Stability Constants Of Complexes Research Articles

Not just a background: pH buffers do interact with lanthanide ions\u2014a Europium(III) case study

The interaction between Eu(III) ion and different pH buffers, popular in biology and biochemistry, viz. HEPES, PIPES, MES, MOPS, and TRIS, has been studied by solution nuclear magnetic resonance spectroscopy (NMR) and time-resolved laser-induced fluorescence spectroscopy (TRLFS) techniques. The Good’s buffers reveal non-negligible interaction with Eu(III) as determined from their complex stability constants, where the sites of interaction are the morpholine and piperazine nitrogen atoms, respectively. In contrast, TRIS buffer shows practically no affinity towards Eu(III). Therefore, when investigating lanthanides, TRIS buffer should be preferred over Good’s buffers.Graphical abstract

Isothermal titration calorimetry investigation of the interactions between vitamin B6-derived hydrazones and bovine and human serum albumin

The binding of low molecular weight compounds with the transport proteins of blood is an essential step of their delivery into living cells and thus the accurate investigation of the interactions occurring in solution at physiological conditions is crucial for the development of efficient biologically active molecules. In this work, we report on the complex species, stability constants and thermodynamic parameters for the binding reactions of hydrazones derived from pyridoxal-5ʹ-phosphate (PLP) with bovine and human serum albumin (BSA and HSA) in neutral aqueous solution. The study has been carried out using isothermal titration calorimetry which allowed to directly obtain both binding constant and enthalpy change values for the systems investigated. The thermodynamic characterization in solution revealed that the PLP-hydrazone derivatives are able to effectively interact with both bovine and human serum albumin and enabled the determination of the driving forces for the molecular recognition process. The formation of the 1:1 complex was found to be always enthalpy favored and driven due to the insertion of the hydrazone moieties into the hydrophobic pockets of BSA or HSA.

Complexation of Amino Acids with Cadmium and Their Application for Cadmium-Contaminated Soil Remediation

The interaction of amino acids with toxic heavy metals influences their immobilization and bioavailability in soils. However, the complexation ability of amino acids with Cd has not been well studied. The complexes of amino acids and cadmium were investigated by density functional theory (DFT) calculations and Fourier transform infrared spectrometry (FTIR) analyses. The complex structures were found to be [COc, COc] for fatty amino-cadmium and PheCd2+, [COc, COc, COs] for GluCd2+ and ThrCd2+, respectively. The complex energy of these conformers followed the order PheCd2+> AlaCd2+ > LeuCd2+ > GluCd2+ > GlyCd2+ > ThrCd2+. Importantly, all of the complex energy values were less than zero, indicating that these complexes could be easily dissolved in water. The Cd2+ concentration decreased with increasing amino acid concentration in aqueous solution. The complex stability constants (logβ) followed the order PheCd2+> AlaCd2+ > LeuCd2+ > GluCd2+ > GlyCd2+ > ThrCd2+, consistent with the order of the calculated complex energy values. The Cd removal efficiencies by Thr, Glu, Gly, Ala, Leu, and Phe were 38.88%, 37.47%, 35.5%, 34.72%, 34.04%, and 31.99%, respectively. In soil batch tests, the total Cd concentration in soil decreased in the presence of amino acids, while the Cd concentration in water increased from 231.97 μg/L to 652.94~793.51 μg/L. The results of sequential extraction showed that the acid-extractable fraction and the reducible fraction of Cd sharply decreased. Consequently, the significant features of amino acids along with their biocompatibility make them potentially applicable chelators in Cd-contaminated soil remediation processes.

