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 element ...

with the

formula In science, a formula is a concise way of expressing information symbolically, as in a mathematical formula or a ''chemical formula''. The informal use of the term ''formula'' in science refers to the general construct of a relationship betwee ...

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 State of matter#Four fundamental states, four fundamental states of matter (the others being liquid, gas, and Plasma (physics), plasma). The molecules in a solid are closely packed together and contain the least amount o ...

is stable to air, 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 phases ...

and

sublime Sublime may refer to: Entertainment * SuBLime, a comic imprint of Viz Media for BL manga * Sublime (band), an American ska punk band ** ''Sublime'' (album), 1996 * ''Sublime'' (film), a 2007 horror film * SubLime FM, a Dutch radio station dedic ...

s readily. Cr(CO)₆ is

zerovalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with other atoms when it forms chemical compounds or molecules. Description The combining capacity, or affinity of an ...

, 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. C ...

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 electr ...

s are identical. The complex is octahedral 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 bromide Phenylmagnesium bromide, with the simplified formula , is a magnesium-containing organometallic compound. It is commercially available as a solution in diethyl ether or tetrahydrofuran (THF). Phenylmagnesium bromide is a Grignard reagent. It is of ...

s 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 ma ...

(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 satur ...

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 plana ...

Fischer carbenes and carbynes

Alkyl and aryl organolithium reagents (RLi) add to Cr(CO)₆ to give anionic acyl 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 al ...

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 pi ...

gives NaCr(CO)₃(C₅H₅). Oxidation of this salt affords

cyclopentadienylchromium tricarbonyl dimer Cyclopentadienylchromium tricarbonyl dimer is the organochromium compound with the formula Cp2Cr2(CO)6, where Cp is C5H5. A dark green crystalline solid. It is the subject of research it exists in measureable equilibrium quantities with the mono ...

(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 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 to ...

), 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 substan ...

. 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