Challenges in modelling and optimisation of stability constants in the study of metal complexes with monoprotonated ligands: Part I. A glass electrode potentiometric and polarographic study of a Cu-TAPSO-OH system

The behaviour of solutions containing 3-(N-tris[hydroxymethyl]methylamine)-2-hydroxypropanesulfonic acid (TAPSO) and copper(II) was studied by two analytical techniques, direct current polarography (DCP) and glass electrode potentiometry (GEP), at fixed total TAPSO to total copper(II) concentration ratios and various pH values, at 25 °C and ionic strength 0.1 M KNO₃. DCP and GEP, when used independently, were not able to provide a final metal-ligand model. Combined interpretation of data from DCP and GEP indicated the formation of six main species, CuL⁺, CuL(OH), CuL(OH)₂⁻, CuL₂, CuL₂(OH)⁻ and CuL₂(OH)₂²⁻ for which stability constants (as log β) were found to be 4.41, 11.43, 17.55, 8.08, 14.3 and 20.3, respectively. Five of these complexes, CuL(OH), CuL(OH)₂⁻, CuL₂, CuL₂(OH)⁻ and CuL₂ (OH)₂²⁻ are reported for the first time.     

Graphic data analysis and complex formation curves as modeling and optimization tools for characterization of Cu–(buffer)x–(OH)y systems involving BTP or BES in aqueous solution

Bis-tris propane or 1,3-bis(tris(hydroxymethyl)methylamino)propane (BTP) and N,N-bis(2-hydroxyethyl)-2-aminoethanesulfonic acid (BES) are pH buffers which have been used in biological experiments. To characterize BTP and BES complexation properties with Cu(II), glass electrode potentiometry and direct current polarography were conducted using total ligand to total copper concentration ratios of different orders of magnitude and pH values at 25 °C and 0.1 M KNO₃ ionic strength. The graphic analysis is a very powerful tool in the prediction and refinement operations of both systems. For the Cu-BTP system, six species were found to describe totally the system, CuL, CuL(OH), CuL(OH)₂, CuL₂, CuL₂(OH), and CuL₂(OH)₂, with respective stability constants determined as 10.7 ± 0.1, 19.4 ± 0.4, 24.3 ± 0.2, 18.8 ± 0.1, 24.7 ± 0.2, and 29.8 ± 0.2. CuL₂, CuL₂(OH), and CuL₂(OH)₂ were described for the first time. In the case of the Cu-BES system, complexation behavior was described by the model constituted by CuL, CuL(OH), and CuL(OH)₂, the latter two described for the first time, with respective stability constants determined as 3.24 ± 0.08, 10.9 ± 0.2, and 16.0 ± 0.3, respectively. UV–vis results allowed us to establish coordination modes for the Cu-BTP and Cu-BES complexes.     

Complex Formation in the Region of Metal Hydrolysis and M(OH)2 Precipitation. A Glass Electrode Potentiometric and Polarographic Study of Cd–(AMPSO)x–(OH)y and Zn–(AMPSO)x–(OH)y Systems

The interaction between cadmium or zinc and AMPSO was investigated by DCP and GEP, at fixed total ligand to total metal concentration ratios and various pH values, at 25.0 °C and 0.1 M KNO₃ ionic strength. For Cd–(AMPSO)_x–(OH)_y system, CdL and CdL(OH) species, were identified, with stability constants values set to (as log β): 2.1±0.1 and 6.2±0.2, respectively. For Zn–(AMPSO)_x–(OH)_y system, the proposed final model with stability constants set to (as log β) is: ZnL=2.5±0.1 and ZnL(OH)₂=12.9±0.2. For both systems, the fact that AMPSO deprotonation occurs in the metal hydrolysis and M(OH)₂ precipitation and the complexes formed are not too strong added a real challenge to data interpretation.     

