Formation and Stability of Binary Complexes of Divalent Ecotoxic Ions (Ni, Cu, Zn, Cd, Pb) with Biodegradable Aminopolycarboxylate Chelants (dl-2-(2-Carboxymethyl)Nitrilotriacetic Acid, GLDA, and 3-Hydroxy-2,2′-Iminodisuccinic Acid, HIDS) in Aqueous Solutions

The protonation and complex formation equilibria of two biodegradable aminopolycarboxylate chelants {dl-2-(2-carboxymethyl)nitrilotriacetic acid (GLDA) and 3-hydroxy-2,2??-iminodisuccinic acid (HIDS)} with Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺ and Pb²⁺ ions were investigated using the potentiometric method at a constant ionic strength of I?=?0.10?mol·dm^?3 (KCl) in aqueous solutions at 25?±?0.1?°C. The stability constants of the proton?Cchelant and metal?Cchelant species for each metal ion were determined, and the concentration distributions of various complex species in solution were evaluated for each ion. The stability constants (log₁₀ K _ML) of the complexes containing Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺ and Pb²⁺ ions followed the identical order of log₁₀ K _CuL?>?log₁₀ K _NiL?>?log₁₀ K _PbL?>?log₁₀ K _ZnL?>?log₁₀ K _CdL for either GLDA (13.03?>?12.74?>?11.60?>?11.52?>?10.31) or HIDS (12.63?>?11.30?>?10.21?>?9.76?>?7.58). In each case, the constants obtained for metal?CGLDA complexes were larger than the corresponding constants for metal?CHIDS complexes. The conditional stability constants (log₁₀ $ K_{\text{ML}}^{'} $ ) of the metal?Cchelant complexes containing GLDA and HIDS were calculated in terms of pH, and compared with the stability constants for EDTA and other biodegradable chelants.     

Effect of hydrogen bonding of solvent on the thermodynamic stability of cadmium ethylenediamine complexes

Changes in the stability of the cadmium(ii) ethylenediamine complexes in mixed water—DMSO solvents were studied by pH-metry and calorimetry. Complex cations [Cd(en)]²⁺, [Cd(en)₂]²⁺, and [Cd(en)₃]²⁺ are formed in aqueous solutions, and the [Cd(en)₄]²⁺ complex with a partially dentate ligand is stable in DMSO. An increase in the DMSO content in a solvent increases the stability of the complexes. The maximum increase in logK is observed for coordinatively saturated compounds. The thermodynamics of complexation is discussed from the viewpoint of solvation approach. Principal differences in the influence of aqueous-alcohol and aqueous-aprotic solvents on the stability of the metal amino complexes were revealed. Protolytic solvents exert a destabilizing effect on the multiligand complexes, because the coordination sphere is involved in H bonding.     

具有平面结构的四齿肼基硫代甲酸苄酯席夫碱镍配合物的合成,晶体结构和三阶非线性光学性质研究

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Hydrogen Bonding and Solvent Effects on Complexation of Alkali Metal Cations by Lower Rim Calix[4]arene Tetra(O-[N -acetyl-D -phenylglycine methyl ester]) Derivative

Complexation of alkali metal cations with 5,11,17,23-tetra-tert-butyl-26,28,25,27-tetrakis(O-methyl-D-α-phenylglycylcarbonylmethoxy)calix[4]arene (L) was studied by means of spectrophotometric, conductometric and potentiometric titrations at 25 °C. The solvent effect on the binding ability of L was examined by using two solvents with different affinities for hydrogen bonding, viz. methanol and acetonitrile. Despite the presence of intramolecular NH···O=C hydrogen bonds in L, which need to be disrupted to allow metal ion binding, this calix[4]arene amino acid derivative was shown to be an efficient binder for smaller Li⁺ and Na⁺ cations in acetonitrile (lg K_LiL > 5, lg K_NaL = 7.66), moderately efficient for K⁺ (lg K_KL = 4.62), whereas larger Rb⁺ and Cs⁺ did not fit in its hydrophilic cavity. The complex stabilities in methanol were significantly lower (lg K_NaL = 4.45, lg K_KL = 2.48). That could be explained by different solvation of the cations and by competition between the cations and methanol molecules (via hydrogen bonds) for amide carbonyl oxygens. The influence of cation solvation on complex stability was most pronounced in the case of Li⁺ for which, contrary to the quite stable LiL⁺ complex in acetonitrile, no complexation was observed in methanol under the conditions used.     

