# Metallocene

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A **metallocene** is a compound typically consisting of two [cyclopentadienyl anions](/source/Cyclopentadienyl_anion) (C5H5−, abbreviated Cp) bound to a [metal](/source/Metallic_element) center (M) in the [oxidation state](/source/Oxidation_state) II, with the resulting general formula (C5H5)2M. Closely related to the metallocenes are the metallocene derivatives, e.g. [titanocene dichloride](/source/Titanocene_dichloride) or [vanadocene dichloride](/source/Vanadocene_dichloride). Certain metallocenes and their derivatives exhibit [catalytic](/source/Catalysis) properties, although metallocenes are rarely used industrially. Cationic group 4 metallocene derivatives related to [Cp2ZrCH3]+ catalyze [olefin polymerization](/source/Ziegler%E2%80%93Natta_catalyst).

Some metallocenes consist of metal plus two [cyclooctatetraenide anions](/source/Cyclooctatetraenide_anion) (C8H82−, abbreviated cot2−), namely the lanthanocenes and the [actinocenes](/source/Actinocene) ([uranocene](/source/Uranocene) and others).

Metallocenes are a subset of a broader class of compounds called [sandwich compounds](/source/Sandwich_compound).[1] In the structure shown at right, the two pentagons are the cyclopentadienyl anions with circles inside them indicating they are [aromatically](/source/Aromaticity) stabilized. Here they are shown in a [staggered conformation](/source/Staggered_conformation).

## History

The first metallocene to be classified was [ferrocene](/source/Ferrocene), and was discovered simultaneously in 1951 by Kealy and Pauson,[2] and Miller et al.[3] Kealy and Pauson were attempting to synthesize [fulvalene](/source/Fulvalene) through the oxidation of a [cyclopentadienyl](/source/Cyclopentadienyl_anion) salt with anhydrous FeCl3 but obtained instead the substance C10H10Fe[2] At the same time, Miller *et al* reported the same iron product from a reaction of [cyclopentadiene](/source/Cyclopentadiene) with iron in the presence of aluminum, potassium, or molybdenum oxides.[3] The structure of "C10H10Fe" was determined by [Geoffrey Wilkinson](/source/Geoffrey_Wilkinson) et al.[1] and by [Ernst Otto Fischer](/source/Ernst_Otto_Fischer) et al.[4] These two were awarded the [Nobel Prize in Chemistry](/source/Nobel_Prize_in_Chemistry) in 1973 for their work on sandwich compounds, including the structural determination of ferrocene.[1] They determined that the carbon atoms of the cyclopentadienyl (Cp) [ligand](/source/Ligand) contributed equally to the bonding and that bonding occurred due to the metal [d-orbitals](/source/D-orbital) and the π-[electrons](/source/Electron) in the [p-orbitals](/source/P-orbital) of the Cp ligands. This complex is now known as ferrocene, and the group of [transition metal](/source/Transition_metal) dicyclopentadienyl compounds is known as metallocenes. Metallocenes have the general formula [(*η*5-C5H5)2M]. Fischer et al. first prepared the ferrocene derivatives involving Co and Ni. Often derived from substituted derivatives of [cyclopentadienide](/source/Cyclopentadienide), metallocenes of many elements have been prepared.[5]

One of the very earliest commercial manufacturers of metallocenes was Arapahoe Chemicals in Boulder, Colorado[6]

## Definition

The general name metallocene is derived from [ferrocene](/source/Ferrocene), (C5H5)2Fe or Cp2Fe, systematically named bis(*η*5-[cyclopentadienyl](/source/Cyclopentadienyl_complex))iron(II). According to the [International Union of Pure and Applied Chemistry](/source/International_Union_of_Pure_and_Applied_Chemistry) definition, a metallocene contains a [transition metal](/source/Transition_metal) and two cyclopentadienyl ligands coordinated in a sandwich structure, i.e., the two cyclopentadienyl anions are on parallel [planes](/source/Plane_(geometry)) with equal [bond lengths](/source/Bond_length) and strengths. Using the nomenclature of "[hapticity](/source/Hapticity)", the equivalent bonding of all 5 carbon atoms of a cyclopentadienyl ring is denoted as *η*5, pronounced "pentahapto". There are exceptions, such as [uranocene](/source/Uranocene), which has two [cyclooctatetraene](/source/Cyclooctatetraene) rings sandwiching a [uranium](/source/Uranium) atom.

