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 e xcited states of the FeO42- cluster an calculated by means of the Rest ricted Active Space self-consistent-held method (RASSCF). The molecula r orbitals of the cluster with main character Fe(3d) define a complete active space; all single, double, triple, and quadruple excitations f rom the molecular orbitals of main character O(2p) to those of main ch aracter Fe(3d) are allowed. The multiconfigurational expansions result ing from these ligands-to-metal excitations include between 50000 to 1 00000 configuration state functions. The results of the calculations l ead to a new interpretation of the bonding and of the absorption spect ra 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) l evels, which is ultimately due to the high and unstable oxidation stat e of Fe(VI) in the FeO42- complex. The analysis of the ground and exci ted 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 extens ions of it which take into account ligands-to-metal 2p-->3d single exc itations, nor to any other simple image; on the contrary, it correspon ds to the superposition of a large number of configurations with a ver y large weight of high-order ligands-to-metal excitations, which indic ates a remarkable intra-cluster inwards delocalization of electron den sity away from the closed-shell ligands, impelled by the unstable high formal charge of Fe(VI). The calculated absorption spectrum allows fo r a thorough interpretation of the features observed in the experiment al spectra measured in Fe(VI)-doped K2MO4 (M=S, Cr) and in 9 M KOH sol ution (absorption maxima, intensities, electronic origins, band shapes ), which implies completely new assignments. This is particularly so f or the broad intense bands observed between 10000-25000 cm(-1), which, according to our calculations, are found to be associated to electron ic transitions from the (3)A(2) ground state to increasingly dense set s of excited states that include not only spin singlet and triplet sta tes (as expected for a d(2) configuration from Ligand Field Theory), b ut also spin quintet electronic states, all of which can be understood as direct effects of the above-mentioned oxygens(2p)-iron(3d) near de generacy. (C) 1998 American Institute of Physics. [S0021-9606(98)30339 -6].