Metallocene

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In chemistry, and in particular, in organometallic chemistry, a metallocene is a compound with the general formula (C₅R₅)₂M consisting of two cyclopentadienyl anions (Cp) bound to a metal center in the oxidation state II. Metallocenes are a subset of a broader class of organometallic compounds called sandwich compounds.

1 Definition
2 Features of metallocenes
3 Uses of metallocenes
4 Derivatives
5 References

[edit] Definition

The general name metallocene is derived from ferrocene, systematically bis(η⁵-cyclopentadienyl)iron. According to the definition by IUPAC, 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. The nature of the bonding of the organic ligand to the metal is referred to as its "hapticity" and is indicated by the Greek letter eta (η). The equivalent bonding of all 5 carbon atoms of each cyclopentadienyl ring is denoted as η⁵, pronounced "pentahapto".

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.

[edit] Features of metallocenes

A notable feature of all metallocenes is their high thermal stability (up to and over 500°C). All neutral metallocenes are soluble in common organic solvents and can be purified by vacuum sublimation. Metallocenes of all 3d-row elements are known, but with the exception of ferrocene all of these are sensitive to air, paramagnetic and do not possess a closed-shell electron structure (see 18-Electron rule).

X-ray diffraction studies have shown that the cyclopentadienyl rings might be either eclipsed or staggered. For non-substituted metallocenes the energy difference between the staggered and eclipsed conformations is only 8 kJ/mol. Ferrocene and osmocene take on an eclipsed conformation at low temperatures, while pentamethylcyclopentadienyl ligands are always in a staggered conformation to minimize steric hindrance between the methyl groups.

[edit] Uses of metallocenes

Many metallocenes have proven to be useful for catalysis. One of the more important commercial uses for metallocenes is olefin polymerization with cationic group 4 metallocene derivatives related to [Cp₂ZrCH₃]⁺.

[edit] Derivatives

Derivatives of metallocenes include structures with an intramolecular bridge between the two cyclopentadienyl rings (ansa-metallocenes), compounds with just one facially-bound planar organic ligand (half-sandwich compounds), compounds with three Cp anions and two metal cations in alternating order (tripledecker configuration) and compounds where one cyclopentadienyl ligand is replaced by three other ligands (piano stool configuration).