Kinetic study of uranium speciation in model solutions and in natural waters using Competitive Ligand Exchange Method

Kinetic speciation of uranium in model solutions containing uranium and humic acid (HA) and in natural waters has been investigated by Competitive Ligand Exchange Method (CLEM). In alkaline freshwaters, most of uranium species were uranium-carbonate species, which were labile in the CLEM experiment. The uranium speciation of every sample was characterized either as “labile” or “non-labile” uranium complexes depending on the dissociation rate coefficients of the complexes. The results showed that as the U(VI)/HA ratio was decreased, the dissociation rate coefficients decreased and the labile fraction decreased as well. When the U(VI)/HA ratio was 0.1, the labile fraction of the U(VI)-HA increased with increasing pH; however, there was no pH effect on the dissociation of U(VI)-HA complexes at lower U(VI)/HA ratios. Chelex-100 had some limitations in its use for the study of dissociation of U(VI)-HA complex at very low U(VI)/HA ratios. By developing an analytical method and procedure for quantitative determination of kinetic parameters for the dissociation of uranium-HA complexes in model solutions and natural waters, this work has made a substantial contribution to analytical chemistry.     

Kinetic speciation of nickel in mining and municipal effluents

This study presents the results of kinetic speciation of nickel in undiluted mining and municipal effluents and effluents diluted with receiving freshwaters from the surrounding environment. The dilution ratios used for the dilution of the effluents were arbitrarily chosen, but were representative of the prevailing mining practices. The purpose of the this dilution was to mimic dilution with natural waters that result from dilution of the mining and municipal effluents with receiving freshwaters, so that this study would reveal environmental realities that are of concern to the managers and regulators of water resources. Ligand exchange kinetics using the competing ligand exchange method (CLEM) was studied using two independent techniques: graphite furnace atomic absorption spectrometry (GFAAS) with Chelex 100 resin as the competing ligand, and adsorptive cathodic stripping voltammetry (AdCSV) with dimethylglyoxime (DMG) as the competing ligand to determine the percentage of Ni metal released from Ni(II)–DOC complexes and the rate of dissociation of Ni(II)–DOC complexes. Using a sample containing a mixture of 30% Copper Cliff Mine effluent, 40% Sudbury municipal effluent and 30% Vermillion River water, both techniques gave results showing that the dilution of the effluent samples increased the percentage of nickel released from Ni(II)–DOC complexes. This increase in the release of nickel from the Ni(II)–DOC complexes may be of concern to managers and regulators of water resources. Agreement between the results of these two techniques has enhanced the validity of the competing ligand exchange method used by both techniques.     

Amorphous silica as a versatile supermolecular ligand for Ni(II) amine complexes: toward interfacial molecular recognition.

Selective adsorption of Ni(II) amine complexes used as precursors for supported catalysts was studied on amorphous silica surfaces. The nature of the adsorption sites was probed by [Ni(en)(dien) (H2O)]2+, [Ni(en)2(H2O)2]2+, and [Ni(dien)(H2O)3]2+ (en = ethylenediamine, dien = diethylenetriamine), which respectively contain one, two, and three labile aqua ligands. The silica surface acts as a mono- or polydentate ligand that can substitute the aqua ligands of the Ni(II) complexes in an inner-sphere adsorption mechanism. Room-temperature adsorption isotherms indicate that each nickel complex selects a limited number of adsorption sites; different sites are recognised by the three complexes, even though they have the same charge and comparable sizes. Several spectroscopic techniques (UV/Vis/NIR, EXAFS, and 29Si NMR) were used to confirm the selective character of the interaction of Ni(II) amine complexes with the silica surface. The specific sites include both silanol/silanolate groups in the same number as the original labile ligands and other surface groups that probably act as hydrogen-bond acceptors. These two types of groups cooperate to result in interfacial molecular-recognition phenomena with interactional complementarity.     

