
A metallocene is a compound typically consisting of two
cyclopentadienyl anion
Sodium cyclopentadienide is an organosodium compound with the formula C5H5Na. The compound is often abbreviated as NaCp, where Cp− is the cyclopentadienide anion. Sodium cyclopentadienide is a colorless solid, although samples often are pin ...
s (, abbreviated Cp) bound to a
metal
A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, electricity and thermal conductivity, heat relatively well. These properties are all associated wit ...
center (M) in the
oxidation state
In chemistry, the oxidation state, or oxidation number, is the hypothetical Electrical charge, charge of an atom if all of its Chemical bond, bonds to other atoms are fully Ionic bond, ionic. It describes the degree of oxidation (loss of electrons ...
II, with the resulting general formula Closely related to the metallocenes are the metallocene derivatives, e.g.
titanocene dichloride
Titanocene dichloride is the organotitanium compound with the formula (hapticity, ''η''5-C5H5)2TiCl2, commonly abbreviated as Cp2TiCl2. This metallocene is a common reagent in organometallic and organic synthesis. It exists as a bright red solid t ...
or
vanadocene dichloride. Certain metallocenes and their derivatives exhibit
catalytic
Catalysis () is the increase in reaction rate, rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst ...
properties, although metallocenes are rarely used industrially. Cationic group 4 metallocene derivatives related to
2ZrCH3">p2ZrCH3sup>+ catalyze
olefin polymerization.
Some metallocenes consist of metal plus two
cyclooctatetraenide anions (, abbreviated cot
2−), namely the lanthanocenes and the
actinocenes (
uranocene
Uranocene, U(C8H8)2, is an organouranium compound composed of a uranium atom sandwiched between two cyclooctatetraene, cyclooctatetraenide rings. It was one of the first Organoactinide chemistry, organoactinide compounds to be synthesized. It is a ...
and others).
Metallocenes are a subset of a broader class of compounds called
sandwich compound
In organometallic chemistry, a sandwich compound is a chemical compound featuring a metal bound by hapticity, haptic, covalent bonds to two arene compound, arene (ring) ligands. The arenes have the formula , substituted derivatives (for example ...
s.
In the structure shown at right, the two pentagons are the cyclopentadienyl anions with circles inside them indicating they are
aromatically stabilized. Here they are shown in a
staggered conformation.
History

The first metallocene to be classified was
ferrocene
Ferrocene is an organometallic chemistry, organometallic compound with the formula . The molecule is a Cyclopentadienyl complex, complex consisting of two Cyclopentadienyl anion, cyclopentadienyl rings sandwiching a central iron atom. It is an o ...
, and was discovered simultaneously in 1951 by Kealy and Pauson,
and Miller et al.
Kealy and Pauson were attempting to synthesize
fulvalene through the oxidation of a
cyclopentadienyl Cyclopentadienyl can refer to
* Cyclopentadienyl anion, or cyclopentadienide,
** Cyclopentadienyl ligand
* Cyclopentadienyl radical, •
* Cyclopentadienyl cation,
See also
* Pentadienyl
{{Chemistry index ...
salt with anhydrous FeCl
3 but obtained instead the substance C
10H
10Fe
At the same time, Miller ''et al'' reported the same iron product from a reaction of
cyclopentadiene
Cyclopentadiene is an organic compound with the chemical formula, formula C5H6. It is often abbreviated CpH because the cyclopentadienyl anion is abbreviated Cp−.
This colorless liquid has a strong and unpleasant odor. At room temperature, ...
with iron in the presence of aluminum, potassium, or molybdenum oxides.
The structure of "C
10H
10Fe" was determined by
Geoffrey Wilkinson et al.
and by
Ernst Otto Fischer et al. These two were awarded the
Nobel Prize in Chemistry
The Nobel Prize in Chemistry () is awarded annually by the Royal Swedish Academy of Sciences to scientists in the various fields of chemistry. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895, awarded for outst ...
in 1973 for their work on sandwich compounds, including the structural determination of ferrocene.
They determined that the carbon atoms of the cyclopentadienyl (Cp)
ligand
In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...
contributed equally to the bonding and that bonding occurred due to the metal and the in the of the Cp ligands. This complex is now known as ferrocene, and the group of
transition metal
In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. The lanthanide and actinid ...
dicyclopentadienyl compounds is known as metallocenes. Metallocenes have the general formula Fischer et al. first prepared the ferrocene derivatives involving Co and Ni. Often derived from substituted derivatives of
cyclopentadienide, metallocenes of many elements have been prepared.
One of the very earliest commercial manufacturers of metallocenes was Arapahoe Chemicals in Boulder, Colorado
Definition

The general name metallocene is derived from
ferrocene
Ferrocene is an organometallic chemistry, organometallic compound with the formula . The molecule is a Cyclopentadienyl complex, complex consisting of two Cyclopentadienyl anion, cyclopentadienyl rings sandwiching a central iron atom. It is an o ...