Challenges in modelling and optimisation of stability constants in the study of Cu-(TAPS)x-(OH)y system by polarography

This work describes the application of polarography, a technique scarcely used for modelling and optimisation of stability constants, in the study of copper complexes with [(2-hydroxy-1,1-bis(hydroxymethyl)ethyl)amino]-1-propanesulfonic acid (TAPS). Direct current polarography (DCP), using low total copper ion and large total ligand to total copper concentration, enabled the full characterization of Cu-(TAPS)_x-(OH)_y system, whose complexation occurs in the pH range of copper hydrolysis and Cu(OH)₂ precipitation. Cu-(TAPS)_x-(OH)_y system was studied by DCP and glass electrode potentiometry (GEP) in aqueous solution at fixed total ligand to total metal concentrations ratios and varied pH values (25.0 °C; I = 0.1 M, KNO₃). The predicted model, as well as the overall stability constants values, are (as log β): CuL⁺ = 4.2, CuL₂ = 7.8, CuL₂(OH)⁻ = 13.9 and CuL₂(OH)₂²⁻ = 18.94. GEP only allowed confirming the stability constants for CuL⁺ and CuL₂ and was used to determine the pKa of TAPS, 8.342.Finally, a briefly comparative analysis between TAPS and other structural related buffers was done. Evaluation based on log β_CuL versus pKa revealed that TES, TRIS, TAPS and AMPSO coordinated via amino and hydroxymethylgroups forming a five-membered chelate ring. For BIS-TRIS and TAPSO, and possibly DIPSO, one or more five-membered chelate rings involving additional hydroxyl groups are also likely formed.     

Modelling and Optimization of Stability Constants of Cadmium or Zinc with Biological Buffers (DIPSO or TAPS) in Aqueous Solutions by Electrochemical Techniques

The complexation behavior of four systems involving cadmium(II) or zinc(II) in aqueous solutions with the biological buffers 3-[N,N-bis(2-hydroxyethyl)amino]-2-hydroxypropanesulfonic acid (DIPSO), and [(2-hydroxy-1,1-bis(hydroxymethyl)ethyl)amino]-1-propanesulfonic acid (TAPS) was studied by direct current polarography (DCP) and glass electrode potentiometry (GEP), at 25.0 ± 0.1 °C and ionic strength 0.1 mol·dm^?3 KNO₃. Except for the Cd–TAPS system, for which full characterization of the system was possible either by DCP or GEP, full characterization of the other metal-buffer systems (Zn–DIPSO, Zn–TAPS and Cd–DIPSO) was only possible using DCP. For Zn-buffers systems, ZnL⁺ and $ {\text{ZnL(OH)}}_{2}^{ - } $ ZnL(OH) 2 ? (where L stands for buffer) were identified. For the Zn–DIPSO system, the overall stability constant values (as log₁₀ β) are 2.1 ± 0.2 and 13.4 ± 0.2, respectively. For the Zn–TAPS system, the overall stability constants values (as log₁₀ β) are 2.4 ± 0.1 and 12.9 ± 0.3, respectively. For the Cd–DIPSO system, the overall stability constants values (as log₁₀ β) of CdL⁺ and CdL(OH) are 2.9 ± 0.1 and 6.9 ± 0.3, respectively. For the Cd–TAPS system, only the species CdL⁺ was identified with log₁₀ β = 2.5 ± 0.1.     

Potentiometric and spectrophotometric titration of Cu2+ complexes with N-isopropyl-2-methyl-1,2-propanediamine

The complexation of Cu²⁺ by N-isopropyl-2-methyl-1,2-propanediamine (L) has been studied by potentiometric and spectrophotometric titration. The dominant complexes formed in this system are [CuL]²⁺, [CuL₂]²⁺, [Cu₂L₂(OH)₂]²⁺, and [CuL(OH)₂]. The data were thoroughly tested for different models with [CuL(OH)]⁺, [CuL(OH)]⁺, [Cu(OH)]⁺, and [Cu₂(OH)₂]²⁺ as additional species. The importance of steric factors is indicated by the d-d* spectra: for [CuL₂]²⁺, (λ_max = 499 nm) the absorption maximum is shifted by 50 nm to high energies relative to [Cu(en)₂]²⁺, (λ_max = 549 nm), whereas the opposite is true for the 1:1 complexes ([CuL]²⁺ : λ_max = 712 nm,s [Cu(en)]²⁺ : λ_max = 660 nm).     

