Chromium hexacarbonyl (IUPAC name: hexacarbonylchromium) is a chromium(0) organometallic compound with the formula Cr(CO)6. It is homoleptic complex, which means that all the ligands are identical. It is a white, air-stable solid with a high vapor pressure.

Preparation
Like many metal carbonyls, Cr(CO)6 is generally prepared by "reductive carbonylation", which involves reduction of a metal halide with under an atmosphere of carbon monoxide. As described in a 2023 survey of methods "most cost-effective routes for the synthesis of group 6 hexacarbonyls are based on the reduction of the metal chlorides (CrCl3, MoCl5 or WCl6) with magnesium, zinc or aluminium powders... under CO pressures".
CrCl3 ->[\text{CO}][\text{Reductant}] Cr(CO)6
Early work on methods included controbutions from luminaries such as Walter Hieber, his student Ernst Otto Fischer, and Giulio Natta. Using specially produced chromium metal]] will react with CO gas to give Cr(CO)6 directly, although the method is not used commercially.

Electronic structure and bonding 
In chromium hexacarbonyl, the oxidation state for chromium is assigned as zero, because Cr-C bonding electrons come from the C atom and are still assigned to C in the hypothetical ionic bond which determines the oxidation states. The formula conforms to the 18-electron rule and the complex adopts octahedral geometry with six carbonyl ligands.

The bonding between d6 chromium metal and neutral carbonyl ligands is described by the Dewar-Chatt-Duncanson model.It involves donation of electrons in HOMO of CO to empty d orbitals of the Cr metals while back-bonding from other d orbitals to the pi* orbital of the ligands reinforces the interactions synergistically.

The crystallographic studies on this compound have discovered the Cr–C and C–O distances of 1.916 and 1.171 Å, respectively. On one hand, there has been continuous efforts to calculate the electronic structures (including HOMO and LUMO) as well as its molecular geometry on the chromium hexacarbonyl compound with various approaches. According to one of the most recent studies, the ground state configuration of Cr(CO)6 turns out (2t2g)6(9 t1u)0(2t2u)0.

Reactions and applications

Photochemical reactions

Pentacarbonyl derivatives 
When heated or UV-irradiated in tetrahydrofuran (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.

Photodimerization of norbornadiene 
Norbornadiene was dimerized photochemically in the presence of Cr(CO)6, similarly to other metal complexes like Fe(CO)6, Ni(CO)4, and Co(CO)8NO.

Arene derivatives
Heating a solution of Cr(CO)6 in an aromatic 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.

Fischer carbenes 
Alkyl and aryl organolithium reagents (RLi) add to Cr(CO)6 to give anionic acyl complexes.  These anionic species in turn react with alkylating agents such as Me3O+ to form (OC)5Cr=C(OMe)R to give Fischer carbene complexes:

Cyclopentadienyl derivatives
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 measurable equilibrium with the monometallic Cr(I) radical CpCr(CO)3.

Ligand-transfer reactions 
A unique double ligand-transfer reaction was reported with using chromium trichloride and chromium hexacarbonyl. In reactions, potassium perrhenate(VII) is reduced and carbonylated by the chromium reagents and undergoes Cp ligand-transfer to afford CpRh(CO)3 complex derivatives.

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,  at 36 °C).

Historic literature

References

External links
 National Pollutant Inventory - Chromium (III) and compounds fact sheet

Carbonyl complexes
Chromium complexes
Organochromium compounds
Octahedral compounds