Chromium carbonyl
From Wikipedia, the free encyclopedia
| Chromium carbonyl | |
|---|---|
 |
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| General | |
| Systematic name | chromium hexacarbonyl |
| Other names | chromium carbonyl |
| Molecular formula | C6CrO6 |
| SMILES | ? |
| Molar mass | ?? g/mol |
| Appearance | colorless crystals |
| CAS number | [13007-92-6] |
| Properties | |
| Density and phase | 1.77 g/cm3, solid |
| Solubility in water | insoluble |
| Other solvents | soluble in benzene, THF |
| Melting point | ? °C (? K) |
| Boiling point | sublimes |
| Structure | |
| Coordination geometry |
octahedral |
| Crystal structure | ??? |
| Dipole moment | 0 D |
| Hazards | |
| MSDS | External MSDS |
| Main hazards | CO source |
| NFPA 704 | |
| R/S statement | R: 22 S: 53-36-45 |
| RTECS number | GB5075000 |
| Supplementary data page | |
| Structure and properties |
Thermodynamic data |
| Spectral data | UV, IR, NMR, MS |
| Related compounds | |
| Related compounds | V(CO)6, Fe(CO)5]-, CrCl3 |
| Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references |
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Chromium carbonyl, also known as chromium hexacarbonyl, is the chemical compound Cr(CO)6. It is a white crystalline solid with a high vapour pressure. At room temperature the solid is stable to air.
Cr(CO)6 is zerovalent, meaning that Cr has no formal charge, and it is called a homoleptic metal carbonyl, which means that all the ligands are the same.
[edit] Reactions
When heated or photolyzed in tetrahydrofuran (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. The products adopt a "piano-stool" structure. These species are typically yellow solids which dissolve well in common organic solvents. One way to remove the chromium tricarbonyl groups from the arenes is to use iodine. Photolysis in air will also give a clean product — one needs only to filter the mixture to remove the chromium oxide before removing the solvent. In general, substituted derivatives of Cr(CO)6 decompose upon exposure to air.
The reaction forming the chromium arene tricarbonyls often works well when the solvent system is a mixture of THF and dibutyl ether, the latter to increase the boiling point of the mixture. Representative results have been described by Dr. David A. Widdowson at Imperial College, London.
Alkyl and aryl organolithium reagents 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.
[edit] Safety
In common with many of the other homoleptic metal carbonyls (e.g. nickel carbonyl 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). The Aberdeen chemist Jörg Feldmann has detected many metal carbonyls, such as molybdenum hexacarbonyl in landfill and sewage gas.