Composition and stability constants of copper(II) complexes with succinic acid determined by capillary electrophoresis

The advantage of capillary electrophoresis was demonstrated for studying a complicated system owing to the dependence of direction and velocity of the electrophoretic movement on the charge of complex species. The stability constants of copper(II) complexes with ions of succinic acid were determined by capillary electrophoresis, including the 1?:?2 metal to ligand complexes which are rarely mentioned. The measurements were carried out at 25 °C and ionic strength of 0.1, obtained by mixing the solutions of succinic acid and lithium hydroxide up to pH 4.2–6.2. It was shown that while pH was more than 4.5 the zone of copper(II) complexes with succinate moves as an anion. It is impossible to treat this fact using only the complexes with a metal-ligand ratio of 1?:?1 (CuL⁰, CuHL⁺). The following values of stability constants were obtained: log β(CuL) = 2.89 ± 0.02, log β(CuHL⁺) = 5.4 ± 0.5, log β(CuL₂^2?) = 3.88 ± 0.05, log β(CuHL₂^?) = 7.2 ± 0.3.     

Determination of the stability constants of Pb–(DIPSO)x–(OH)y and Pb–(AMPSO)x–(OH)y systems

In this work, complexation between lead ion and the ligands 3-[N,N-bis(2-hydroxyethyl)amino]-2-hydroxypropanesulfonic acid (DIPSO) and N-(1,1-dimethyl-2-hydroxyethyl)-3-amino-2-hydroxypropanesulfonic acid (AMPSO), which are commercial pH buffers, is presented. Both ligands form complexes with lead in their pH buffer range (between pH 6.5 and 8.5 for DIPSO and between pH 8.0 and 9.0 for AMPSO). The final models and the overall stability constants, which are reported here, were determined by direct current polarography and glass electrode potentiometry [only for the Pb–(DIPSO)_x–(OH)_y system] at 25.0 °C and 0.1 M KNO₃ ionic strength. For the Pb–(DIPSO)_x–(OH)_y system, the proposed final model contains PbL, PbL₂, PbL₂(OH), and PbL₂(OH)₂ with stability constants, as log β, of 3.4 ± 0.1, 6.35 ± 0.15, 12.8 ± 0.2, and 18.0 ± 0.3, respectively. For the Pb–(AMPSO)_x–(OH)_y system, the species observed are PbL, PbL(OH), and PbL(OH)₂ with stability constants, as log β, of 2.9 ± 0.5, 9.4 ± 0.1, and 14.5 ± 0.2, respectively. For AMPSO, the possible adsorption of the ligand at the mercury electrode surface was evaluated by alternating current polarography through calculation of the capacitance of the double layer.     

Copper(II) complexes of potentially terdentate N2-[(R)-2-hydroxypropyl]- and N2-[(S)-2-hydroxypropyl]-(S)-phenylalaninamide for chiral recognition: Synthesis of the Ligands and Formation Constants

Copper(II) complexes of the ligands N²-[(R)-2-hydroxypropyl]- and N²-[(S)-2-hydroxypropyl]-(S)-phenylalaninamide performed chiral separation of N-dansyl-protected and unmodified amino acids in HPLC (reversed phase). With the aim of investigating which species are potentially involved in the discrimination mechanism, the two ligands were synthesized and their complexation equilibria with Cu²⁺ studied by potentiometry and spectrophotometry in aqueous solution up to pH 11.7. The formation constants of the species observed, [CuL]²⁺, [CuL₂]²⁺, [CuLH_–1]⁺, [CuL₂H_–1]⁺, [CuL₂H_–2], and [CuL₂H_–3]^?, were quite similar for both compounds and were compared to those of (S)-phenylalaninamide. Most probably, in [CuL₂H_–3]^? the ligands behave as terdentate, with the deprotonated OH group occupying an apical position.     

