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 into the following three categories: a) π-complexes, b) σ-complexes, and c) 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 M-Cp interaction is typically drawn as a single line from the metal center to the center of the Cp ring.[1]
Examples of famous metal-Cp complexes include ferrocene (FeCp2) and many analogues 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.[2]
Mixed ligand Cp complexes are more numerous than the 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 of Cp-ligands
All 5 carbon atoms of Cp-ligands are bound to the metal in the vast majority of metal-Cp complexes. This bonding mode is called η5-coordination. The M-Cp bonding arises from overlap of the five pi-MO's on the Cp ligand with the s, p, and d orbitals on the metal. 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.[1]
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).[1] Specialized alternatives to NaCp include trimethylsilylcyclopentadiene and thallium cyclopentadienide (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.
.
Comparison of Cp* with Cp
The pentamethylcyclopentadienyl ligand (Cp*) is a important ligand in organometallic compounds arising from the binding of the five ring-carbon atoms in C
5Me−
5, or Cp*−, to metals. Relative to the more common cyclopentadiene (Cp) ligand, Cp* offers certain features that are often advantageous. 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.
Synthesis of Cp* complexes
| Compound | Cp | Cp* |
|---|---|---|
| Cp2Fe | yellow | yellow |
| CpTiCl3 | yellow | red |
| [Cp*Fe(CO)2]2 | red-violet | |
| [Cp*IrCl2]2 | yellow | |
| Cp*Re(CO)3 | colorless | |
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.
References
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]