Lanthanide oxalate complexes

As the size of the chelating ligand increases, a maximum in stability is normally obtained for 5 or 6 membered rings. For lanthanide complexes, oxalate forms a 5-membered ring and is more stable than the malonate complexes with 6-membered rings. In turn, the latter are more stable than the 7-membered chelate rings formed by succinate anions. 

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)... 

Most lanthanide compounds are sparingly soluble. Among those that are analytically important are the hydroxides, oxides, fluorides, oxalates, phosphates, complex cyanides, 8-hydroxyquinolates, and cup-ferrates. The solubility of the lanthanide hydroxides, their solubility products, and the pH at which they precipitate, are given in Table 2. As the atomic number increases (and ionic radius decreases), the lanthanide hydroxides become progressively less soluble and precipitate from more acidic solutions. The most common water-soluble salts are the lanthanide chlorides, nitrates, acetates, and sulfates. The solubilities of some of the chlorides and sulfates are also given in Table 2. 

The measurement of stability constants of complexes of yttrium, lanthanide, and actinide ions with oxalate, citrate, edta, and 1,2-diaminocyclohexanetetra-acetate ligands has revealed that there is a slight increase in the stability of complexes of the /-electron elements, relative to the others. A series of citric acid (H cit) complexes of the lanthanides have been investigated by ion-exchange methods and the species [Ln(H2cit)]", [Ln(H2cit)2] , [Ln-(Hcit)], and [Ln(Hcit))2] were detected. Simple and mixed complexes of dl- and jeso-tartaric acid have been obtained with La " and Nd ions, and the stability constants of lactate, pyruvate, and x-alaninate complexes of Eu and Am " in water have been determined. 

Another lanthanide carboxylate complex that also contains nonacoordinated lanthanide ion in a monocapped square antiprismatic arrangement of nine oxygens is the erbium oxalate trihydrate with the composition Er(OOCCOO) (HOOCCOO) (OH2)3. This complex crystaUizes (767) in space group P4/ with 0 = 8.664, c =6.4209 A and Z=2, when the precipitate of erbium oxalate, ob-... 

Now the pressure-jump technique will be described with a system consisting of lanthanide oxalate complexes [17]. The technique used a sudden change in pressure to perturb the equilibrium and a conductivity bridge to detect and follow the changes in the system. The course of re-equilibration is recorded by the use of an oscilloscope and a camera. 

The rate constants for the reaction of murexide with lanthanides obtained by Geier [21] are of similar magnitude as those of oxalate complexes. The forward rate is insensitive to the nature of the entering ligand which provides support for the operation of the four-step mechanism proposed by Eigen [18]. The substitution reactions of lanthanides appear to be governed by the four-step mechanism. The rate determining step in this mechanism is the... 

Formation kinetics of some lanthanide oxalate complexes [22], ... 

A plot of log Ln vs log K for the malonate system is shown in Fig. 7.31. The catalyzed isomerization of the malonate complex proceeds by a different mechanism from that of the oxalato analogue because (i) lack of linear free energy relationship between the catalytic rate constants and the complex formation constants, (ii) linear relationship between catalytic rate constants (log A n) and formation constants of lanthanide malonate complexes (log K ), (iii) catalytic rate constants for the malonate system are smaller than the corresponding rate constants for the oxalate system. The proposed mechanism for... 

The solution reactions of Cm3+ closely resemble those of the lanthanide and actinide +3 ions, and the fluoride, oxalate, phosphate, iodate, and hydroxide are insoluble. Complexes appear to be weaker than those of preceding elements. 

Usually, Pm is separated from other lanthanides by ion exchange in the presence of complexing agents in solution. This method was also applied by Marinsky, Glendenin and Coryell in 1947 after oxalate precipitation of the rare-earth fraction, the precipitate was treated with carbonate solution to remove the main part of Y, dissolved and passed as 5% citrate solution (pH 2.5) through a cation-exchange column. The result is shown in Fig. 14.2. 

Although it might be assumed quite reasonably that the coordination chemistry of the some 15 elements known during Werner s lifetime would have developed significantly during that period, the literature contains only a few accounts of synthesis and characterization. Indeed, the 1920 edition of Werner s classic monograph (50) records essentially the same information as the 1908 edition, namely the compositions of a limited number of double nitrates, sulfates, and oxalates. Both Spencer ( 5) and Little in their comprehensive compilations of data at about the same time, describe only certain lanthanide double salts and adducts, but not within the framework of complex compounds or coordination chemistry. 

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