Lanthanides: Coordination Chemistry

Abstract The simple aqueous coordination chemistry of Ln 3+ ions is predominantly that of O‐ and O,N‐donor ligands, as amines are too basic to give isolable complexes from solution; the high hydration energy of Ln 3+ means that such syntheses have to be done in nonpolar solvents. Coordination numbers are determined by steric, not ligand‐field, factors. With uncongested ligands (e.g., H 2 O, thf), coordination numbers from 7 to 9 are typical; but with bulky ligands such as‐N(SiMe 3 ) 2 , it is the repulsions between bulky substituent groups that determine how many ligands enter the coordination sphere. The use of small bidentate ligands can lead to coordination numbers up to 12. Coordination numbers often decrease with increasing atomic number of the lanthanide, but there are exceptions to this rule. Because coordination numbers are mainly determined by packing around the metal ion center, solvent molecules can be included in the coordination sphere, and X‐ray diffraction studies are the ultimate reliable way of determining the coordination number. Apart from its aqua ion, there are no cases of simple Sc 3+ complexes greater than six coordinate, except when nitrate is present in the coordination sphere, while Y 3+ complexes resemble those of the heavier lanthanides. A rich variety of simple complexes exists, exemplified by the thf complexes LnCl 3 (thf) x (e.g., x = 2, 2.5, 3, 3.5, 4), which are also useful starting materials in synthesis.