hapticities

Hapticities are a concept in coordination chemistry and organometallic chemistry describing how ligands bind to a central metal atom or ion. The binding mode is indicated by the eta notation (η) followed by a number, written as η^n, where n is the number of contiguous atoms in the ligand that are bound to the metal.

Hapticity distinguishes the binding mode from denticity, which counts donor atoms in a ligand regardless of how they attach. A single ligand can exhibit different hapticities in different complexes or under different conditions. Common examples include η^5 binding of the cyclopentadienyl ligand to a metal in ferrocene-like complexes, η^6 binding of benzene, and η^3 binding of an allyl ligand.

The concept helps explain geometry, reactivity, and electron distribution in organometallic systems. In many electron-counting schemes, an η^n ligand contributes a set number of electrons to the metal, often related to 2n, though the exact accounting can depend on the ligand and the chosen model. Hapticity also influences fluxional behavior, where a ligand may rearrange its binding mode with temperature or other conditions.

Commonly cited examples include η^5 cyclopentadienyl ligands in sandwich compounds and η^6 benzene ligands in various metal-benzene complexes. Understanding hapticities aids in predicting catalytic activity, stabilizing structures, and interpreting spectroscopic and crystallographic data.