Boron triiodide is a chemical compound of boron and iodine with chemical formula BI<sub>3</sub>. It has a trigonal planar molecular geometry.
Boron triiodide can be prepared by the reaction of boron with iodine at 209.5 ðC or 409.1 ðF. It can also be prepared by reacting hydroiodic acid with boron trichloride:
Another method is by reacting lithium borohydride with iodine. As well as boron triiodide, this reaction also produces lithium iodide, hydrogen and hydrogen iodide:
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 ñ 0.02 pm, c = 736.42 ñ 0.03 pm, space group P6<sub>3</sub>/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:
Its dielectric constant is 5.38 and its heat of vaporization is 40.5 kJ/mol. At extremely high pressures, BI<sub>3</sub> becomes metallic at ~23âÂÂGPa and is a superconductor above ~27âÂÂGPa.
Boron triiodide can be used to produce other chemical compounds and as a catalyst (for example in coal liquefaction).