Determination of Dynamic Metal Complexes and their Diffusion Coefficients in the Presence of Different Humic Substances by Combining Two Analytical Techniques

The combined use of a competing ligand exchange (CLE) method and a diffusive gradient in thin films (DGT) technique in a quasi-labile system provides a better understanding of dynamic metal (Cu and Ni) complexes in the presence of humic substances of different origins. The CLE and DGT techniques provide total labile (dynamic) metal complexes (Cu and Ni) and their dissociation rate constants in environmental systems. DGT was found to estimate lower concentrations of labile metal complexes than CLE. These discrepancies were caused by diffusion controlled metal flux (towards the binding resin gel) in the diffusive gel of DGT. The interactions of Cu and Ni with humic acids are stronger than their interactions with fulvic acid and natural organic matter. Changes in the lability of Ni and Cu complexes (complexed with humic substances of different origins) with the changing analytical detection window indicate that the complexes of these metals were formed with different binding sites with diverse binding energies in the humic substances. The combination of these two techniques was found to be very useful in determining diffusion coefficients of labile metal-humate complexes in quasi-labile systems. The values of diffusion coefficients of labile Ni and Cu complexes determined in this study are in good agreement with limited results from the literature. This finding is novel and can be very useful in further improving our understanding of the metal-humate interactions in natural environments.     

配合物生成反应速率常数与平衡常数之间的直线自由能关系IV:5位取代-1,10-邻菲 啉-铜(II)与镍(II)的金属交换反应动力学和机理研究

The kinetics of the metal exchange reactions between (5-R-phen)copper(II) (R = Me, H, Cl, and NO2) and Ni(II) was studied at 25?and ionic strength 1.0 mol dm-3 or pH 2.3-3.5. The rate of the exchange reactions was measured by a spectrophotometer. The reactions appeared to proceed through 3 different pathways which involved H+ attack and Ni attack as well as a pH- and Ni-independent dissociation of the complexes. The kinetics conforms to the following rate law: d[Ni(5-R-phen)]/dt = (kp + kH[H+] + kNi[Ni2+])[(Cu(5-R-phen)2+]. The reaction rate of the 3 pathways increased with decreasing basicity of the ligand. Some linear free energy relationships were found to exist between the reactivity of these Cu(II) complexes and the base strength of the ligand 5-R-phen. The mechanisms of the reactions are discussed.     

Mechanism and kinetics of ligand exchange between ferric citrate and desferrioxamine B

The kinetics of ligand exchange between ferric citrate and desferrioxamine B (DFB) was investigated at pH 8.0 and high citrate/Fe molar ratios (500-5000) with particular attention given to understanding the precise mechanism of ligand exchange. Ferric citrate complexes present in a test solution and therefore involved in the reaction with the incoming ligand (DFB) were initially examined by evaluating ferric citrate speciation on the basis of published thermodynamic constants. The speciation analysis indicated that mononuclear (mono- and dicitrate) ferric complexes are the major species responsible for the ligand exchange with DFB under the conditions examined in the present work. Given the tendency of DFB to adjunctively associate with the ferric citrate complexes, we propose a kinetic model containing the following three mechanisms: (i) direct association of DFB to the ferric dicitrate complex prior to any dissociation of citrate molecules from the Fe center, (ii) adjunctive association of DFB toward ferric monocitrate complex following dissociation of one molecule of citrate from the parent complex, and (iii) complexation of hydrated Fe by DFB after sequential dissociation of two molecules of citrate from the Fe center. Overall rates for the ligand exchange were determined by spectrophotometrically monitoring the formation of ferrioxamine B. Further analysis in quantifying the rate of each mechanism by use of published and determined rate constants of relevant elemental reactions suggested that the first and second mechanisms were significant under our experimental conditions where [Cit] ? [DFB] with the relative importance of these two pathways depending on citrate concentration.     

Cobalt(II), nickel(II), copper(II), and zinc(II) complexes with [3(5)]adamanzane, 1,5,9,13-tetraazabicyclo[7.7.3]nonadecane, and [(2.3)(2).2(1)] adamanzane, 1,5,9,12-tetraazabicyclo[7.5.2]hexadecane

