Cyclopentadienyl complex

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.

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.

Other metals form bent structures, such as zirconocene dichloride, [ZrCp2Cl2], which is a catalyst for ethylene polymerization.

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].

a) π-complexes Metals and cyclopentadienyl moieties connected by π-bonds are called π-complexes. π-Complexes, especially in the η5-type coordination mode, are the most typical of the three types of complexes. Almost all of the transition metals, that is, group 4 to 10 metals, employ this coordination mode.

b) σ-complexes σ-Complexes have a direct σ-bond between the metal and one of the carbons of the cyclopentadienyl group. Typical examples of this type of complex are group 14 metal complexes such as CpSiMe3, Cp2Sn, and CpPb. CpSiMe3 is commonly used as the starting material for the synthesis of group 4 metal cyclopentadienyl complexes.

c) Ionic complexes Ionic complexes primarily involve alkali metal cations and alkali earth metals cations connected to cyclopentadienyl anions. Of course, these complexes are not ionic, but the bonding is probably highly polar and often η-1 bonding is indicated.   Ionic type complexes are generally synthesized directly by reaction of cyclopentadiene and the metal in a non-aromatic solvent. These complexes can be good starting materials for several π-type cyclopentadienyl 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:
 * Cp*H + C4H9Li  →   Cp*Li  +  C4H10
 * Cp*Li + TiCl4 →   Cp*TiCl3  +  LiCl


 * 2 Cp*H +  2 Fe(CO)5   →   [Cp*Fe(CO)2]2  +  H2

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.