Study of Equilibria in the Aluminium(III)-Glycine and -Alanine Systems

The formation of various hydrolytic and mixed hydrolytic complexes of the aluminium(III) ion in the presence of glycine and L-alanine, has been studied in 0.5 mol dm^?3 (Na)NO₃ medium at 25deg;C, by emf method. The concentration ratios of amine acids to aluminium(III) were varied from 1 : 1 to 10 : 1. The least-squares treatment of the data obtained, in the absence of the amino acids, indicates the formation of the dimer, [Al₂(OH)₂]⁴⁺, and monomer, [AlOH]²⁺, with the stability constants log β₂₂ = ?7.03 ± 0.03 and log β₁₁ = ?5.65 ± 0.09, respectively. At pH values higher then ～4.0 formation of the trimer [Al₃(OH)₄]⁵⁺ (log β₃₄ = ?12.60 ± 0.08) becomes significant. In the presence of amino acids the evidence has been found for the formation of [Al₂(OH)₄]²⁺ (log β₂₄ = ?15.65 ± 0.09). Besides the formation of the pure hydrolytic complexes, equilibria in the title systems can be explained by assuming the main reaction products to have the compositions [Al(OH)₃Gly] (log β₁₃₁ = ?7.53 ± 0.04), [Al₂(OH)₂(Gly)₂] (log β₂₂₂ = 6.56 ± 0.09) and [Al(OH)₃Ala] (log β₁₃₁ = ?7.70 ± 0.03), [Al₂(OH)₂Ala₂] (log β₂₂₂ = 7.23 ± 0.07).     

Three copper(II) complexes constructed from 4-(2-pyridyl)-<Emphasis Type="Italic">1H</Emphasis>-1,2,3-triazole ligands: syntheses,structures, optical,and electrochemical properties

Three Cu(II) complexes constructed from 4-(2-pyridyl)-1H-1,2,3-triazole (L), namely, [CuL₂Cl₂]·H₂O, [CuL₂(CH₃OH)(NO₃)]NO₃ and [CuL₂(H₂O)]SO₄, have been synthesized and characterized. X-ray crystal structure analyses revealed that [CuL₂Cl₂]·H₂O and [CuL₂(CH₃OH)(NO₃)]NO₃ have octahedral geometries, whilst [CuL₂(H₂O)]SO₄ adopts a square-pyramidal coordination geometry. In all three complexes, the Cu(II) atoms are chelated by two L ligands in the basal plane, whilst the apical positions are occupied by Cl^?, NO₃^?, CH₃OH or H₂O. The bandgaps between the HOMO and LUMO were estimated by cyclic voltammetry (CV) and diffuse reflectance spectroscopy (DRS) to be 3.43, 3.12, and 3.74 eV, respectively. Theoretical calculations on each structure gave similar results to the experiments. Frontier molecular orbital analyses showed that the higher electron density on the apical ligand, the lower the bandgap.     

Homo- and heteroleptic mercury(II) complexes with monoamino complexones in aqueous solution

