Hapticity

Hapticity, denoted by η, counts the contiguous atoms of a ligand that bind simultaneously to a metal center. It distinguishes bonding modes such as η¹, η², and η⁵.

Denticity: Denticity counts the donor atoms in a ligand that coordinate to a central atom. Unlike hapticity, it counts coordinating donor atoms whether or not they are contiguous.
η notation: A notation using η and a superscript to specify how many contiguous ligand atoms bind to a metal. It is the standard way to state a ligand’s hapticity.
Dewar–Chatt–Duncanson model: A bonding model for metal–alkene complexes involving alkene donation and metal-to-alkene back-donation. It explains the bonding in the common η² mode of olefin ligands.
Chelation: Coordination in which a multidentate ligand binds one metal through multiple donor atoms. Chelation concerns multiple attachment points, not whether they form a contiguous sequence.
Electron-counting rule: A method for totaling valence electrons at a metal center in a coordination compound. A ligand’s hapticity helps determine how many electrons it contributes to the metal center.
Organometallic chemistry: The study of compounds containing bonds between metals and carbon atoms. Hapticity is chiefly used to describe bonding in organometallic compounds.
Ferrocene: An organometallic compound with an iron atom between two η⁵-cyclopentadienyl rings. Its sandwich structure made η⁵ bonding a prominent structural example.
Bridging ligand: A ligand that coordinates to two or more metal centers at once. Bridging describes how many metals a ligand connects, rather than its hapticity at one metal.
Haptotropic rearrangement: A change in which contiguous atoms of a ligand coordinate to a metal center. It changes a ligand’s hapticity while the ligand remains associated with the metal.
Cyclopentadienyl ligand: The C₅H₅⁻ ligand, which commonly binds a metal through all five ring atoms. Its η⁵ bonding is a canonical example of high hapticity.

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