The bonding of ferrous iron to sulfur and oxygen in tetrahedral coordination: A comparative study using SCF Xα scattered wave molecular orbital calculations

Molecular quantum mechanical calculations have been performed on high-spin ferrous iron tetrahedrally coordinated to sulfur and oxygen, respectively. The molecular orbital energies obtained from the calculations are compared with experimental optical and X-ray emission spectra. Good agreement was found between calculated and experimental spectral transition energies for the optical absorption spectra of Fe²⁺ in sphalerite, of Fe²⁺ in FeAl₂O₄, staurolite and (Zn, Fe)O, and for the FeKβ X-ray emission spectra of FeCr₂O₄. This both clarified interpretation of the spectra and established the validity of the calculations. Distinct differences occur in the molecular orbital structures of the sulfide and oxide clusters. In the sulfide, the crystal field type (mainly Fe 3d) molecular orbitals lie within the nonbonding (mainly S 3p) orbitais in energy, whereas in the oxide, they lie well above the 02p nonbonding orbitals. This also results in a wider valence band in the oxide than in the sulfide. The crystal field type (Fe 3d) molecular orbitais have more ligand character in the sulfide than the oxide and the chalcophilic properties of iron are partly attributed to this observation.