Isolation of the free bicyclic tetraamine, [3(5)]adamanzane.H(2)O (1,5,9,13-tetraazabicyclo[7.7.3]nonadecane.H(2)O), is reported along with the synthesis and characterization of a copper(II) complex of the smaller macrocycle [(2.3)(2).2(1)]adamanzane (1,5,9,12-tetraazabicyclo[7.5.2]hexadecane) and of three cobalt(II), four nickel(II), one copper(II), and two zinc(II) complexes with [3(5)]adamanzane. For nine of these compounds (2-8, 10b, and 12) the single-crystal X-ray structures were determined. The coordination geometry around the metal ion is square pyramidal in [Cu([(2.3)(2).2(1)]adz)Br]ClO(4) (2) and trigonal bipyramidal in the isostructural structures [Cu([3(5)]adz)Br]Br (3), [Ni([3(5)]adz)Cl]Cl (5), [Ni([3(5)]adz)Br]Br (6), and [Co([3(5)]adz)Cl]Cl (8). In [Ni([3(5)]adz)(NO(3))]NO(3) (4) and [Ni([3(5)]adz)(ClO(4))]ClO(4) (7) the coordination geometry around nickel(II) is a distorted octahedron with the inorganic ligands at cis positions. The coordination polyhedron around the metal ion in [Co([3(5)]adz)][ZnCl(4)] (10b) and [Zn([3(5)]adz)][ZnCl(4)] (12) is a slightly distorted tetrahedron. Anation equilibrium constants were determined spectrophotometrically for complexes 2-6 at 25 and 40 degrees C and fall in the region 2-10 M(-1) for the halide complexes and 30-65 M(-1) for the nickel(II) nitrate complex (4). Rate constants for the dissociation of the macrocyclic ligand from the metal ions in 5 M HCl were determined for complexes 2, 3, 5, 8, 10, and 12. The reaction rates vary from half-lives at 40 degrees C of 14 min for the dissociation of the Zn([3(5)]adz)(2+) complex (12) to 14-15 months for the Ni([3(5)]adz)Cl(+) ion (5).     

Kinetics of non-isothermal decomposition of acid maleato complexes of some bivalent metals

McGinn et al. [1] and Misra and Naik [2] studied the complexing capacity of maleic acid with metals. Edge [3] reported his findings on the pyrolysis of normal nickel maleato. This note is concerned with the estimation of kinetic parameters of the non-isothermal decomposition of acid maleato complexes of Co(II), Cu(II), Ni(II) and Cd(II).     

Impact of ligand protonation on higher-order metal complexation kinetics in aqueous systems

The impact of ligand protonation on the complexation kinetics of higher-order complexes is quantitatively described. The theory is formulated on the basis of the usual situation for metal complex formation in aqueous systems in which the exchange of water for the ligand in the inner coordination sphere is rate-determining (Eigen mechanism). We derive expressions for the general case of lability of ML(n) species that account for the contributions from all outer-sphere complexes to the rate of complex formation. For dynamic complexes, dissociation of ML is usually the rate-determining step in the overall process ML(n) --> M. Under such conditions, it is the role of ligand protonation in the step ML --> M that is relevant for the kinetic flux. 1:2 complexes of Cd(II) with pyridine-2,6-dicarboxylic acid fall into this category, and their lability at a microelectrode is reasonably well predicted by the differentiated approach. For non-dynamic systems, the kinetic flux arising from dissociation of higher-order complexes contributes to the rate-determining step. In this case, the weighted contribution of protonated and unprotonated outer-sphere complexes in all contributing dissociation reactions must be taken into account. The kinetic flux arising from the dissociation of 1:2 complexes of Ni(II) with bicine at a conventional electrode was quite well described by this combined approach. The results establish the generic role of ligand protonation within the overall framework of metal complexation kinetics in which complexes may be dynamic to an extent that depends on the operational time scale of the measurement technique.     

Protolytic dissociation mechanisms and comparative acid stabilities of palladium(II), zinc(II), copper(II), and nickel(II) complexes of alkylated dipyrrins

Complexes of Pd(II), Cu(II), Ni(II), and Zn(II) with alkylated dipyrrins (Hdpm) were synthesized and characterized by physicochemical and spectroscopic methods. Protolytic dissociation kinetics of these complexes in benzene in the presence of acetic and trichloroacetic acid was studied. A protonated dipyrrin is the reaction product of protolytic dissociation of the complexes in acid solutions. The observed and true dissociation rate constants, as well as activation reaction parameters, were calculated. Kinetic models of the processes are proposed, and the patterns of influence of the ligand nature on dissociation kinetics were determined. The Pd(II) complexes proved to be much more stable than other those of the other metals, according to the results of the kinetic studies. The lability of the complexes strongly depends on the length and position of the alkyl substituent of the ligand. The dissociation of the Ni(II) complex gives a heteroligand complex at low concentrations of acid, but the complex undergoes full protolytic dissociation at higher concentrations of acid. The dissociation of the complex of Cu(II) is an equilibrium process, involving formation of the protonated form of the ligand.     

