Chromium carbonyl, also known as chromium hexacarbonyl, is the

chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one ele ...

with the formula Cr( CO)₆. At

room temperature Colloquially, "room temperature" is a range of air temperatures that most people prefer for indoor settings. It feels comfortable to a person when they are wearing typical indoor clothing. Human comfort can extend beyond this range depending on ...

the

solid Solid is one of the four fundamental states of matter (the others being liquid, gas, and plasma). The molecules in a solid are closely packed together and contain the least amount of kinetic energy. A solid is characterized by structura ...

is stable to

air The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing for ...

, although it does have a high

vapor pressure Vapor pressure (or vapour pressure in English-speaking countries other than the US; see spelling differences) or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phase ...

and sublimes readily. Cr(CO)₆ is zerovalent, meaning that Cr has an

oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. ...

of zero, and it is a homoleptic complex, which means that all the

ligand In coordination chemistry, a ligand is an ion or molecule ( 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 ele ...

s are identical. The complex is

octahedral In geometry, an octahedron (plural: octahedra, octahedrons) is a polyhedron with eight faces. The term is most commonly used to refer to the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet at ea ...

with Cr–C and C–O distances of 1.91 and 1.14 Å, respectively.

History

The synthesis of Cr(CO)₆ was described in a series of papers published in 1926–7. The procedure involves treatment of Cr(III) salts with high pressures of carbon monoxide using phenyl magnesium bromides as a reductant.

Reactions

Pentacarbonyl derivatives

When heated or UV-irradiated in

tetrahydrofuran Tetrahydrofuran (THF), or oxolane, is an organic compound with the formula (CH2)4O. The compound is classified as heterocyclic compound, specifically a cyclic ether. It is a colorless, water- miscible organic liquid with low viscosity. It is ...

(THF) solution, Cr(CO)₆ converts to Cr(CO)₅(THF) with loss of one CO ligand. The THF ligand is readily displaced. Often the THF complex is generated and used in situ. UV-irradiation of frozen solutions of chromium hexacarbonyl affords a variety of labile adducts, including labile but complexes with some noble gases.

Arene derivatives

Heating a solution of Cr(CO)₆ in an

aromatic In chemistry, aromaticity is a chemical property of cyclic (ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to sat ...

solvent results in replacement of three CO ligands. The reactions are especially favorable for electron-rich arenes: :Cr(CO)₆ + C₆H₅R → Cr(CO)₃(C₆H₅R) + 3 CO The products are " piano-stool" complexes. These species are typically yellow solids. One example is

(benzene)chromium tricarbonyl (Benzene)chromium tricarbonyl is an organometallic compound with the formula . This yellow crystalline solid compound is soluble in common nonpolar organic solvents. The molecule adopts a geometry known as “piano stool” because of the planar ...

Fischer carbenes and carbynes

Alkyl and aryl

organolithium reagent In organometallic chemistry, organolithium reagents are chemical compounds that contain carbon–lithium (C–Li) bonds. These reagents are important in organic synthesis, and are frequently used to transfer the organic group or the lithium atom ...

s (RLi) add to Cr(CO)₆ to give anionic

acyl In chemistry, an acyl group is a moiety derived by the removal of one or more hydroxyl groups from an oxoacid, including inorganic acids. It contains a double-bonded oxygen atom and an alkyl group (). In organic chemistry, the acyl group ( I ...

complexes. These anionic species in turn react with

alkylating agent Alkylation is the transfer of an alkyl group from one molecule to another. The alkyl group may be transferred as an alkyl carbocation, a free radical, a carbanion, or a carbene (or their equivalents). Alkylating agents are reagents for effecting ...

s such as Me₃O⁺ to form (OC)₅Cr=C(OMe)R to give Fischer carbene complexes: : FischerSyn1

Cyclopentadienyl derivatives

Treatment of chromium hexacarbonyl with

sodium cyclopentadienide 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 p ...

gives NaCr(CO)₃(C₅H₅). Oxidation of this salt affords cyclopentadienylchromium tricarbonyl dimer (Cp₂Cr₂(CO)₆). This complex is distinctive because it exists in measureable equilibrium with the monometallic Cr(I) radical CpCr(CO)₃.

Safety

In common with many of the other homoleptic metal carbonyls (e.g.

nickel carbonyl Nickel carbonyl (IUPAC name: tetracarbonylnickel) is a nickel(0) organometallic compound with the formula Ni(CO)4. This colorless liquid is the principal carbonyl of nickel. It is an intermediate in the Mond process for producing very high-pu ...

and

iron carbonyl Iron pentacarbonyl, also known as iron carbonyl, is the compound with formula . Under standard conditions Fe( CO)5 is a free-flowing, straw-colored liquid with a pungent odour. Older samples appear darker. This compound is a common precursor t ...

), chromium hexacarbonyl is toxic and thought to be

carcinogenic A carcinogen is any substance, radionuclide, or radiation that promotes carcinogenesis (the formation of cancer). This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes. Several radioactive sub ...

. Its vapor pressure is relatively high for a metal complex, at 36 °C).

References

External links

National Pollutant Inventory - Chromium (III) and compounds fact sheet
{{metal carbonyls Carbonyl complexes Chromium complexes Octahedral compounds Organochromium compounds