Copper sulfide

There are a number of copper sulfides like Cu₂S, Cu_2-xS CuS and even CuS₂, although the latter is a high pressure phase. From the stoichiometry it would seem that Cu₂S is Cu⁺ (d¹⁰) monovalent cuprous sulfide whereas CuS is cupric Cu²⁺ (d⁹). However, this is not the case. Because the Cu²⁺ ion is a relatively strong oxidizer[?], it is able to oxidize sulfide to disulfide ions:

    2Cu2+ + 2S2-  --> 2Cu+  + 2S- --> 2Cu+ + (S2)2-

In this process the one missing d-electron ("hole") of the d⁹ configuration is transferred to the sulfur. Sometimes that leads to sulfur pair formation sometimes not.

The result is that Cu is essentially monovalent in all these compounds. What lies near the Fermi level is a valence band predominantly S 2p character

Most of them are metallic because if the pairformation is incomplete the remaining holes in the valence band lead to metallic conductivity. E.g. in CuS only two thirds of the sulfurs occur as pairs (disulfide ions).

CuS₂ is a special case. It has the pyrite structure. In this structure all sulfurs occur as pairs (disulfide ions). However because Cu is essentially Cu+, the disulfide ions are essentially (S-S)^-1 rather than (S-S)^-2. In other words there is an extra hole on the disulfide ion. As it is delocalized, this pyrite is metallic. By contrast the mineral pyrite FeS₂ consists of Fe²⁺ ions and (S-S)^-2 ions and is a semiconductor.