Ethylenediaminetetraacetic acid complexation with lanthanide

The lanthanides form many compounds with organic ligands. Some of these compounds ate water-soluble, others oil-soluble. Water-soluble compounds have been used extensively for rare-earth separation by ion exchange (qv), for example, complexes form with citric acid, ethylenediaminetetraacetic acid (EDTA), and hydroxyethylethylenediaminetriacetic acid (HEEDTA) (see Chelating agents). The complex formation is pH-dependent. Oil-soluble compounds ate used extensively in the industrial separation of rate earths by tiquid—tiquid extraction. The preferred extractants ate catboxyhc acids, otganophosphoms acids and esters, and tetraaLkylammonium salts. 

Coordination Complexes. The abiUty of the various oxidation states of Pu to form complex ions with simple hard ligands, such as oxygen, is, in order of decreasing stabiUty, Pu + > PuO " > Pu + > PuO Thus, Pu(Ill) forms relatively weak complexes with fluoride, chloride, nitrate, and sulfate (105), and stronger complexes with oxygen ligands (Lewis-base donors) such as carbonate, oxalate, and polycarboxylates, eg, citrate, and ethylenediaminetetraacetic acid (106). The complexation behavior of Pu(Ill) is quite similar to that of the light lanthanide(Ill) ions, particularly to Nd(Ill)... 

Hambright et al. (1988) have also studied the kinetics of displacement of the Gd " ion from the gadolinium(III) complex of TSPP by ethylenediaminetetraacetate (EDTA) giving Gd(EDTA) and H2(TSPP) as the main products. This represents the first example of metal removal from a metalloporphyrin by a chelating ligand. A mechanism has been proposed to account for the kinetic data. The water-soluble lanthanide complexes of TMPyP also undergo demetallation in the presence of EDTA (Haye and Hambright 1991). Similar to the acid solvolysis reactions, a linear relationship between log k and the ionic radius of the metal center can be established, and complexes with smaller metal center are more stable toward demetallation by EDTA. 

In view of the structural and functional relationship between amino acids and polyaminopolycarboxylic acids, the coordination of some lanthanide ions (La, Pr, Er) with ethylenediaminetetraacetate (EDTA) has also been examined under much higher pH conditions. The formation of lanthanide-hydroxo complexes formulated as K2R(0H)EDTA 4H20 was reported as early as in 1955 (Djordjevic and Vuletic, 1980), but it is not until recent studies that the first crystal structure of a lanthanide-EDTA hydroxo species was reported. Efforts in this field were also stimulated by the recognition that the R(OH2)s array present in previously known mononuclear EDTA complexes is dimensionally close to the R(/is-0H)3 fragment of [R4(/i3-OH)4] + clusters and by the desire to understand the mechanism of formation of the clusters. A proposal for the latter is outlined in Figure 90. 

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