In metallocene names, the prefix before the '**ending indicates what [metallic element](/source/Metallic_element) is between the Cp groups. For example, in ferrocene, iron(II), ferrous iron is present.**

In contrast to the more strict definition proposed by International Union of Pure and Applied Chemistry, which requires a d-block metal and a sandwich structure, the term metallocene and thus the denotation '**, is applied in the chemical literature also to non-transition metal compounds, such as [barocene](/source/Barocene) (Cp2Ba), or structures where the aromatic rings are not parallel, such as found in [manganocene](/source/Manganocene) or [titanocene dichloride](/source/Titanocene_dichloride) (Cp2TiCl2).**

Some metallocene complexes of [actinides](/source/Actinide) have been reported where there are three cyclopentadienyl ligands for a monometallic complex, all three of them bound η5.[7]

## Classification

There are many (*η*5-C5H5)–metal complexes and they can be classified by the following formulas:[8]

Formula Description [(η5-C5H5)2M] Symmetrical, classical 'sandwich' structure [(η5-C5H5)2MLx] Bent or tilted Cp rings with additional ligands, L [(η5-C5H5)MLx] Only one Cp ligand with additional ligands, L ('piano-stool' structure)

Cp-based complexes can also be classified by type:[8]

1. Parallel
1. Multi-decker
1. [Half-sandwich compound](/source/Half-sandwich_compound)
1. [Bent metallocene](/source/Bent_metallocene) or tilted
1. More than two Cp ligands

## Synthesis

Three main routes are normally employed in the formation of these types of compounds:[8]

### Using a metal salt and cyclopentadienyl reagents

Sodium cyclopentadienide (NaCp) is the preferred reagent for these types of reactions. It is most easily obtained by the reaction of molten sodium and dicyclopentadiene.[9] Traditionally, the starting point is the cracking of [dicyclopentadiene](/source/Dicyclopentadiene), the dimer of cyclopentadiene. Cyclopentadiene is deprotonated by strong bases or alkali metals.

- MCl2 + 2 NaC5H5 → (C5H5)2M + 2 NaCl (M = V, Cr, Mn, Fe, Co; solvent = THF, DME, NH3)

- CrCl3 + 3 NaC5H5 → [(C5H5)2Cr] + "C10H10" + 3 NaCl

NaCp acts as a reducing agent and a ligand in this reaction.

### Using a metal and cyclopentadiene

This technique provides using metal atoms in the gas phase rather than the solid metal. The highly reactive atoms or molecules are generated at a high temperature under vacuum and brought together with chosen reactants on a cold surface.

- M + C5H6 → MC5H5 + H2 (M = Li, Na, K)
- M + 2 C5H6 → [(C5H5)2M] + H2 (M = Mg, Fe)

### Using cyclopentadienyl reagents

A variety of reagents have been developed that transfer Cp to metals. Once popular was [thallium cyclopentadienide](/source/Thallium_cyclopentadienide). It reacts with metal halides to give thallium chloride, which is poorly soluble, and the [cyclopentadienyl complex](/source/Cyclopentadienyl_complex). Trialkyl[tin](/source/Tin) derivatives of Cp− have also been used.

