Effective Stability Constants

Table 2 illustrates how these adjustments influence the value of the conditional constants for complexation of DMA by Fe +, Zn +, Cu + and Ni +. With Fe +, the predominance of FeDMA(H j) leads to an effective stability constant that is —10" greater than the stability constant for FeDMA formation (10 vs. 10 " ). In comparison, the other three metals form only 1 1 complexes with DMA, hence there is no adjustment for the MCT. The second adjustment accounts for competition from hydroxyl ligands. Thus, because of the highly hydrolytic character of Fe(III), the conditional constant for Fe(III)DMA is lowered by about 10 compared with the other metals being adjusted by a factor of... 

An 1,8-naphthyridine q-aminonitrile moiety serves both as an effective donor-acceptor array for complexation of creatinine and as an intrinsic chromophore and fluorophore. In the pH range of 4.1-4.6 the monoprotonated form apparently predominates in 70 % aqueous methanol, producing the absorption spectrum shown in Figure 14. Under these conditions creatinine exists as a mixture of protonated and unprotonated forms, since its pK is approximately 4.2 in this solvent mixture. Such proton-transfer equilibria complicate the calculation of specific stability constants, but under buffered conditions absorption and emission changes result only from complexation, not from proton transfer. As shown in Figure 14, addition of creatinine to a buffered solution decreases the intensity of the 442 nm absorption band attributed to the protonated receptor. Creatinine complexation also quenches the yellow-green fluorescence of the protonated receptor and titration experiments in progress may yield the effective stability constant of the complex. This receptor exemplifies the manner in which intrinsic chromophores and fluorophores may be incorporated into hosts for reversible complexation of clinically important analytes (26). 

Kmi is the apparent, conditional, or effective stability constant. It is not a real constant since it depends on experimental conditions (here pH). From eqn [5],... 

A comparison of values for bi- (n = 3) and hexadentate (n = 1) chelators can be misleading. For example, log P of deferiprone is 35.9 but the log of the third stepwise formation constant given by log(P /p2) is only 9.7 (Motekaitis and Martell 1991). Also, this definition of stability constant does not take into account the different acidities of the ligands and the ability of iron to compete for them with proton. Protonation of the ligand and hydrolysis of the metal, as well as competition with other metals and ligands in biological systems, complicate the interpretation of stability constants. Therefore, in comparing the stability of iron chelates it is useful to introduce the additional terms iTeff and pM. Martell has defined an effective stability constant for Fe complexes based on competition for the... 

Figure 3.11 Effective stability constants K", as functions of coordinate x. The thickness of the hypothetical reaction layer, 6, is indicated with crosses. Its dependence on fc, determined by Eq. (3.38) is shown in the inset (logarithmic presentation).

Figure 3.12 Effective stability constants IC in the case of different charge transfer reactions the reduction of M+ aqua complexes (a) and that of ML+ species (b).

Figure 3.13 Effective stability constants P and concentration profiles of M ions (inset). The thickness of the reaction layer is indicated with crosses. The thickness of the...

NMR signals of the amino acid ligand that are induced by the ring current of the diamine ligand" ". From the temperature dependence of the stability constants of a number of ternary palladium complexes involving dipeptides and aromatic amines, the arene - arene interaction enthalpies and entropies have been determined" ". It turned out that the interaction is generally enthalpy-driven and counteracted by entropy. Yamauchi et al. hold a charge transfer interaction responsible for this effect. 

In view of the magnitude of crystal-field effects it is not surprising that the spectra of actinide ions are sensitive to the latter s environment and, in contrast to the lanthanides, may change drastically from one compound to another. Unfortunately, because of the complexity of the spectra and the low symmetry of many of the complexes, spectra are not easily used as a means of deducing stereochemistry except when used as fingerprints for comparison with spectra of previously characterized compounds. However, the dependence on ligand concentration of the positions and intensities, especially of the charge-transfer bands, can profitably be used to estimate stability constants. 

By considering the stability constant and the lipophilicity of host molecules, Fyles et al. synthesized a series of carboxylic ionophores having a crown ether moiety and energetically developed the active transport of alkali metal cations 27-32). Ionophores 19-21 possess appropriate stability constants for K+ and show effective K+-selective transports (Fig. 5). Although all of the corresponding [15]crown-5 derivatives (22-24) selectively transport Na+, their transport rates are rather slow compared with... 

In equation (q) only the fully ionised form of EDTA, i.e. the ion Y4 , has been taken into account, but at low pH values the species HY3, H2Y2, H3 Y and even undissociated H4Y may well be present in other words, only a part of the EDTA uncombined with metal may be present as Y4. Further, in equation (q) the metal ion M"+ is assumed to be uncomplexed, i.e. in aqueous solution it is simply present as the hydrated ion. If, however, the solution also contains substances other than EDTA which can complex with the metal ion, then the whole of this ion uncombined with EDTA may no longer be present as the simple hydrated ion. Thus, in practice, the stability of metal-EDTA complexes may be altered (a) by variation in pH and (b) by the presence of other complexing agents. The stability constant of the EDTA complex will then be different from the value recorded for a specified pH in pure aqueous solution the value recorded for the new conditions is termed the apparent or conditional stability constant. It is clearly necessary to examine the effect of these two factors in some detail. 

The extent of hydrolysis of (MY)(n 4)+ depends upon the characteristics of the metal ion, and is largely controlled by the solubility product of the metallic hydroxide and, of course, the stability constant of the complex. Thus iron(III) is precipitated as hydroxide (Ksal = 1 x 10 36) in basic solution, but nickel(II), for which the relevant solubility product is 6.5 x 10 l8, remains complexed. Clearly the use of excess EDTA will tend to reduce the effect of hydrolysis in basic solutions. It follows that for each metal ion there exists an optimum pH which will give rise to a maximum value for the apparent stability constant. 

Complexation of Pu is discussed in terms of the relative stabilities of different oxidation states and the "effective" ionic charge of Pu0 and Pu02+2. An equation is proposed for calculating stability constants of Pu complexes and its correlation with experimental values demonstrated. The competition between inner v outer sphere complexation as affected by the oxidation state of Pu and the pKa of the ligand is reviewed. Two examples of uses of specific complexing agents for Pu indicate a useful direction for future studies. 

One of several studies of this system shows remarkable agreement with the present results. Fardy and Pearson (3) investigated this system by cation exchange at 2 M acidity and ionic strength and reported uncorrected stability constants 8i = 278 (+ 8) and B2 = 6.8 (+0.2) x 103 (K2 24). Assuming the difference in the medium (2 M HClOi, vs. 1 M NaClO, 1.0 M HClOi ) has a minimal effect on the activities of the various complexes,... 

Transition-metal complexes span an enormous range of stabilities. One of the principal aims of this chapter is to attempt to understand some of the factors which control these, and to determine the importance of ligand-field effects. Very extensive compilations of stability constants are available. 

The Chelate Effect and Polydentate Ligands 147 Table 8-1. Stability constants for some nickel(ii) complexes of ammonia and 1,2-diaminoethane. 

Some typical stability constant data are presented in Fig. 8-14, whilst Fig. 8-15 shows some biological manifestations that illustrate the ubiquity of the effect. The sequence is known as the Irving-Williams series. 

Table 6. Free calcium concentrations in equilibrium with common complexing agents. A low free calcium concentration implies effective complexation, whether the complex formed is soluble or insoluble. The data were derived from either stability constants (soluble complexes) or solubility products (insoluble complexes).

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