A. C. Stückl, C. A. Daul and H. U. Güdel
Excited-state energies and distortions of d⁰ transition metal tetraoxo complexes: A density functional study
J. Chem. Phys. 107, 4606-4617 (1997)                 

Abstract: Excitation energies and excited-state distortions of 9 tetrahedral transition metal tetraoxo complexes with a formal d⁰ electron configuration have been investigated using density functional theory. A symmetry based calculation scheme was applied for the ³T₂, ³T₁, ¹T₂, and ¹T₁ states deriving from the first excited electron configurations. The multiplet method was combined with a transition state approach for the calculation of the excitation energies. The results are compared with those from experiments, and with other calculations. The experimental ground-state properties are very well reproduced. The calculated absorption energies are slightly overestimated, but with an overall very good agreement. Potential-energy curves are calculated for both the ground and first excited states. The experimentally determined expansion of the excited state as well as the reduction in the vibrational frequencies are reproduced by the calculation. The bonding in this series of complexes is characterized by their strong capability of electron redistribution. As a consequence, the formal charge at the metal center of 7+ in MnO₄^– is reduced to about 2+. The negative charge effectively transferred from the oxygens to the metal in the formal LMCT ligand-to-metal charge transfer excitation process is 0.06 for MnO₄^–. Thus there is not much physical meaning in the label LMCT.