Theoretical studies of the electronic structure of copper in tetrahedral and triangular coordination with sulfur

Abstract

Molecular orbital calculations are presented for the copper-sulfur polyhedral clusters CuS 4 7− , CuS 4 6− , CuS 3 5− and CuS 3 4− , which occur in many minerals. Calculated and experimental optical and X-ray energies are found to be in good agreement. The crystal field orbitals of Cu+ in tetrahedrally coordinated sulfides are found to be less tightly bound than the S3p nonbonding orbitals by about 2–3 eV whereas the e and t 2 crystal field orbitals are split by about 1 eV. The crystal field splitting of Cu2+ in tetrahedral coordination is about 0.7–0.8 eV while the separation of the S3p nonbonding orbitals and the partially filled t 2 crystal field orbital is about 2 eV. In triangular coordination both the Cu+ and Cu2+ crystal field orbitals are more stable than in tetrahedral coordination, more widely split and more strongly mixed with the S3p orbitals. CuS is shown to be unstable as the mixed oxidation state compound Cu2+III (Cu+IV)2S2−(S 2 2− ); rather each Cu is predicted to have a fractional oxidation state and partially-empty crystal field orbitals.