Complexation, lanthanide aminopolycarboxylates

To improve the separations with aminoce oxylates, researchers turned their efforts toward understanding the nature and characteristics of aminopolycarboxylate-lanthanide complexation. l e ensuing wealdi of information resolved fundamental questions about the structure of the complexes, ligand effects, bonding characteristics, and hydration. 

Aluminum, coordination compounds, 446 Aminopolycarboxylates, lanthanide, complexation, 346-358 Ammine cupric ions, complexity constant determination, 101-106 Ammine cuprous ions, complexity constant determination, 106-107 Ammonium ion-hydrogen-bonding receptor complexes, schematic representation, 150,151/... 

Mainly as a result of the inaccessibility of the f-electrons, directed bonding typically observed in the transition elements is not seen in the lanthanides. In many respects, the lanthanides behave more like the alkaline earth elements than like the transition elements. The lanthanides behave as hard-acid cations in solution, preferring interaction with hard-base donors like oxygen and fluoride to that with sulfur or heavy halide donors. Significant lanthanide interaction with nitrogen donors is observed only when steric factors force the interaction (as in aminopolycarboxylic acid complexes). 

Another aminopolycarboxylic acid, closely related to IMDA and forming strong complexes with the lanthanide ions, is nitrilotriacetic acid (NTA). The hy-... 

Proton relaxation NMR using Gd(IH) support these conclusions. Spin-lattice relaxation (Ti) values are inversely related to the average number of inner sphere hydration waters of the lanthanide cation (50-52). Chang reported hydration values of 3 and 1 for Gd complexes of EDTA and DTPA, respectively. Both NMR relaxation studies and luminescence lifetime data indicate a total coordination number between 8 and 10 for the lanthanides in aminopolycarboxylate complexes. 

Thermodynamic studies of lanthanide-aminopolycarboxylate complexation have been quite valuable in the understanding of lantiianide coordination chemistry. The same experimental conditions of Grenthe (42,43) were utilized by Choppin,... 

Goedken, and Gritmon(J3) in 1977 to investigate the thermodynamic parameters of a variety of aminopolycarboxylate ligands across the lanthanide series. Variations in the enthalpy and entropy curves vs. Z were attributed to different degrees and patterns of dehydration as well as the increasing polydentate nature of the complexation. The linear relationship between log 6 and ZpKa of... 

The exchange of a radioisotopic lanthanide for a stable lanthanide metal chelated to an aminopolycarboxylate ligand has been found to proceed via two mechanistic pathways(tiO). Below pH 6, an acid-catalyzed mechanism predominates in which protonation catalyzes decomposition of die lanthanide-aminopolycarboxylate complex. The uncomplexed ligand chelates with free lanthanide cations in solution. The mechanism below illustrates the acid-catalyzed pathway, where A = aminopolycarboxylate ligand (t denotes the activated complex Ln is die radioactive tracer). 

Formation constants of complexes formed by Am(iii) with aminopolycarbo-xylic acids are larger than for the comparable complexes of the light lanthanides (Z = 57-61). Thus, addition of an aminopolycarboxylic add to a lithium or aluminum nitrate solution containing Am(iii) and rare earths enhances TBP extraction of the lanthanides relative to ameridum [42],... 

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