In the lanthanide series, we have 14 elements that result from the addition of electrons into the f orbitals. For the lanthanide series, the electron configurations are 4f (1-14). In the lanthanide series, the majority of the compounds are formed by trivalent M3+ ions. From a spectroscopic point of view, all the rare earth ions have a large spin–orbit coupling constants, resulting in electronic STATES being defined by the angular MOMENTUM values J. The lanthanides form compounds that are primarily ionic in character, with coordination numbers of 8 or 9. Recently, complexes with coordination numbers of 11 and 12 have been synthesized. The compounds of lanthanides are substitution labile in aqueous solutions [91].

The influence of the ligand field on the electronic states of lanthanides is small and generally of the ORDER of ∼ 200 cm−1. Because the ligand field perturbation of J states are minimal, the f–f electronic transitions are sharp. In addition to f–f transitions, both 4f → 5d and charge TRANSFER transitions are also observed in the spectra of lanthanides [92]. Lanthanide ions exhibit emission in the solid state, and in some cases in aqueous solutions. Energy transfer from the ligand or intermolecularly from an excited state can give rise to the emission from lanthanide ions.

The photochemistry of lanthanide ions is limited to photoredox reactions. Photo substitution has not been studied because of the lability of metal ions, and since ligand substitution reactions are rapid even under thermal conditions.