Thermodynamics of mixed-ligand complexation of mercury(II) ethylenediaminetetraacetate with histidine and lysine in aqueous solution

The formation of mixed-ligand complexes HgEdtaIm²⁻, HgEdtaL³⁻, HgEdtaHL²⁻, and (HgEdta)₂L⁵⁻ (L is histidine, lysine; Im is imidazole) was studied by calorimetry, pH-metry, and NMR spectroscopy. The thermodynamic parameters (logK, ΔrG ⁰, ΔrH, Δr _S) for the reactions of complex formation at 298.15 K and ion strength of 0.5 (KNO₃) were determined. The most likely coordination mode for the complexone and amino acid in the mixed complexes was identified.     

Electrochemical studies of Tl(I) crown-ether complexes in nonaqueous media

Summary Complexation equilibria of the Tl(I) ion with 18-crown-6 and dibenzo-18-crown-6 were studied polarographically in 10 nonaqueous solvents. The stability of the complexes is strongly influenced by the nature of solvents and varies with their Lewis basicities. It has been found that the logK _s value (K _s is the stability constant of the complex) can be well described by empirical relation logK _s=a DN+b, whereDN stands for the Gutmann donor number anda andb mark the regression coefficient. Addition of the second explanatory parameter, the acceptor number, is not statistically significant. This result is in agreement with the predominant role of Tl(I) ion solvation.

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Elektrochemische Untersuchungen von Tl(I)-Kronenetherkomplexen in nichtwäßrigen Medien

Zusammenfassung Es wurden die Komplexierungsgleichgewichte des Tl(I)-Ions mit 18-Krone-6 und Dibenzo-18-krone-6 polarographisch in 10 nichtwäßrigen Lösungsmitteln untersucht. Die Stabilität der Komplexe wird sehr stark vom Solvens beeinflußt, wobei eine starke Abhängigkeit von der Lewis-Basizität beobachtet wird. Es wurde festgestellt, daß die logK _s-Werte (K _s ist die Komplexstabilitätskonstante) gut mit der empirischen Beziehung logK _s=a DN+b beschrieben werden können, wobeiDN die Gutmann'sche Donorzahl unda undb die Regressionskonstanten bedeuten. Hinzunahme der Akzeptorzahl als zweiten Parameter bleibt statistisch insignifikant. Dieses Ergebnis stimmt mit dem dominierenden Einfluß der Tl(I)-Ionensolvatation überein.

     

ACE applied to the quantitative characterization of benzo‐18‐crown‐6‐ether binding with alkali metal ions in a methanol–water solvent system

ACE was applied to the quantitative evaluation of noncovalent binding interactions between benzo‐18‐crown‐6‐ether (B18C6) and several alkali metal ions, Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺, in a mixed binary solvent system, methanol–water (50/50 v/v). The apparent binding (stability) constants (K_b) of B18C6–alkali metal ion complexes in the hydro‐organic medium above were determined from the dependence of the effective electrophoretic mobility of B18C6 on the concentration of alkali metal ions in the BGE using a nonlinear regression analysis. Before regression analysis, the mobilities measured by ACE at ambient temperature and variable ionic strength of the BGE were corrected by a new procedure to the reference temperature, 25°C, and the constant ionic strength, 10 mM . In the 50% v/v methanol–water solvent system, like in pure methanol, B18C6 formed the strongest complex with potassium ion (log K_b=2.89±0.17), the weakest complex with cesium ion (log K_b=2.04±0.20), and no complexation was observed between B18C6 and the lithium ion. In the mixed methanol–water solvent system, the binding constants of the complexes above were found to be about two orders lower than in methanol and about one order higher than in water.     

Complexation of (N, N-diethylthiocarbamoylmethyl) diphenylphosphine sulfide with silver nitrate. The structure of the [Ag{Ph2P(S)CH2C(S)NEt2}2]NO3 complex

(N, N-Diethylthiocarbamoylmethyl)diphenylphosphine sulfide, Ph₂P(S)CH₂C(S)NEt₂, forms a distorted tetragonal bisligand complex [AgL₂]NO₃ with Ag⁺. The stability constant of the complex in acetonitrile was estimated by spectrophotometry (logK=3.7). Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 846–852, April, 1997.     

