hapticity

Hapticity is a concept in coordination and organometallic chemistry that describes the number of contiguous atoms within a ligand that bind to a single metal center. The binding mode is denoted by the hapticity symbol η^n, where n is the number of donor atoms involved in the metal–ligand interface. Hapticity is related to, but distinct from, denticity, which counts separate donor sites on a ligand.

Common examples illustrate the idea. η^5 cyclopentadienyl (Cp−) ligands bind through five adjacent carbon atoms; η^6 arenes such as benzene bind through six carbons; η^2 ligands include simple alkenes that bind through both carbons of a C=C bond; η^3 ligands include allyl or related three-atom π systems. Metallocenes like iron(II) with two η^5-C5H5 ligands, Fe(η^5-C5H5)2, exemplify the η^5 binding mode.

Hapticity has important implications for electron counting and geometry. In the 18-electron rule, an η^n ligand typically donates n electrons to the metal center, influencing electron configuration and preference for certain oxidation states and structures. Ligands can alter their hapticity with changes in metal, oxidation state, or fluxional behavior, and some ligands can bridge two or more metals while exhibiting different η values.

Overall, hapticity provides a concise way to describe how a ligand engages a metal, guiding predictions of reactivity, stereochemistry, and electronic structure in organometallic complexes.