Redox method for the determination of stability constants of some trivalent metal complexes

Stability constants determination of very stable metal complexes using the redox method, based on the equilibrium of Fe (3+)Fe (2+) followed by a couple of platinum/reference electrodes, was undertaken and tested to complexes of some trivalent metal ions with well known polyaminopolycarboxymethylated linear ligands (edta, nta, cdta, dtpa and ttha) and also to some new macrocyclic ligands. SUPERQUAD program was used for the calculations, after adaptation of the experimental data. The method proved to be very useful for Fe(3+) and In(3+) complexes, if no polynuclear complexes are formed or/and if the kinetics of the complexation reaction is not very slow. However, for the Ga(3+) complexes the applicability of this method is very limited and the competition with OH(-) using the displacement reaction which occurs at pH higher than 6 with formation of Ga(OH)(4)(-) seems to give more accurate results. A complete data of stability constants for the case of the complexes of ttha with In(3+) is given.     

Stability constants determination of successive metal complexes by hyphenated CE‐ICPMS

The study of radionuclides speciation requires accurate evaluation of stability constants, which can be achieved by CE‐ICPMS. We have previously described a method for 1:1 metal complexes stability constants determination. In this paper, we present its extension to the case of successive complexations and its application to uranyl‐oxalate and lanthanum‐oxalate systems. Several significant steps are discussed: analytical conditions choice, mathematical treatment by non‐linear regression, ligand concentration and ionic strength corrections. The following values were obtained: at infinite dilution, log(β₁°(UO₂Oxa))=6.93±0.05, log(β₂°(UO₂(Oxa)₂^2?))=11.92±0.43 and log(β₃°(UO₂(Oxa)₃^4?))=15.11±0.12; log(β₁°(LaOxa⁺))=5.90±0.07, log(β₂°(La(Oxa)₂^?))=9.18±0.19 and log(β₃°(La(Oxa)₃^3?))=9.81±0.33. These values are in good agreement with the literature data, even though we suggest the existence of a new lanthanum‐oxalate complex: La(Oxa)₃^3?. This study confirms the suitability of CE‐ICPMS for complexation studies.     

An investigation into the determination of stability constants of metal complexes by convolution-deconvolution cyclic voltammetry

A technique is described for calculating stability constants of metal-ligand complexes from convolution-deconvolution voltammetry. Semi-integration of the cyclic voltammetric currents with respect to time allows calculation of E(1 2 ) values in a manner comparable to the use in polarography of the Heyrovsky-Ilkovic equation. The technique described also allows determination of the ratio of the diffusion coefficients of the free and complexed metal ions and provides a second check of the stoichiometry. A reliable route to the metal-complex stability constants by the equations of Lingane and DeFord and Hume is therefore obtained. Advantages of this technique compared with the use of polarography, differential pulse polarography and pH titrations are discussed, with the complexes formed by cadmium with glycine, alanine, valine and aspartic acid as examples.     

Stability constants of 2-mercaptohistamine metal complexes

Summary Stability constants of 2-mercaptohistamine (2-MH) with a number of transition metal ions were determined by a potentiometric titration method and the coordination mode of (2-MH) metal complexes were thereby characterized. With Mn^II, Fe^ll, Co^II, and Ni^II, coordination occurred through both the imidazole-nitrogen and amino groups, while with Zn^II, Cd^II, Pb^II, and Hg^II, both thiolate and amino groups, contribute to the coordination. in the 2-MH copper complex, polymer formation is suggested on the basis of visible absorption and e.p.r. spectra.     

Evaluation of unimolecular rate constants for some outer-sphere electrochemical reactions

Unimolecular rate constants, k_et (s^?1), and transfer coefficients, α_cet, for the outer-sphere electroreduction of several Co(III) ammine and ethylenediamine complexes have been evaluated by electrostatically adsorbing the reactants at silver electrodes coated with chloride or bromide monolayers. Sufficiently strong diffuse-layer adsorption is thereby produced so to enable k_cet and α_cet to be determined by means of linear sweep voltammetry, employing sufficiently fast sweep rates (10–100 V s^?1) and dilute reactant concentrations (≤ 50 μM) so that the bulk solution reactant contributes negligibly to the observed faradaic transients. Comparison with corresponding rate constants and transfer coefficients for the solution reactants enables the influence of precursor-state stability upon the latter rate parameters to be assessed.     

