text: Coordination numbers become ambiguous when dealing with polyhapto ligands.
  For π-electron ligands such as the cyclopentadienide ion [CH], alkenes and the cyclooctatetraenide ion [CH], 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-CH). Various ways exist for assigning the contribution made to the coordination number of the central iron atom by each cyclopentadienide ligand. The contribution could be assigned as one since there is one ligand, or as five since there are five neighbouring atoms, or as three since there are three electron pairs involved. Normally the count of electron pairs is taken.