The electronic structures of manganese oxide minerals

Molecular orbital calculations, using the Xa scattered wave method, were done for the clusters MnOl,0-, MnO?-, and MnOB- corresponding to Mn2*, Mn3*, and Mna* in octahedral coordination with O2-. Bond lengths representative of those observed in manganese oxides were chosen and 05 symmetry was used for each cluster. The calculated orbital energies and charge distributions are used to describe the nature of chemical bonding in manganese oxides. Spectroscopic transition energies are calculated and these are compared with experimental optical, X-ray emission and photoelectron (Esca) spectra of manganese oxides. The agreement between the calculated and experimental spectroscopic transition energies is fairly good and indicates that isolated clusters can be used to model the localized aspects of the electronic structure of manganese oxide minerals. The electronic structure ofthese clusters can also be related to the crystal chemistry of the manganese oxides. In spite of radius ratio considerations, Mna* is found to be considerably more stable in octahedral rather than tetrahedral coordination. This is demonstrated by comparing the electronic structure of MnOS- with that of a tetrahedral MnOX- cluster. A large degree of covalency in the MnOS- cluster is consistent with the strong Lewis acidity of Mna*. In contrast, the bonding in MnOl,O- is mostly ionic. The calculated exchange splittings of the crystal field orbitals show that low-spin Mn2* should not exist even under high-pressure, but that low spin Mn3* may substitute for Mna* in manganese (IV) oxides.