Stability constants hydroxycarboxylate ligands

Stability constants for calcium complexes of a selection of hydroxycarboxylate ligands are listed in Table VII (239,246,272-274). For tartrate, malate, and citrate stabilities decrease in the expected order Ca2+> Ba2+> Ra2+ (231,275). The stability constant for the complex of pyruvate (logiOifi 0.8 (273)) is similar to that for acetate calcium complexes of a-ketoglutarate and of oxaloacetate are somewhat more stable (logio-Ki = 1.3, 1.6 respectively (273)). The sequence logio-Ki = 3.0, 1.4, 1.1, 0.6 for the dicarboxylate ligands oxalate, malonate, succinate,... 

Examination of Table 8.9 reveals that aminopolycarboxylic adds complex Am(iii) more strongly than do either hydroxycarboxylic or aminoalkylpolyphos-phoric adds (e.g. ethyIenediaminebis(methyIene)phosphonic add). Keller [3] observes that in the series of a-hydroxycarboxylic adds (e.g. glycolic and lactic) the stability of the ameridum complex decreases with increasing number of carbon atoms. The logarithm of the stability constant of the complexes of Am with aminopolycarboxylic adds increases linearly (Fig. 8.9) with the number of bound donor atoms of the ligand. 

Stability constants have been measured in solution for a large number of ligands (Moeller et al., 1%5, and Sillen and Martell, 1971). Howeyer, because the rare earth ions are hard acids or class (a) metals, most of these involve at least one oxygen as a donor atom. The carboxylic acids, hydroxycarboxylic acids, and various substituted aminopolycarboxylic acids have been most widely studied. Inorganic ligands, in general, form only very weak complexes or, as in the case of a number of oxyanions, form precipitates. By a judicious choice of experimental conditions the stability constants for a number of these have been measured, however. 

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