Stability constants dependence

The factors that determine the value of the stability constant depend on the nature of the metal and ligand. Attempts at introducing a systematic approach in predicting the stability of complexes have shown that certain ligand atoms prefer to bind particular metal ions. The various concepts for the rationalisation of this preference have been developed by Sidgwick Ahrland et al. and more recently Pearson Although these concepts contain some unresolved contradictions, they are nonetheless useful in giving an idea of the type of complex expected and the possible sequence of the stability. 

Malley and Mellor (17) have shown that for a given ligand the stability constants depend on the central metal. For convenience the values of the constants are frequently quoted on a logarithmic scale. Thus, for 8-hydroxyquinoline, the logarithmic stability constants, log K2, of some divalent, first-row transition metal oxinates are ... 

The stability constants of some CyD complexes estimated by HPLC and GC are collected in Table 5.1. It is clear that the stability constants depend on the structure of the guest compound, on the kind of CyD, and the solvent and additives applied. Very interesting information was obtained from HPLC systems with the CyD added to the mobile phase. The change in retention factors of each compound... 

Chemists use the stability constant to characterize the strength and stability of coordination complexes. The stability constant depends on two factors, and they are thermodynamic and kinetic stability. 

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 metal chelate adsorption chromatography a metal is immobilised by partial chelation on a column which contains bi- or tri- dentate ligands. Its application is in the separation of substances which can complex with the bound metals and depends on the stability constants of the various ligands (Porath, Carlsson, Olsson and Belfrage Nature 258 598 I975 Loennerdal, Carlsson and Porath FEES Lett 75 89 1977). 

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. 

The factor at can be calculated from the known dissociation constants of EDTA, and since the proportions of the various ionic species derived from EDTA will be dependent upon the pH of the solution, a will also vary with pH a plot of log a against pH shows a variation of logoc = 18 at pH = 1 to loga = 0 at pH = 12 such a curve is very useful for dealing with calculations of apparent stability constants. Thus, for example, from Table 2.4, log K of the EDTA complex of the Pb2+ ion is 18.0 and from a graph of log a against pH, it is found that at a pH of 5.0, log a = 7. Hence from equation (30), at a pH of 5.0 the lead-EDTA complex has an apparent stability constant given by ... 

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. 

The physical nature of the sulfate complexes formed by plutonium(III) and plutonium(IV) in 1 M acid 2 M ionic strength perchlorate media has been inferred from thermodynamic parameters for complexation reactions and acid dependence of stability constants. The stability constants of 1 1 and 1 2 complexes were determined by solvent extraction and ion-exchange techniques, and the thermodynamic parameters calculated from the temperature dependence of the stability constants. The data are consistent with the formation of complexes of the form PuSOi,(n-2)+ for the 1 1 complexes of both plutonium(III) and plutonium(IV). The second HSO4 ligand appears to be added without deprotonation in both systems to form complexes of the form PuSOifHSOit(n"3) +. ... 

The low TTA dependence at 35.0°C probably is attributable to dissolution of TTA in the aqueous phase. Observation of fourth-power dependence on acidity argues against any change in the extraction mechanism (e.g., Pu(IV) reduction or NO3 involvement). An aqueous Pu(TTA)3+ complex has been reported (14, 15) and this possibility has been considered in the error analysis of the Pu(IV)-sulfate stability constants. 

Cathodic electrodeposition of microcrystalline cadmium-zinc selenide (Cdi i Zn i Se CZS) films has been reported from selenite and selenosulfate baths [125, 126]. When applied for CZS, the typical electrocrystallization process from acidic solutions involves the underpotential reduction of at least one of the metal ion species (the less noble zinc). However, the direct formation of the alloy in this manner is problematic, basically due to a large difference between the redox potentials of and Cd " couples [127]. In solutions containing both zinc and cadmium ions, Cd will deposit preferentially because of its more positive potential, thus leading to free CdSe phase. This is true even if the cations are complexed since the stability constants of cadmium and zinc with various complexants are similar. Notwithstanding, films electrodeposited from typical solutions have been used to study the molar fraction dependence of the CZS band gap energy in the light of photoelectrochemical measurements, along with considerations within the virtual crystal approximation [128]. 

The transport of EDTA into a bacterial strain capable of its degradation has been examined (Witschel et al. 1997). Inhibition was observed with DCCD (ATPase inhibitor), nigericin (dissipates ApH), but not valinomycin (dissipates Av /), and was dependent on the stability constant of metal-EDTA complexes. 

The reactivity of (367) and (368) towards phosphine complexes of Ni11 has been investigated. Depending on the nature of the phosphine and the dithiocarbamate used, different complexes were obtained 974 The stability constants for the Ni11 complexes of (369), (370), and (371) have been determined in aqueous solution. Both 1 1 and 1 2 complexes were observed.975... 

The stability constant is dependent, amongst other things, on the solvating medium. For example, for a simple crown ether kc is usually very large and kd also large, but in nonpolar solvents kd is much smaller than kc, so that Ks increases with decreasing polarity of the solvating medium. 

Phase transfer catalysis. As well as their use in homogeneous reactions of the type just described, polyethers (crowns and cryptands) may be used to catalyse reactions between reagents contained in two different phases (either liquid/liquid or solid/liquid). For these, the polyether is present in only catalytic amounts and the process is termed phase transfer catalysis . The efficiency of such a process depends upon a number of factors. Two important ones are the stability constant of the polyether complex being transported and the lipophilicity of the polyether catalyst used. 

Electrochemical experiments allow the determination of complex stability constants for Fe2+ by measuring complex redox potentials over a range of pH values. The Fe34YFe2+ redox potential of the siderophore complex, as with the spectral characteristics of the complex, is dependent on the inner coordination environment of the iron. These considerations will be addressed later (Section III.D). 

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