Reactions of mercury(II) with iminodiacetic (H₂Ida), 2-hydroxyethyliminodiacetic (H₂Heida), and nitrilotriacetic acids (H₃Nta) were studied by spectrophotometry and pH potentiometry. The resulting complexes included [HgIda], [Hg(OH)Ida]^?, [HgIda₂]^2?, [HgHeida], [Hg(OH)Heida]^?, [Hg(Heida)₂]^2?, [HgNta]^?, [HgNta₂]^4?, [Hg(Ida)Heida]^2?, [Hg(Ida)Nta]^3?, and [Hg(Heida)Nta]^3?. The logarithms of their stability constants calculated for I = 0.1 (NaClO₄) and T = 20 ± 2°C were 11.14 ± 0.07, 20.33 ± 0.08, 19.40 ± 0.10, 11.42 ± 0.04, 19.68 ± 0.11, 18.48 ± 0.09, 13.42 ± 0.05, 20.80 ± 0.08, 19.05 ± 0.06, 20.64 ± 0.11, and 20.53 ± 0.16, respectively. The experimental data were analyzed in terms of the mathematical models that predict the existence of a wide spectrum of complex species in solution and allow one to consider only those species that are sufficient for accurate reproduction of the observed pattern.     

Modelling of Pb-(TAPS)x-(OH)y system and refinement of stability constants in the region of lead hydrolysis and lead hydroxide precipitation

The influence of [(2-hydroxy-1,1-bis(hydroxymethyl)ethyl)amino]-1-propanesulfonic acid (TAPS) on solutions containing lead(II) was studied by direct current polarography (DCP) and glass electrode potentiometry (GEP). The readings were taken at fixed total TAPS to total lead(II) concentration ratios and various pH values, at 25.0 ± 0.1 °C and ionic strength 0.1 M KNO₃.Due to the basic pK_a of the ligand, which occurs in the pH range where large amount of lead polynuclear species are formed, and the occurrence of ligand adsorption, that disabled the use of high concentrations of TAPS on DCP experiments, GEP and DCP experimental conditions were put to the limit in order to provide the correct Pb-TAPS-OH model and reliable stability constants.The proposed final model is: PbL, PbL₂, PbL₂(OH) and PbL₂(OH)₂ with overall stability constants values, as log β, 3.27 ± 0.06, 6.5 ± 0.1, 12.7 ± 0.1 and 17.27 ± 0.06, respectively.A comparative analysis of the strength of complexation of TAPS and a structural related buffer, 2-hydroxy-3-[tris(hydroxymethyl)methylamino]-1-propanesulfonic acid (TAPSO), with lead is also discussed.     

Voltammetry as Virtual Potentiometric Sensor in Modelling of a Metal/Ligand System and Refinement of Stability Constants. Part 5

The concept of virtual potential (employed here in modelling operations), a unique experimental setup designed and built in our laboratories, and new regression equations derived for nonlinear fitting of quasi‐reversible direct‐current polarograms were combined with the existing rigorous treatment and refinement of polarographic data to establish reliable metal/ligand models and accurate stability constants for the lead(II)/glycine/OH^? and lead(II)/sarcosine/OH^? systems (sarcosine = N‐methylglycine). In the case of glycine, the complexes [M(HL)], [ML], [ML₂], and [ML₃] were identified, and their stability constants (as log β) were established to be 10.51 ± 0.06, 4.58 ± 0.02, 7.19 ± 0.10, and 9.27 ± 0.02, respectively, the complex [ML₃] being reported here for the first time (Table 2). The system with sarcosine involving [M(HL)], [ML], [ML₂], [ML₃], and [ML₂(OH)₂], with the stability constants (as log β) 11.01 ± 0.04, 4.18 ± 0.03, 7.23 ± 0.03, 9.1 ± 0.3, and 15.97 ± 0.07, respectively, is reported for the first time (Table 3). The log K₁ value for Pb^II with sarcosine is a fraction of a log unit smaller when compared with the Pb^II complex with glycine, in agreement with the literature data for Cu^II, Ni^II, and Zn^II showing the same trend for these two ligands. The proposed nonlinear curve‐fitting operations expand the applicability of polarography to study reliably and conveniently quasi‐reversible, on the polarographic time scale, metal/ligand systems (systems with involved heterogeneous kinetics).     

