Preparation:

Structure:

BX₃ are all planar monomeric species due to the extra B(pπ)-X(pπ) bonding (compare AlCl₃, which dimerizes to give Al₂Cl₆, as the inter-molecular Lewis acid-Lewis base bonds are stronger than the intra-molecular bonds in BX₃).

Reactions:

All BX₃ are Lewis acids, with acid strength BF₃<BCl₃<BBr₃<BI₃.This reflects the loss of pπ-bonding when the BX₃ distorts from planar to pyramidal on reaction with the Lewis base; the B(pπ)-F(pπ) bond is the strongest due to the small size of the F^- ion, and therefore the least reactive.

Competition exists between different halides, and rapid equilibrium is set up.

Hydrolyzed to give Boric Acid [B(OH)3]

Formation of lower halides, (BX)_n. These all have B-B bonds (though boron catenates less than carbon), and the dimers are all planar with D_2h symmetry, except B₂Cl_4(g), which is D_2d. Larger, cage-like structures have [3c,2e] bonds, eg. B₄Cl₄, which is tetrahedral, with [3c,2e] bonds making up the faces.