Selective complexation of metal ions is a matter of fundamental science and of practical application. Natureprovides many examples of selectivity in metal complexation and transport that can be used to design selectivechelators. However, the lanthanides, because of their variable coordinationnumber, highly ionic nature and lability, provide a particular challenge to thechemist. Throughout much of his career this chemist has sought to put thelanthanides in bondage, as illustrated to the right. The donor atoms in the figureare nitrogen, because some early chemistry involved using the Ln metal ion as atemplate to make nitrogen macrocycle ligands.1 However, the lanthanides areoxophiles and so selective and strong chelators for them should contain oxygendonors. Before describing some of the ligand families that this has led to, a briefdiscussion of arguably the most fundamental lanthanide complexes, the aquoions, will be presented as part of a discussion of the lanthanide ionic radiicontraction.2
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