LanthanidIons

Lanthanide ions, usually present as Ln3+ in solution, are the ionic forms of the lanthanide elements, which span atomic numbers 57 through 71. The chemistry is dominated by a pronounced and gradual lanthanide contraction: the ionic radii decrease steadily from La3+ to Lu3+, influencing complex stability and solubility. These ions are hard Lewis acids and preferentially bind oxygen- or fluorine-donor ligands. In aqueous solution they are highly hydrated, commonly existing as eight- or nine-coordinated aqua ions such as [Ln(H2O)9]3+; ligand binding often displaces water to form Ln–ligand complexes.

Chemically, the lanthanide ions show relatively similar behavior across the series due to shielding of the 4f electrons, enabling systematic chelation and predictable complex formation with carboxylates, phosphates, and polyaminocarboxylates like EDTA or DOTA. Their coordination chemistry supports various applications in separation, catalysis, and material science.

Spectroscopically, lanthanide ions are known for sharp, line-like f–f transitions, producing characteristic emissions used in phosphors, lasers, and bioimaging when sensitized by nearby dopants or ligands. The most common oxidation state encountered is +3; Ce can access +4 in certain oxides and solutions, while other oxidation states are rare under ordinary conditions.

Occurrence and extraction: lanthanide ions are obtained from minerals such as monazite and bastnäsite, and their separation is challenging due to chemical similarities. Modern processing relies on solvent extraction, ion exchange, and selective complexation to separate adjacent lanthanides.

Applications span lighting and displays, scintillators, catalysts, and analytical chemistry as tracers and luminescent probes. See also: lanthanides, f-block elements, phosphors.