Complexation Equilibria and Determination of Stability Constants of Some Divalent Metal Ion Complexes of L-Cysteine and Diphenylamine in Aqueous Media

<i>Background. </i>Metal complexes of biologically active ligands had considerable interests. L-cysteinate residue, L-Cys, is a biologically abundant and important versatile binding site of proteins. Diphenylamine, DPA, is an important aromatic amine containing two phenyl groups. Complexation equilibria of the divalent metal ions, Ca²⁺ and Zn²⁺ with the bio- relevant α-amino acid, L-cysteine and the nitrogen-containing diphenylamine ligand were investigated by means of the potentiometric technique at 25.0 ± 0.1°C and constant ionic strength of 0.200 ± 0.001 mol·dm^-3 NaNO₃. <i>Objective.</i> The stability constants and standard free energy changes of the α-amino acid, L-cysteine and diphenylamine complex species were determined at 0.200 ± 0.001 mol·dm^-3ionic strength. <i>Methods.</i> The formation of the different 1:1 and 1:2 binary complexes and 1:1:1 ternary complexes were inferred from the potentiometric titrations. <i>Results.</i> The concentration distribution of L-cysteine species formed in solution was evaluated. The dissociation constants of the α-amino acid and diphenylamine were determined at different ionic strength. The stability constants of these binary and ternary systems were calculated. The values of ∆ log₁₀ K, percent relative stabilization, %R.S. and log₁₀ X for the ternary systems were evaluated and discussed. <i>Concussion.</i> The ternary complex formation occurred in a stepwise manner with L-cysteine acting as the primary ligand. The obtained values of ∆ G⁰ indicated that Complex formation reactions are spontaneous. Also, for all systems studied, the ternary complexes formed are more thermodynamically stable than the binary complexes.

Влияние катамина АБ на комплексообразование скандия с пирокатехиновым фиолетовым

Regularities of the complex formation of scandium with catechol violet in the presence of the cationic surfactant catamine AB in aqueous solutions are considered. The optimum pH interval of scandium complexation with the reagent in systems with and without catamine AB has been determined. The absorption spectra of pyrocatechin vio- let and its complexes with scandium in the double system and in the catamin AB presence at different pH values and surfactant concentrations were taken. The ratio of metal ion and reagent in the forming complex in double and triple systems has been established by methods of saturation and isomolar series. Calibration graphs were constructed, molar light absorption coefficients were calculated, and conditional stability constants of scandium complexes with pyrocate- chin violet were determined by the Babko method.

Спектрофотометрическое определение ионовCo(II) сN-(2-гидроксибензоил)-N'-(п-тозил)гидразином в аммиачных средах

A technique has been developed for the spectrophotometric determination of Co(II) ions with N-(2-hydroxybenzoyl)-N'-(p-tosyl)hydrazine (GBSH) in an ammonia medium. The absorption spectra of the reagent and its complex with Co(II) ions have been recorded and analyzed. The optimal conditions for the formation of the complex compound were found: the pH range of complex formation; optimal time for color development; amount of photometric reagent. Under optimal conditions for the complexation of GBSG with Co(Ⅱ) ions, a calibration graph was constructed. The Bouguer-Lambert-Beer law is fulfilled in the range from 0.3 to 2.4 mg Co(II) in 25 ml of solution. The true and apparent molar light absorption coefficients were calculated. The true molar coefficient of light absorption of the complex compound GBSG with Co(Ⅱ) ions in an ammonia medium is 750. The molar ratios in the solution of the complex compound [Co(Ⅱ)]:[GBSG] = 1:1 and 1:2. The conditional stability constant of the GBSG complex with Co(Ⅱ) ions was calculated using the Babko method and it is 3.24∙1012. The convergence and correctness of the spectrophotometric technique were determined by the method of mathematical statistics.

POTENTIOMETRIC DETERMINATION OF STABILITY CONSTANTS OF METFORMIN COMPLEXES WITH METALS IN AQUEOUS MEDIA

The stability constant of coordination complexes of metformin i.e 1,1- Dimethyl guanidine hydrochlorides with metals such as Sm+2, La+3, Cu+2, and Mn+2 has been studied metrically and in the presence of NaClO4, 27C±0.5. The findings revealed that the sequence of the stabilization constant of the formal complexes increased from La to Cu as La&lt; Sm&lt; Mn&lt; Cu.

