1. Figgis, B. N., and Lewis, J., Modern Co-ordination Chemistry, Chap. 6 (Interscience, New York, 1960).
2. Lewis, J., Sci. Prog., 50, 419 (1962).
3. Figgis, B. N., Nature, 182, 1568 (1958).
4. Griffith, J. S., Theory of Transition Metal Ions (Cambridge Univ. Press, New York, 1961).
5. Penney, W. G., and Schlapp, R., Phys. Rev., 42, 666 (1932).