Metallocenes are a type of organometallic complex, i.e., a chemical compound in which a metal atom is bonded to an organic structure. With the metallocenes, one or more aromatic rings are bonded to the metal ion by their pi electrons (electrons that occur in a certain type of orbit around the atom or molecule).

Ferrocene was the first metallocene to be discovered. This reddish-orange crystalline solid melts at 343°F (173°C). It is also one of a subgroup called the "sandwich compounds" because its two aromatic rings are parallel to each other--one above and one below the plane containing the metal atom (in this case, iron).

Metallocenes are formed by the combination of ionic cyclopentadiene, a reactive but aromatic organic anion, with derivatives of the transition metals or metal halides. (Transition metals are divided into three main series and include chromium, cobalt, hafnium, iron, titanium, vanadium...