Cyclodextrin-Based Supramolecular Ternary Complexes: Emerging Role of Ternary Agents on Drug Solubility, Stability, and Bioavailability.

The aqueous solubility of active drug moiety plays a crucial role in the development of an efficacious formulation. The poor aqueous solubility of BCS class II and IV drugs is manifested as poor bioavailability. Preparation of cyclodextrin inclusion complex to improve the solubility, stability, and bioavailability is a well-established technique. The latest trend in cyclodextrin research is focused on ternary complexes wherein an auxiliary agent such as water-soluble polymers, organic ions, metals, or amino acids is incorporated in the inclusion complex. The cyclodextrin-based supramolecular ternary complex offers significant advantages over binary complex specifically for oral drug delivery. Compared with the binary complex, the ternary complex exhibits better complexation efficiency and stability constant. Moreover, the ternary complex has a major advantage of reducing the concentration of cyclodextrin required to achieve maximum solubility and stability. Lately, in silico molecular modeling has gained tremendous attention as a preliminary tool to evaluate the cyclodextrin-based ternary or binary complex which has been discussed. This review gives an insight into various ternary agents explored worldwide, significant observations, safety, and clinical studies carried out on ternary cyclodextrin complexes.

Determination of Stability Constant of The Complexes of Metal Ion Fe(III), Mn(II) And Cr(III) With Some Substituted 2-Oxo-2H-Chromene-3-Carbohydrazide Derivatives at 0.1 M Ionic Strength at 42oC

Proton ligand and metal ligand stability constant are measured for Fe(III) and 1, 2-dihydroxy benzene and 1, 5-disulphonic acid complexes . Determination of stability constant of substituted of pyrazoles with rare earth metals form complexes are reported . Effect of temperature on formation constants of 2 acetylpyridine(N benzoyl)glycine hydrazone with lanthanide(III) ions at different ionic strengths are reported. Irving and Rossotti, Herson and Gilbert , Wilkins and Lewis and Rossotti and Rossotti have determined stability constant by Bjerrum-Calvin titration technique . Kabadi , Jahagirdar and Narwade have determined pK values of salicylaldehyde, salicylic acid and sulphonic acid respectively by similar procedure. The metal ligand stability constant of some β-diketones are reported . Stability constants have investigated for some substituted pyrazolines, isoxalline and diketone . The method most frequently applied for study of complex equilibria is pH-metric titration technique . Stepwise formation of mononuclear binary complexes is described by set of equilibrium constants. For pH-metric measurements an electrode must be selected. According to Bjerrum, Martell and Calvin , the formation of complex MLN is stepwise process and one has to deal with a series of equilibria of the type: Irving and Rossotti, Herson and Gilbert , Wilkins and Lewis and Rossotti and Rossotti have determined stability constant by Calvin-Bjerrum titration technique . The value of nA,n, pK, log K1 and log K2 are evaluated by Irving and Rossotti's equation. The stability constant and thermodynamic parameters of complexes containing lighter lanthanides , copper(II) , calcium and magnesium are reported. It is also important in environmental studies , medicinal and industrial chemistry . Effects of transition metal on stability constant of complexes have studied by pH metric method . Bendi , Janrao and Ramteke have studied stability constant of ligands with lanthanide metals complexes.The present work involves pH metric study of substituted 2-oxo-2H-chromene-3-carbohydrazide derivatives with metal ions Fe(III), Mn(II), Cr(III) and Ti(III) at temperature42oC. The Bjerrum-Calvin titration technique as modified by Irving and Rossotti has been employed in the present study. The following three sets of titrations are carried out in sequence.