Determination of ion-pair formation constants of univalent metal–crown ether complex ions with anions in water using ion-selective electrodes: application of modified determination methods to several salts

Ion-pair formation constants (mol^–1 dm³ unit), K_MX for a univalent metal salt (MX) and K_MLX for its ion-pair complex (ML⁺X^–) with a crown ether (L) in water, were determined at various ionic strengths (I) and 25°C by potentiometry with ion-selective electrodes for MX=NaPic, NaMnO₄, NaBPh₄, KPic, and KMnO₄; and MLX=Na(18C6)Pic, K(18C6)Pic, and Na(18C6)BPh₄, where Pic^– and 18C6 denote a picrate ion and 18-crown-6 ether, respectively. Equations for analyzing I-dependence of logK_MLX and logK_MX were derived and fitted well to the I-dependence using a non-linear regression analysis. The equilibrium constants at I=0 mol dm^–3, K_MLX° and K_MX°, were simultaneously obtained from the analysis. The experimental values of K_MLX and K_MX were only in agreement with the values calculated from K_MLX° and K_MX°, respectively, in the ranges of higher I.     

Facilitated Transfer of Alkali Metal Ions by a Tetraester Derivative of Thiacalix[4]arene at the Liquid–Liquid Interface

The facilitated transfer of alkali metal ions (Na⁺, K⁺, Rb⁺, and Cs⁺) by 25,26,27,28‐tetraethoxycarbonylmethoxy‐thiacalix[4]arene across the water/1,2‐dichloroethane interface was investigated by cyclic voltammetry. The dependence of the half‐wave transfer potential on the metal and ligand concentrations was used to formulate the stoichiometric ratio and to evaluate the association constants of the complexes formed between ionophore and metal ions. While the facilitated transfer of Li⁺ ion was not observed across the water/1,2‐dichloroethane interface, the facilitated transfers were observed by formation of 1 : 1 (metal:ionophore) complex for Na⁺, K⁺, and Rb⁺ ions except for Cs⁺ ion. In the case of Cs⁺ a 1 : 2 (metal:ionophore) complex was obtained from its special electrochemical response to the variation of ligand concentrations in the organic phase. The logarithms of the complex association constants, for facilitated transfer of Na⁺, K⁺, Rb⁺, and Cs⁺, were estimated as 6.52, 7.75, 7.91 (log β₁°), and 8.36 (log β₂°), respectively.     

CRYSTAL STRUCTURES OF Cd(H2O)2Cu(CN)3·2H2O,K[Cd(H2O)2Cu2(CN)5]·2H2O AND K2[Cd(H2O)Cu4(CN)8]·1.5H2O. STRUCTURES OF MULTI-DIMENSIONAL FRAMEWORKS FORMED WITH CADMIUM(II), COPPER(I) AND BRIDGING CYANO GROUPS

Abstract

In an attempt to form new multi-dimensional structures of cyano complexes including cadmium(II) and copper(I), four new complexes were obtained successively from an aqueous solution at intervals of from a few days to a few months. The complex 1 obtained first was unstable in the atmosphere. The crystal structures of the other complexes (2–4) obtained from second to fourth were determined by single crystal X-ray structure determinations. Their crystal data are as follows: 2 Cd(H₂O)₂ Cu(CN)₃·2H₂O, monoclinic, C2/m, a = 14.038(1), b = 9.944(1), c = 7.738(1) Å, β = 116.019(7)°, Z = 4; 3 K[Cd(H₂O)₂Cu₂(CN)₅]·2H₂O, triclinic, PI, a = 17.429(9), b = 16.519(7), c = 10.085(5) Å, α = 128.60(3), β = 137.44(2), γ = 45.82(2)°, Z = 4; 4 K₂[Cd(H₂O)Cu₄(CN)₈]·1.5H₂O, monoclinic, C2/c, a = 19.387(2), b = 16.056(3), c = 12.663(2)Å, β = 110.419(9)°, Z = 4. The main structural feature found in the complexes is that the whole framework consists of two networks, a Cd-Cu(I)-CN complex network that has an infinite network formed with bridging cyano groups between the metal atoms and a network formed with hydrogen bonding among water molecules. The second network is connected to Cd in the Cd-Cu(I)-CN complex network via a water ligand coordinated to Cd. In 2 a planar network of [CdCu(CN)₃] complexes are stacked along the c axis and the second network links the stacked complexes. 3 has a stacked structure of [Cd(H₂O)₂Cu(CN)₅]^2? in a bi-layered structure. The second network of 3, which includes K⁺ ions with an electrostatic interaction, spreads over the crystal, penetrating vacant spaces of the metal complex network. 4 has a double lattice structure with a pair of enantiomeric three-dimensional [Cd(H₂O)Cu₄(CN)₈]^2? complexes inter-penetrating each other. There are three structural factors for forming these framwork structures: (1) a non-planar coordination structure for Cd(II) that extends the planar structure of the Cu(I)-CN complex to a three-dimensional structure for the Cd-Cu(I)-CN complex; (2) a trigonal planar coordination structure for Cu(I) that generates vacant space in the metal complex network and makes possible hydrogen bonds to form the second network; (3) structural distortions of bridging cyano groups and a coordination structure of Cu(I) that cause variations of the metal complex network structure.     

