1. Williams, R. J. P., Hale, J. D.: Struct. Bonding 1, 249 (1966).
2. Kleve, B.: Acta Chem. Scand. 21, 1530 (1967).
3. Dyke, M., Sass, R. L.: J. Phys. Chem. 72, 266 (1968).
4. Pauling, L.: The nature of the chemical bond, 3rd edit. Ithaca, N. Y.: Cornell University Press 1960.
5. e. g. Truter, M. R., Vickery, B. L.: J. Chem. Soc. (A) 1971, 2077 and references therein.