A new interpretation of the bonding and spectroscopy of the tetraoxoferrate(VI) FeO42- ion

In this paper we present an ab initio study of the absorption spectrum of the FeO42- ion. The wavefunctions and energies of the ground and excited states of the FeO42- cluster an calculated by means of the Restricted Active Space self-consistent-held method (RASSCF). The molecular orbitals of the cluster with main character Fe(3d) define a complete active space; all single, double, triple, and quadruple excitations from the molecular orbitals of main character O(2p) to those of main character Fe(3d) are allowed. The multiconfigurational expansions resulting from these ligands-to-metal excitations include between 50000 to 100000 configuration state functions. The results of the calculations lead to a new interpretation of the bonding and of the absorption spectra of FeO42- (which were observed in the solid state and in solution), both of them stem from the near degeneracy between Fe(3d) and O(2p) levels, which is ultimately due to the high and unstable oxidation state of Fe(VI) in the FeO42- complex. The analysis of the ground and excited state wavefunctions reveals that the electronic structure of FeO42- does not correspond to the ionic image of Ligand Field Theory [d(2)-Fe(VI)+closed-shell O2- ions], nor does it correspond to simple extensions of it which take into account ligands-to-metal 2p-->3d single excitations, nor to any other simple image; on the contrary, it corresponds to the superposition of a large number of configurations with a very large weight of high-order ligands-to-metal excitations, which indicates a remarkable intra-cluster inwards delocalization of electron density away from the closed-shell ligands, impelled by the unstable high formal charge of Fe(VI). The calculated absorption spectrum allows for a thorough interpretation of the features observed in the experimental spectra measured in Fe(VI)-doped K2MO4 (M=S, Cr) and in 9 M KOH solution (absorption maxima, intensities, electronic origins, band shapes), which implies completely new assignments. This is particularly so for the broad intense bands observed between 10000-25000 cm(-1), which, according to our calculations, are found to be associated to electronic transitions from the (3)A(2) ground state to increasingly dense sets of excited states that include not only spin singlet and triplet states (as expected for a d(2) configuration from Ligand Field Theory), but also spin quintet electronic states, all of which can be understood as direct effects of the above-mentioned oxygens(2p)-iron(3d) near degeneracy. (C) 1998 American Institute of Physics. [S0021-9606(98)30339-6].