The boron trihalides are highly volatile; BF₃ (b. p., 173 K) and BCI₃, (b. p., 260.5 K are gases, BBr₃ a volatile liquid (b. p., 319 K) and Bl₃ a low melting solid (m. p., 323 K). Boron halides are all hydrolysed by water giving H₃B0₃, and hydrohalic acids, HX or hydrofluoboric acid, HBF₄:

Bx₃+ 3H₂O-------------------------------->H₃B0₃ + 3HX, (X = CI, Br, I)

4BF₃ + 3H₂0-------------------------------> H₃BO₃ + 3HBF₄

In addition to the trihalides, boron forms lower halides of formula, B₂X₄. But only B₂F₄ and B₂Cl₄ have been studied in some detail. B₂F₄ is a colourless gas whereas 'B₂Cl₄ is a colourless liquid at room temperature. These halides are much less stable than the corresponding trihalides. B₂F₄ is the most stable of all the B₂X₄ compounds.  These are spontaneously inflammable in air and react with H₂, H₂0, ROH, CI₂, etc.

Unlike BH₃ which is unstable, boron trihalides are monomeric molecular compounds and have no tendency to dime rise. In this respect boron trihalides resemble organoboranes, BR₃, but differ from diborane, B₂,H₆, and the aluminium halides,  AI₂X,. Thus, boron trihalides having three electron pair B₂-X₆ bonds are electron deficient. However, the interatomic distances, B-X, are substantially shorter than those expected for single bonds. For example, the B-F₂ bond length in BF₃ is 130 pm which is shorter than the sum of the covalent radii of B (80 pm) and F (72 pm). This shortening of bonds has been explained in terms of appreciable p?-p? bonding

Between an empty p orbital of the sp² hybridised boron atom and filled p orbitals of one of the fluorine atoms.

All the four trihalides are trigonal planar molecules. This can be explained on the  basis of sp² hybridisation of the boron atom.

The three hybrid orbitals of boron overlap with singly filled 2p orbitals of three halogen atoms giving rise to three B-X bonds. The empty 2p orbital of boron  which is not involved in hybridisation in perpendicular to the plane of the triangle. Its energy is comparable to that of the filled 2p orbitals of halogen atom. Thus it can accept a pair of electrons from a filled 2p orbital of any one of the three halogen atoms, forming a dative ? bond. This makes an octet of electrons around the boron atom. The Bx₃ molecule exists as a resonance hybrid of the following three structures as shown in

Because of lack of efficient overlapping, the extent of ? bonding decreases as the size of atoms involved in bonding increases. Thus, the extent of ? bonding in boron halides decreases from BF₃, to BI₃.