STRUCTURE AND THERMODYNAMICS OF LANTHANIDE AND ACTINIDE COMPLEXES IN SOLUTION

Several questions of importance in the study of lanthanide and actinide coordi-nation compounds are reviewed. There is considerable evidence that in aqueous solution the primary coordination number is nine for the ions La(ni) through Nd(m) and eight for the ions heavier than Gd(m). While it seems that some degree of covalency exists in the metal—ligand bonding a model of electrostatic bonding seems satisfactory for explaining the structure and formation of the complexes. The dehydration of the lanthanide ions upon complexation largely determines the enthalpy and entropy data. However, there seems to be a compensation effect in the hydration parts of those terms such that the free energy changes seem to reflect the metal—ligand reaction unobscured by hydra-tion factors. A number of individual inorganic and organic ligand systems are reviewed from these viewpoints. The lanthanide and actinide elements constitute two families of metals which often exhibit very similar chemical behaviour. This similarity is mos