The boron trihalides boron trifluoride, BF
3
, boron trichloride, BCl
3
, and boron tribromide, BBr
3
, are important industrial chemicals having increased usage as Lewis acid catalysts and in chemical vapor deposition (CVD) processes. Boron halides are widely used in the laboratory as catalysts and reagents in numerous types of organic reactions and as starting material for many organoboron and inorganic boron compounds. Reactions of boron trihalides that are of commercial importance are those of BCl
3
and, to a lesser extent, BBr
3
, with gases in chemical vapor deposition (CVD). Boron trichloride is prepared on a large scale by the reaction of Cl
2
and a heated mixture of borax,
, and crude oil residue in a rotary kiln heated to 1038°C. Boron trihalides, BX
3
, are trigonal planar molecules which are (
sp
)
2
hybridized. The X–B–X angles are 120°. The boron trihalides are strong Lewis acids; however, the order of relative acid strengths,
, is contrary to that expected based on the electronegativities and atomic sizes of the halogen atoms. Boron tribromide is produced on a large scale by the reaction of Br
2
and granulated B
4
C at 850–1000°C or by the reaction of HBr with CaB
6
at high temperatures. Approximately 75–95% of the BCl
3
consumed in the United States is used to prepare boron filaments by CVD. Another important use of BCl
3
is as a Friedel‐Crafts catalyst in various polymerization, alkylation, and acylation reactions, and in other organic syntheses. BCl
3
is also used for the production of halosilanes, in the preparation of many boron compounds and in the production of optical wave guides. BBr
3
is used in the manufacture of isotopically enriched crystalline boron, as a Friedel‐Crafts catalyst in various polymerization, alkylation, and acylation reactions, and in semiconductor doping and etching. Boron subhalides are binary compounds of boron and the halogens, where the atomic ratio of halogen to boron is less than 3. The boron monohalides and boron dihalide radicals have been studied. Diboron tetraflouride, B
2
F
4
, diboron tetrachloride, B
2
Cl
4
, diboron tetrabromide, B
2
Br
4
, and diborontetraiodide, B
2
I
4
, are well‐known but thermally unstable compounds.
Fluoroboric acid and the fluoroborates are commercially important to a variety of industries. The acid is used in plating circuits, in metal finishing, in the production of aluminum, and as an acid catalyst. Main group metal and ammonium fluoroborates find use as fluxes, as catalysts, and in flame‐retardant manufacture. Transition‐ and other heavy‐metal fluoroborate salts are used in the plating industry and as catalysts. Properties and manufacture are described.
