Hexadentate EDTA complex

In this section it will be useful to discuss some common experimental methods used in the determination of stability constants of rare earth complexes. Ethylenediamine tetraacetate anion (EDTA) is a hexadentate and forms complexes with trivalent rare earth ion readily. The pioneering studies of Schwarzenbach [4] on the determination of stability constants of rare earth EDTA complexes by potentiometry and polarography can be considered to illustrate the principles involved in the determination of stability constants. 

It should be noted that most structurally characterized dinuclear complexes are products of adventitious lanthanide hydrolysis, and are not reproducible. With the ligand-controlled hydrolytic approach, they can now be rationally synthesized starting from a mononuclear aqua complex. Shown in Figure 6.30 is the synthesis of [(EDTA)Er (ti-OH)2Er(EDTA)] (EDTA=ethylenediaminetetraacetate) the dinuclear core doubly bridged by the fi-OH groups is encapsulated by two hexadentate EDTA ligands, one on each metal atom [69],... 

Figure 17-3 Structure of a metal/EDTA complex. Note that EDTA behaves here as a hexadentate ligand in that six donor atoms are involved in bonding the divalent metal cation.

Figure 7.1 Complex formed between hexadentate EDTA anion and an octahedral six-coordinate metal ion". The six dative covalent bonds point towards the comers of a regular octahedron. The complex ion formed will, in all probability, be water soluble because of the residual negative charge left on the complex, i.e. (4-n) , becoming aquated...

Hydration of lanthanide complexes. X-ray diffraction studies of the solid complexes of KLn(EDTA)(H20),c showed the number of water molecules in the coordination sphere to be three for the lighter lanthanides and two for the heavier ones for total coordination numbers of nine and eight, respectively, since EDTA is hexadentate (Hoard et al. 1967). Ots (1973) measured a maximum at europium for the heat capacity change AC° for the formation of lanthanide-EDTA complexes. This maximum was taken as a strong evidence for hydration equilibrium between the complexed species,... 

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