Copper(I) iodide

Copper(I) iodide is the inorganic compound with the formula CuI. It is also known as cuprous iodide. It is useful in a variety of applications ranging from organic synthesis to cloud seeding.

Copper(I) iodide
IUPAC name
Copper(I) iodide
Other names
Cuprous iodide
3D model (JSmol)
ECHA InfoCard 100.028.795
Molar mass 190.45 g/mol
Appearance White powder
when impure: tan or brownish
Odor odorless
Density 5.67 g/cm3 [1]
Melting point 606 °C (1,123 °F; 879 K)
Boiling point 1,290 °C (2,350 °F; 1,560 K) (decomposes)
0.000042 g/100 mL
1 x 10−12 [2]
Solubility soluble in ammonia and iodide solutions
insoluble in dilute acids
Vapor pressure 10 mm Hg (656 °C)
-63.0·10−6 cm3/mol
Tetrahedral anions and cations
Safety data sheet Sigma Aldrich[3]
GHS pictograms
GHS Signal word Danger
H302, H315, H319, H335, H400, H410
P261, P273, P305+351+338, P501
NFPA 704 (fire diamond)
Flash point Non-flammable
NIOSH (US health exposure limits):
PEL (Permissible)
TWA 1 mg/m3 (as Cu)[4]
REL (Recommended)
TWA 1 mg/m3 (as Cu)[4]
IDLH (Immediate danger)
TWA 100 mg/m3 (as Cu)[4]
Related compounds
Other anions
Other cations
silver iodide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Pure copper(I) iodide is white, but samples are often tan or even, when found in nature as rare mineral marshite, reddish brown, but such color is due to the presence of impurities. It is common for samples of iodide-containing compounds to become discolored due to the facile aerobic oxidation of the iodide anion to molecular iodine.[5]


Copper(I) iodide, like most binary (containing only two elements) metal halides, is an inorganic polymer. It has a rich phase diagram, meaning that it exists in several crystalline forms. It adopts a zinc blende structure below 390 °C (γ-CuI), a wurtzite structure between 390 and 440 °C (β-CuI), and a rock salt structure above 440 °C (α-CuI). The ions are tetrahedrally coordinated when in the zinc blende or the wurtzite structure, with a Cu-I distance of 2.338 Å. Copper(I) bromide and copper(I) chloride also transform from the zinc blende structure to the wurtzite structure at 405 and 435 °C, respectively. Therefore, the longer the copper – halide bond length, the lower the temperature needs to be to change the structure from the zinc blende structure to the wurtzite structure. The interatomic distances in copper(I) bromide and copper(I) chloride are 2.173 and 2.051 Å, respectively.[6]



Copper(I) iodide can be prepared by heating iodine and copper in concentrated hydriodic acid, HI. In the laboratory however, copper(I) iodide is prepared by simply mixing an aqueous solution of sodium or potassium iodide and a soluble copper(II) salt such copper sulfate.

Cu2+ + 2I → CuI2

The CuI2 immediately decomposes to iodine and insoluble copper(I) iodide, releasing I2.[7]

2 CuI2 → 2 CuI + I2

This reaction has been employed as a means of assaying copper(II) samples, since the evolved I2 can be analyzed by redox titration. The reaction in itself may look rather odd, as using the rule of thumb for a proceeding redox reaction, Eooxidator  Eoreductor > 0, this reaction fails. The quantity is below zero, so the reaction should not proceed. But the equilibrium constant[8] for the reaction is 1.38*10−13. By using fairly moderate concentrates of 0.1 mol/L for both iodide and Cu2+, the concentration of Cu+ is calculated as 3*10−7. As a consequence, the product of the concentrations is far in excess of the solubility product, so copper(I)iodide precipitates. The process of precipitation lowers the copper(I) concentration, providing an entropic driving force according to Le Chatelier's principle, and allowing the redox reaction to proceed.


CuI is poorly soluble in water (0.00042 g/L at 25 °C), but it dissolves in the presence of NaI or KI to give the linear anion [CuI2]. Dilution of such solutions with water reprecipitates CuI. This dissolution–precipitation process is employed to purify CuI, affording colorless samples.[5]

Copper(I) iodide can be dissolved in acetonitrile, yielding a solution of different complex compounds. Upon crystallization, molecular[9] or polymeric[10][11] compounds can be isolated. Dissolution is also observed when a solution of the appropriate complexing agent in acetone or chloroform is used. For example, thiourea and its derivatives can be used. Solids that crystallize out of those solutions are composed of hybrid inorganic chains.[12]


CuI has several uses:

  • CuI is used as a reagent in organic synthesis. In combination with 1,2- or 1,3 diamine ligands, CuI catalyzes the conversion of aryl-, heteroaryl-, and vinyl-bromides into the corresponding iodides. NaI is the typical iodide source and dioxane is a typical solvent (see aromatic Finkelstein reaction).[13] Aryl halides are used to form carbon–carbon and carbon–heteroatom bonds in process such as the Heck, Stille, Suzuki, Sonogashira and Ullmann type coupling reactions. Aryl iodides, however, are more reactive than the corresponding aryl bromides or aryl chlorides. 2-Bromo-1-octen-3-ol and 1-nonyne are coupled when combined with dichlorobis(triphenylphosphine)palladium(II), CuI, and diethylamine to form 7-methylene-8-hexadecyn-6-ol.[14]
  • CuI is used in cloud seeding,[15] altering the amount or type of precipitation of a cloud, or their structure by dispersing substances into the atmosphere which increase water's ability to form droplets or crystals. CuI provides a sphere for moisture in the cloud to condense around, causing precipitation to increase and cloud density to decrease.
  • The structural properties of CuI allow CuI to stabilize heat in nylon in commercial and residential carpet industries, automotive engine accessories, and other markets where durability and weight are a factor.
  • CuI is used as a source of dietary iodine in table salt and animal feed.[15]
  • CuI is used in the detection of mercury. Upon contact with mercury vapors, the originally white compound changes color to form copper tetraiodomercurate, which has a brown color.
  • CuI is used in designing and synthesizing Cu(I) clusters,[16] which is polymetal complex compounds.


