Boron Halides


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 acidLewis base bonds are stronger than the intra-molecular bonds in BX3).Structure:


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 D2h symmetry, except B2Cl4(g), which is D2d. Larger, cage-like structures have [3c,2e] bonds, eg. B4Cl4, which is tetrahedral, with [3c,2e] bonds making up the faces.