Thermodynamics of Complexation Reactions of Lithium Ion with Dibenzo-14-Crown-4 and Its Analogs in Nonaqueous Solvents

Formation constants of Li⁺ complexes with 4-substituted dibenzo-14-crown-4 (DB14C4; 4-substituted group: methyl-, tert-butyl-, H-, bromo-, chloro-, formyl-, nitro-) ligands were determined by ⁷Li NMR spectrometry for solutions in nitromethane (NM), acetonitrile (ACN), propylene carbonate (PC), acetone (AC), Pyridine (Py), tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), and N,N′-dimethyl formamide (DMF). Only a 1:1 complex was formed in solvents with a small or medium donor number. The formation constants of these complexes are strongly influenced by the size of the metal ion – the effect of the size of the cavity, by the solvent and by substituent. The stability of lithium ion with different substituents on DB14C4 decreases in the order methyl- > tert-butyl- > H- > bromo- > chloro- > formyl- > nitro- in various solvents. A good Hammett correlation was found by plotting log K_f vs. ∑σ in PC and AC. The extent of the substituent effect increases as the donor number of solvent decreases. The complexes were both enthalpy and entropy stabilized. The same magnitude of ΔS° value for different substituents indicates formation with a similar configuration upon complexation between crown ether and lithium ion. A slight variation in entropy contribution was observable depending on the nature of the alkyl substituent, whereas a large variation in enthalpic contribution shows a remarkable substituent effect upon complexation; the effect can reach 70% in magnitude.     

Synthesis,Characterization and Spectral Studies of Triethanolamine Complexes of Metal Saccharinates. Crystal Structures of [Co(TEA)2](SAC)2 AND [Cu2(μ-TEA)2(SAC)2]·2(CH3OH)

The novel transition metal saccharinate complexes of triethanolamine (TEA) have been synthesized and characterized by elemental analyses, magnetic moments, UV-Vis and IR spectra. Mn(II), Co(II), Ni(II), Zn(II), Cd(II) and Hg(II) form mononuclear complexes of [M(TEA)₂](SAC)₂, where SAC is the saccharinate ion, while the Cu(II) complex is dimeric. The TEA ligand acts as a tridentate N,O,O'-donor ligand and one ethanol group is not involved in coordination. The SAC ion does not coordinate to the metal ions and is present as the counter-ion in the Mn(II), Co(II), Ni(II), Zn(II), Cd(II) and Hg(II) complexes, but coordinates to the Cu(II) ion as a monodentate ligand. The crystal structures of the [Co(TEA)₂](SAC)₂ and [Cu₂(μ-TEA)₂(SAC)₂]·2(CH₃OH) complexes were determined by single crystal x-ray diffraction. The Co(II) ion has a distorted octahedral coordination by two TEA ligands. The Cu(II) complex crystallizes as a dimethanol solvate and has doubly alkoxo-bridged centrosymmetric dimeric molecules involving two tridentate triethanolaminate (deprotonated TEA) and two monodentate SAC ligands. The geometry of each Cu(II) ion is a distorted square pyramid. Both crystal structures are stabilized by hydrogen bonds to form a three-dimensional network.     

