Q. Complexation Behaviour of the f-block?

Ions of actinide and lanthanide elements have a strong tendency to form complexes with a variety of oxygen and nitrogen donor ligands. Perhaps, because of their comparatively higher charge to size ratio, the actinide ions have a greater tendency to form complexes than the lanthanides. Also, because of the existence of a large number of oxidation states, the complexation behaviour of actinides is more varied. The lanthanide and actinide ions form the most stable complexes with chelating ligands such as oxalic acid, tartaric acid, citric acid, nitric acid, ethylenediamine tetraacetic acid (EDTA) and 0-diketones. In these complexes the metal ions have very high coordination numbers. For case, the coordination number of the metal ion in [Th(acac)₄]. [Ce(N0₃)₄.(OPPh₃)₂] and  [Ce(N0₃)₆]^2- is 8, 10, 12, respectively. In these complexes, the acetyl acetonate (acac) and the nitrate ligands are operating as bidentate ligands occupying two coordination sites around the metal ion. These metal ions form water soluble complexes with citric acid, tartaric acid and EDTA. The formation of water soluble complexes with these ligands facilitates separation of the metal ions by ion exchange chromatography.