A
metallocene is a compound with the general formula (C
5H
5)
2M consisting of two
cyclopentadienylCyclopentadiene is a chemical compound with the formula C5H6. This colorless liquid organic chemical has a strong and unpleasant odor. At room temperature, this cyclic diene dimerizes over the course of hours to give dicyclopentadiene via a Diels-Alder reaction...
anions (Cp, which is C
5H
5-) bound to a metal center (M) in the
oxidation stateIn chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
II. Closely related to the metallocenes are the metallocene derivatives, e.g.
titanocene dichlorideTitanocene dichloride, or dicyclopentadienyl titanium dichloride is 2TiCl2 ; this metallocene is widely used in organometallic and organic synthesis both as a reagent and as a catalyst...
. Certain metallocenes and their derivatives exhibit
catalyticCatalysis is the process in which the rate of a chemical reaction is either increased or decreased by means of a chemical substance known as a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. The catalyst may participate...
properties, although metallocenes are rarely used industrially.
A
metallocene is a compound with the general formula (C
5H
5)
2M consisting of two
cyclopentadienylCyclopentadiene is a chemical compound with the formula C5H6. This colorless liquid organic chemical has a strong and unpleasant odor. At room temperature, this cyclic diene dimerizes over the course of hours to give dicyclopentadiene via a Diels-Alder reaction...
anions (Cp, which is C
5H
5-) bound to a metal center (M) in the
oxidation stateIn chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
II. Closely related to the metallocenes are the metallocene derivatives, e.g.
titanocene dichlorideTitanocene dichloride, or dicyclopentadienyl titanium dichloride is 2TiCl2 ; this metallocene is widely used in organometallic and organic synthesis both as a reagent and as a catalyst...
. Certain metallocenes and their derivatives exhibit
catalyticCatalysis is the process in which the rate of a chemical reaction is either increased or decreased by means of a chemical substance known as a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. The catalyst may participate...
properties, although metallocenes are rarely used industrially. Cationic group 4 metallocene derivatives related to [Cp
2ZrCH
3]
+ catalyze
olefin polymerizationA Ziegler-Natta catalyst is a catalyst used in the production of polymers of 1-alkenes . Ziegler-Natta catalysts are typically based on titanium compounds and organometallic aluminium compounds, such as the undefined methylaluminoxane or well defined triethylaluminium,...
. Metallocenes are a subset of a broader class of organometallic compounds called sandwich compounds.
In the structure shown at right, the two pentagons are the cyclopentadienyl anions with circles inside them indicating they are
aromaticallyIn organic chemistry, the structures of some rings of atoms are unexpectedly stable. Aromaticity is a chemical property in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibit a stabilization stronger than would be expected by the stabilization of conjugation alone...
stabilized. Here they are shown in a staggered conformation.
Definition
The general name metallocene is derived from
ferroceneFerrocene is an organometallic compound with the formula Fe
2. It is the prototypical metallocene, a type of organometallic chemical compound consisting of two cyclopentadienyl rings bound on opposite sides of a central metal atom. Such organometallic compounds are also known as...
, (C
5H
5)
2Fe or Cp
2Fe , systematically named bis(η
5-
cyclopentadienylA cyclopentadienyl complex is a metal complex with one or more cyclopentadienyl groups...
)iron(II). According to the IUPAC definition, a metallocene contains a
transition metalThe term transition metal has two possible meanings:*In the past it referred to any element in the d-block of the periodic table, which includes groups 3 to 12 on the periodic table...
and two cyclopentadienyl ligands coordinated in a
sandwich structure, i. e., the two cyclopentadienyl anions are co-
planarIn computer graphics, planar is the method of representing pixel colours with several bitplanes of RAM. Each bit in a bitplane is related to one pixel on the screen...
with equal
bond lengthIn molecular geometry, bond length or bond distance is the average distance between nuclei of two bonded atoms in a molecule.- Explanation :...
s and strengths. Using the nomenclature of "
hapticityThe term hapticity is used to describe how a group of contiguous atoms of a ligand are coordinated to a central atom. Hapticity of a ligand is indicated by the Greek character 'eta', η. A superscripted number following the η denotes the number of contiguous atoms of the ligand that are bound to...
," the equivalent bonding of all 5 carbon atoms of a cyclopentadienyl ring is denoted as η
5, pronounced "pentahapto." There are exceptions, such as
uranoceneUranocene U2 is the most notable cyclooctatetraenide of the f elements and one of the first organouranium compounds to be synthesized. Uranocene is a member of the actinocenes, a group of metallocenes incorporating elements from the actinide series...
, which has two
cyclooctatetraene1,3,5,7-Cyclooctatetraene is an unsaturated derivative of cyclooctane, with the formula C
8H
8. It is also known as [8]annulene. This polyunsaturated hydrocarbon is a colorless to light yellow flammable liquid at room temperature...
rings sandwiching a
uraniumUranium is a silvery-white metallic chemical element in the actinide series of the periodic table that has the symbol U and atomic number 92. Besides its 92 protons, a uranium nucleus can have between 141 and 146 neutrons. The most common uranium isotopes are U-238 and U-235 . A uranium atom has...
atom.
In metallocene names, the prefix before the
-ocene ending indicates what metallic element is between the Cp groups. For example in ferrocene, iron(II) or
ferrousFerrous, in chemical science, indicates a bivalent iron compound , as opposed to ferric, which indicates a trivalent iron compound ....
is present.
In contrast to the more strict definition proposed by IUPAC, which requires a d-block metal and a sandwich structure, the term metallocene and thus the denotation -ocene, is applied in the chemical literature also to non-transition metal compounds, such as Cp
2Ba, or structures where the aromatic rings are not co-planar, such as found in manganocene or titanocene dichloride, Cp
2TiCl
2.
Physical properties and structures of metallocenes
A notable feature of some metallocenes is their high thermal stability. Charge-neutral metallocenes are soluble in common organic solvents and can generally be purified by vacuum sublimation.
In metallocenes of the type (C
5R
5)
2M, the two cyclopentadienyl rings can be either eclipsed or staggered, as indicated by single crystal X-ray diffraction studies. For non-substituted metallocenes the energy difference between the staggered and eclipsed conformations is only a few kJ/mol. Ferrocene and osmocene exhibit eclipsed conformations at low temperatures, whereas in the related bis(pentamethylcyclopentadienyl) complexes the rings are in a staggered conformation to minimize steric hindrance between the
methyl groupIn chemistry, a methyl group is a hydrophobic alkyl functional group named after methane . It has the formula -3 and is often abbreviated -Me...
s.
Derivatives
- ansa metallocenes: Derivatives of metallocenes include structures with an intramolecular bridge between the two cyclopentadienyl rings (ansa-metallocenes)
- Triple decker complexes: compounds with three Cp anions and two metal cations in alternating order, e.g. [Ni2Cp3]+.
- metallocenium cations: the most famous example is ferrocenium, [Fe(C5H5)2]+, derived from oxidation of ferrocene.