Graphic Data Analysis and Complex Formation Curves as Modelling and Optimization Tools in Potentiometric and Voltammetric Speciation Studies of a Pb?(TAPSO)x?(OH)y System

The formation of complex species between lead and TAPSO, a commercialized biological buffer, was evidenced and discussed. Formation constants of Pb? (TAPSO)_x? (OH)_y system have been studied by direct current polarography and by glass electrode potentiometry at fixed total–ligand and total–metal concentration ratio and varied pH, at 25.0 °C and ionic strength set to 0.1 M KNO₃. The graphic analysis revealed to be a very powerful tool in the prediction and refinement operations of Pb–(TAPSO)_x? (OH)_y system. The proposed final model for this system is: PbL, PbL₂, PbL₂(OH) and PbL₂(OH)₂, with stability constants values, as log β, of 3.7±0.1, 6.6±0.1, 12.9±0.1 and 17.8±0.1, respectively.     

Heteroligand mercury(II) complexes with aspartic,tartaric, and citric acids

Mercury(II) complexes with aspartic (H₂Asp) and tartaric acids (H₂Tart) and heteroligand mercury(II) complexes with H₂Asp, H₂Tart, and citric acids (H₃Cit) were studied by spectrophotometry in aqueous solutions with I = 0.1(NaClO₄) at 20 ± 2°C. It was found that the complexation in both binary and ternary systems depends on the concentrations of the reagents and the pH of the medium. The resulting complexes included [HgAsp], [Hg(OH)Asp]^?, [HgAsp₂]^2?, [HgTart], [Hg(OH)Tart]^?, [Hg(OH)₂Tart]^2?, [HgAspCit]^3?, [HgAspTart]^2?, and [Hg(OH)AspTart]^3?. The logarithms of their stability constants were 11.74 ± 0.12, 20.18 ± 0.17, 20.11 ± 0.10, 5.40 ± 0.11, 15.52 ± 0.09, 24.70 ± 0.12, 19.19 ± 0.12, 14.55 ± 0.16 and 23.80 ± 0.14, respectively. The experimental data were analyzed in terms of the mathematical models that predict the existence of a wide spectrum of complex species in solution and allow one to consider only those species that are sufficient for accurate reproduction of the observed pH-dependence of the optical density.     

氢过碘酸）合银（III）配离子氧化L-脯氨酸的反应动力学及机理

L-脯氨酸独有的亚胺基使其在生物医药领域具有许多独特的功能,并广泛用作不对称有机化合物合成的有效催化剂。本文在碱性介质中研究了二（氢过碘酸）合银（III）配离子氧化 L-脯氨酸的反应。经质谱鉴定,脯氨酸氧化后的产物为脯氨酸脱羧生成的 γ-氨基丁酸盐;氧化反应对脯氨酸及Ag(III) 均为一级;二级速率常数 k′ 随 [IO₄^-] 浓度增加而减小,而与 [OHˉ] 的浓度几乎无关;推测反应机理应包括 [Ag(HIO₆)₂]^5-与 [Ag(HIO₆)(H₂O)(OH)]^2-之间的前期平衡,两种Ag（III）配离子均作为反应的活性组分,在速控步被完全去质子化的脯氨酸平行地还原,两速控步对应的活化参数为： k₁ (25 ^oC)＝1.87±0.04（mol·L^-1)^-1s^-1,∆ H₁^≠＝45±4 kJ · mol^-1, ∆ S₁^≠＝-90±13 J· K^-1·mol^-1 and k₂ (25 ^oC) ＝3.2±0.5（mol·L^-1)^-1s^-1, ∆ H₂^≠＝34±2 kJ · mol^-1, ∆ S₂^≠＝-122 ±10 J· K^-1·mol^-1。本文第一次发现 [Ag(HIO₆)₂]^5-配离子也具有氧化反应活性。     

Structure and Magnetism of two Polynuclear Complexes Containing both Macrocyclic and Azido Ligands

