Chromium hexacarbonyl

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 an oxidation state of zero, and it is a homoleptic complex, which means that all the ligands are identical. The complex is octahedral with Cr–C and C–O distances of 1.91 and 1.14 Å, respectively.[2]

Chromium hexacarbonyl
Names
IUPAC name
Hexacarbonylchromium
Other names
Chromium carbonyl
Identifiers
3D model (JSmol)
ChEBI
ChemSpider
ECHA InfoCard 100.032.579
RTECS number
  • GB5075000
UNII
Properties
Cr(CO)6
Molar mass 220.057 g/mol
Appearance colorless crystals
Density 1.77 g/cm3, solid
Melting point 90 °C (194 °F; 363 K)
Boiling point 210 °C (410 °F; 483 K) (decomposes)
insoluble
Solubility soluble in organic solvents
Structure
orthrhombic
octahedral
0 D
Hazards
Main hazards Toxic
Safety data sheet Oxford MSDS
NFPA 704 (fire diamond)
Flammability code 1: Must be pre-heated before ignition can occur. Flash point over 93 °C (200 °F). E.g. canola oilHealth code 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g. chloroformReactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
1
2
0
Flash point 210 °C (410 °F; 483 K)
Lethal dose or concentration (LD, LC):
150 mg/kg (oral, mouse)
230 mg/kg (oral, rat)
NIOSH (US health exposure limits):
PEL (Permissible)
 TWA 1 mg/m3[1]
REL (Recommended)
 TWA 0.5 mg/m3[1]
IDLH (Immediate danger)
 250 mg/m3[1]
Related compounds
Other cations
 Molybdenum hexacarbonyl
Tungsten hexacarbonyl
Related compounds
 Vanadium hexacarbonyl
Dimanganese decacarbonyl
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Reactions

When heated or UV-irradiated 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

UV-irradiation of frozen solutions of chromium hexacarbonyl affords a variety of labile adducts.[3]

Such reactions proceed well with electron-rich arenes such as anisole. The products adopt a "piano-stool" structure. These species are typically yellow solids.

Alkyl and aryl organolithium reagents (RLi) add to a carbonyl ligand to give anionic acyl complexes.[4] These species in turn react with alkylating agents such as Me3O+ to form (OC)5Cr=C(OMe)R,[5]

Treatment of chromium hexacarbonyl with sodium cyclopentadienide 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.[6]

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 mmHg (130 Pa) at 36 °C).[7]

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References
  1. NIOSH Pocket Guide to Chemical Hazards. "#0141". National Institute for Occupational Safety and Health (NIOSH).
  2. Whitaker, A.; Jeffery, J. W. (1967). "The Crystal Structure of Chromium Hexacarbonyl". Acta Crystallogr. 23: 977–984. doi:10.1107/S0365110X67004153.
  3. Perutz, Robin N.; Turner, James J. (1975). "Photochemistry of the Group 6 Hexacarbonyls in Low-Temperature Matrixes. III. Interaction of the Pentacarbonyls with Noble Gases and Other Matrixes". Journal of the American Chemical Society. 97: 4791–800. doi:10.1021/ja00850a001.CS1 maint: uses authors parameter (link)
  4. Elschenbroich, C. (2006). Organometallics. Weinheim: Wiley-VCH. ISBN 978-3-527-29390-2.
  5. Herndon, James W. (2001). "Pentacarbonyl(methoxyphenylcarbene)chromium(0)". e-EROS Encyclopedia of Reagents for Organic Synthesis.
  6. Manning, A. R.; Hacket, Paul; Birdwhistell, Ralph (1990). "Hexacarbonylbis(η5‐Cyclopentadienyl)Dichromium, Molybdenum, and Tungsten and their Analogs, M2(η5‐C5H4R)2(CO)6 (M = Cr, Mo, and W; R = H, Me or PhCH2)". Inorganic Syntheses. 28: 148–149. doi:10.1002/9780470132593.ch39.
  7. Patnaik, Pradyot (2003). "Chromium hexacarbonyl". Handbook of Inorganic Chemicals. McGraw-Hill Professional. pp. 222–223. ISBN 978-0-07-049439-8.
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