Many other methods have been developed. [Chromocene](/source/Chromocene) can be prepared from [chromium hexacarbonyl](/source/Chromium_hexacarbonyl) by direct reaction with cyclopentadiene in the presence of [diethylamine](/source/Diethylamine); in this case, the formal deprotonation of the cyclopentadiene is followed by [reduction](/source/Redox) of the resulting protons to [hydrogen](/source/Hydrogen) gas, facilitating the [oxidation](/source/Oxidation) of the metal centre.[10]

- Cr(CO)6 + 2 C5H6 → Cr(C5H5)2 + 6 CO + H2

Metallocenes generally have high thermal stability. Ferrocene can be sublimed in air at over 100 °C with no decomposition; metallocenes are generally purified in the laboratory by vacuum [sublimation](/source/Sublimation_(chemistry)). Industrially, sublimation is not practical so metallocenes are isolated by crystallization or produced as part of a hydrocarbon solution. For Group IV metallocenes, donor solvents like ether or THF are distinctly undesirable for polyolefin catalysis. Charge-neutral metallocenes are soluble in common organic solvents. Alkyl substitution on the metallocene increases the solubility in hydrocarbon solvents.

## Structure

A structural trend for the series MCp2 involves the variation of the M-C bonds, which elongate as the valence electron count deviates from 18.[11]

M(C5H5)2 rM–C (pm) Valence electron count Fe 203.3 18 Co 209.6 19 Cr 215.1 16 Ni 218.5 20 V 226 15

In metallocenes of the type (C5R5)2M, the cyclopentadienyl rings rotate with very low barriers. Single crystal [X-ray diffraction](/source/X-ray_diffraction) studies reveal both [eclipsed](/source/Eclipsed_conformation) or [staggered](/source/Staggered_conformation) rotamers. For non-substituted metallocenes the energy difference between the staggered and eclipsed conformations is only a few [kJ/mol](/source/Kilojoule_per_mole). Crystals of ferrocene and osmocene exhibit eclipsed conformations at low temperatures, whereas in the related bis(pentamethylcyclopentadienyl) complexes the rings usually crystallize in a staggered conformation, apparently to minimize [steric hindrance](/source/Steric_effect) between the [methyl groups](/source/Methyl_group).

## Spectroscopic properties

### Vibrational (infrared and Raman) spectroscopy of metallocenes

[Infrared](/source/Infrared_spectroscopy) and [Raman](/source/Raman_spectroscopy) spectroscopies have proved to be important in the analysis of cyclic polyenyl metal sandwich species, with particular use in elucidating covalent or ionic M–ring bonds and distinguishing between central and coordinated rings. Some typical spectral bands and assignments of iron group metallocenes are shown in the following table:[8]

Spectral frequencies of group 8 metallocenes Ferrocene (cm−1) Ruthenocene (cm−1) Osmocene (cm−1) C–H stretch 3085 3100 3095 C–C stretch 1411 1413 1405 Ring deformation 1108 1103 1096 C–H deformation 1002 1002 995 C–H out-of-plane bend 811 806 819 Ring tilt 492 528 428 M–ring stretch 478 446 353 M–ring bend 170 185 –

### NMR (1H and 13C) [spectroscopy](/source/Spectroscopy) of metallocenes

[Nuclear magnetic resonance](/source/Nuclear_magnetic_resonance) (NMR) is the most applied tool in the study of metal sandwich compounds and organometallic species, giving information on nuclear structures in solution, as liquids, gases, and in the solid state. 1H NMR chemical shifts for paramagnetic organotransition-metal compounds is usually observed between 25 and 40 ppm, but this range is much more narrow for diamagnetic metallocene complexes, with chemical shifts usually observed between 3 and 7 ppm.[8]

### Mass spectrometry of metallocenes

[Mass spectrometry](/source/Mass_spectrometry) of metallocene complexes has been very well studied and the effect of the metal on the fragmentation of the organic moiety has received considerable attention and the identification of metal-containing fragments is often facilitated by the [isotope](/source/Isotope) distribution of the metal. The three major fragments observed in mass spectrometry are the molecular ion peak, [C10H10M]+, and fragment ions, [C5H5M]+ and M+.[8]

## Derivatives

Main article: [Sandwich compound](/source/Sandwich_compound)

After the discovery of ferrocene, the synthesis and characterization of derivatives of metallocene and other sandwich compounds attracted researchers’ interests.

