What is "metallocene"

n. (context chemistry English) any of a class of organic compound containing two cyclopentadienyl anions and a transition metal whose bonding involves overlap of ''ns'' and ''np'' orbitals of the metal with molecular orbitals of appropriate symmetry of each cyclopentadienyl ring; the rings are either parallel to each other, or sometimes canted

A metallocene is a compound typically consisting of two cyclopentadienyl anions (Cp, which is ) bound to a metal center (M) in the oxidation state II, with the resulting general formula (CH)M. Closely related to the metallocenes are the metallocene derivatives, e.g. titanocene dichloride, vanadocene dichloride. Certain metallocenes and their derivatives exhibit catalytic properties, although metallocenes are rarely used industrially. Cationic group 4 metallocene derivatives related to [CpZrCH] catalyze olefin polymerization. Metallocenes are a subset of a broader class of organometallic compounds called sandwich compounds.

In the structure shown at right, the two pentagons are the cyclopentadienyl anions with circles inside them indicating they are aromatically stabilized. Here they are shown in a staggered conformation.