The aqua Al(III) ion is the "hardest" of the trivalent ions commonly found in the environment and in biological systems. Its effective ionic radius is 54 Å, which is smaller than other commonly encountered trivalent metal ions. Therefore the aqua Al(III) ion has the highest charge/size ratio and high affinity for hard anions. As a result, it has a strong tendency to hydrolyze in aqueous solution and its coordination equilibria contain rather complicated hydroxo complexes. 1 2,5) investigated the relations between the basicities of these oxygen donors and the stabilities of their Al(III) complexes. They concluded that the logarithm of protonation constants of monodentate donors and pK sum of bi-or terdentate donors provide a quantative measure for the hardness of each donor. The order of decreasing basicities along with the summation of their approximate logarithms of protonation constants are proposed to be: (catecholate ca. 22)Ͼ(aliphatic monohydroxy acid anion ca. 18) Ͼ(aromatic hydroxy acid anion ca. 16)Ͼ(alkoxide ca. 14)Ͼ(phenoxide ca. 10)Ͼ(carboxylate ca. 4). Therefore the complexes of Al(III) ion that are sufficiently stable to hydrolytic reactions and consequent precipitation in aqueous solution should contain ligands with the highest pK sum.The complexes of catechol derivatives with Al(III) ion including metal ions such as Fe(II), Fe(III), Mg(II), Ca(II), Mn(II), Co(II), Ni(II), Cu(II), Zn(II), Pb(II), Cd(II), UO 2 (II), and VO(II) were studied by various researchers. 1,2,5-26) They determined that two or three moles of catechol derivatives are coordinated with one mole of these metal ions in their complexes. In particular, the constants of formation equilibria of Al(III) complexes of catechol derivatives have been the subject of a number of investigations. 1) Some of the hydroxy aromatic ligands are interesting three-protic ligands (H 3 L), 2,3-dihydroxybenzoic acid (2,3-DHBA), 3,4-dihydroxybenzoic acid (3,4-DHBA), 3,4-dihydroxyhydrocinnamic acid (3,4-DHHCA), and 3,4-dihydroxyphenylacetic acid (3,4-DHPA) are typical examples and they have affinities for metal ions to form stable complexes. 19,20,21,26) Some of their complexes do not hydrolyze and precipitate in aqueous solution, since 3,4-DHBA, 3,4-DHHCA, and 3,4-DHPA have one carboxylate binding site besides the catecholate sites that are in chelatable positions. 2,3-DHBA contains three potential binding sites on three adjacent ring carbons. It is a good model of the competitive salicylate (COO Ϫ , O Ϫ ) and catecholate type (O Ϫ , O Ϫ ) chelation in the same molecule. 5) Kennedy and Powell 7) investigated Al(III): catechol and 3,4-DHBA complexes using potentiometry in aqueous solution in Iϭ0.1 mol · l Ϫ KCl ionic medium at 25°C. They reported the stability constants for the mononuclear diphenolate complexes of Al(III) with catechol such as AlL, AlL 2 , and AlL 3 . They also indicated that monoand bis-complexes of catechol become hydrolyzed and then AlL(OH) Ϫ , and AlL 2 (OH) 4Ϫ hydroxo complexes formed. In the case of 3,4-DHBA, the existence of Al(HL)...

The equilibrium reactions of scandium(III) with some triprotic catechol derivatives (H 3 L) were studied. The selected ligands that are 2,3-dihydroxybenzoic acid (2,3-DHBA), 3,4-dihydroxybenzoic acid (3,4-DHBA), 3.4-dihydroxyhydrocinnamic acid (3,4-DHHCA), and 3,4-dihydroxyphenylacetic acid (3,4-DHPA) were investigated in aqueous solution by means of potentiometry in 0.1 M ionic medium at 25 ° C. The stability constants are reported for the ScL and ScL ( H 2 L ) -mononuclear complexes. 2,3-DHBA can bind to Sc 3+ ion strongly and the salicylate mode (COO -, O -) is effective over the acidic pH range. But in higher pH range, 3,4-DHBA, 3,4-DHHCA, and 3,4-DHPA act more efficiently through catecholate groups (O -, O -).

The binaryÀligand complexes formed between acetohydroxamic acid (Aha) and benzohydroxamic acid (Bha) ligands and selected transition metals (Sc(III), Y(III), and La(III)) were investigated potentiometrically in aqueous solution at (25 ( 0.1) °C and I = 0.10 M NaCl to determine the protonation constants of the free ligands and stability constants of the binary complexes. The complexation model for each metalÀligand system was established from the potentiometric data using the software program BEST. The concentration distributions of the various metalÀligand complexes formed in solution were also evaluated. The complex stability was found to follow the trend Sc(III) > Y(III) > La(III) based on the ionic radius of the metal ions.

The interactions of Sc 3+ and Y 3+ ions with disodium 1,2-dihydroxybenzene-3,5-disulfonate (Na 2 H 2 L, where H 2 L 2-= Tiron) were investigated in aqueous solution by means of potentiometric and spectroscopic methods. The coordination of Tiron to Sc 3+ and Y 3+ takes place through two phenolic oxygen atoms of catecholate ion in different stoichiometries. Thus, the binding of Tiron to Sc 3+ occurs either in 1 : 1 or 1 : 2 molar ratios; they have resulted by the formation of [ ScL ] -and [ScL 2 ] 5-type complexes, respectively. On the other hand, Y 3+ ion behaves like Th 4+ and Ln 3+ ions toward Tiron. It forms [YL] -type complex in 1 : 1 molar ratio; but in 1 : 2 or in higher molar ratios, only unique [ Y 2 L 3 ] 6-type complex formation takes place. The formation constants of [ ScL ] -, [ ScL 2 ] 5-, [ YL ] -, and [ Y 2 L 3 ] 6-complexes were determined by analysis of the potentiometric data in ionic medium of 0.1 M KNO 3 or NaClO 4 at 25° C. The hydrolytic reactions of Sc(III) and Y(III) complexes with Tiron were determined from potentiometric data and the formation constants of [ ScL ( OH )] 2-and [ YL ( OH )] 2-were also calculated.g°

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