Electronic Structures of the Tris(acetylacetonato) Complexes of the Iron-Series Transition-Metal Ions. I. General Theory and Its Application to Simple Complexes

A theoretical treatment of the π‐electron structure of metal complexes containing organic unsaturated molecules as ligands is developed and applied to the tris(acetylacetonato) complexes of Al(III), Sc(III), and Co(III) ions. The π‐electron structure of each complex is studied by considering the configuration interaction among various kinds of electron configurations obtained by putting $\pi$ electrons in the $\text{3dπ}$ atomic orbitals of the central metal ion and in the π‐molecular orbitals of three ligands which are obtained by the Pariser–Parr–Pople method in such a way as SCF MO's can be obtained under the electrostatic field of the central metal cation. The ground, locally excited, and metal–ligand charge‐transfer configurations are taken for all the complexes. The theoretically obtained electronic transition energies and intensities are discussed in detail and are compared with the observed values. The present theoretical method including explicitly the electron interaction gives a satisfactory explanation of the electronic absorption bands observed with the complexes. In particular, the following results seem to be noticeable: (1) the magnitude of splitting of the locally excited band into the $A_2$ and $E$ components of the $D_3$ symmetry can be predicted quantitatively; (2) the moderately strong absorption bands appearing in the visible region for cobalt complex can be assigned to the metal–ligand charge‐transfer transition.