the earliest and best known of the so-called sandwich compounds; these are derivatives of transition metals in which two organic ring systems are bonded symmetrically to the metal atom. Its molecular formula is (C₅H₅)₂Fe.

First prepared in 1951 by the reaction of sodium cyclopentadienide with iron(+2) chloride, ferrocene occurs as highly stable orange crystals with a melting point of 174° C (345° F). Chemically, ferrocene behaves like benzene and other aromatic compounds in that it undergoes substitution reactions. The removal of one electron from the molecule raises the iron atom to the next-higher oxidation state (i.e., from +2 to +3), leading to the formation of salts containing the blue ferricinium cation, (C₅H₅)₂Fe⁺.