CuS structure and bonding[edit]
Copper sulfide crystallizes in the hexagonal crystal system, and this is the form of the mineral covellite. There is also an amorphous high pressure form [8] which on the basis of the Raman spectrum has been described as having a distorted covellite structure. An amorphous room temperature semiconducting form produced by the reaction of a Cu(II) ethylenediamine complex with thiourea has been reported, which transforms to the crystalline covellite form at 30 °C.[9]
The crystal structure of covellite has been reported several times,[10][11][12] and whilst these studies are in general agreement on assigning the space group P63/mmc there are small discrepancies in bond lengths and angles between them. The structure was described as "extraordinary" by Wells[13] and is quite different from copper(II) oxide, but similar to CuSe (klockmannite). The covellite unit cell contains 6 formula units (12 atoms)in which:
4 Cu atoms have tetrahedral coordination (see illustration).
2 Cu atoms have trigonal planar coordination (see illustration).
2 pairs of S atoms are only 207.1 pm apart [12] indicating the existence of an S-S bond (a disulfide unit).
the 2 remaining S atoms form trigonal planar triangles around the copper atoms, and are surrounded by five Cu atoms in a pentagonal bipyramid (see illustration).
The S atoms at each end of a disulfide unit are tetrahedrally coordinated to 3 tetrahedrally coordinated Cu atoms and the other S atom in the disulfide unit (see illustration).
The formulation of copper sulfide as CuIIS (i.e. containing no sulfur-sulfur bond) is clearly incompatible with the crystal structure, and also at variance with the observed diamagnetism [14] as a Cu(II) compound would have a d9 configuration and be expected to be paramagnetic.[5]
Studies using XPS[15][16][17][18] indicate that all of the copper atoms have an oxidation state of +1. This contradicts a formulation based on the crystal structure and obeying the octet rule that is found in many textbooks (e.g.[5][19]) describing CuS as containing both CuI and CuII i.e. (Cu+)2Cu2+(S2)2−S2−. An alternative formulation as (Cu+)3(S2−)(S2)− was proposed and supported by calculations.[20] The formulation should not be interpreted as containing radical anion, but rather that there is a delocalized valence "hole".[20][21] Electron paramagnetic resonance studies on the precipitation of Cu(II) salts indicates that the reduction of Cu(II) to Cu(I) occurs in solution.[