Sandwich compound
Encyclopedia
Organometallic chemistry
Organometallic chemistry is the study of chemical compounds containing bonds between carbon and a metal. Since many compounds without such bonds are chemically similar, an alternative may be compounds containing metal-element bonds of a largely covalent character...
, a sandwich compound is a chemical compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
featuring a metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
bound by haptic
Hapticity
The 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...
covalent bond
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....
s to two arene ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
s. The arenes have the formula CnHn, substituted derivatives (for example Cn(CH3)n) and heterocyclic derivatives (for example BCnHn+1). Because the metal is usually situated between the two rings, it is said to be "sandwiched." A special class of sandwich complexes are the metallocenes.
The term sandwich compound was introduced in organometallic nomenclature in during the mid-1950s in a report by J.D. Dunitz, L.E. Orgel and R.A. Rich, who confirmed the structure of ferrocene
Ferrocene
Ferrocene is an organometallic compound with the formula Fe2. 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 sandwich compounds...
by X-ray crystallography
X-ray crystallography
X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...
. The correct structure had been proposed several years previously by Robert Burns Woodward
Robert Burns Woodward
Robert Burns Woodward was an American organic chemist, considered by many to be the preeminent organic chemist of the twentieth century...
. The structure helped explain puzzles about ferrocene's conformers
Conformational isomerism
In chemistry, conformational isomerism is a form of stereoisomerism in which the isomers can be interconverted exclusively by rotations about formally single bonds...
, the molecule features an iron atom sandwiched between two parallel cyclopentadienyl
Cyclopentadienyl
In organic chemistry, cyclopentadienyl is a cyclic group of atoms with the formula C5H5. Cyclopentadienyl are closely related to cyclopentadiene. Cyclopentadienyl have five carbon atoms bonded together in a pentagonal planar ring, all five of which are bonded to individual hydrogen atoms...
rings. This result further demonstrated the power of X-ray crystallography
X-ray crystallography
X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...
and accelerated the growth of organometallic chemistry
Organometallic chemistry
Organometallic chemistry is the study of chemical compounds containing bonds between carbon and a metal. Since many compounds without such bonds are chemically similar, an alternative may be compounds containing metal-element bonds of a largely covalent character...
.
Classes of sandwich compounds
The best known members are the metalloceneMetallocene
A metallocene is a compound typically consisting of two cyclopentadienyl anions bound to a metal center in the oxidation state II, with the resulting general formula 2M. Closely related to the metallocenes are the metallocene derivatives, e.g. titanocene dichloride, vanadocene dichloride...
s of the formula M(C5H5)2 where M = Cr
Chromocene
Chromocene is an organochromium compound with the formula Cr2, often abbreviated Cp2Cr. This sandwich compound is structurally similar to ferrocene but does not follow the 18-electron rule because it only has 16 electrons. It also paramagnetic and highly reducing...
, Fe
Ferrocene
Ferrocene is an organometallic compound with the formula Fe2. 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 sandwich compounds...
, Co
Cobaltocene
Cobaltocene, known also as biscobalt or even "bis Cp cobalt", is an organocobalt compound with the formula Co2. It is a dark purple solid that sublimes readily slightly above room temperature. Cobaltocene was discovered shortly after ferrocene, the first metallocene...
, Ni
Nickelocene
Nickelocene is the organonickel compound with the formula Ni2. Also known as bisnickel or NiCp2, this bright green paramagnetic solid is of enduring academic interest, although it yet has no practical applications....
, Zr
Organozirconium chemistry
Organozirconium compounds are organometallic compounds containing a carbon to zirconium chemical bond. Organozirconium chemistry is the corresponding science exploring properties, structure and reactivity of these compounds. In general organozirconium compounds are stable and non-toxic...
, Ru
Ruthenocene
Ruthenocene is an organoruthenium compound with the formula 2Ru. This pale yellow, volatile solid is classified as a sandwich compound and more specifically, as a metallocene.-Structure and bonding:...
, Rh
Rhodocene
Rhodocene, formally known as bisrhodium, is a chemical compound with the formula [Rh2]. Each molecule contains an atom of rhodium bound between two planar systems of five carbon atoms known as cyclopentadienyl rings in a sandwich arrangement. It is an organometallic compound as it has covalent...
