page_title: Metallocene
section_title: Definition
hierarchical_section_title: Metallocene / Definition
caption_attribution_description: Ball-and-stick model of the ferrocene molecule, [Cp2Fe], as found in the monoclinic crystal structure. X-ray crystallographic data from Acta Cryst. (1997). B53, 928-938.
context_page_description: A metallocene is a compound typically consisting of two cyclopentadienyl anions bound to a metal center in the oxidation state II, with the resulting general formula (C₅H₅)₂M. 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 [Cp₂ZrCH₃]⁺ catalyze olefin polymerization.
  Some metallocenes consist of metal plus two cyclooctatetraenide anions, namely the lanthanocenes and the actinocenes.
  Metallocenes are a subset of a broader class of 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.
context_section_description: The general name metallocene is derived from ferrocene, (C₅H₅)₂Fe or Cp₂Fe, systematically named bis(η⁵-cyclopentadienyl)iron(II). According to the IUPAC 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 planes with equal bond lengths and strengths. Using the nomenclature of "hapticity", the equivalent bonding of all 5 carbon atoms of a cyclopentadienyl ring is denoted as η⁵, pronounced "pentahapto". There are exceptions, such as uranocene, which has two cyclooctatetraene rings sandwiching a uranium atom.
  In metallocene names, the prefix before the -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 IUPAC, which requires a d-block metal and a sandwich structure, the term metallocene and thus the denotation -ocene, is applied in the chemical literature also to non-transition metal compounds, such as barocene (Cp₂Ba), or structures where the aromatic rings are not parallel, such as found in manganocene or titanocene dichloride (Cp₂TiCl₂).
  Some metallocene complexes of actinides have been reported where there are three cyclopendadienyl ligands for a monometallic complex, all three of them bound η⁵.
caption: 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.
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