sentences:  Coordination numbers become ambiguous when dealing with polyhapto ligands.For π-electron ligands such as the cyclopentadienide ion [C5H5]−, alkenes and the cyclooctatetraenide ion [C8H8]2−, the number of adjacent atoms in the π-electron system that bind to the central atom is termed the hapticity.In ferrocene the hapticity, "η", of each cyclopentadienide anion is five, Fe("η"5-C5H5)2.