Cyclopentadienyl complex

Zirconocene dichloride, a cyclopentadienyl complex

A cyclopentadienyl complex is a metal complex with one or more cyclopentadienyl groups (C
₅H⁻
₅, abbreviated as Cp⁻). Based on the type of bonding between the metals and the cyclopentadienyl moieties, cyclopentadienyl complexes are classified as π-complexes, σ-complexes, or ionic complexes.

Cyclopentadienyl ligand

The alkali-metal cyclopentadienyl complexes react with various transition metal compounds to form a variety of complexes that are found throughout chemistry. The Cp ligand typically coordinates metals through the donation of π-electrons. The metal–cyclopentadienyl interaction is typically drawn as a single line from the metal center to the center of the Cp ring.

Biscyclopentadienyl complexes are called metallocenes. A famous example of this type of complex is ferrocene (FeCp₂), which has many analogues for other metals, such as 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 developed in the 1950s. Metallocenes are often thermally stable, and find use as catalysts in various types of reactions.

Mixed-ligand Cp complexes, containing one Cp ligand and one or more other ligands, are more numerous than the dual-Cp sandwich compounds. One early and still widely studied example being the Fp dimer, (Cp₂Fe₂(CO)₄). Monometallic compounds featuring only one Cp ring are known as half sandwich compounds or as piano stool compounds, one example being cyclopentadienylcobalt dicarbonyl (CpCo(CO)₂). Other metals form bent complexes, e.g., zirconocene dichloride, [ZrCp₂Cl₂], a catalyst for ethylene polymerization.

Bonding modes

All 5 carbon atoms of a Cp ligand are bound to the metal in the vast majority of M–Cp complexes. This bonding mode is called η⁵-coordination. The M–Cp bonding arises from overlap of the five π molecular orbitals of the Cp ligand with the s, p, and d orbitals on the metal. This π bonding is significant, hence these complexes are referred to a π-complexes. Almost all of the transition metals, that is, group 4 to 10 metals, employ this coordination mode.

In relatively rare cases, Cp binds to metals via only one carbon center. These types of interactions are described as σ-complexes because they only have a σ bond between the metal and 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. It is probable that η¹-Cp complexes are intermediates in the formation of η⁵-Cp complexes.

Still rarer, the Cp unit can bond to the metal via a three-carbons. In these η³-Cp complexes, the bonding resembles that in allyl ligands. Such complexes are invoked as intermediates in ring slipping reactions.

Synthesis of Cp complexes

Simple cyclopentadienyl complexes are prepared by treating a metal halide with sodium cyclopentadienide (NaCp). Specialized alternatives to NaCp include trimethylsilyl cyclopentadiene and cyclopentadienylthallium (CpTl). For the preparation of some particularly robust complexes, e.g. nickelocene, cyclopentadiene is employed in the presence of a conventional base such as KOH. When only a single Cp ligand is installed, the other ligands typically carbonyl, halogen, alkyl, and hydride.

Most Cp complexes are prepared by substitution of preformed Cp complexes by replacement of halide, CO, and other simple ligands.

Variations of Cp complexes

• Variations of Cyclopentadienyl complexes
• 
  decamethylcobaltocene, a powerful reducing agent
• 
  A constrained geometry organotitanium complex, related to commercial catalysts
• 
  An ansa-metallocene]]

Ansa Cp ligands

Main article: Ansa-metallocene

A pair of cyclopentadienyl ligands can be covalently linked giving rise to so-call ansa metallocenes. The angle between the two Cp rings is fixed. Rotation of the rings about the metal-centroid axis is stopped as well. A related class of derivatives give rise to the constrained geometry complexes. In these cases, a Cp ligand as linked to a non-Cp ligand. Such complexes have been commercialized for the production of polypropylene.

=Cp* complexes

Some representative reactions leading to such Cp*-metal complexes include:

• Cp*H + C₄H₉Li → Cp*Li + C₄H₁₀
• 2 Cp*Li + TiCl₄ → Cp*₂TiCl₂ + 2 LiCl
• Cp*₂TiCl₂ + TiCl₄ → 2 Cp*TiCl₃
• Cp*Li + Me₃SiCl → Cp*SiMe₃ + LiCl
• Cp*SiMe₃ + TiCl₄ → Cp*TiCl₃ + Me₃SiCl
• 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.

Applications

Cp metal complexes are mainly used as stoichiometric reagents in chemical research. Ferrocenium reagents are oxidants. Cobaltocene is a strong, soluble reductant.

Derivatives of Cp₂TiCl₂ and Cp₂ZrCl₂ are the basis of some reagents in organic synthesis. Upon treatment with aluminoxane, these dihalides give catalysts for olefin polymerization. Such species are called Kaminsky-type catalysts.

Further reading

• Yamamoto, A. (1986). Organotransition Metal Chemistry: Fundamental Concepts and Applications. New York, NY: Wiley-Interscience. p. 105.
• Shriver, D.; Atkins, P. W. (1999). Inorganic Chemistry. New York, NY: W. H. Freeman.
• King, R. B.; Bisnette, M. B. (1967). "Organometallic chemistry of the transition metals XXI. Some π-pentamethylcyclopentadienyl derivatives of various transition metals". J. Organomet. Chem. 8: 287–297. doi:10.1016/S0022-328X(00)91042-8. [Initial examples of the synthesis of Cp*-metal complexes]