Lanthanides are defined as lanthanum (La) and 14 other elements (Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu). They form a separate series in the periodic table. The electron filling numbers in the inner 4f orbitals of the lanthanide atoms are different from each other, so the 15 lanthanides have different physical and chemical properties. In the 1930s and early 1940s, Freed and coworkers  discovered that the relative intensities of the absorption bands of Eu (III) were differed in different solvents. This pioneered the spectroscopic study of lanthanide ions in solution. Subsequently, Weissman  found in his experiments that complexes of Eu (III) with certain UV absorbing ligands were highly luminescent upon UV light excitation. Since Eu (III) itself shows only a few very weak absorption bands, the solution of this ion does not offer very bright luminescence. Therefore, it is highly likely that this highly luminescent phenomenon originates from organic ligands. These studies have triggered a strong interest in lanthanide metal complexes among researchers. In turn, more and more attention has been paid to this unique class of complexes. Lanthanide metal complexes are constructed by forming strong ionic coordination bonds between lanthanide ions and organic ligands. These complexes not only retain the properties of organic ligands, but also have the properties of lanthanide ions. Because of its special 4f-layer electronic structure, lanthanide ions have strong coordination ability and a variety of coordination modes. In the process of forming complexes, new materials with different structures and properties can be obtained by changing the coordination number and coordination sites. In addition, the expected regulation of the product can be achieved under certain conditions.
Coordination chemical properties of lanthanide ions
The general electronic configuration of lanthanide is changed from [Xe]4f05d16s2 of lanthanum to [Xe]4f145d16s2 of lutetium. The difference in the internal electronic configuration leads to the difference in the physical and chemical properties of these lanthanide elements.
- Radius of lanthanide ions
The radius of lanthanide ions ranges from 0.085 to 0.106 nm. The radius of lanthanide ions decreases with increasing atomic number, which is called “lanthanide contraction” phenomenon. The chemical bond of lanthanide metal complexes is mainly ionic bond, so the structure and properties of lanthanide metal complexes are closely related to their ionic radius.
- Coordination number of lanthanide ions
The coordination number of general lanthanide ions is 6-10, and the number of lanthanide metal complexes with coordination number of 8 or 9 has reached more than half of the total. The lanthanide ions have larger radius, and the chemical bond of the complexes is mainly ionic, so the lanthanide ions have higher coordination number.
- Structure of lanthanide metal complexes
Lanthanum metal complexes have rich and diverse structures, which is mainly due to the fact that lanthanide ions and coordination atoms can form a variety of coordination modes, thus forming a variety of polyhedrons.
Lanthanide metal complexes have tremendous applications in many fields by virtue of their excellent properties. The following are a few simple applications.
- Lanthanide metal complex-based medicine is proving successful in therapeutics, especially in drug delivery, chemotherapy, and photodynamic therapy.
- Based on the intrinsic magnetic properties of lanthanide metals, lanthanide metal complexes have become the frontier of research. This is attributed to the discovery of magnetic bistability on mononuclear complexes, which may pave the way for the use of these systems as magnetic memory molecular units.
- Lanthanide metal complexes with attached organic ligands are regarded as attractive luminescent materials because of their characteristic narrow emission bands and long emission lifetimes over a wide range of wavelengths (ultraviolet/visible/near-infrared (UV/vis/near-IR)).
- Lanthanide metal complexes not only serve as new drug candidates for catalytic processes, but also constitute a significant part of enantioselective catalysis.
- A great deal of research is being conducted on the application of lanthanide metal complexes in quantum technology. It has been reported that they show great potential for spin-based quantum information processing.
Figure 1. Application of lanthanide metal complexes
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- Freed, S.; et al. Ions of europium distributed between different configurations in homogeneous solutions. J. Chem. Phys.1939, 7: 824-828.
- Weissman, S. I. Intramolecular energy transfer the fluorescence of complexes of europium. J. Chem. Phys. 1942, 10: 214-217.