Metal ligand stability constants

The metal-ligand stability constants in aqueous solutions for different metal ions, including Am(III) were determined262 using some neutral donors in combination with HTTA as the extraction agent for the uncomplexed metal ion. 

A linear relationship has been established414 between the pKa s and the metal-ligand stability constants for a series of VO24 complexes with substituted acetophenones. 

Now equations 4.46 and 4.48 can be used to reexpress the equation for the metal-ligand stability constant, ATml> in a form that directly considers pH effects. It then becomes evident that, if the pH is raised to an alkaline value, the rise in OH activity and fall in H activity have opposing effects on metal complexation. The lowered metal ion activity, (M ), resulting from precipitation as the hydroxide, disfavors complexation. Conversely, the raised free ligand activity, (L ), resulting from dissociation at higher pH favors complexation. At extreme pH, complexation diminishes because of metal hydroxide precipitation (high pH) and association of the... 

Now we are ready to consider the practical application of metal-ligand stability constants to metal adsorption on organic matter. First we will take the case of a metal-humus system that is undersaturated with respect to the metal hydroxide. 

Agrawal YK and Patel DR, Thermodynamic proton-ligand and metal-ligand stability constants of some drugs, /. Pharm. Sci.,... 

The thermodynamic proton-ligand (pJ a) metal-ligand stability constants of clioquinol, clofibrate, nitrofurazone, and tetracycline with Cu", Zn, Mn", Mg" and Ca" have been determined at 35 °C in 50% ethanol-water media. An empirical pH correction for... 

USE Metal-ligand stability constants of complexes of 2-hydroxy-4-methylpro-piophenone,... 

Conditional Metal—Ligand Formation Constants Recognizing EDTA s acid-base properties is important. The formation constant for CdY in equation 9.11 assumes that EDTA is present as Y . If we restrict the pH to levels greater than 12, then equation 9.11 provides an adequate description of the formation of CdY . for pH levels less than 12, however, K overestimates the stability of the CdY complex. 

The ligand pATa values and transition metal chelate stability constants of arylisoxazoles were detected photometrically and the stability of the complexes studied (79JlCi25i). 

Compartmental pyrazole ligands (14) can be used to form bimetallic complexes (15).157 This ligand enforces a separation distance between the metal ions. Stability constants were calculated showing a stability for both mononuclear and dinuclear compounds that was less than Cu11 but greater than Nin. 

Since the strength of the metal-ligand bond is directly related to the metal-ligand stretching constant, it is expected that the v(M—O) vibrations will be in the order Al, Ga, In > alkali earth > alkali metals . Correlations were found between the stability constants... 

It is important to note that although equation 7 can provide much greater calculated M,j,/[M] values than equation 1, It is, nevertheless, expected that equation 7 will provide underestimates of the total to free metal ratio. This conclusion is based on the observation ( 1) that equation 6, in the vast majority of cases, underestimates the magnitude of mixed ligand stability constants. 

Munze (19) has used a Born-type equation to calculate stability constants of In(III) and An(III) complexes of carboxylates as well as other ligands which agreed well with experimental values. His approach was modified by allowing the dielectric constant to be a parameter (the "effective" dielectric constant, De) in an analysis of fluoride complexation by M(II), M(III) and M(IV) cations (20). A value of De = 57 was found satisfactory to calculate trivalent metal fluoride stability constants which agreed with experimental values for Ln(III), An(III) and group IIIB cations (except Al(III). Subsequently, the equation was used... 

The relative ability of the transition metal ions to form complex ions is Mn2+ < Fe2+ < Co2+ < Ni2+ < Cu2+ > Zn2+ for the divalent cations and Cr3+ — Mn3+ > Fe3+ < Co3+ for the trivalent cations. The strongest complexing divalent cation is Cu(II). Fe(III) is the weakest complexing trivalent transition metal ion, but is stronger than other trivalent cations such as Al3+ and the lanthanides. The heats of hydration (Table 3.2), strengths of EDTA complexes (Table 3.5), and solubility products of metal hydroxyoxides (Table 3.3) also follow this general order, with water, EDTA, and OH" as the respective ligands. Stability constants less than I09 indicate the weaker ion-ion interaction of ion pairs. 

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