Boron triiodide

Boron triiodide is a chemical compound of boron and iodine with chemical formula BI3. It has a trigonal planar molecular geometry.

Preparation
Boron triiodide can be prepared by the reaction of boron with iodine at 209.5 °C or 409.1 °F. It can also be prepared by reacting hydroiodic acid with boron trichloride:


 * 3HI + BCl3 -> BI3 + 3HCl (reaction requires high temperature)

Another method is by reacting lithium borohydride with iodine. As well as boron triiodide, this reaction also produces lithium iodide, hydrogen and hydrogen iodide:


 * 3LiBH4 + 8I2 -> 3LiI + 3BI3 + 4H2 + 4HI

Properties
In its pure state, boron triiodide forms colorless, otherwise reddish, shiny, air and hydrolysis-sensitive crystals, which have a hexagonal crystal structure (a = 699.09 ± 0.02 pm, c = 736.42 ± 0.03 pm, space group P63/m (space group no. 176)). Boron triiodide is a strong Lewis acid and soluble in carbon disulfide.

Boron triiodide reacts with water and decomposes to boric acid and hydriodic acid:


 * BI3 + 3H2O <-> B(OH)3 + 3HI

Its dielectric constant is 5.38 and its heat of vaporization is 40.5 kJ/mol. At extremely high pressures, BI3 becomes metallic at ~23 GPa and is a superconductor above ~27 GPa.

Applications
Boron triiodide can be used to produce other chemical compounds and as a catalyst (for example in coal liquefaction).