A key feature of metallocenes is planar aromatic organic ligands in which multiple of the carbon atoms in the ring form metal-carbon bonds that are equivalent.
The prototypical metallocene is ferrocene, consisting of two cyclopentadienyl rings bound on opposite sides of a central iron atom, forming an organometallic sandwich compound. In the case of ferrocene, for instance, the length and strength and bond angles about the metal-carbon bonds are identical. The nature of the bonding of the organic ligand to the metal is referred to as its "hapticity" and is indicated by the Greek letter eta. The equivalent bonding of all 5 carbon atoms of each cyclopentadienyl ring in ferrocene is denoted as eta-5.
The systematic name of ferrocene is, therefore, bis(eta-5-cyclopentadienyl)iron.
A similar structure to metallocenes, including just one facially-bound planar organic ligand instead of two, are the piano stool compounds.