  1. Lide, David R., ed. (2006). CRC Handbook of Chemistry and Physics (87th ed.). Boca Raton, FL: CRC Press. ISBN 0-8493-0487-3.
  2. Skoog DA, West DM, Holler FJ, Crouch SR (2004). Fundamentals of Inorganic Chemistry. Brooks/Cole. p. A-6. ISBN 978-0-03-035523-3.
  3. Sigma-Aldrich Co., Copper(I) iodide.
  4. NIOSH Pocket Guide to Chemical Hazards. "#0150". National Institute for Occupational Safety and Health (NIOSH).
  5. Kauffman GB, Fang LY (1983). Purification of Copper(I) Iodide. Inorg. Synth. Inorganic Syntheses. 22. pp. 101–103. doi:10.1002/9780470132531.ch20. ISBN 978-0-470-13253-1.
  6. Wells AF (1984). Structural Inorganic Chemistry (5th ed.). Oxford: Oxford University Press. pp. 410 and 444.
  7. Holleman AF, Wiberg E (2001). Inorganic Chemistry. San Diego: Academic Press. ISBN 0-12-352651-5.
  8. The value depends on the specific half-reaction for iodine. The value itself is calculated by using the formula: Kredox=10^{(nox*nred/0.0591)*(Eooxidator  Eoreductor)} which in itself is easily derived from the Nernst equations for the specific half reactions. Using Eoox=EoCu2+/Cu+ = 0.15; nox = 1 for copper; Eored=EoI/I2 = 0.52; nred = 2 for iodine
  9. Barth ER, Golz C, Knorr M, Strohmann C (November 2015). "Crystal structure of di-μ-iodido-bis-[bis(aceto-nitrile-κN)copper(I)]". Acta Crystallographica Section E. 71 (Pt 11): m189–90. doi:10.1107/S2056989015018149. PMC 4645014. PMID 26594527.
  10. Healy PC, Kildea JD, Skelton BW, White AH (1989). "Lewis-Base Adducts of Group 11 Metal(I) Compounds. XL. Conformational Systematics of [(N-base)1(CuX)1]∞ Orthogonal' Stair' Polymers (N-base = 'One-Dimensional Aceto-nitrile, Benzo-nitrile Ligand)". Australian Journal of Chemistry. 42 (1): 79. doi:10.1071/CH9890079. ISSN 0004-9425.
  11. Arkhireeva TM, Bulychev BM, Sizov AI, Sokolova TA, Belsky VK, Soloveichik GL (1990). "Copper(I) complexes with metal-metal (d10–d10) bond. Crystal and molecular structures of adducts of tantalocene trihydride with copper(I) iodide of composition: (η5-C5H5)2TaH[(μ2-H)Cu(μ2-I)2Cu(μ2-H)]2HTa(η5-C5H5)2, (η5-C5H4But)2TaH(μ2-H)2Cu(μ2-I)2Cu(μ2-H)2HTa(η5-C5H4But)2·CH3CN and {Cu(μ3-I)·P[N(CH3)2]3}4". Inorganica Chimica Acta. 169 (1): 109–118. doi:10.1016/S0020-1693(00)82043-5.
  12. Rosiak D, Okuniewski A, Chojnacki J (December 2018). "Copper(I) iodide ribbons coordinated with thiourea derivatives". Acta Crystallographica Section C. 74 (Pt 12): 1650–1655. doi:10.1107/S2053229618015620. PMID 30516149.
  13. Klapars A, Buchwald SL (December 2002). "Copper-catalyzed halogen exchange in aryl halides: an aromatic Finkelstein reaction". Journal of the American Chemical Society. 124 (50): 14844–5. doi:10.1021/ja028865v. PMID 12475315.
  14. Marshall JA, Sehon CA. "Isomerization of Β-Alkynyl Allylic Alcohols to Furans Catalyzed by Silver Nitrate on Silica Gel: 2-Pentyl-3-methyl-5-heptylfuran". Organic Syntheses. 76: 263.
  15. Zhang J, Richardson HW (June 2000). "Copper compounds". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. pp. 1–31. doi:10.1002/14356007.a07_567.
  16. Yu M, Chen L, Jiang F, Zhou K, Liu C, Sun C, Li X, Yang Y, Hong M (2017). "Cation-Induced Strategy toward an Hourglass-Shaped Cu6I7– Cluster and its Color-Tunable Luminescence". Chemistry of Materials. 29 (19): 8093–8099. doi:10.1021/acs.chemmater.7b01790.

Further reading

  • Macintyre J (1992). Dictionary of Inorganic Compounds. 3. London: Chapman and Hall. p. 3103.
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