This article is about the co-ordination of a ligand to a central atom. For the sense of touch in technology, see Haptic technology.
Ferrocene contains two η5-cyclopentadienyl ligands

Hapticity is the coordination of a ligand to a metal center via an uninterrupted and contiguous series of atoms.[1] The hapticity of a ligand is described with the Greek letter η ('eta'). For example, η2 describes a ligand that coordinates through 2 contiguous atoms. In general the η-notation only applies when multiple atoms are coordinated (otherwise the κ-notation is used). In addition, if the ligand coordinates through multiple atoms that are not contiguous then this is considered denticity [2] (not hapticity), and the κ-notation is used once again.[3] When naming complexes care should be taken not to confuse η with μ ('mu'), which relates to bridging ligands.[4][5]


The need for additional nomenclature for organometallic compounds became apparent in the mid-1950s when Dunitz, Orgel, and Rich described the structure of the "sandwich complex" ferrocene by X-ray crystallography[6] where an iron atom is "sandwiched" between two parallel cyclopentadienyl rings. Cotton later proposed the term hapticity derived from the adjectival prefix hapto (from the Greek haptein, to fasten, denoting contact or combination) placed before the name of the olefin,[7] where the Greek letter η (eta) is used to denote the number of contiguous atoms of a ligand that bind to a metal center. The term is usually employed to refer to ligands containing extended π-systems or where agostic bonding is not obvious from the formula.

Historically important compounds where the ligands are described with hapticity


The η-notation is encountered in many co-ordination compounds:

Note that with some bridging ligands, an alternative bridging mode is observed, e.g. κ11, like in (Me3SiCH2)3V(μ-N21(N),κ1(N'))V(CH2SiMe3)3 contains a bridging dinitrogen molecule, where the molecule is end-on coordinated to the two metal centers (see hapticity vs. denticity).

Electrons donated by "π- ligands" vs. hapticity

Ligand Electrons
(neutral counting)
(ionic counting)
η1 Allyl 1 2
3 4
η3-Allenyl 3 4
η2-Butadiene 2 2
η4-Butadiene 4 4
η1-cyclopentadienyl 1 2
η3-cyclopentadienyl 3 4
5 6
η2-Benzene 2 2
η4-Benzene 4 4
η6-Benzene 6 6
η7-Cycloheptatrienyl 7 6
η8-Cyclooctatetraenyl 8 10

Changes in hapticity

The hapticity of a ligand can change in the course of a reaction.[11] E.g. in a redox reaction:

Here one of the η6-benzene rings changes to a η4-benzene.

Similarly hapticity can change during a substitution reaction:

Here the η5-cyclopentadienyl changes to an η3-cyclopentadienyl, giving room on the metal for an extra 2-electron donating ligand 'L'. Removal of one molecule of CO and again donation of two more electrons by the cyclopentadienyl ligand restores the η5-cyclopentadienyl. The so-called indenyl effect also describes changes in hapticity in a substitution reaction.

Hapticity vs. denticity

Hapticity must be distinguished from denticity. Polydentate ligands coordinate via multiple coordination sites within the ligand. In this case the coordinating atoms are identified using the κ-notation, as for example seen in coordination of 1,2-bis(diphenylphosphino)ethane (Ph2PCH2CH2PPh2), to NiCl2 as dichloro[ethane-1,2-diylbis(diphenylphosphane)-κ2P]nickel(II). If the coordinating atoms are contiguous (connected to each other), the η-notation is used, as e.g. in titanocene dichloride: dichlorobis(η5-2,4-cyclopentadien-1-yl)titanium.[12]

Hapticity and fluxionality

Molecules with polyhapto ligands are often fluxional, also known as stereochemically non-rigid. Two classes of fluxionality are prevalent for organometallic complexes of polyhapto ligands:


  1. "IUPAC Gold Book - η (eta or hapto) in inorganic nomenclature". Goldbook.iupac.org. 2014-02-24. Retrieved 2016-01-18.
  2. "IUPAC Gold Book - denticity". Goldbook.iupac.org. 2014-02-24. Retrieved 2016-01-18.
  3. "IUPAC Gold Book - κ (kappa) in inorganic nomenclature". Goldbook.iupac.org. 2014-02-24. Retrieved 2016-01-18.
  4. "IUPAC Gold Book - bridging ligand". Goldbook.iupac.org. 2014-02-24. Retrieved 2016-01-18.
  5. "IUPAC Gold Book - µ- (mu) in inorganic nomenclature". Goldbook.iupac.org. 2014-02-24. Retrieved 2016-01-18.
  6. J. Dunitz; L. Orgel; A. Rich (1956). "The crystal structure of ferrocene". Acta Crystallographica. 9 (4): 373–5. doi:10.1107/S0365110X56001091.
  7. F. A. Cotton (1968). "Proposed nomenclature for olefin-metal and other organometallic complexes". J. Am. Chem. Soc. 90 (22): 6230–6232. doi:10.1021/ja01024a059.
  8. 1 2 Kubas, Gregory J. (March 1988). "Molecular hydrogen complexes: coordination of a σ bond to transition metals". Accounts of Chemical Research. 21 (3): 120–128. doi:10.1021/ar00147a005.
  9. 1 2 Kubas, Gregory J. (2001). Metal Dihydrogen and σ-Bond Complexes - Structure, Theory, and Reactivity (1 ed.). New York: Kluwer Academic/Plenum Publishers. ISBN 978-0-306-46465-2. LCCN 00059283.
  10. D. Sutton (1993). "Organometallic diazo compounds". Chem. Rev. 93 (3): 995–1022. doi:10.1021/cr00019a008.
  11. Huttner, Gottfried; Lange, Siegfried; Fischer, Ernst O. (1971). "Molecular Structure of Bis(Hexamethylbenzene)-Ruthenium(0)". Angewandte Chemie International Edition in English. 10 (8): 556–557. doi:10.1002/anie.197105561.
  12. "IR- Coordination Compounds: Describing the Constitution of Coordination Compounds: Specifying donor atoms: General" (PDF). Nomenclature of Inorganic Chemistry – Recommendations 1990 (the 'Red Book') (Draft March 2004 ed.). IUPAC. 2004. p. 16.
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