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Cyclopentadienyl complex
A cyclopentadienyl complex is a coordination complex of a metal and cyclopentadienyl groups (C
5H−
5, abbreviated as Cp−). Cyclopentadienyl ligands almost invariably bind to metals as a pentahapto (η5-) bonding mode. 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. Bent metallocenes are represented by compounds of the type [MCp2Lx]. Some are catalysts for ethylene polymerization. Metallocenes are often thermally stable, and find use as catalysts in various types of reactions.
Mixed-ligand Cp complexes containing Cp ligand and one or more other ligands. They are more numerous. One widely studied example is the Fp dimer, (Cp2Fe2(CO)4). Monometallic compounds featuring only one Cp ring are often known as half sandwich compounds or as piano stool compounds, one example being methylcyclopentadienylmanganese tricarbonyl (CpMn(CO)3).
In the vast majority of M–Cp complexes, all 5 carbon atoms of a Cp ligand bind the metal (η5-coordination). The M–Cp bonding arises from overlap of the five molecular orbitals in the Cp π system with the metal s, p, and d orbitals; hence these complexes are referred to as π-complexes. Equivalent in an electron counting to η5-Cp anion are three CO ligands as well as scorpionate ligands. The electronic properties of these ligand sets differ significantly. For example, bis(trispyrazolyl)iron(II) is high spin, whereas ferrocene is low spin.
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. An example of both is (Cp2Fe(CO)2). 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 three carbons. In these η3-Cp complexes, the bonding resembles that in allyl ligands. Such complexes, sometimes called "slipped Cp complexes", are invoked as intermediates in ring slipping reactions.
The compounds are generally prepared by salt metathesis reactions of alkali-metal cyclopentadienyl compounds with transition metal chlorides. Sodium cyclopentadienide (NaCp) and lithium cyclopentadienide are commonly used. Trimethylsilylcyclopentadiene and cyclopentadienylthallium (CpTl) are alternative sources. 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 are typically carbonyl, halogen, alkyl, or hydride.
Most Cp complexes are prepared by substitution of preformed Cp complexes by replacement of halide, CO, and other simple ligands.
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Cyclopentadienyl complex
A cyclopentadienyl complex is a coordination complex of a metal and cyclopentadienyl groups (C
5H−
5, abbreviated as Cp−). Cyclopentadienyl ligands almost invariably bind to metals as a pentahapto (η5-) bonding mode. 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. Bent metallocenes are represented by compounds of the type [MCp2Lx]. Some are catalysts for ethylene polymerization. Metallocenes are often thermally stable, and find use as catalysts in various types of reactions.
Mixed-ligand Cp complexes containing Cp ligand and one or more other ligands. They are more numerous. One widely studied example is the Fp dimer, (Cp2Fe2(CO)4). Monometallic compounds featuring only one Cp ring are often known as half sandwich compounds or as piano stool compounds, one example being methylcyclopentadienylmanganese tricarbonyl (CpMn(CO)3).
In the vast majority of M–Cp complexes, all 5 carbon atoms of a Cp ligand bind the metal (η5-coordination). The M–Cp bonding arises from overlap of the five molecular orbitals in the Cp π system with the metal s, p, and d orbitals; hence these complexes are referred to as π-complexes. Equivalent in an electron counting to η5-Cp anion are three CO ligands as well as scorpionate ligands. The electronic properties of these ligand sets differ significantly. For example, bis(trispyrazolyl)iron(II) is high spin, whereas ferrocene is low spin.
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. An example of both is (Cp2Fe(CO)2). 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 three carbons. In these η3-Cp complexes, the bonding resembles that in allyl ligands. Such complexes, sometimes called "slipped Cp complexes", are invoked as intermediates in ring slipping reactions.
The compounds are generally prepared by salt metathesis reactions of alkali-metal cyclopentadienyl compounds with transition metal chlorides. Sodium cyclopentadienide (NaCp) and lithium cyclopentadienide are commonly used. Trimethylsilylcyclopentadiene and cyclopentadienylthallium (CpTl) are alternative sources. 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 are typically carbonyl, halogen, alkyl, or hydride.
Most Cp complexes are prepared by substitution of preformed Cp complexes by replacement of halide, CO, and other simple ligands.