Metallocene

Definition

The general name metallocene is derived from 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.^{[citation needed]} 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.

Derivatives

Main article: sandwich compound

• 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.

References

1. A. Salzer (1999). "Nomenclature of Organometallic Compounds of the Transition Elements". Pure Appl. Chem. 71 (8): 1557 - 1585.
2. Robert H. Crabtree The Organometallic Chemistry of the Transition Metals 4th ed. Wiley-Interscience: 2005.