### Metallocenophanes

[Metallocenophanes](/source/Metallocenophanes) feature linking of the cyclopentadienyl or polyarenyl rings by the introduction of one or more heteroannular bridges. Some of these compounds undergo thermal [ring-opening polymerizations](/source/Ring-opening_polymerization) to give soluble high molecular weight polymers with transition metals in the polymer backbone. [Ansa-metallocenes](/source/Ansa-metallocene) are derivatives of metallocenes with an intramolecular [bridge](/source/Bridge_(chemical)) between the two cyclopentadienyl rings.

### Polynuclear and heterobimetallic metallocenes

- Ferrocene derivatives: biferrocenophanes have been studied for their mixed [valence](/source/Valence_(chemistry)) properties. Upon one-electron oxidation of a compound with two or more equivalent ferrocene moieties, the electron vacancy could be localized on one ferrocene unit or completely [delocalized](/source/Delocalized).
- [Ruthenocene](/source/Ruthenocene) derivatives: in the solid state biruthenocene is disordered and adopts the transoid conformation with the mutual orientation of Cp rings depending on the intermolecular interactions.
- [Vanadocene](/source/Vanadocene) and [rhodocene](/source/Rhodocene) derivatives: vanadocene complexes have been used as starting materials for the synthesis of heterobimetallic complexes. The 18 [valence electron](/source/Valence_electron) ions [Cp2Rh]+ are very stable, unlike the neutral monomers Cp2Rh which [dimerize](/source/Dimer_(chemistry)) immediately at room temperature and they have been observed in [matrix isolation](/source/Matrix_isolation).

### Multi-decker sandwich compounds

Triple-decker complexes are composed of three Cp anions and two metal cations in alternating order. The first triple-decker sandwich complex, [Ni2Cp3](+), was reported in 1972.[12] Many examples have been reported subsequently, often with [boron-containing rings](/source/Carborane).[13]

### Metallocenium ions

The most famous example is [ferrocenium](/source/Ferrocenium), [Fe(C5H5)2](+), the blue iron(III) complex derived from oxidation of orange iron(II) ferrocene. The lithocene anion, [Li(C5H5)2]–,[14] is the best-documented example of a metallocene anion; otherwise such ions are little known.

### Lanthanide and actinide derivatives

Many sandwich complexes of lanthanide and actinides have been prepared, although simple molecular derivatives of the type M(C5H5)2 are lacking. Numerous complexes are derived from [pentamethylcyclopentadiene](/source/Pentamethylcyclopentadiene) (Cp*H), e.g. Cp*2MX. [Cyclooctatetraene](/source/Cyclooctatetraene) (COT) as its planar dianionic derivative forms many complexes, such as [uranocene](/source/Uranocene) U(COT)2 and [Ce(COT)2]-.[15] Myriad modifications of COT form bis complexes, including dibenzoCOT and bis([[trimethylsilyl)COT.[16]

## Applications

Many derivatives of early metal metallocenes are active catalysts for [olefin polymerization](/source/Olefin_polymerization). Unlike traditional and still dominant heterogeneous [Ziegler–Natta](/source/Ziegler%E2%80%93Natta) catalysts, metallocene catalysts are homogeneous.[8] Early metal metallocene derivatives, e.g. [Tebbe's reagent](/source/Tebbe's_reagent), [Petasis reagent](/source/Petasis_reagent), and [Schwartz's reagent](/source/Schwartz's_reagent) are useful in specialized organic synthetic operations.

### Potential applications

The ferrocene/[ferrocenium](/source/Ferrocenium) [biosensor](/source/Biosensor) has been discussed for determining the levels of glucose in a sample electrochemically through a series of connected [redox](/source/Redox) cycles.[8]

Metallocene dihalides [Cp2MX2] (M = Ti, Mo, Nb) exhibit anti-tumor properties, although none have proceeded far in clinical trials.[17]

## See also

- [Jemmis *mno* rules](/source/Jemmis_mno_rules)
- [Actinocenes](/source/Actinocenes)
- [f-block metallocene](/source/F-block_metallocene)

