The hapticity of an organometallic ligand is a pretty simple concept—it just tells how many atoms from a given ligand are coordinated to the metal center. This is typically denoted in chemistry literature with the Greek letter “eta.” A η² thus involves a ligand with two atoms coordinated to the metal center, a η¹ one atom, and so on. The most common hapticity for a ligand is η¹, but it is not uncommon to find ligands, such as the cyclopentadienyl anion ligand (usually η⁵), with higher coordination numbers.