note that with some bridging ligands, an alternative bridging mode is observed, e.g. κ,κ, like in (MeSiCH)V(μ-N-κ(N),κ(N′))V(CHSiMe) contains a bridging dinitrogen molecule, where the molecule is end-on coordinated to the two metal centers (see hapticity vs. denticity).
*The bonding of π-bonded species can be extended over several atoms, e.g. in allyl, butadiene ligands, but also in cyclopentadienyl or benzene rings can share their electrons.
*Apparent violations of the 18-electron rule sometimes are explicable in compounds with unusual hapticities:
**The 18-VE complex (η-CH)Fe(η-CH)(CO) contains one η bonded cyclopentadienyl, and one η bonded cyclopentadienyl.
**Reduction of the 18-VE compound [Ru(η-CMe)] (where both aromatic rings are bonded in an η-coordination), results in another 18-VE compound: [Ru(η-CMe)(η-CMe)].
*Examples of polyhapto coordinated heterocyclic and inorganic rings: Cr(η-CHS)(CO) contains the sulfur heterocycle thiophene and Cr(η-BNMe)(CO) contains a coordinated inorganic ring (BN ring).