Spherical structures, lanthanide-coordination

Halides are negatively charged spherical structures which normally bind to anion receptors in a non-directional fashion. One of the first examples of the use of halides for binding to lanthanide complexes, by direct metal coordination within a coordinatively unsaturated environment, was that of Charbonniere et al. [55] who developed lanthanide complexes of a bis-bipyridine-phosphine oxide ligand (Scheme 6.9a), as luminescent anion sensors for halides. In this design, the bipyridine (bpy) units were expected to coordinate to the... 

The spectroscopic levels correspond to the electronic levels for the free ion in a spherical symmetry. When this ion is coordinated to a ligand, all directions are not identical anymore, but depends on the symmetry of the coordination sphere. The coordination sites of the ligands can be represented as point charges around the lanthanide ion generating a static electrostatic ligand field around the metal ion. The spectroscopic levels are then further split into spectroscopic sublevels due to the ligand field. A (2>5+i)p jgygi g split into (2 / + 1) sublevels. But not all sublevels are uncovered upon coordination. The number of sublevels that a spectroscopic level exhibits actually depends on the symmetry of the coordination sphere. Therefore, the fine structure of the spectroscopic levels (i.e., the sublevels) is a tool to determine the symmetry around the ion. 

---