A metallocene is a compound typically consisting of two cyclopentadienyl anions (Cp, which is C5H5-) bound to a metal center (M) 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. 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.
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 co-planar 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) or ferrous 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 Cp2Ba, or structures where the aromatic rings are not co-planar, such as found in manganocene or titanocene dichloride (Cp2TiCl2).
Synthesis, properties, and structures of metallocenes
Most metallocenes are prepared by the reaction of sodium cyclopentadienide with metal dihalides:
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