, Ti
Titanocene dichloride
Titanocene dichloride is the organotitanium compound with the formula 2TiCl2, commonly abbreviated as Cp2TiCl2. This metallocene is a common reagent in organometallic and organic synthesis. It exists as a bright red solid that slowly hydrolyzes in air...
, V
Vanadocene dichloride
Vanadocene dichloride, dichloro bisvanadium is 2VCl2 . It is a structural analoque of titanocene dichloride but with vanadium instead of titanium....
, Mo, W, Zn. These species are also called bis(cyclopentadienyl)metal complexes.
- Mixed cyclopentadienyl complexes: M(C5H5)(CnHn). An example is Ti(C5H5)(C7H7)
- Bis(benzene) complexes: M(C6H6)2, the best known example being Bis(benzene)chromiumBis(benzene)chromiumBischromium is the organometallic compound with the formula Cr2. It is sometimes called dibenzenechromium. The compound played an important role in the development of sandwich compounds in organometallic chemistry and is the prototypical complex containg two arene ligands.-Preparation:The...
. - Bis(cyclooctatetraenyl) complexes: U(C8H8)2UranoceneUranocene 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...
, the Th derivative is also known.
Sandwich complexes are even known containing purely inorganic ligands, such as Fe(C5Me5)(P5) and [(P5)2Ti]2-.
Monometallic half sandwich compounds
Metallocene
A metallocene is a compound typically consisting of two cyclopentadienyl anions bound to a metal center in the oxidation state II, with the resulting general formula 2M. Closely related to the metallocenes are the metallocene derivatives, e.g. titanocene dichloride, vanadocene dichloride...
s including just one facially-bound planar organic ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
instead of two gives rise to a still larger family of half sandwich compounds. The most famous example is probably methylcyclopentadienyl manganese tricarbonyl
Methylcyclopentadienyl manganese tricarbonyl
Methylcyclopentadienyl manganese tricarbonyl is an organomanganese compound with the formula Mn3. Marketed initially in 1958 as a supplement to the gasoline additive tetraethyl lead to increase the fuel's octane rating, MMT was later used in unleaded gasoline...
. Such species are occasionally referred to as piano stool compounds, at least when there are three diatomic ligands in addition to the hydrocarbon "seat" of the piano stool. The name derives from the similarity of the structure to such a "stool" with the seat being a facial planar organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
, e.g. benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
or cyclopentadiene
Cyclopentadiene
Cyclopentadiene is an organic compound with the formula C5H6. This colorless liquid 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...
, and the legs being ligands such as CO
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
or allyl
Allyl
An allyl group is a substituent with the structural formula H2C=CH-CH2R, where R is the connection to the rest of the molecule. It is made up of a methylene , attached to a vinyl group . The name is derived from the Latin word for garlic, Allium sativum. Theodor Wertheim isolated an allyl...
.
Dimetallic half sandwich compounds
Compounds such as the cyclopentadienyliron dicarbonyl dimerCyclopentadienyliron dicarbonyl dimer
Cyclopentadienyliron dicarbonyl dimer is an organometallic compound with the formula 2Fe24, also abbreviated Cp2Fe24. It is called Fp2 or "fip dimer." It is a dark reddish-purple crystalline solid, which is readily soluble in moderately polar organic solvents such as chloroform and pyridine, but...
and cyclopentadienylmolybdenumtricarbonyl dimer can be considered a special case of half-sandwiches, except that they are dimetallic. A structurally related species is [Ru(C6H6)Cl2]2.
Multidecker sandwiches
The first multidecker sandwich complex was the dicationic triscyclopentadienylCyclopentadienyl
In organic chemistry, cyclopentadienyl is a cyclic group of atoms with the formula C5H5. Cyclopentadienyl are closely related to cyclopentadiene. Cyclopentadienyl have five carbon atoms bonded together in a pentagonal planar ring, all five of which are bonded to individual hydrogen atoms...
dinickel complex [Ni2Cp3](BF4
Tetrafluoroborate
Tetrafluoroborate is the anion BF4−. This tetrahedral species is isoelectronic with tetrafluoromethane, CF4 and tetrafluoroammonium NF4+, and is valence isoelectronic with many stable and important species including the closely related anion perchlorate, ClO4−...
