Copper monosulfide

Copper monosulfide is a chemical compound of copper and sulfur with the formula. It occurs in nature as the dark indigo blue mineral covellite. It is one of a number of binary compounds of copper and sulfur, and has attracted interest because of its potential uses in catalysis and photovoltaics. It is a moderate conductor of electricity.

Manufacturing

A black colloidal precipitate of copper monosulfide is formed when hydrogen sulfide,, is bubbled through solutions of Cu salts. This is the basis of its industrial production.
Special forms of CuS for certain applications can be prepared by melting an excess of sulfur with copper(I) sulfide or by precipitation with hydrogen sulfide from a solution of anhydrous copper(II) chloride in anhydrous ethanol. Nanoparticles of CuS can be prepare by reaction of dissolved copper(II) chloride with thioglycolic acid.
The reaction of copper with molten sulfur followed by boiling sodium hydroxide and the reaction of sodium sulfide with aqueous copper sulfate will also produce copper sulfide.

CuS structure and bonding

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 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 ethylenediamine complex with thiourea has been reported, which transforms to the crystalline covellite form at.

The crystal structure of covellite has been reported several times, 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 and is quite different from Copper(II) oxide, but similar to Copper(II) selenide. The covellite unit cell contains 6 formula units in which:

4 Cu atoms have tetrahedral coordination.
2 Cu atoms have trigonal planar coordination.
2 pairs of S atoms are only apart indicating the existence of an S-S bond.
the 2 remaining S atoms form trigonal planar triangles around the copper atoms, and are surrounded by five Cu atoms in a pentagonal bipyramid.
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.

The formulation of copper sulfide as is clearly incompatible with the crystal structure, and also at variance with the observed diamagnetism as a Cu compound would have a d⁹ configuration and be expected to be paramagnetic.
Studies using XPS 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 describing CuS as containing both and i.e.. An alternative formulation as was proposed and supported by calculations. The formulation should not be interpreted as containing radical anion, but rather that there is a delocalized valence "hole".
Electron paramagnetic resonance studies on the precipitation of Cu salts indicates that the reduction of Cu to Cu occurs in solution.

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ball-and-stick model of part of the crystal structure of covellite	trigonal planar coordination of copper	tetrahedral coordination of copper	trigonal bipyramidal coordination of sulfur	tetrahedral coordination of sulfur-note disulfide unit