Chromium carbonyl
Encyclopedia
Chromium carbonyl, also known as chromium hexacarbonyl, is the chemical compound
with the formula
Cr(CO)6. At room temperature
the solid is stable to air, although it does have a high vapor pressure
and sublimes readily. Cr(CO)6 is zerovalent
, meaning that Cr has a formal charge
of zero, and it is called a homoleptic
complex, which means that all the ligand
s are the same. The complex is octahedral with Cr-C and C-O distances of 1.91 and 1.14 Å, respectively.
(THF) solution, Cr(CO)6 converts to Cr(CO)5(THF) with loss of one CO ligand. Similarly, heating a solution of Cr(CO)6 in an aromatic solvent results in replacement of three CO ligands:
Such reactions proceed particularly well with electron-rich arenes such as anisole
, either as the neat reagent or using a mixture of THF and dibutyl ether
, the latter to increase the boiling point
of the mixture. The products adopt a "piano-stool
" structure. These species are typically yellow
solids, which dissolve well in common organic solvent
s. The arene can be liberated from the chromium with iodine
or by photolysis in air. In general, substituted derivatives of Cr(CO)6 decompose upon exposure to air.
Alkyl and aryl organolithium reagent
s RLi add to a carbonyl ligand to give anionic acyl
complexes. These species react with alkylating agents such as Me3O+
to form (OC)5Cr=C(OMe)R, an example of a Fischer carbene
. If the R group is a vinyl or an aryl group, then the resulting carbene complex can react with an acetylene
to form a new benzene ring to which is bonded the chromium tricarbonyl fragment. The two acetylene carbon atoms become part of the new ring, as does a carbon from one of the carbonyl ligands. Also the three carbons from the vinyl
carbene
become part of the new benzene ring.
and iron carbonyl), chromium hexacarbonyl is toxic and thought to be carcinogenic. Its vapor pressure is relatively high for a metal complex, 1 mm Hg ( 36 °C).
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...
with the formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
Cr(CO)6. At room temperature
Room temperature
-Comfort levels:The American Society of Heating, Refrigerating and Air-Conditioning Engineers has listings for suggested temperatures and air flow rates in different types of buildings and different environmental circumstances. For example, a single office in a building has an occupancy ratio per...
the solid is stable to air, although it does have a high vapor pressure
Vapor pressure
Vapor pressure or equilibrium vapor pressure is the pressure of a vapor in thermodynamic equilibrium with its condensed phases in a closed system. All liquids have a tendency to evaporate, and some solids can sublimate into a gaseous form...
and sublimes readily. Cr(CO)6 is zerovalent
Valence (chemistry)
In chemistry, valence, also known as valency or valence number, is a measure of the number of bonds formed by an atom of a given element. "Valence" can be defined as the number of valence bonds...
, meaning that Cr has a formal charge
Formal charge
In chemistry, a formal charge is the charge assigned to an atom in a molecule, assuming that electrons in a chemical bond are shared equally between atoms, regardless of relative electronegativity....
of zero, and it is called a homoleptic
Homoleptic
In inorganic chemistry, a homoleptic chemical compound is a metal compound with all ligands identical. The term uses a homo prefix to indicate that something is the same for all....
complex, which means that all the 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 are the same. The complex is octahedral with Cr-C and C-O distances of 1.91 and 1.14 Å, respectively.
Reactions
When heated or photolyzed in tetrahydrofuranTetrahydrofuran
Tetrahydrofuran is a colorless, water-miscible organic liquid with low viscosity at standard temperature and pressure. This heterocyclic compound has the chemical formula 4O. As one of the most polar ethers with a wide liquid range, it is a useful solvent. Its main use, however, is as a precursor...
(THF) solution, Cr(CO)6 converts to Cr(CO)5(THF) with loss of one CO ligand. Similarly, heating a solution of Cr(CO)6 in an aromatic solvent results in replacement of three CO ligands:
- Cr(CO)6 + C6H5R → Cr(CO)3(C6H5R) + 3 CO
Such reactions proceed particularly well with electron-rich arenes such as anisole
Anisole
Anisole, or methoxybenzene, is the organic compound with the formula CH3OC6H5. It is a colorless liquid with a smell reminiscent of anise seed, and in fact many of its derivatives are found in natural and artificial fragrances...