The two complexes of formula [Cu₂(CuL)₂(μ‐N₃)₄] · 2CH₃OH ( 1 ) and [Cu₂(NiL)₂(μ‐N₃)₄] · 2CH₃OH ( 2 ) (CuL and NiL, H₂L = 2,3‐dioxo‐5,6,14,15‐dibenzo‐1,4,8,12‐tetraazacyclo‐pentadeca‐7,13‐dien), were synthesized and structurally determined. The magnetic susceptibility data of 1 and 2 were analyzed. For complex 1 , magnetic measurements show alternating ferromagnetic and antiferromagnetic exchange couplings J₁ = 23.67 cm^–1, J₂ = –189.11 cm^–1, zJ’ = –0.62 cm^–1. For complex 2 , the doubly bridged asymmetric EO promotes a ferromagnetic interaction between Cu^II and Cu^II ions(J = 40.764 cm^–1).     

Hydrolytic Behavior of La<Superscript>3+</Superscript> and Sm<Superscript>3+</Superscript> at Various Temperatures

The hydrolytic species of lanthanide ions, La³⁺ and Sm³⁺, in water at I = 0.1 mol·dm^?3 KCl ionic strength and temperatures of 298.15, 310.15 and 318.15 K were investigated by potentiometry. The hydrolytic species were modeled by the HySS simulation program. From the results, the hydrolytic species of each metal ion at different temperatures were calculated using the program HYPERQUAD2013. The hydrolysis constants (log₁₀ β) of [La(OH)]²⁺ and La(OH)₃ were calculated as ?8.52 ± 0.46, ?26.84 ± 0.48, and log₁₀ β values of [Sm(OH)]²⁺, [Sm(OH)₂]⁺, Sm(OH)₃ were calculated as ?7.11 ± 0.21, ?15.84 ± 0.25 and ?23.44 ± 0.52 in aqueous media at 298.15 K, respectively. The dependence of the hydrolysis constants on the temperature allowed us to calculate the enthalpy, entropy, and Gibbs energy of hydrolysis values of each species.     

Complexation of M–(buffer) x –(OH) y Systems Involving Divalent Ions (Cobalt or Nickel) and Zwitterionic Biological Buffers (AMPSO, DIPSO, TAPS and TAPSO) in Aqueous Solution

The complexation behavior of eight M–(buffer) _x –(OH) _y systems involving two divalent ions (cobalt and nickel) and four zwitterionic biological buffers (AMPSO, DIPSO, TAPS and TAPSO) were characterized. Complex formation was detected for all eight M–(buffer) _x –(OH) _y systems studied, but fully defined final models were obtained for only four of these systems. For systems involving cobalt or nickel with AMPSO or TAPS, a complete characterization of the systems was not possible in the studied buffer pH-range. Metal complexation was studied by glass-electrode potentiometry (GEP) and UV-Vis spectroscopy at 25.0 °C and I=0.1 mol⋅dm⁻³ KNO₃ ionic strength. For the Ni–(L) _x –(OH) _y and Co–(L) _x –(OH) _y systems, with L = TAPSO or DIPSO, the proposed final models and overall stability constants were obtained by combining results from both techniques. For the Ni–(L) _x –(OH) _y systems, the measured values of the stability constants are log ₁₀ β _NiL=3.0±0.1 and log ₁₀ β _NiL2=4.8±0.1 for L = TAPSO, and log ₁₀ β _NiL=2.7±0.1 and log ₁₀ β _NiL2=4.6±0.1 for L = DIPSO. For the Co–(L) _x –(OH) _y systems, the overall stability constants are log ₁₀ β _CoL=2.2±0.1, log ₁₀ β _CoL2=3.6±0.2 and log ₁₀ β _CoL(OH)=7.6±0.1 for L = TAPSO, and log ₁₀ β _CoL=2.0±0.1 and log ₁₀ β _CoL(OH)=7.8±0.1 for L = DIPSO. For both buffers, the CoL(OH) species is characterized by a major structural rearrangement.