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. It is a white, air-stable solid with a high vapor pressure. 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. 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.