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A cyclopentadienyl complex is a metal complex with one or more cyclopentadienyl groups (C5H5-, abbreviated as Cp-). Based on the type of bonding between the metals and the cyclopentadienyl moieties, cyclopentadienyl complexes are classified into the following three categories: a) π-complexes, b) σ-complexes, and c) ionic complexes.
Additional recommended knowledge
The Cyclopentadienyl Ligand
The alkali-metal cyclopentadienyl complexes react with various transition metal compounds to form a variety of complexes that are throughout chemistry. The Cp ligand typically bonds to metals through all five carbon atoms, like in ferrocene, [FeCp2]; chromocene, [CrCp2]; cobaltocene, [CoCp2]; and nickelocene, [NiCp2]. When the Cp rings are mutually parallel the compound is known as a sandwich complex. This area of organometallic chemistry was first discovered in 1954 with the realization of the structure of ferrocene.
Bonding of Cp-ligands
Cp-ligands are generally bound via all 5 carbon atoms to a metal centre (η5-coordination). In rare cases, the Cp unit can bond through three carbon atoms, like in [(η3-Cp)WCp(CO)2]; or through one carbon atom, as in [(η1-Cp)FeCp(CO)2].
c) Ionic complexes
Synthesis of Cp complexes
Most cyclopentadienyl complexes are prepared by treating a metal halide with sodium cyclopentadienide (NaCp). For the preparation of some particularly robust complexes, cyclopentadiene is employed the presence of a conventional base such as NaOH. Specialized alternatives to NaCp include trimethylsilyl cyclopentadiene (CpSiMe3) and thallium cyclopentadienide (CpTl) in an etherial solvent. Most Cp complexes have various other ligands besides a cyclopentadienyl ligand, such as carbonyl, halogen, alkyl, and so on. Biscyclopentadienyl complexes are called metallocenes. It is often the case that these complexes are thermally stable, and they are used for various catalysts. For example, some of early transition metal complexes, such as Cp2TiCl2 and Cp2ZrCl2 with aluminoxane as a co-catalyst, can catalyze for olefin polymerization. Such species are called Kaminsky-type catalysts.
Synthesis of Cp* complexes
Some representative reactions leading to such Cp*-metal complexes follow:
Some Cp* complexes are prepared using hexamethyl Dewar benzene as the precursor. This method was traditionally used for [Rh(C5Me5)Cl2]2.
Comparison Cp* with Cp
Being more electron-rich, Cp* is a stronger donor and is less easily removed from the metal. Consequently its complexes exhibit increased thermal stability. Its steric bulk allows the isolation of complexes with fragile ligands. Its bulk also attenuates intermolecular interactions, decreasing the tendency to form polymeric structures. Its complexes also tend to be highly soluble in non-polar solvents.
Organometallic Chemistry volume, 8 (1967) pp. 287-297.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Cyclopentadienyl_complex". A list of authors is available in Wikipedia.|