Structure and Thermodynamics of Lanthanide and Actinide Complexes in Solution

Several questions of importance in the study of lanthanide and actinide coordination 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 hydration 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 most easily observed with the cations in oxidation state iii, the most common one for all the elements of the lanthanide series and for the transpiutonium elements of the actinide series. This paper reviews the present state of understanding of the complexes formed in aqueous solution by the trivalent ions of these two families ofelements. The coordination chemistry of these elements has been studied neither as intensively nor as extensively as that of the transition metal ions. One reason for this is that these elements have been less available generally than the transition metal elements. Although they are fairly abundant, a lack of industrial application until a relatively few years ago caused many of the lanthanide elements to be fairly expensive. Increased interest in the use of these elements in phosphors and lasers and as catalysts as well as improvements in the methods of separation has caused a substantial reduction in their cost. The availability of the transuranium elements of the actinide series was limited to a few nuclear energy laboratories where greater emphasis was placed on research of either a more nuclear or more applied nature. This latter situation has changed as the transpiutonium programme of the US Atomic Energy Commission has resulted in production of relatively large amounts of the elements as heavy as californium (Z = 98). However, the 23