## References

1. Wilkinson, G.; Rosenblum, M.; Whiting, M. C.; Woodward, R. B. (1952). "The Structure of Iron Bis-Cyclopentadienyl". *[J. Am. Chem. Soc.](/source/J._Am._Chem._Soc.)*. **74** (8): 2125–2126. [Bibcode:1952JAChS..74.2125W](https://ui.adsabs.harvard.edu/abs/1952JAChS..74.2125W). [doi:10.1021/ja01128a527](https://doi.org/10.1021/ja01128a527)

1. Kealy, T. J. & Pauson, P. L. (1951). "A New Type of Organo-Iron Compound". *[Nature](/source/Nature_(journal))*. **168** (4285): 1039. [Bibcode:1951Natur.168.1039K](https://ui.adsabs.harvard.edu/abs/1951Natur.168.1039K). [doi:10.1038/1681039b0](https://doi.org/10.1038/1681039b0). [S2CID 4181383](https://api.semanticscholar.org/CorpusID:4181383)

1. Miller, S. A.; Tebboth, J. A.; Tremaine, J. F. (1952). "114. Dicyclopentadienyliron". *[J. Chem. Soc.](/source/J._Chem._Soc.)*. **1952**: 632–635. [doi:10.1039/JR9520000632](https://doi.org/10.1039/JR9520000632)

1. Fischer, E. O. & Pfab, W. (1952). "Zur Kristallstruktur der Di-Cyclopentadienyl-Verbindungen des zweiwertigen Eisens, Kobalts und Nickels" [On the crystal structure of the di-cyclopentadienyl compounds of divalent iron, cobalt and nickel]. *[Z. Naturforsch. B](/source/Z._Naturforsch._B)*. **7** (7): 377–379. [doi:10.1515/znb-1952-0701](https://doi.org/10.1515/znb-1952-0701)

1. Chirik, Paul J. (2010). "Group 4 Transition Metal Sandwich Complexes: Still Fresh after Almost 60 Years". *Organometallics*. **29** (7): 1500–1517. [doi:10.1021/om100016p](https://doi.org/10.1021/om100016p)

1. .ARAPAHOE CHEMICALS, INC (1962-11-01). "Arapahoe Chemicals, Inc". *Analytical Chemistry*. **34** (12): 122A. [doi:10.1021/ac60192a828](https://doi.org/10.1021/ac60192a828). [ISSN 0003-2700](https://www.worldcat.org/issn/0003-2700)

1. Brennan, J. G.; Andersen, R. A.; Zalkin, A. (1986). "Chemistry of trivalent uranium metallocenes: Electron-transfer reactions. Synthesis and characterization of [(MeC5H4)3U]2E (E= S, Se, Te) and the crystal structures of hexakis(methylcyclopentadienyl)sulfidodiuranium and tris(methylcyclopentadienyl)(triphenylphosphine oxide)uranium". *[Inorg. Chem.](/source/Inorg._Chem.)*. **25** (11): 1761–1765. [doi:10.1021/ic00231a008](https://doi.org/10.1021/ic00231a008)

1. Long, N. J. (1998). *Metallocenes: Introduction to Sandwich Complexes*. London: [Wiley-Blackwell](/source/Wiley-Blackwell). ISBN 978-0632041626.

1. Panda, T. K.; Gamer, M. T.; Roesky, P. W. (2003). "An Improved Synthesis of Sodium and Potassium Cyclopentadienide". *Organometallics*. **22** (4): 877. [doi:10.1021/om0207865](https://doi.org/10.1021/om0207865)

1. Fischer, E. O. & Hafner, W. (1955). "Cyclopentadienyl-Chrom-Tricarbonyl-Wasserstoff" [Cyclopentadienylchromium tricarbonyl hydride] (in German). *[Z. Naturforsch. B](/source/Z._Naturforsch._B)*. **10** (3): 140–143. [doi:10.1515/znb-1955-0303](https://doi.org/10.1515/znb-1955-0303). [S2CID 209650632](https://api.semanticscholar.org/CorpusID:209650632)

1. Flower, K. R. & Hitchcock, P. B. (1996). "Crystal and molecular structure of chromocene (*η*5-C5H5)2Cr". *[J. Organomet. Chem.](/source/J._Organomet._Chem.)*. **507** (1–2): 275–277. [doi:10.1016/0022-328X(95)05747-D](https://doi.org/10.1016/0022-328X(95)05747-D) Discusses all metallocene structures available at that time.

