The electronic structure of a chromium tricarbonyl complex Cr CO 3 and an iron tricarbonyl complex Fe CO 3 can be determined by ab initio calculations, which are computational methods based on quantum mechanics to solve the electronic structure of molecules.Chromium tricarbonyl complex Cr CO 3 :Chromium has the electron configuration [Ar] 3d5 4s1. In the Cr CO 3 complex, chromium is in the +3 oxidation state, which results in the electron configuration [Ar] 3d3. The 3d orbitals of chromium interact with the * orbitals of the carbonyl ligands CO , leading to the formation of metal-to-ligand -backbonding. This backbonding stabilizes the complex and results in a low-spin d3 configuration for the chromium center.Iron tricarbonyl complex Fe CO 3 :Iron has the electron configuration [Ar] 3d6 4s2. In the Fe CO 3 complex, iron is in the +3 oxidation state, which results in the electron configuration [Ar] 3d5. Similar to the chromium complex, the 3d orbitals of iron interact with the * orbitals of the carbonyl ligands CO , leading to the formation of metal-to-ligand -backbonding. This backbonding stabilizes the complex and results in a high-spin d5 configuration for the iron center.Comparison:Both chromium and iron tricarbonyl complexes exhibit metal-to-ligand -backbonding, which stabilizes the complexes. However, the main difference between the two complexes lies in their electronic configurations. The chromium tricarbonyl complex has a low-spin d3 configuration, while the iron tricarbonyl complex has a high-spin d5 configuration. This difference in electronic structure can lead to variations in their chemical properties, such as reactivity and magnetic behavior.