Metallocene

A metallocene is a compound typically consisting of two cyclopentadienyl anions (Cp, which is C
5H−
5) 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 first metallocene to be classified was ferrocene, and was discovered simultaneously in 1951 by Kealy and Pauson, and Miller et al. Keally and Pauson were attempting to synthesize fulvalene through the oxidation of a cyclopentadienyl salt with anhydrous FeCl₃ but obtained instead the substance C₁₀H₁₀Fe At the same time, Miller et al reported the same iron product from a reaction of cyclopentadiene with iron in the presence of aluminum, potassium, or molybdenum oxides. The structure of "C₁₀H₁₀Fe" was determined by Wilkinson et al. and by Fischer et al. These two were awarded the Nobel Prize in Chemistry in 1973 for their work on sandwich compounds, including the structural determination of ferrocene. They determined that the carbon atoms of the cyclopentadienyl (Cp) ligand contributed equally to the bonding and that bonding occurred due to the metal d-orbitals and the π-electrons in the p-orbitals of the Cp ligands. This complex is now known as ferrocene and the group of transition metal dicyclopentadienyl compounds is known as metallocenes and have the general formula [(η⁵-C₅H₅)₂M]. Fischer et al. first prepared the ferrocene derivatives involving Co and Ni. Often derived from substituted derivatives of cyclopentadienide, metallocenes of many elements have been prepared.

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