Question

why do boron halides form addition compounds with amines??

Answer 1

Boron halides are considered electron deficient. As a Group III element with a relatively low electronegativity, boron tends not to fill its valence shell in simple trivalent compounds. In simple terms, it uses only 3 of its 4 valence orbitals to form bonds to, for example, three halogen atoms, in trihalide compounds. Ammonia, and amines in general, have a lone pair of electrons in the N atom's valence shell, which are available for forming a bond with an electron deficient atom, such as boron. Thus the relative ease of forming addition compounds between boron halides and amines.