Editing Metallocene (section)

==Definition==
[[File:Ferrocene-from-xtal-3D-balls.png|thumb|150px|[[Ball-and-stick model]] of a metallocene [[molecule]] where the cyclopentadienyl anions are in a [[staggered conformation]]. The purple ball in the middle represents the metal cation.]]
The general name metallocene is derived from [[ferrocene]], (C<sub>5</sub>H<sub>5</sub>)<sub>2</sub>Fe or Cp<sub>2</sub>Fe, systematically named {{nowrap|bis(''η''<sup>5</sup>-[[cyclopentadienyl complex|cyclopentadienyl]])iron(II).}} According to the [[International Union of Pure and Applied Chemistry]] definition, a metallocene contains a [[transition metal]] and two cyclopentadienyl ligands coordinated in a sandwich structure, i.e., the two cyclopentadienyl anions are on parallel [[Plane (geometry)|planes]] with equal [[bond length]]s and strengths. Using the nomenclature of "[[hapticity]]", the equivalent bonding of all 5 carbon atoms of a cyclopentadienyl ring is denoted as ''η''<sup>5</sup>, pronounced "pentahapto". There are exceptions, such as [[uranocene]], which has two [[cyclooctatetraene]] rings sandwiching a [[uranium]] atom.

In metallocene names, the prefix before the ''{{not a typo|-ocene}}'' ending indicates what [[metallic element]] is between the Cp groups. For example, in ferrocene, iron(II), ferrous iron is present.

In contrast to the more strict definition proposed by International Union of Pure and Applied Chemistry, which requires a d-block metal and a sandwich structure, the term metallocene and thus the denotation ''{{not a typo|-ocene}}'', is applied in the chemical literature also to non-transition metal compounds, such as [[barocene]] (Cp<sub>2</sub>Ba), or structures where the aromatic rings are not parallel, such as found in [[manganocene]] or [[titanocene dichloride]] (Cp<sub>2</sub>TiCl<sub>2</sub>).

Some metallocene complexes of [[actinide]]s have been reported where there are three cyclopentadienyl ligands for a monometallic complex, all three of them bound η<sup>5</sup>.<ref>{{cite journal|journal= [[Inorg. Chem.]]|doi= 10.1021/ic00231a008|title= Chemistry of trivalent uranium metallocenes: Electron-transfer reactions. Synthesis and characterization of [(MeC<sub>5</sub>H<sub>4</sub>)<sub>3</sub>U]<sub>2</sub>E (E= S, Se, Te) and the crystal structures of hexakis(methylcyclopentadienyl)sulfidodiuranium and tris(methylcyclopentadienyl)(triphenylphosphine oxide)uranium|year= 1986|last1= Brennan|first1= J. G.|last2= Andersen|first2= R. A.|last3= Zalkin|first3= A.|volume= 25|issue= 11|pages= 1761–1765}}</ref>