, (C
5H
5)
2Fe or Cp
2Fe, systematically named According to the
International Union of Pure and Applied Chemistry
The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
definition, a metallocene contains a
transition metal
In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. The lanthanide and actinid ...
and two cyclopentadienyl ligands coordinated in a sandwich structure, i.e., the two cyclopentadienyl anions are on parallel
planes with equal
bond length
In molecular geometry, bond length or bond distance is defined as the average distance between Atomic nucleus, nuclei of two chemical bond, bonded atoms in a molecule. It is a Transferability (chemistry), transferable property of a bond between at ...
s and strengths. Using the nomenclature of "
hapticity
In coordination chemistry, hapticity is the coordination complex, coordination of a ligand to a metal center via an uninterrupted and contiguous series of atoms. The hapticity of a ligand is described with the Greek letter eta (letter), η ('eta ...
", 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
Uranocene, U(C8H8)2, is an organouranium compound composed of a uranium atom sandwiched between two cyclooctatetraene, cyclooctatetraenide rings. It was one of the first Organoactinide chemistry, organoactinide compounds to be synthesized. It is a ...
, which has two
cyclooctatetraene
1,3,5,7-Cyclooctatetraene (COT) is an unsaturated derivative of cyclooctane, with the formula C8H8. It is also known as nnulene. This polyunsaturated hydrocarbon is a colorless to light yellow flammable liquid at room temperature. Because of ...
rings sandwiching a
uranium
Uranium is a chemical element; it has chemical symbol, symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Ura ...
atom.
In metallocene names, the prefix before the ' ending indicates what
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 (Cp
2Ba), or structures where the aromatic rings are not parallel, such as found in
manganocene or
titanocene dichloride
Titanocene dichloride is the organotitanium compound with the formula (hapticity, ''η''5-C5H5)2TiCl2, commonly abbreviated as Cp2TiCl2. This metallocene is a common reagent in organometallic and organic synthesis. It exists as a bright red solid t ...
(Cp
2TiCl
2).
Some metallocene complexes of
actinide
The actinide () or actinoid () series encompasses at least the 14 metallic chemical elements in the 5f series, with atomic numbers from 89 to 102, actinium through nobelium. Number 103, lawrencium, is also generally included despite being part ...
s have been reported where there are three cyclopentadienyl ligands for a monometallic complex, all three of them bound η
5.
Classification
There are many (''η''
5-C
5H
5)–metal complexes and they can be classified by the following formulas:
Cp-based complexes can also be classified by type:
#Parallel
#Multi-decker
#
Half-sandwich compound
#
Bent metallocene or tilted
#More than two Cp ligands
Synthesis
Three main routes are normally employed in the formation of these types of compounds:
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. Traditionally, the starting point is the cracking of
dicyclopentadiene
Dicyclopentadiene, abbreviated DCPD, is a chemical compound with formula . At room temperature, it is a white brittle wax, although lower purity samples can be straw coloured liquids. The pure material smells somewhat of soy wax or camphor, with ...
, the dimer of cyclopentadiene. Cyclopentadiene is deprotonated by strong bases or alkali metals.
:MCl
2 + 2 NaC
5H
5 → (C
5H
5)
2M + 2 NaCl (M = V, Cr, Mn, Fe, Co; solvent = THF, DME, NH
3)
:CrCl
3 + 3 NaC
5H
5 →
5H5)2Cr">C5H5)2Cr+ "C
10H
10" + 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 + C
5H
6 → MC
5H
5 + H
2 (M = Li, Na, K)
:M + 2 C
5H
6 →
5H5)2M">C5H5)2M+ H
2 (M = Mg, Fe)
Using cyclopentadienyl reagents
A variety of reagents have been developed that transfer Cp to metals. Once popular was
thallium cyclopentadienide. It reacts with metal halides to give thallium chloride, which is poorly soluble, and the
cyclopentadienyl complex. Trialkyl
tin
Tin is a chemical element; it has symbol Sn () and atomic number 50. A silvery-colored metal, tin is soft enough to be cut with little force, and a bar of tin can be bent by hand with little effort. When bent, a bar of tin makes a sound, the ...
derivatives of Cp
− have also been used.
Many other methods have been developed.
Chromocene can be prepared from
chromium hexacarbonyl by direct reaction with cyclopentadiene in the presence of
diethylamine
Diethylamine is an organic compound with the formula . It is classified as a secondary amine. It is a flammable, volatile weakly alkaline liquid that is miscible with most solvents. It is a colorless liquid, but commercial samples often appear br ...
; in this case, the formal deprotonation of the cyclopentadiene is followed by
reduction of the resulting protons to
hydrogen
Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...
gas, facilitating the
oxidation
Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is ...
of the metal centre.
:Cr(CO)
6 + 2 C
5H
6 → Cr(C
5H
5)
2 + 6 CO + H
2
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. 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 MCp
2 involves the variation of the M-C bonds, which elongate as the valence electron count deviates from 18.