Phosphinate Analogues of Ida and Nta with Low Basicity of Nitrogen Atom: Acid-Base and Complexation Properties

Abstract

Analogues of iminodiacetic acid (H₂IDA) and nitrilotriacetic acid (H₃NTA) bearing two (hydroxomethyl)methylphosphinate or (2-carboxyethyl)methylphosphinate groups were synthesized. Their acid-base and coordination properties with divalent metal ions (Cu²⁺, Ni²⁺ and Zn²⁺) were studied by potentiometry. The compounds exhibit very low basicity of the amino groups (pK_a = 6.2–7.6) due to presence of two electron-withdrawing methylphosphinate groups and, consequently, a low stability of the complexes. In the case of IDA-analogues, the low complex stability results in precipitation of metal hydroxides in neutral region. As expected, NTA-analogues form more stable complexes and, thus, they were also studied in the alkaline region. Presence of additional carboxylate in the 2-carboxyethylphosphinic acid groups results in the formation of dinuclear complexes. Solid-state structures of two compounds were determined by X-ray diffraction analysis. The compounds are protonated on the nitrogen atom and the structures are stabilized by rich hydrogen bond network.     

PROTONATION CONSTANTS AND SOLVENT DEPENDENT SPIN-CROSSOVER BEHAVIOUR OF IRON(II)-2-(2′-PYRIDYL)BENZIMIDAZOLE COMPLEXES

Abstract

2-(2′-Pyridyl)benzimidazole (pybzim = LH) coordinates to iron(II) as a bidentate and forms the tris-ligated complex, [Fe(pybzim)₃]²⁺ as isolated in the solid. Titration of [Fe(pybzim)₃]²⁺ with base demonstrates the successive deprotonation of the imino hydrogens of the coordinated ligands. Protonation constants for the free ligand, pybzim (Iog₁₀ K ^H = 11.33) and the complex, [Fe(pybzim)₃]²⁺ (log₁₀ K ^H ₁ = 9.58, log₁₀ K ^H ₂ = 8.13 and log₁₀ K ^H ₃ = 6.97) were measured in 30% (v/v) H₂O/EtOH. Results show that coordination to iron(II) increases the acidity of the imino hydrogen of the ligand. Spin-crossover behaviour of the complex were studied in different solvents ME, AC, AN, NM, NB, DMF, DMSO and ANL. The complex shows strong spin-crossover behaviour which is solvent dependent. Values of the spin-equilibrium constant (K _sc) and the associated thermodynamic parameters (ΔH _sc = 18.1–21.3 kJ mol^?1 and δS _sc = 69.6–84.4JK^?1 mol^?1) were calculated. An increase of the enthalpy is observed with increasing donor number (DN) of the solvent.     

HYBRID PHOSPHINE-CROWN ETHER LIGANDS: THE STUDY OF BENZO-15-CROWN-5 AND N-PHENYLAZA-15-CROWN-5 AND -18-CROWN-6 FUNCTIONALIZED WITH Ph2P-

Abstract

Methods for the preparation of the 4-diphenylphosphino derivatives of N-phenylaza-15-crown-5 and -18-crown-6 are described. The properties of these systems and the 4′-diphenylphosphino derivative of benzo-15-crown-5 have been examined by way of picrate ion extraction abilities and IR spectra of their Ni(CO)₃L (L = these phosphines) complexes. All three have abilities to extract Na⁺ and K⁺ that are comparable to benzo-15-crown-5. The IR studies (ν_CO, A₁ band) indicate that the azacrown systems have better ability than the benzocrown system to increase the electron density on the nickel center. Further, the addition of alkali metal ions, Na⁺ and K⁺, to the Ni(CO)₃L solutions results in maximum shifts of ca 1.5 cm^?1 for the former systems and 0.7 cm^?1 for the latter system. A rationale for this observation is presented in terms of Hammett substituent constants. Finally, an X-ray structure of the phosphine oxide of the phenylaza-15-crown-5 derivative is presented. A prominent feature of the structure is that the nitrogen atom is essentially planar with the result that the crown ether ring is large and not preorganized for coordination of spherical ions.     

