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. Closely related to the metallocenes are the metallocene derivatives, e.g. titanocene dichloride. Metallocenes of the type Cp2M are not useful for catalysis. One of the more important commercial uses for metallocenes is olefin polymerization with cationic group 4 metallocene derivatives related to [Cp2ZrCH3]+. Metallocenes are a subset of a broader class of organometallic compounds called sandwich compounds.
The general name metallocene is derived from ferrocene, systematically bis(η5-cyclopentadienyl)iron(II). 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.
Physical properties and structures of metallocenes
A notable feature of some metallocenes is their high thermal stability (often > 500 °C). 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 8 kJ/mol. Ferrocene and osmocene are eclipsed conformation at low temperatures, wherea in the related bis(pentamethylcyclopentadienyl) complexes the rings are in a staggered conformation to minimize steric hindrance between the methyl groups.
- ansa metallocenes: Derivatives of metallocenes include structures with an intramolecular bridge between the two cyclopentadienyl rings (ansa-metallocenes)
- half-sandwich compounds: derivatives with just one planar organic ligand, these include the so-called "piano stool complexes."
- 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.
- A. Salzer (1999). "Nomenclature of Organometallic Compounds of the Transition Elements". Pure Appl. Chem. 71 (8): 1557–1585.
- Robert H. Crabtree The Organometallic Chemistry of the Transition Metals 4th ed. Wiley-Interscience: 2005.