1. L. C. W. Baker: in Advances in the Chemistry of Coordination Compounds ,ed. S. Kirschner, MacMillan, New York (1961), p. 608 ff.
2. P. Souchay: Polyanions et Polycations ,Gauthier-Villars, Paris (1963)
3. P. Souchay: Ions Mineraux Condensés ,Masson et Cie, Paris (1969).
4. H. T. Evans, Jr.: Perspect. Struct. Chem. 4, 1 (1971).
5. T. J. R. Weakley: Struct. Bonding 18, 131 (1974).