Hapticity is a term describing how a group of contiguous atoms
to a central atom of a coordination compound
. Each atom of the contiguous group is bound once and only once to the central atom.
The hapticity of a compound is the number of contiguous atoms bound to the central atom and is denoted by the greek letter η.
Ferrocene is a molecule with two cyclopentadienyl rings bound to a central iron molecule. Since five contiguous carbon atoms are bound to the central iron atom, the hapticity is five. Ferrocene's molecular formula is Fe(η5