由2-(4'-羧基苯基)咪唑-4,5-二羧酸构筑的过渡金属配位聚合物的合成、结构及性质

在溶剂热条件下,由2-(4′-羧基苯基)咪唑-4,5-二羧酸(H_4L,C_(12)H_8N_2O_6),合成了4个配位聚合物{[M(H_3L)_2]·2H_2O}_n(M=Zn(1),Cd(2),Co(3)),[Cd(H_2L)(H_2O)]_n(4)。用元素分析、红外光谱、热重分析和单晶X射线衍射对配合物进行了表征和结构分析。结构分析结果表明:1～3是异质同晶。配体失去1个质子以H_3L~-的形式通过单齿和N,O-双齿螯合的配位模式与中心金属离子配位,构成一个略有变形的八面体结构。对于配合物4来说,配体失去2个质子以H_2L~(2-)的形式分别通过单齿和N,O-双齿螯合的配位方式与Cd~(2+)配位,中心离子采取扭曲的七配位五角双锥配位模式,并且通过配体苯环上羧基氧原子的双齿桥联作用连接2个中心离子,形成四元环的双核结构;同时呈现双节点(3,6)-连接的二维拓扑网络(4.4.4)(4.4.4.4.4.4.5.6.6.6.6.6)。测定了产物的固体荧光光谱;用EtBr荧光探针法研究了配体及配合物与ct-DNA的相互作用。     

On The Effect of Dielectric Constant on Formation and Stability Constants of Vanadium(v) with Leucine in Mixed Solvent of Methanol + Water at 298.15 K

Abstract

Equilibria in aqueous solutions in the system leucine + VO₂ ⁺ has been studied by a combination of potentiometric and spectrophotometric methods, in the p^H range 3–4.4 with high ligand-to-metal ratios. An equilibrium model, MY, is assumed, where M and Y represent the metal ion and fully dissociated amino acid anion, respectively. The results showed that formation and stability constants of complex system increased with decrease of dielectric constants. Linear relationships were observed when log K_P was plotted vs. 1/D, where K_P and D represent stability and dielectric constants of the system, respectively.     

Crystal Structure of the Complex of 1,4,7,10,13,16-Hexaoxacyclooctadecane with Lithium Picrate Dihydrate

Abstract

A lithium dihydrate picrate complex with 18-crown-6 (18C6) has been synthesized. Solvent extraction in the CH₂Cl₂/H₂O system was studied (log K_ex = 2.04). Crystals of Li(H₂O)₂(18C6)Picrate are triclinic, space group P1, with a = 7.744(2), b = 11.848(3), c = 14.532(4), α = 76.33(2)°, β = 75.80(2)°, γ = 77.81(2)°, Z = 2; final R = 0.039, wR² = 0.094 (GOOF = 1.035) for 3218 independent reflections. The coordination polyhedron is a distorted trigonal bipyramid. The coordinated water molecules are involved in a hydrogen bond network with the oxygen atoms of 18C6 and the picrate ion. This network stabilizes the metal ion complex with the crown ether and changes the selectivity of the extraction process.     

Investigation of the spectral properties of a squarylium near-infrared dye and its complexation with Fe(III) and Co(II) ions

The spectral features of the squarylium near-infrared (NIR) dye NN525 in different solutions and its complexation with several metal ions were investigated. The absorbance maximum of the dye is λ=663 nm in methanol. This value matches the output of a commercially available laser diode (650 nm), thus making use of such a source practical for excitation. The emission wavelength of the dye in methanol is λ_em=670 nm. The addition of either Fe(III) ion or Co(II) ion resulted in fluorescence quenching of the dye. The Stern–Volmer quenching constant, K_SV, was calculated from the Stern–Volmer plot to be K_SV=2.70×10⁷ M⁻¹ for Co(II) ion. The K_SV value for Fe(III) ion could not be established due to the non-linearity of the Stern–Volmer plot and the modified Stern–Volmer plot for this ion. The detection limit is 6.24×10⁻⁸ M for Fe(III) ion and 1.55×10⁻⁵ M for Co(III) ion. The molar ratio of the metal to the dye was established to be 1:1 for both metal ions. The stability constant, K_S, of the metal–dye complex was calculated to be 3.14×10⁶ M⁻¹ for the Fe–dye complex and 2.64×10⁵ M⁻¹ for the Co–dye complex.