Enhanced transport of heavy metal ions by low-molecular-weight organic acids in saturated porous media: Link complex stability constants to heavy metal mobility

Environment-ubiquitous low-molecular-weight organic acids (LMWOAs) can interact with heavy metal ions and thus affect their mobility in subsurface aquifers. Herein, the effects of LMWOAs (including acetic acid, tartaric acid, malonic acid, oxalic acid, and citric acid) on the mobility of heavy metal ions (including Cd2+, Zn2+, Ni2+, Mn2+, and Co2+) in porous media were investigated to reveal the role of the stability constants of metal-LMWOA complexes in the mobility of heavy metal ions in porous media. The results showed that the mobility of different metal ions followed the order of Cd2+ < Zn2+ < Ni2+ < Mn2+ < Co2+ despite of LMWOAs-free or LMWOAs-addition. For each heavy metal, all the organic acids enhanced its transport by forming stable non-adsorbing metal-LMWOA complexes and the enhanced ability followed the order of citric acid > oxalic acid > malonic acid > tartaric acid > acetic acid. An interesting finding was that there was a significantly positive correlation between the enhanced abilities of LMWOAs to metal mobility and the complex stability constants (log K) (R2 = 0.801–0.961, p < 0.05), indicating that the complex stability of metal-LMWOA was the dominant factor responsible for the enhanced transport of heavy metal ions. Meanwhile, the linear slope indicated the intensity of enhancement of LMWOAs on heavy metal mobility was heavy metal type-dependent. This study proposed that the complex stability of metal-LMWOA could be an indicator to quantify and predict the impact of LMWOAs on the mobility of heavy metals.

New-Generation Carbon-Capture Ionic Liquids Regulated by Metal-Ion Coordination.

Development of efficient carbon capture-and-release technologies with minimal energy input is a long-term challenge in mitigating CO2 emissions, especially via CO2 chemisorption driven by engineered chemical bond construction. Herein, taking advantage of the structural diversity of ionic liquids (ILs) in tuning their physical and chemical properties, precise reaction energy regulation of CO2 chemisorption was demonstrated deploying metal-ion-amino-based ionic liquids (MAILs) as absorbents. The coordination ability of different metal sites (Cu, Zn, Co, Ni, and Mg) to amines was harnessed to achieve fine-tuning on stability constants of the metal ion-amine complexes, acting as the corresponding cations in the construction of diverse ILs coupled with CO2 -philic anions. The as-afforded MAILs exhibited efficient and controllable CO2 release behavior with great reduction in energy input and minimal sacrifice on CO2 uptake capacity. This coordination-regulated approach offers new prospects for the development of ILs-based systems and beyond towards energy-efficient carbon capture technologies.

Stability constants of complexes of metal ions with peatsoil humic acids under non-acid-conditions

Stability constants of complexes of four divalent metal ions viz. Cu2+, Pb2+,Mg2+ and Cd2+ with humic acids (HA) were determined by potentiometric titration of humic acids with the corresponding salt of the divalent metals in aqueous media under non-acid-condition. The log K (logarithm of the stability constant) ranged from 1.0942 to 2.7471 for metal-humic acid complexes were determined using point-wise computational method. The order of stability constants were obtained as follows: Cu &gt;Pb&gt; Cd &gt; Mg for metal -HA complexes respectively, indicating a higher degree of complexation with Cu metal ion.

Synthesis, Magnetic Susceptibility, Thermodynamic Study and Bio-Evaluation of Transition Metal Complexes of New Schiff Base Incorporating INH Pharmacophore

Transition metal complexes of synthesized organic ligands incorporating INH were successfully investigated by potentiometrically. The stability constants of these binary complexes were evaluated and order of stability constant found as above Zn(II) > Co(II) > Ni(II) > Mn(II) > Cu(II). Magnetic moment value clearly indicates that newly prepared transition metal complexes of N-[(E)-(2-Hydroxyphenyl)methylen]isonicotinohydrazid are paramagnetic in nature. The stability constants for complexes decrease with an increasing temperature, indicating that the composition equilibrium is exothermic in nature. Also the change in enthalpy (ΔH) and change in Gibb’s free energy (ΔG) were negative for all of the systems which suggested that all of the reactions were exothermic. The change in entropy (ΔS) found positive for most of the complexes which indicate in metal–ligand binding process is entropy favourable. Metal complexes of organic ligand shows moderate antibacterial activity against gram positive and gram negative bacteria. It also possesses prominent antifungal activity for Nickel (II) complex. Biological evaluation results of transition metal complexes show superior pharmacological applications as compared to organic ligand.

