Metallocene

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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. Certain metallocenes and their derivatives are 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.

Contents 1 Definition 2 Physical properties and structures of metallocenes 3 Derivatives 4 References

[edit] Definition

The general name metallocene is derived from ferrocene, 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."

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] Physical properties and structures of metallocenes

Main article: sandwich compound

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.

[edit] Derivatives

Main article: sandwich compound

• 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.

[edit] References

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

v • d • e Concepts in organometallic chemistry Principles Electron counting, 18-Electron rule, Polyhedral skeletal electron pair theory, Isolobal principle, π backbonding, Hapticity Reactions Oxidative addition, Reductive elimination, Beta-hydride elimination, Transmetalation, Carbometalation Types of compounds Gilman reagent, Grignard reagents, Cyclopentadienyl complexes, Metallocenes, Sandwich compound, Transition metal carbene complexes Applications Monsanto process, Ziegler-Natta catalyst, Olefin metathesis Related branches of chemistry Organic chemistry - Inorganic chemistry - Bioinorganic chemistry