Metal-ligand complexes stability constants

Table 3.3 shows some stability constants for hard and soft types of metal ligand complexes. The constants are experimental values [1,2] and, as such, contain some uncertainties. Nevertheless they support well the HSAB principle. 

Twenty years ago the main applications of electrochemistry were trace-metal analysis (polarography and anodic stripping voltammetry) and selective-ion assay (pH, pNa, pK via potentiometry). A secondary focus was the use of voltammetry to characterize transition-metal coordination complexes (metal-ligand stoichiometry, stability constants, and oxidation-reduction thermodynamics). With the commercial development of (1) low-cost, reliable poten-tiostats (2) pure, inert glassy-carbon electrodes and (3) ultrapure, dry aptotic solvents, molecular characterization via electrochemical methodologies has become accessible to nonspecialists (analogous to carbon-13 NMR and GC/MS). 

EDTA Must Compete with Other Ligands To maintain a constant pH, we must add a buffering agent. If one of the buffer s components forms a metal-ligand complex with Cd +, then EDTA must compete with the ligand for Cd +. For example, an NH4+/NH3 buffer includes the ligand NH3, which forms several stable Cd +-NH3 complexes. EDTA forms a stronger complex with Cd + and will displace NH3. The presence of NH3, however, decreases the stability of the Cd +-EDTA complex. 

Complex stability constants are most often determined by pH-potentiometric titration of the ligand in the presence and absence of the metal ion.100 This method works well when equilibrium is reached rapidly (within a few minutes), which is usually the case for linear ligands. For macrocyclic compounds, such as DOTA and its derivatives, complex formation is very slow, especially for low pH values where the formation is not complete, therefore a batch method is... 

Martell, A. E. 1964. Section II Organic ligands in stability constant of metal-ion complexes. Special Publication 17. London The Chemical Society. 

Formally, complexation stability constants in water (log K) and extraction constants (logA j.J can be related via partitioning coefficients of the free ligand and its complex between the aqueous and organic phases.16 However, the latter are rarely available, and therefore the relationships between log/f and log Ktx are not widely used. Nevertheless, in many cases, the binding ability of ligands to metal in complexation and extraction processes follows the same trend. In this respect, information... 

For species distribution calculations stability constants of metal-ligand complexes are required (e.g. Stiff, 1971 Bilinski et al., 1976 Wedborg, 1979 5 Simoes Goncalves et al., 1981). The accuracy of many data is insufficient and moreover not all data of the potentially important equilibria are known, resulting in large discrepancies in species distribution suggested by various authors (Tables 2 and 3). 

How Does the Nature of the Metal Ion Influence Complexation If the metal-ligand complex is the result of electrostatic attraction between the metal ion and the ligand, then both charge and metal ion size are influential. For example, the stability constants (KM]) for phosphate complexes are as follows ... 

P. W. Linder, Experimental Techniques for Determining Complex Stability Constants, in Handbook of Metal-Ligand Interactions in Biological Fluids , ed. G. Berthon, Marcel Dekker Inc., New York, Basel, Hong Kong, 1995, Vol. 1, p. 524. 

O Sullivan, W., and Smithers, G. W. (1979), Stability constants for biologically important metal-ligand complexes. In "Method.s in Enzymology" (D. L, Lynch, ed-), Vol- 63, pp. 294-336. Academic Press, San Diego. 

In the case of a polyprotic acid for which the individual ionizations are well separated (ideally, by at least 3 log units), values for the individual constants can be calculated from data points in the appropriate regions of the titration curve. If the individual ionizations overlap, the Bjerrum fi (n-bar) method may be used. This mathematical approach was introduced by Bjerrum for the calculation of stability constants of metal-ligand complexes, but it can also be applied to the determination of proton-ligand equilibrium constants. 

Figure 6.10. Complex formation of Fe(III) and of Cu(II) by various ligands The Lability cannot be predicted alone from complex stability constants but competitive effects of (with metal ions) and of OH (with ligands) need to be considered. Multidentate complex formers form more stable complexes, especially at high dilutions, than monodentate ligands (e.g., F, NH3). To solutions of TOTFe(III) = 10 M and TOTCu(II) = 10 M, respectively, complex formers (at the concentrations indicated in the figures) were added (points are calculated). (Cit = citrate, gly = gly

Many studies have been carried out concerning the stability constants of humic and fulvic acid complexes. Stability constants vary considerably with pH and ionic strength and this, together with the variable nature of the ligands involved, accounts for the range of values reported for individual metal ions in the literature. However, the stabilities of divalent metal complexes generally follow the well-known Irving-Williams order Mg < Ca < Mn < Co < Zn Ni < Cu < Hg. 

The selectivity of a ligand for one metal ion over another refers to the stability of the complexes formed. The higher the formation constant of the metal-ligand complex, the better the selectivity of the ligand for the metal relative to similar complexes formed with other metals. 

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