Pentahapto

Pentahapto is a term in organometallic chemistry used to describe a mode of ligand binding known as hapticity. The notation η5 indicates that five atoms of the ligand engage with a central metal. The name combines pente, meaning five, with hapto, meaning to seize or bind.

In an η5 interaction, the five atoms form a continuous π-system that donates electron density to the metal, creating a delocalized bond rather than five separate bonds. This binding mode is common for five-atom rings or fragments capable of delocalization, providing a single, concerted interaction with the metal center.

The canonical η5 ligand is the cyclopentadienyl anion (Cp−). In many metal complexes, Cp binds in η5 fashion, as in ferrocene, where two η5-C5H5 rings sandwich an iron atom. Notation for such ligands typically uses η5-C5H5 or η5-C5H5−, and the overall complex is described accordingly, for example [Fe(η5-C5H5)2].

Ligands capable of η5 binding are less common than those with other haptities, but several five-atom π-systems can, in principle, adopt this mode under suitable electronic and geometric conditions. The concept of pentahapto is part of a broader framework that describes how many atoms of a ligand coordinate to a metal center.

Relation to other haptities: ηn denotes the number of atoms involved in the metal–ligand interaction. Common values include η3, η4, η5, and η6, and the chosen hapticity influences the complex’s geometry, electron count, and reactivity.