Copper monosulfide explained

Copper monosulfide is a chemical compound of copper and sulfur. It was initially thought to occur in nature as the dark indigo blue mineral covellite. However, it was later shown to be rather a cuprous compound, formula Cu⁺₃S(S₂).^[1] CuS is a moderate conductor of electricity.^[2] A black colloidal precipitate of CuS is formed when hydrogen sulfide, H₂S, is bubbled through solutions of Cu(II) salts. It is one of a number of binary compounds of copper and sulfur (see copper sulfide for an overview of this subject), and has attracted interest because of its potential uses in catalysis^[3] and photovoltaics.^[4]

Manufacturing

Copper monosulfide can be prepared by passing hydrogen sulfide gas into a solution of copper(II) salt.

Alternatively, it 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.

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^[5] 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.^[6]
The crystal structure of covellite has been reported several times,^{[7] [8] [9]} 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^[10] 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^[9] 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 Cu^IIS (i.e. containing no sulfur-sulfur bond) is clearly incompatible with the crystal structure, and also at variance with the observed diamagnetism^[11] as a Cu(II) compound would have a d⁹ configuration and be expected to be paramagnetic.
Studies using XPS^{[12] [13] [14] [15]} 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.) describing CuS as containing both Cu^I and Cu^II i.e. (Cu⁺)₂Cu²⁺(S₂)²⁻S²⁻. An alternative formulation as (Cu⁺)₃(S²⁻)(S₂)⁻ was proposed and supported by calculations.^[16] The formulation should not be interpreted as containing radical anion, but rather that there is a delocalized valence "hole".^{[16] [17]} Electron paramagnetic resonance studies on the precipitation of Cu(II) salts indicates that the reduction of Cu(II) to Cu(I) occurs in solution.^[18]

See also

• Copper sulfide for an overview of all copper sulfide phases
• Copper(I) sulfide, Cu₂S
• Covellite

Notes and References

1. Liang, W., Whangbo, M.H. (1993) Conductivity anisotropy and structural phase transition in Covellite CuS Solid State Communications, 85(5), 405-408
2. Wells A.F. (1962) Structural Inorganic Chemistry 3d edition Oxford University Press
3. Kuchmii. S.Y.. Korzhak A.V.. Raevskaya A.E.. Kryukov A.I.. 2001. Catalysis of the Sodium Sulfide Reduction of Methylviologene by CuS Nanoparticles. Theoretical and Experimental Chemistry. 37 . 1. 36–41. Springer . New York. 10.1023/A:1010465823376. 91893521.
4. Mane. R.S.. Lokhande C.D.. June 2000. Chemical deposition method for metal chalcogenide thin films. Materials Chemistry and Physics . 65. 1. 1–31. 10.1016/S0254-0584(00)00217-0 .
5. Peiris. M. Sweeney, J.S. . Campbell, A.J. . Heinz D. L. . 1996. Pressure-induced amorphization of covellite, CuS. J. Chem. Phys.. 104. 1. 11–16. 10.1063/1.470870 . 1996JChPh.104...11P .
6. Grijalva. H. . Inoue, M. . Boggavarapu, S. . Calvert, P. . 1996. Amorphous and crystalline copper sulfides, CuS. J. Mater. Chem. . 6. 7. 1157–1160. 10.1039/JM9960601157.
7. Oftedal. I.. Die Kristallstruktur des Covellins (CuS). 1932. Z. Kristallogr.. 83. 1–6. 9–25. 10.1524/zkri.1932.83.1.9. 101164006.
8. Berry . L. G.. 1954. The crystal structure of covellite CuS and klockmannite CuSe. American Mineralogist. 39 . 504.
9. Evans. H.T. Jr.. Konnert J.. 1976. Crystal structure refinement of covellite. American Mineralogist. 61. 996–1000.
10. Wells A.F. (1984) Structural Inorganic Chemistry 5th edition Oxford Science Publications
11. http://www-d0.fnal.gov/hardware/cal/lvps_info/engineering/elementmagn.pdf Magnetic susceptibility of the elements and inorganic compounds
12. Nakai. I. . Sugitani, Y. . Nagashima, K. . Niwa, Y. . 1978. X-ray photoelectron spectroscopic study of copper minerals. Journal of Inorganic and Nuclear Chemistry. 40. 5. 789–791. 10.1016/0022-1902(78)80152-3.
13. Folmer. J.C.W.. Jellinek F.. 1980. The valence of copper in sulfides and selenides: An X-ray photoelectron spectroscopy study. Journal of the Less Common Metals. 76. 1–2. 789–791. 10.1016/0022-5088(80)90019-3.
14. Folmer. J.C.W.. Jellinek F.. Calis G.H.M. 1988. The electronic structure of pyrites, particularly CuS₂ and Fe_1−xCu_xSe₂: An XPS and Mössbauer study. Journal of Solid State Chemistry. 72. 1. 137–144. 10.1016/0022-4596(88)90017-5. 1988JSSCh..72..137F .
15. Goh. S.W.. Buckley A.N.. Lamb R.N.. February 2006. Copper(II) sulfide?. Minerals Engineering. 19. 2. 204–208. 10.1016/j.mineng.2005.09.003.
16. Liang. W.. Whangbo M, -H. February 1993. Conductivity anisotropy and structural phase transition in Covellite CuS. Solid State Communications. 85. 5. 405–408. 10.1016/0038-1098(93)90689-K. 1993SSCom..85..405L .
17. Nozaki. H . Shibata, K . Ohhashi,N.. April 1991. Metallic hole conduction in CuS . Journal of Solid State Chemistry . 91. 2. 306–311. 10.1016/0022-4596(91)90085-V. 1991JSSCh..91..306N .
18. Luther . GW. Theberge SM. Rozan TF. Rickard D. Rowlands CC. Oldroyd A.. February 2002. Aqueous copper sulfide clusters as intermediates during copper sulfide formation.. Environ. Sci. Technol.. 36. 3. 394–402. 10.1021/es010906k. 11871554. 2002EnST...36..394L .