Why do boron hallides form addition compound with ammonia ammines?
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. In a very general sense, this is a kind of acid/base reaction, with the boron halide acting as a Lewis acid (electron acceptor) and the amine acting as a Lewis base (electron donor).
