Chromium hexacarbonyl


Chromium carbonyl, also known as chromium hexacarbonyl, is the chemical compound with the formula Cr6. At room temperature the solid is stable to air, although it does have a high vapor pressure and sublimes readily. Cr6 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.

Reactions

When heated or UV-irradiated in tetrahydrofuran solution, Cr6 converts to Cr5 with loss of one CO ligand. Similarly, heating a solution of Cr6 in an aromatic solvent results in replacement of three CO ligands:
UV-irradiation of frozen solutions of chromium hexacarbonyl affords a variety of labile adducts.
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 add to a carbonyl ligand to give anionic acyl complexes. These species in turn react with alkylating agents such as Me3O+ to form 5Cr=CR,
Treatment of chromium hexacarbonyl with sodium cyclopentadienide gives NaCr3. Oxidation of this salt affords cyclopentadienylchromium tricarbonyl dimer. This complex is distinctive because it exists in measureable equilibrium with the monometallic Cr radical CpCr3.

Safety

In common with many of the other homoleptic metal carbonyls.