A new fluorescence probe for simultaneous determination of Fe2+ and Fe3+ by orthogonal signal correction-principal component regression

In this research, a newly reported fluorescence molecular probe [N, N’-bis(methylsalicylaldehyde)-4,4 bipyridinium] dichloride (DALD) was used for selective detection and determination of Fe2+ and Fe3+ in aqueous solutions. The complexation reaction between DALD and Fe2+ and Fe3+ is the basis of the variation in the emission intensity and their quantitative fluorescence determination. The stoichiometries and formation constants of DALD and both ions were obtained by appropriate chemical model and numerical refinement of spectrofluorometric-mole ratio titration data. The selectivity of the probe towards Fe2+ and Fe3+ was tested in the presence of Mn+ (n = 1, 2, 3) cations. None of the other ions show any observable fluorescence signal quenching at the excitation and emission wavelengths (345/456 nm). The emission profile of said iron species was in the range of 340–560 nm and has shown a high degree of overlap. The univariate calibration ranges and detection limits of Fe2+ and Fe3+ were 0.7–4.1 μM, 0.7–2.7 μM and 1.3 μM, and 0.69 μM, respectively. The orthogonal signal correction-principal component regression (OSC-PCR) as a factor-based and data compression method was used to determine both cations in aqueous solutions simultaneously. OSC-PCR correlates the fluorescence spectra to the concentration of metal ions very well, and the coefficients of determination were 0.9530 and 0.9919 for Fe2+ and Fe3+, respectively. The solution equilibria of interaction between the probe and metal ions were studied and the stability constants of complexes were 10.68 (± 0.05), 1:2 and 4.46 (± 0.12) 1:1 for Fe2+ and Fe3+ respectively. The factors that influenced sensitivity and selectivity were recognized and optimized.

Cyclic Analogs of Desferrioxamine E Siderophore for 68Ga Nuclear Imaging: Coordination Chemistry and Biological Activity in Staphylococcus aureus.

As multidrug-resistant bacteria are an emerging problem and threat to humanity, novel strategies for treatment and diagnostics are actively sought. We aim to utilize siderophores, iron-specific strong chelating agents produced by microbes, as gallium ion carriers for diagnosis, applying that Fe(III) can be successfully replaced by Ga(III) without losing biological properties of the investigated complex, which allows molecular imaging by positron emission tomography (PET). Here, we report synthesis, full solution chemistry, thermodynamic characterization, and the preliminary biological evaluation of biomimetic derivatives (FOX) of desferrioxamine E (FOXE) siderophore, radiolabeled with 68Ga for possible applications in PET imaging of S. aureus. From a series of six biomimetic analogs, which differ from FOXE with cycle length and position of hydroxamic and amide groups, the highest Fe(III) and Ga(III) stability was determined for the most FOXE alike compounds–FOX 2-4 and FOX 2-5; we have also established the stability constant of the Ga-FOXE complex. For this purpose, spectroscopic and potentiometric titrations, together with the Fe(III)–Ga(III) competition method, were used. [68Ga]Ga-FOXE derivatives uptake and microbial growth promotion studies conducted on S. aureus were efficient for compounds with a larger cavity, i.e., FOX 2-5, 2-6, and 3-5. Even though showing low uptake values, Fe-FOX 2-4 seems to be also a good Fe-source to support the growth of S. aureus. Overall, proposed derivatives may hold potential as inert and stable carrier agents for radioactive Ga(III) ions for diagnostic medical applications or interesting starting compounds for further modifications.

