Structures of lanthanide-edta complexes
Master Thesis
1983
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University of Cape Town
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Abstract
The structures of six lanthanide-edta complexes were elucidated by X-ray diffraction, so as to ascertain exactly at which point the change from the nine-coordinate [Ln(edta)(H₂O)₃ ]- complex to the eight-coordinate [Ln(edta)(H₂O)₂]-complex occurs. The transition was found to occur at Erbium. The structures of the complexes K⁺[Ln(edta)(H₂O)₃ ].5H₂O for Ln=Praseodymium, Europium, Gadolinium and Holmium were elucidated and found to be isomorphous crystallizing in the orthorhombic space group Fdd1 with Z = 16. In each case , the central metal ion has a coordination mnnber of nine, being bonded to the hexadentate edta ligand and three water molecules. For each structure, anomalous dispersion corrections were applied since Fdd2 is a polar space group. The molecules of water of crystallization were seen to be partially disordered. The structures of the complexes K⁺[Ln(edta)(H₂O)₂ ].4H₂O for Ln =Erbium and Ytterbium were elucidated and found to be isomorphous. A space group problem arose with these two structures as there was a problem of pseudo-synunetry, in that the oolecules were arranged in a P2₁/n pattern, but the motif in fact resembled that of C2/c or Cc. The structures were finally refined in the centrosynnnetric, oonoclinic space group C2/c with Z = 4, but were found to be disordered. In fact, in each case, two independent structures were identified, one 'good' and one 'lesser' structure. A study of the 'good' structure in each case showed that the central metal ion has a coordination number of eight, being bonded to the hexadentate edta ligand and two water molecules. The technique of thermogravirretric analysis was employed in the study of the molecules of water of crystallization.
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Bibliography: pages. 188-192.
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Reference:
Mullins, J. 1983. Structures of lanthanide-edta complexes. University of Cape Town.