Copper(I) iodide

Copper(I) iodide is the chemical 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 is white, but samples are often tan or even, when found in nature as mineral marshite, reddish brown, but such color is due to impurities. It is common for iodides to become discolored because of the easy oxidation of the iodide anion to iodine.

Preparation
In the laboratory, copper(I) Iodide is prepared by simply mixing an aqueous solutions 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.
 * 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. Copper(I) iodide can also prepared by heating iodine and copper in concentrated hydriodic acid, HI.

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.

Structure
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 (ɣ-CuI), a wurtzite structure between 390 and 440 °C (β-CuI), and a cubic 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.

Uses
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.  Aryl halides are used to form carbon-carbon and carbon-heteroatom bonds in process such as the Heck, Stille, Suzuki, 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.
 * CuI is used in cloud seeding1, 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.1