1. Werner, Helmut & Salzer, Albrecht (1972-01-01). "Die Synthese Eines Ersten Doppel-Sandwich-Komplexes: Das Dinickeltricyclopentadienyl-Kation". *Synthesis and Reactivity in Inorganic and Metal-Organic Chemistry*. **2** (3): 239–248. [doi:10.1080/00945717208069606](https://doi.org/10.1080/00945717208069606). [ISSN 0094-5714](https://www.worldcat.org/issn/0094-5714)

1. Grimes, R. N. (2004). "Boron clusters come of age". *[J. Chem. Educ.](/source/J._Chem._Educ.)*. **81** (5): 657–672. [Bibcode:2004JChEd..81..657G](https://ui.adsabs.harvard.edu/abs/2004JChEd..81..657G). [doi:10.1021/ed081p657](https://doi.org/10.1021/ed081p657)

1. Harder, Sjoerd & Prosenc, Marc Heinrich (16 September 1994). "The Simplest Metallocene Sandwich: the Lithocene Anion". *Angewandte Chemie International Edition in English*. **33** (17): 1744-1746. [doi:10.1002/anie.199417441](https://doi.org/10.1002/anie.199417441)

1. Zhou, Zheng; Babson, Aaron J.; Wei, Zheng; Petrukhina, Marina A. (2025). "Construction of Bimetallic and Trimetallic Aggregates with a [Y(COT)2]− Building Block: Solid-State Structures and Solution Behavior". *Organometallics*. **44** (18): 2092–2098. [doi:10.1021/acs.organomet.5c00233](https://doi.org/10.1021/acs.organomet.5c00233). [PMC 12458972](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC12458972). [PMID 41000223](https://pubmed.ncbi.nlm.nih.gov/41000223)

1. Castellanos, Ernesto & Demir, Selvan (2023). "Linear, Electron-Rich, Homoleptic Rare Earth Metallocene and Its Redox Activity". *Inorganic Chemistry*. **62** (5): 2095–2104. [doi:10.1021/acs.inorgchem.2c03735](https://doi.org/10.1021/acs.inorgchem.2c03735). [PMID 36689470](https://pubmed.ncbi.nlm.nih.gov/36689470)

1. Kuo, L. Y.; Kanatzidis, M. G.; Sabat, M.; Marks, T. J.; Marks, Tobin J. (1991). "Metallocene antitumor agents. Solution and solid-state molybdenocene coordination chemistry of DNA constituents". *[J. Am. Chem. Soc.](/source/J._Am._Chem._Soc.)*. **113** (24): 9027–9045. [Bibcode:1991JAChS.113.9027K](https://ui.adsabs.harvard.edu/abs/1991JAChS.113.9027K). [doi:10.1021/ja00024a002](https://doi.org/10.1021/ja00024a002)

### Additional references

- Salzer, A. (1999). "Nomenclature of Organometallic Compounds of the Transition Elements". *[Pure Appl. Chem.](/source/Pure_Appl._Chem.)*. **71** (8): 1557–1585. [doi:10.1351/pac199971081557](https://doi.org/10.1351/pac199971081557). [S2CID 14367196](https://api.semanticscholar.org/CorpusID:14367196)
- Crabtree, Robert H. (2005). *The Organometallic Chemistry of the Transition Metals*. 4th ed. Wiley-Interscience.ISBN 0470257628
- Miessler, Gary L. & Tarr, Donald A. (2004). [*Inorganic Chemistry*](https://archive.org/details/inorganicchemist03edmies). Upper Saddle River, NJ: Pearson Education. ISBN 978-0-13-035471-6.
- Cotton, F. A. & Wilkinson, G. (1988). *Inorganic Chemistry*. 5th ed. Wiley. pp. 626–7.
- Togni, A. & Halterman, R. L. (1998). *Metallocenes*. Wiley-VCH.

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