Electrospray-Induced Mass Spectrometry Is Not Suitable for Determination of Peptidic Cu(II) Complexes.

The toolset of mass spectrometry (MS) is still expanding, and the number of metal ion complexes researched this way is growing. The Cu(II) ion forms particularly strong peptide complexes of biological interest which are frequent objects of MS studies, but quantitative aspects of some reported results are at odds with those of experiments performed in solution. Cu(II) complexes are usually characterized by fast ligand exchange rates, despite their high affinity, and we speculated that such kinetic lability could be responsible for the observed discrepancies. In order to resolve this issue, we selected peptides belonging to the ATCUN family characterized with high and thoroughly determined Cu(II) binding constants and re-estimated them using two ESI-MS techniques: standard conditions in combination with serial dilution experiments and very mild conditions for competition experiments. The sample acidification, which accompanies the electrospray formation, was simulated with the pH–jump stopped-flow technique. Our results indicate that ESI-MS should not be used for quantitative studies of Cu(II)–peptide complexes because the electrospray formation process compromises the entropic contribution to the complex stability, yielding underestimations of complex stability constants.

Formulation of ternary genistein β-cyclodextrin inclusion complex: In vitro characterization and cytotoxicity assessment using breast cancer cell line

Genistein (GT) is a poorly water-soluble flavonoid and reported for different biological activity. The present work was designed to prepare the inclusion complex using β-Cyclodextrins (β-CDs) as complexing agent and D -α-Tocopherol polyethylene glycol 1000 succinate (TPGS) as ternary substance to enhance the solubility and in vitro activity. The inclusion complex was prepared by solvent evaporation method and characterized for different physicochemical parameters. The selected GT ternary inclusion complex was further evaluated for in vitro antioxidant and cell viability study against breast cancer cell line. The phase solubility study result data revealed significantly marked enhancement in the solubility of GT. The complexation efficiency (CE) and stability constant (Ks) of GTBIC and GTTIC2 was found to 0.31% and 0.48% as well as 419.67 M-1 and 654.98 M-1, respectively. The saturation solubility results depicted a significant (p < 0.05) enhancement in the saturation solubility (2.61 and 5.27-fold) of GT. Genistein ternary inclusion complex (GTTIC2) prepared with TPGS (0.05%, w/w) showed the maximum release pattern 96.5 ± 3.98% in 90 min. The DSC and XRD spectra showed the transformation of crystalline GT into the amorphous state after complexation. The infrared (IR) and nuclear magnetic resonance (NMR) spectra confirms the formation of inclusion complex by showing changes in the characteristic peaks. GTTIC2 formulation exhibited significantly greater in-vitro antioxidant and cell viability activity against MCF7 cell than pure GT. The improved activity can be attributed to the enhanced solubility of Genistein by effective complexation in β-CDs and TPGS.

Binding strength of mercury (II) to different dissolved organic matter: The roles of DOM properties and sources

Dissolved organic matter (DOM) influences the environmental fate and toxic effects of trace metals such as mercury (Hg). However, because of limits in DOM analytical techniques and lack of sample diversity in past studies, it remains unclear whether the binding strength of DOM complexed with Hg(II) is related to the DOM properties. In this study, different DOM isolates (n = 26) from various sources were used to determine the conditional stability constant (logK) of DOM-Hg complexes using the equilibrium dialysis ligand exchange (EDLE) method. UV–Vis and fluorescence spectrometry were used to evaluate the correlation between logK values and DOM properties, such as chromophoric moieties, aromaticity, and molecular weight. Results demonstrated that the DOM from different sources presented an extensive range of binding strengths to Hg(II), because of their heterogeneous properties. Moreover, DOM chromophores, including aromaticity and molecular weight, are critical indicators of the DOM-Hg affinity in ambient-relevant circumstances. Significantly, higher terrestrial DOM led to greater DOM-Hg affinity. Additionally, this study supports that UV–Vis and fluorescence spectroscopy can be used to estimate DOM composition and its binding strength with Hg(II). Furthermore, the observed relationship between logK and DOM properties provided a possible pathway of explanation for the spatial co-variations between Hg(II) concentrations and DOM characters observed in previous field investigations.