Catalytic-thermometric determination of stability constants of complexes using the iodine-azide reaction

A catalytic-thermometric method for the determination of the stoichiometry and stability constants of complexes with sulphur-containing ligands is proposed. The method is based on the inhibition of the catalytic activity of the ligands on the iodine-azide reaction when a metallic ion is present. A study of the errors associated with the method has been performed and used to improve experimental design. The conclusions have been applied to the determination of the stoichiometry and the stability constants of the Ni(II)-DDTC and Ni(II)-PDTC complexes.     

Stability constants of some bivalent metal complexes of N-phenyl-o-tolylbenzohydroxamic acid

The stability constants of the Cu²⁺, Zn²⁺, Ni²⁺ and Mu²⁺ complexes of N-phenyl-o-methoxybenzohydroxamic acid at 30° in 50% v/v aqueous dioxan are: log K₁ 10·45, 8·16, 7·52, 6·33; log K₂ 8·90, 6·70, 6·01, 5·59 (for the ions in the order given).     

Evaluation of stability constants of metal complexes with sulphonated azo-ligands

An ion-exchange procedure is described for the determination of the stability constants for cadmium, copper, nickel and manganese complexes with three sulphonated azo ligands: calmagite, alizarin violet N and palatine chrome black. The results, log K values, show a good agreement with those obtained by a spectrophotometric technique.     

Spectrofluorimetric determination of stoichiometry and association constants of the complexes of harmane and harmine with beta-cyclodextrin and chemically modified beta-cyclodextrins

The association characteristics of the inclusion complexes of the beta-carboline alkaloids harmane and harmine with beta-cyclodextrin (beta-CD) and chemically modified beta-cyclodextrins such as hydroxypropyl-beta-cyclodextrin (HPbeta-CD), 2,3-di-O-methyl-beta-cyclodextrin (DMbeta-CD) and 2,3,6-tri-O-methyl-beta-cyclodextrin (TMbeta-CD) are described. The association constants vary from 112 for harmine/DMbeta-CD to 418 for harmane/HPbeta-CD. The magnitude of the interactions between the host and the guest molecules depends on the chemical and geometrical characteristics of the guest molecules and therefore the association constants vary for the different cyclodextrin complexes. The steric hindrance is higher in the case of harmine due to the presence of methoxy group on the beta-carboline ring. The association obtained for the harmane complexes is stronger than the one observed for harmine complexes except in the case of harmine/TMbeta-CD. Important differences in the association constants were observed depending on the experimental variable used in the calculations (absolute value of fluorescence intensity or the ratio between the fluorescence intensities corresponding to the neutral and cationic forms). When fluorescence intensity values were considered, the association constants were higher than when the ratio of the emission intensity for the cationic and neutral species was used. These differences are a consequence of the co-existence of acid-base equilibria in the ground and in excited states together with the complexation equilibria. The existence of a proton transfer reaction in the excited states of harmane or harmine implies the need for the experimental dialysis procedure for separation of the complexes from free harmane or harmine. Such methodology allows quantitative results for stoichiometry determinations to be obtained, which show the existence of both 1:1 and 1:2 beta-carboline alkaloid:CD complexes with different solubility properties.     

About the calculation of exchange coupling constants in polynuclear transition metal complexes

The application of theoretical methods based on the density functional theory with hybrid functionals provides good estimates of the exchange coupling constants for polynuclear transition metal complexes. The accuracy is similar to that previously obtained for dinuclear compounds. We present test calculations on simple model systems based on H. He and CH(2). He units to compare with Hartree-Fock and multiconfigurational results. Calculations for complete, nonmodeled polynuclear transition metal complexes yield coupling constants in very good agreement with available experimental data.     