In metallocenes of the type (C
5R
5)
2M, the cyclopentadienyl rings rotate with very low barriers. Single crystal
X-ray diffraction
X-ray diffraction is a generic term for phenomena associated with changes in the direction of X-ray beams due to interactions with the electrons around atoms. It occurs due to elastic scattering, when there is no change in the energy of the waves. ...
studies reveal both
eclipsed or
staggered rotamers. For non-substituted metallocenes the energy difference between the staggered and eclipsed conformations is only a few
kJ/mol. 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
Steric effects arise from the spatial arrangement of atoms. When atoms come close together there is generally a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape ( conformation) and reactivi ...
between the
methyl group
In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms, having chemical formula (whereas normal methane has the formula ). In formulas, the group is often abbreviated a ...
s.
Spectroscopic properties
Vibrational (infrared and Raman) spectroscopy of metallocenes
Infrared
Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...
and
Raman 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:
NMR (1H and 13C)
spectroscopy
Spectroscopy is the field of study that measures and interprets electromagnetic spectra. In narrower contexts, spectroscopy is the precise study of color as generalized from visible light to all bands of the electromagnetic spectrum.
Spectro ...
of metallocenes
Nuclear magnetic resonance
Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are disturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with a ...
(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.
Mass spectrometry of metallocenes
Mass spectrometry
Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is used ...
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
Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...
distribution of the metal. The three major fragments observed in mass spectrometry are the molecular ion peak,
10H10M">10H10Msup>+, and fragment ions,
5H5M">5H5Msup>+ and M
+.
Derivatives
After the discovery of ferrocene, the synthesis and characterization of derivatives of metallocene and other sandwich compounds attracted researchers’ interests.
Metallocenophanes
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 to give soluble high molecular weight polymers with transition metals in the polymer backbone.
Ansa-metallocenes are derivatives of metallocenes with an intramolecular
bridge
A bridge is a structure built to Span (engineering), span a physical obstacle (such as a body of water, valley, road, or railway) without blocking the path underneath. It is constructed for the purpose of providing passage over the obstacle, whi ...
between the two cyclopentadienyl rings.
Polynuclear and heterobimetallic metallocenes
*Ferrocene derivatives: biferrocenophanes have been studied for their mixed
valence 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
In chemistry, delocalized electrons are electrons in a molecule, ion or solid metal that are not associated with a single atom or a covalent bond.IUPAC Gold Boo''delocalization''/ref>
The term delocalization is general and can have slightly dif ...
.
*
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
Vanadocene, bis(η5-cyclopentadienyl) vanadium, is the organometallic compound with the formula V(C5H5)2, commonly abbreviated Cp2V. It is a violet crystalline, paramagnetic solid. Vanadocene has relatively limited practical use, but it has been ex ...
and
rhodocene derivatives: vanadocene complexes have been used as starting materials for the synthesis of heterobimetallic complexes. The 18
valence electron
In chemistry and physics, valence electrons are electrons in the outermost shell of an atom, and that can participate in the formation of a chemical bond if the outermost shell is not closed. In a single covalent bond, a shared pair forms with b ...
ions
2Rh">p2Rhsup>+ are very stable, unlike the neutral monomers Cp
2Rh which
dimerize
In chemistry, dimerization is the process of joining two identical or similar Molecular entity, molecular entities by Chemical bond, bonds. The resulting bonds can be either strong or weak. Many symmetrical chemical species are described as dim ...
immediately at room temperature and they have been observed in
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, , was reported in 1972. Many examples have been reported subsequently, often with
boron-containing rings.
Metallocenium ions
The most famous example is
ferrocenium, , the blue iron(III) complex derived from oxidation of orange iron(II) ferrocene. The lithocene anion,
5H5)2">i(C5H5)2sup>–, is the best-documented example of a metallocene anion; otherwise such ions are little known.
Applications
Many derivatives of early metal metallocenes are active catalysts for
olefin polymerization. Unlike traditional and still dominant heterogeneous
Ziegler–Natta catalysts, metallocene catalysts are homogeneous.
Early metal metallocene derivatives, e.g.
Tebbe's reagent
Tebbe's reagent is the organometallic compound with the formula (C5H5)2TiCH2ClAl(CH3)2. It is used in the methylidenation of carbonyl compounds, that is it converts organic compounds containing the R2C=O group into the related R2C=CH2 derivative. ...
,
Petasis reagent
The Petasis reagent, named after Nicos A. Petasis, is an organotitanium compound with the formula Cp2Ti(CH3)2. It is an orange-colored solid.
Preparation and use
The Petasis reagent is prepared by the salt metathesis reaction of methylmagnesiu ...
, and
Schwartz's reagent are useful in specialized organic synthetic operations.
Potential applications
The ferrocene/
ferrocenium biosensor
A biosensor is an analytical device, used for the detection of a chemical substance, that combines a biological component with a physicochemical detector.
The ''sensitive biological element'', e.g. tissue, microorganisms, organelles, cell rece ...
has been discussed for determining the levels of glucose in a sample electrochemically through a series of connected
redox
Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is t ...
cycles.
Metallocene dihalides
2MX2">p2MX2(M = Ti, Mo, Nb) exhibit anti-tumor properties, although none have proceeded far in clinical trials.
See also
*
Jemmis ''mno'' rules
*
Actinocenes
*
f-block metallocene
References
Additional references
*
*
*
*
*
{{Cyclopentadienide complexes