Copper Sulfide - Oxidation States of Copper and Sulfur

Oxidation States of Copper and Sulfur

The bonding in copper sulfides cannot be correctly described in terms of a simple oxidation state formalism because the Cu-S bonds are somewhat covalent rather than ionic in character, and have a high degree of delocalization resulting in complicated electronic band structures. Although many textbooks (e.g. ) give the mixed valence formula (Cu+)₂(Cu2+)(S2–)(S₂)2– for CuS, X-ray photoelectron spectroscopic data give strong evidence that, in terms of the simple oxidation state formalism, all the known copper sulfides should be considered as purely monovalent copper compounds, and more appropriate formulae would be (Cu+)₃(S2–)(S₂)– for CuS, and (Cu+)(S₂)− for CuS₂, respectively. Further evidence that the assignment of the so-called "valence hole" should be to the S₂ units in these two formulae is the length of the S-S bonds, which are significantly shorter in CuS (0.207 nm) and CuS₂ (0.203 nm) than in the "classical" disulfide Fe2+(S₂)2− (0.218 nm). This bond length difference has been ascribed to the higher bond order in (S-S)− compared to (S-S)2− due to electrons being removed from a π* antibonding orbital. NMR studies on CuS show that there are two distinct species of copper atom, one with a more metallic nature than the other. and this apparent discrepancy with the X-ray photo-electron spectrum data simply highlights the problem that NMR has in assigning oxidation states in a mixed-valence compound. The issue of the valence of copper in sulfides (as well as selenides an tellurides) continues to be revisited in the literature. A good example is a 2009 study of the ternary compound CuCo₂S₄ (a spinel mineral known as carrollite) that "was undertaken primarily to establish unequivocally the oxidation state of the Cu in the mineral" and concluded "that the experimental and simulated Cu L2,3 absorption spectra established an unequivocal oxidation state of CuI in the carrollite bulk".