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Chromium hexacarbonyl
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. At room temperature the solid is stable to air, although it does have a high vapor pressure and sublimes readily.

History
The Cr(CO)6 was prepared by the treatment of Cr(III) salts with high pressures of carbon monoxide in the presence of reducing agents such as Grignard reagents or zinc dust.

In 1957, Natta et al revealed that it becomes easier to reduce a chromous salt of organic acids, chromium acetylacetonate with an excess of  powdered magnesium in pyridine as a reaction medium. This change raised the product yield dramatically up to 82%.

After two decades from then, the formation of chromium carbonyls was enabled by the direct reactions of the chromium(0) metal with CO gas, which had previously been regarded viable for only the metals including Ni or Fe, not Cr.

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 is (2t2g)6(9 t1u)0(2t2u)0.

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.

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 carbynes and 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.

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).