1. Phillips, C.S.G., Williams, R.J.P., in: Inorganic Chemistry, Vol. 1. Oxford: Clarendon Press 1965, for a promising but limited and somewhat confused attempt
2. Pauling, P., Pauling, L., in: Chemistry. San Francisco: Freeman 1975
3. Mahan, B.H., in: University Chemistry, p. 19. Reading, Mass.: Addison-Wesley 1970
4. Ingold, C.K., in: Structure and Mechanism in Organic Chemistry, p. 205. London: Bell and Sons 1953
5. Brescia, F., et al., in: Fundamentals of Chemistry. New York: Academic Press 1966