η-bonding

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The Greek letter η (eta) is used to denote the number of atoms of a ligand that bind to a metal center. The superscript number denotes the number of bonding atoms, e.g. η¹ means one atom, η³ means three atoms, and η⁵ means five atoms.

Examples

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Cyclopentadienyl

The cyclopentadienyl ligand shows η¹, η³, and η⁵ bonding modes.

Electrons donated by pi-bonding organic ligands vs. hapticity

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Ligand	Electrons contributed (neutral counting)	Electrons contributed (ionic counting)
η-1 Allyl	1	2
η-3 Allyl	3	4
η-2Butadiene	2	2
η-4Butadiene	4	4
η-1cyclopentadienyl	1	2
η-3cyclopentadienyl	3	4
η-5cyclopentadienyl	5	6
η-2Benzene	2	2
η-4Benzene	4	4
η-6Benzene	6	6

1. Shriver, D.; Atkins, P. (1999). Inorganic Chemistry, New York: W. H. Freeman.