Calculation of stability constants of new metal-thiosemicarbazone complexes based on the QSPR modeling using MLR and ANN methods

In this study, the stability constants (logβ11) of twenty-eight new complexes between several ion metals and thiosemicarbazone ligands were predicted on the basis of the quantitative structure property relationship (QSPR) modeling. The stability constants were calculated from the results of the QSPR models. The QSPR models were built by using the multivariate least regression (QSPRMLR) and artificial neural network (QSPRANN). The molecular descriptors, physicochemical and quantum descriptors of complexes were generated from molecular geometric structure and semi-empirical quantum calculation PM7 and PM7/sparkle. The best linear model QSPRMLR involves five descriptors, namely Total energy, xch6, xp10, SdsN, and Maxneg. The quality of the QSPRMLR model was validated by the statistical values that were R2train = 0.860, Q2LOO = 0.799, SE = 1.242, Fstat = 54.14 and PRESS = 97.46. The neural network model QSPRANN with architecture I(5)-HL(9)-O(1) was presented with the statistical values: R2train = 0.8322, Q2CV = 0.9935 and Q2test = 0.9105. Also, the QSPR models were evaluated externally and achieved good performance results with those from the experimental literature. In addition, the results from the QSPR models could be used to predict the stability constants of other new metal-thiosemicarbazones.

Potentiometer study of the process of complexation of silver (I) ions with thiourea

The process of complexation of silver(I) ions with thiourea (Tu) in the temperature range 287.16&ndash;318.16 K at an ionic strength of 0.11 M (I = 0.1 NaNO3 + 0.01 HNO3) was studied by the potentiometric method using a silver electrode in aqueous solution. The study was performed at a concentration of silver(I) ions CAg+ = 1 &middot; 10&ndash;5 and CAg+ = 1 &middot; 10&ndash;3 M. The values of the stability constants of AgTui + complexes (i = 1&ndash;2) were estimated at CAg+ = 1 &middot; 10&ndash;5 M. The AgTu3+ complex constant was determined at CAg+ = 1 &middot; 10&ndash;3 M and CTu &ge; 0.01 M. The stability constants of complexes were calculated using the Leden method. The values of the complex constants at 298.16 K are equal to: lg&beta;1 = 5.59 &plusmn; 0.10 (AgTu+), lg&beta;2 = 10.62 &plusmn; 0.03 (AgTu2+) and lg&beta;3 = 13.05&plusmn;0.11 (AgTu3+). It is established that the temperature effect does not affect the number of formed particles in the solution; an increase in temperature leads to a decrease in the stability of complexes. The analysis of the values of the stability constants by degrees shows that the step constants AgTu3+ (AgTu2+ + Tu = AgTu3+ + lg&beta;3 = 2.4) are much smaller than the constants for mono- and bis-coordinated complexes. The complexes thermodynamic parameters (&Delta;H0, &Delta;G0, &Delta;S0) were calculated by the temperature coefficient method. In the system under study, all complexation reactions are exothermic. The largest increase in the value of enthalpy (&Delta;H0) is observed in the formation of a complex containing two molecules of thiourea. The change in entropy (&Delta;S0) at the stage of formation of bis- and tris-coordinated complexes is negative, which is most likely due to a decrease in the number of particles in the system under study. It is determined that enthalpy-entropy dependences show a monotonic character. The spontaneous course of complexation reactions is determined by the value of the free Gibbs energy (&Delta;G0). In degrees G0 decreases (&Delta;G0)1 &lt; (&Delta;G0)2 &lt; (&Delta;G0)3) and has a negative value, which indicates a decrease in the affinity of the complexing agent to the ligand.