, either as the neat reagent or using a mixture of THF and dibutyl ether
Dibutyl ether
Dibutyl ether is a colourless, neutral, readily flammable liquid of low volatility with a characteristic odour. It is insoluble in water, but miscible with most common organic solvents...
, the latter to increase the boiling point
Boiling point
The boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....
of the mixture. The products adopt a "piano-stool
Sandwich compound
In organometallic chemistry, a sandwich compound is a chemical compound featuring a metal bound by haptic covalent bonds to two arene ligands. The arenes have the formula CnHn, substituted derivatives and heterocyclic derivatives...
" structure. These species are typically yellow
Yellow
Yellow is the color evoked by light that stimulates both the L and M cone cells of the retina about equally, with no significant stimulation of the S cone cells. Light with a wavelength of 570–590 nm is yellow, as is light with a suitable mixture of red and green...
solids, which dissolve well in common organic solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
s. The arene can be liberated from the chromium with iodine
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
or by photolysis in air. In general, substituted derivatives of Cr(CO)6 decompose upon exposure to air.
Alkyl and aryl organolithium reagent
Organolithium reagent
An organolithium reagent is an organometallic compound with a direct bond between a carbon and a lithium atom. As the electropositive nature of lithium puts most of the charge density of the bond on the carbon atom, effectively creating a carbanion, organolithium compounds are extremely powerful...
s RLi add to a carbonyl ligand to give anionic acyl
Acyl
An acyl group is a functional group derived by the removal of one or more hydroxyl groups from an oxoacid, including inorganic acids.In organic chemistry, the acyl group is usually derived from a carboxylic acid . Therefore, it has the formula RCO-, where R represents an alkyl group that is...
complexes. These species react with alkylating agents such as Me3O+
Meerwein salt
Triethyloxonium tetrafluoroborate is the organic oxonium compound with the formula [3O]BF4. It is often called Meerwein's reagent after its discoverer Hans Meerwein. Also well known and commercially available is the related trimethyloxonium tetrafluoroborate. The compounds are exceptionally...
to form (OC)5Cr=C(OMe)R, an example of a Fischer carbene
Transition metal carbene complex
A transition metal carbene complex is a organometallic compound featuring a divalent organic ligand. The divalent organic ligand coordinated to the metal center is called a carbene. Carbene complexes for almost all transition metals have been reported. Many methods for synthesizing them and...
. If the R group is a vinyl or an aryl group, then the resulting carbene complex can react with an acetylene
Acetylene
Acetylene is the chemical compound with the formula C2H2. It is a hydrocarbon and the simplest alkyne. This colorless gas is widely used as a fuel and a chemical building block. It is unstable in pure form and thus is usually handled as a solution.As an alkyne, acetylene is unsaturated because...
to form a new benzene ring to which is bonded the chromium tricarbonyl fragment. The two acetylene carbon atoms become part of the new ring, as does a carbon from one of the carbonyl ligands. Also the three carbons from the vinyl
Vinyl
A vinyl compound is any organic compound that contains a vinyl group ,which are derivatives of ethene, CH2=CH2, with one hydrogen atom replaced with some other group...
carbene
Carbene
In chemistry, a carbene is a molecule containing a neutral carbon atom with a valence of two and two unshared valence electrons. The general formula is RR'C:, but the carbon can instead be double-bonded to one group. The term "carbene" may also merely refer to the compound H2C:, also called...
become part of the new benzene ring.
Safety
In common with many of the other homoleptic metal carbonyls (e.g. nickel carbonylNickel carbonyl
Nickel carbonyl is the organonickel compound with the formula Ni4. This pale-yellow liquid is the principal carbonyl of nickel. It is an intermediate in the Mond process for the purification of nickel and a reagent in organometallic chemistry...
and iron carbonyl), chromium hexacarbonyl is toxic and thought to be carcinogenic. Its vapor pressure is relatively high for a metal complex, 1 mm Hg ( 36 °C).