The determination of the ionisation constants of polymaleic acid and the stability constants of its complexes with copper

The ionisation constants of unfractionated polymaleic acid, H₃A, a synthetic analogue of fulvic acid, and the stability constants of its copper complexes were determined by differential pulse polarography. The ionisation constants are: pK₁ 5.16, pK₂ 7.04 and pK₃ 9.82 at 20° C and ionic strength 0.12 M. The stability constant of the CuHA complex, which is the predominant species at pH 5–9, lies in the range log (CuHA) = 7.15–8.7; the mean value is 8.17.     

A Method for the polarographic determination of the stability constants of mixed complexes

A new method for determining the stability constants of mixed complexes is proposed which is especially appropriate when only the mixed complex 1:1:1 is formed. In order to verify this method, the coordinate system Pb(II)-SCN^?-NO^?₃ has been studied polarographically in aqueous medium of a constant ionic strength of μ = 1.0 M. Using our knowledge of the stability constants of simple complexes and applying this method we obtain β₁₁ = 8 for the mixed complex.     

Vibrational characteristics of outer-sphere surface complexes: example of sulfate ions adsorbed onto metal (hydr)oxides

The vibrational characteristics of outer-sphere complexes of sulfate at several mineral oxide-water interfaces were investigated by in situ attenuated total reflection Fourier transform infrared (ATR FT-IR) spectroscopy. In the IR spectra obtained from surface outer-sphere complexes, only one peak of the asymmetric stretching vibrational mode υ(3) similar to that of free sulfate ion SO(4)(2-) in aqueous solution is observed. However, on the investigated (hydr)oxide surfaces of Al(3+), Ti(4+), Fe(2+/3+), Cr(3+), Ce(4+), Cu(2+), Y(3+), Zn(2+), and Nd(3+), a shift of up to 14 cm(-1) was found, which was correlated to the polarizing power of the metal cations. A high polarizing power was found to result in a stronger shift of υ(3) compared to that of the aqueous SO(4)(2-) ion. Furthermore, the impact of the metal oxide structure on the characteristics of the formed outer-sphere complex was negligible because different Al and Fe (hydr)oxides did not show any changes in the respective IR spectra. Finally, the ionic strength (1-10(-4) M) and pH (6.8-3.1) have been modified to change the surface potential, showing no direct influence on the spectra (i.e., on the geometry of the outer-sphere complex).     

Proton nuclear magnetic resonance and infra-red studies of the outer-sphere hydration of pentane-2,4-dionato metal complexes in benzene solution

Studies of hydrogen bonding between water and bis(pentane-2,4-dionato)-beryllium(II) (Be(acac)₂), bis(3-methylpentane-2,4-dionato)-beryllium(II) (Be(Meacac)₂), and tris(pentane-2,4-dionato)cobalt(III) (Co(acac)₃) have been undertaken in [²H]₆-benzene solution using¹H NMR and infra-red spectroscopy. Equilibrium constants for 1:1 water-metal complex hydrates, and approximate enthalpies and entropies of reaction, have been measured from the¹H NMR data. The equilibrium constants are larger for Co(acac)₃ than for the Be(II) complexes, in line with previous data for the partition of these chelates between water and aliphatic hydrocarbon solvents.     

Data analysis in the determination of stoichiometries and stability constants of complexes.

The determination of stoichiometries and stability constants of complexes by means of UV-visible spectrophotometry applying traditional methods does not evaluate the quality of the values obtained, since the classic application of these methods does not provide coverage intervals. However, the use of chemometric techniques in different steps of the application of these methods makes it possible to obtain not only a real value of the characteristics of the complexes but a validation of such a value. In this paper a methodology is proposed that combines some traditional methods, three different regression models (LMSR, LSR and LSPR) and a small number of mathematical algorithms. This methodology is able to estimate, in a simple and rigorous way, the stoichiometry and stability constant of a complex and its corresponding uncertainties.