Electronic Structure of Tris(α-diimine)iron(II) Complexes

The electronic structures of tris(α-diimine)iron(II) complexes have been investigated. The theoretical consideration has revealed that a considerable π-back donation takes place in the ground state and as a result unusually stable complexes of iron(II) are formed. The intense band which appears in the visible region has been proved to be due to the charge transfer transition from 3dπ AO (atomic orbital) of iron to the lowest vacant π MO (molecular orbital) of the ligand α-diimine. It was found both theoretically and experimentally that the π→π* transition with the lowest energy in α-diimine is polarized parallel to the nitrogen-nitrogen direction of the molecule, and that transition dipoles in the three ligands of tris(α-diimine)iron(II) ion interact to give resultant A1→A2 and A1→E transitions in the metal complex with D3 symmetry. Details of the electronic structures, including the splitting energy and the intensity of the local excitation in α-diimine molecule and the nature of the charge transfer band, have been discussed.