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

Abstract

In this paper we present an ab initio study of the absorption spectrum of the FeO 4 2Ϫ ion. The wavefunctions and energies of the ground and excited states of the FeO 4 2Ϫ cluster are calculated by means of the Restricted Active Space self-consistent-field method ͑RASSCF͒. 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 FeO 4 2Ϫ ͑which were observed in the solid state and in solution͒, both of them stem from the near degeneracy between Fe(3d) and O(2 p) levels, which is ultimately due to the high and unstable oxidation state of Fe͑VI͒ in the FeO 4 2Ϫ complex. The analysis of the ground and excited state wavefunctions reveals that the electronic structure of FeO 4 2Ϫ does not correspond to the ionic image of Ligand Field Theory ͓d 2 -Fe͑VI͒ϩclosed-shell O 2Ϫ ions͔, nor does it correspond to simple extensions of it which take into account ligands-to-metal 2 p→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 Fe͑VI͒. The calculated absorption spectrum allows for a thorough interpretation of the features observed in the experimental spectra measured in Fe͑VI͒-doped K 2 MO 4 ͑MϭS, Cr͒ and in 9 M KOH solution ͑absorption maxima, intensities, electronic origins, band shapes͒, which implies completely new assignments. This is particularly so for the broad intense bands observed between 10000-25000 cm Ϫ1 , 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 Theory͒, 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.

The Ab Initio Model Potential Method: A Common Strategy for Effective Core Potential and Embedded Cluster Calculations

“…162 In fact, as the oxidation state grows from IV to VI in the series, the ground state wavefunctions reveal a sharply decreasing contribution of the d 1 configuration and an increasing importance of high order ligands-to-metal excitations. 162 This trend corresponds to a notable change in nondynamic electron correlation, which makes it necessary to adapt the level of methodology used from the CASSCF(3d) level to the RASSCF level described above.…”

“…162 In fact, as the oxidation state grows from IV to VI in the series, the ground state wavefunctions reveal a sharply decreasing contribution of the d 1 configuration and an increasing importance of high order ligands-to-metal excitations. 162 This trend corresponds to a notable change in nondynamic electron correlation, which makes it necessary to adapt the level of methodology used from the CASSCF(3d) level to the RASSCF level described above. A numerical experiment 147 using nuclear charge 24 on Fe, so as to set its formal oxidation state to IV, showed that the weight of the d 2 configuration in the RASSCF wavefunction of the Fe(IV) cluster appears to be about 85 %, very different from its 0.5 % contribution in the Fe(VI) cluster wavefunction.…”

Alternative configuration interaction expansions for transition metal ions with intermediate oxidation states in crystals: The structure and absorption spectrum of Cs2GeF6:Mn4+

“…Along this line, a series of investigations conducted in our laboratory has pointed out that considerably different multiconfigurational expansions of the defect cluster wave functions are needed depending, basically, on the formal oxidation state of the impurity. 3,4 This is related to the fact that the interactions between d n and ligand-to-metal charge transfer configurations increase dramatically with formal oxidation state: They are small in V͑II͒, 5,6 considerable in Mn͑IV͒, 4 and very large in Fe͑VI͒ 3 impurities, for instance. Accordingly, the truncations of the configurational space needed to properly account for electron correlation cannot be the same.…”

“…The limit of high oxidation states seems to be essentially controlled by quasidegeneracy effects rather than by dynamic correlation. 3 The details of the calculations and of the truncation procedures as well as their results are presented in Sec. II.…”

“…In this paper we show that the ligand-to-metal charge transfer configurations become so important in this intermediate oxidation state impurity ͑and, possibly, non-negligible ligand-ligand weak bonding interactions͒ that none of the 18 molecular orbitals of F 2p character should be left inactive in the correlation treatment. This requirement can be satisfied in MnF 6 2Ϫ because of the higher oxidation state of manganese, which enhances the ligand field splittings in the Mn 3d 3 configuration manifold so much that one dominant Mn 3d 3 configuration rather than the full Mn 3d 3 active space can be used as a single reference for single and double excitations from all occupied ligand 2p orbitals. The results of this work, together with those of previous studies, outline two different alternative truncation schemes of the valence electron correlation which produce the same, necessary, high accuracy in structural and spectroscopic properties of transition metal ions doped in ionic crystals.…”