)2. Since that discovery, many related multidecker sandwich compounds have been discovered, especially triple deckers. A versatile method involves the attachment of Cp*Ru+ to preformed sandwich complexes.
Inverse sandwiches
In these anti-bimetallic compounds, the metals are found to be bridged by a single carbocyclic ring. Examples include {(THF)3Ca}2(1,3,5-triphenylbenzene) and {(Ar)Sn}2COT
Cyclooctatetraene
1,3,5,7-Cyclooctatetraene is an unsaturated derivative of cyclooctane, with the formula C8H8. It is also known as [8]annulene. This polyunsaturated hydrocarbon is a colorless to light yellow flammable liquid at room temperature...
.
Double and multimetallic sandwich compounds
Another family of sandwich compound, involves more than one metal sandwiched between two carbocyclic rings. Examples of the double sandwich include V2(indenyl)2, Ni2(COTCyclooctatetraene
1,3,5,7-Cyclooctatetraene is an unsaturated derivative of cyclooctane, with the formula C8H8. It is also known as [8]annulene. This polyunsaturated hydrocarbon is a colorless to light yellow flammable liquid at room temperature...
)2 and Cr2(pentalene)2. Depicted below is an example of a multimetallic sandwich compound, which has four palladium
Palladium
Palladium is a chemical element with the chemical symbol Pd and an atomic number of 46. It is a rare and lustrous silvery-white metal discovered in 1803 by William Hyde Wollaston. He named it after the asteroid Pallas, which was itself named after the epithet of the Greek goddess Athena, acquired...
atoms joined in a chain sandwiched between two perylene
Perylene
Perylene or perilene is a polycyclic aromatic hydrocarbon with the chemical formula C20H12, occurring as a brown solid. It or its derivatives may be carcinogenic, and it is considered to be a hazardous pollutant. In cell membrane cytochemistry, perylene is used as a fluorescent lipid probe...
units. The counterion
Counterion
A counterion is the ion that accompanies an ionic species in order to maintain electric neutrality. In table salt the sodium cation is the counterion for the chlorine anion and vice versa.In a charged transition metal complex, a simple A counterion is the ion that accompanies an ionic species in...
s are bulky tetraarylborates.
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Applications
Ferrocene and methylcyclopentadienyl manganese tricarbonylMethylcyclopentadienyl manganese tricarbonyl
Methylcyclopentadienyl manganese tricarbonyl is an organomanganese compound with the formula Mn3. Marketed initially in 1958 as a supplement to the gasoline additive tetraethyl lead to increase the fuel's octane rating, MMT was later used in unleaded gasoline...
have been used as antiknock agent
Antiknock agent
An antiknock agent is a gasoline additive used to reduce engine knocking and increase the fuel's octane rating.The mixture known as gasoline, when used in high compression internal combustion engines, has a tendency to ignite early causing a damaging "engine knocking" noise...
s. Certain bent metallocenes of zirconium and hafnium are effective precatalysts for the polymerization of propylene
Propylene
Propene, also known as propylene or methylethylene, is an unsaturated organic compound having the chemical formula C3H6. It has one double bond, and is the second simplest member of the alkene class of hydrocarbons, and it is also second in natural abundance.-Properties:At room temperature and...
. Many half sandwich complexes of ruthenium, such as those derived from (cymene)ruthenium dichloride dimer
(Cymene)ruthenium dichloride dimer
ruthenium dichloride dimer is the organometallic compound with the formula [RuCl2]2. This red-coloured, diamagnetic solid is a reagent in organometallic chemistry and homogeneous catalysis.-Preparation and reactions:...
catalyse transfer hydrogenation
Transfer hydrogenation
Transfer hydrogenation is the addition of hydrogen to a molecule from a source other than gaseous H2. It is applied in industry and in organic synthesis, in part because of the inconvenience and expense of using gaseous H2...
, a useful reaction in organic synthesis
Organic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...
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