A discussion is commenced in which the electronic properties of tetrahedral intermetallic compounds are described in terms of localized molecular-orbital bonds. The correct definitions of covalency and polarity of these bonds are linked with the net ionicity of the atoms of the crystal. Detailed non-empirical calculations of boron nitride provide a description of the bonds, and associated formal charges. An alternative simple but partially self-consistent analysis is presented whereby the coefficients in the simple molecular-orbital description may be found semi-empirically. The charges thus predicted agree well with those calculated by the earlier non-empirical method, and also with such experimental values as are available.