Metallocene

General chemical structure of a Metallocene compound, where M is some metal cation

A metallocene is a compound with the general formula (C₅H₅)₂M consisting of two cyclopentadienyl anions (Cp, which is C₅H₅^-) bound to a metal center (M) in the oxidation state II. Closely related to the metallocenes are the metallocene derivatives, e.g. titanocene 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.

Definition

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.

Physical properties and structures of metallocenes

A notable feature of some metallocenes is their high thermal stability. Charge-neutral metallocenes are soluble in common organic solvents and can generally be purified by vacuum sublimation.

In metallocenes of the type (C₅R₅)₂M, the two cyclopentadienyl rings can be either eclipsed or staggered, as indicated by single crystal X-ray diffraction studies. For non-substituted metallocenes the energy difference between the staggered and eclipsed conformations is only a few kJ/mol. Ferrocene and osmocene exhibit eclipsed conformations at low temperatures, whereas in the related bis(pentamethylcyclopentadienyl) complexes the rings are in a staggered conformation to minimize steric hindrance between the methyl groups.

Derivatives

• ansa metallocenes: Derivatives of metallocenes include structures with an intramolecular bridge between the two cyclopentadienyl rings (ansa-metallocenes)
• Triple decker complexes: compounds with three Cp anions and two metal cations in alternating order, e.g. [Ni₂Cp₃]⁺.
• metallocenium cations: the most famous example is ferrocenium, [Fe(C₅H₅)₂]⁺, derived from oxidation of ferrocene.

References

1. A. Salzer (1999). "Nomenclature of Organometallic Compounds of the Transition Elements". Pure Appl. Chem. 71 (8): 1557–1585. doi:10.1351/pac199971081557. http://www.iupac.org/reports/1999/7108salzer/index.html. 
2. Robert H. Crabtree The Organometallic Chemistry of the Transition Metals 4th ed. Wiley-Interscience: 2005.