The insertion reaction of acetonitrile on aryl nickel complexes stabilized by bidentate N,N'-chelating ligands

Organometallic complexes to be used as single component precursors in the catalytic dimerization/polymerization of olefins usually must contain a labile ligand that can easily be displaced by the olefin. This is the first step in the activation of the precursor. One commonly used labile ligand is a nitrile. Here we report an example of incompatibility between the nickel or palladium aryl bond and acetonitrile. Neutral [MBr(Mes)NN] complexes in which Mes=2,4,6-Me3C6H2, NN=diazabutadiene (DAD), pyridinylimine (PIM), 2,2'-bipyridine (bipy) or 1,10-phenanthroline (phen) gave the expected [M(Mes)(3,5-lut)(NN)][BF4] compounds and the unexpected [Ni(Mes){NH=C(Me)(2,4,6-Me3C6H2)}(NN)][BF4] complexes in the presence of TlBF4 and 3,5-lutidine or acetonitrile. The sequence of reactions that leads to the imine ligand must include an initial insertion of the nitrile on the sigma(Ni-Mes) bond. These ionic complexes remain stable under 20 bar of ethylene.     

A novel vic-dioxime ligand and its Ni(II), Cu(II) and Co(II) complexes containing calix[4]pyrrole moiety: synthesis, characterization and redox properties

A new calix[4]pyrrole functionalized vic-dioxime, 3-(4-methyl-9,9,14,14,19,19-hexaethylcalix[4]pyrrole)benzoaminoglyoxime (LH₂) was synthesized from anti-chloroglyoxime and 3-aminophenyl-calix[4]pyrrole at room temperature. The mononuclear complexes {nickel(II), copper(II) and cobalt(II)} of this vic-dioxime ligand were prepared and their structures were confirmed by elemental analysis, IR and UV–Vis spectrophotometry, magnetic susceptibility; the MS, ¹H and ¹³C NMR spectra of the LH₂ ligand and its Ni(II) complex were also recorded. The experimental results indicated that the ligand:metal ratio was 2:1 in the cases of Ni(II), Cu(II) and Co(II) complexes as is with most vic-dioximes. Electrochemical properties of the ligand, and its complexes were investigated in DMSO solution by cyclic voltammetry at 200?mV?s^?1 scan rate.     

Square-planar N2S2NiII complexes with an extended pi-conjugated system

The reaction of a mixture of 2-(1-naphthyl)benzothiazoline (HL1) and 2,6-diphenylbenzo[1,2-d:4,5-d']bisthiazoline (H3L2) with nickel(II) acetate tetrahydrate yielded three kinds of square-planar nickel(II) complexes: one nickel(II) complex with innocent ligands ([Ni(L1)2] (1c)) and two nickel(II) complexes with non-innocent ligands ([Ni(L1-L1)] (1a) and [Ni(L1-L2)] (1b)). The complex 1c has two bidentate-N,S ligands, which are formed via ring opening of HL1. On the other hand, the two complexes 1a and 1b contain a tetradentate-N2S2 ligand, which is created via ring opening of HL1 and H3L2, followed by bond formation between imino carbon atoms. Complexes 1a and 1b show very intense absorptions in the near-infrared (NIR) region, characteristic of square-planar complexes with non-innocent ligands. The third nickel(II) complex having a non-innocent tetradentate-N2S2 ligand ([Ni(L2-L2)] (2)) was prepared from H3L2 and nickel(II) acetate tetrahydrate. The electronic spectrum of 2 exhibits a very intense absorption at 981 nm (epsilon = 3.6 x 10(4) M-1 cm-1), which is significantly red-shifted compared with those of 1a (837 nm and 4.4 x 10(4) M-1 cm-1) and 1b (885 nm and 4.5 x 10(4) M-1 cm-1), indicating the presence of an extended pi delocalization. The reaction of 2,6-bis(3,5-dichlorophenyl)benzo[1,2-d:4,5-d']bisthiazoline (H3L3) with nickel(II) acetate tetrahydrate also led to the formation of a nickel(II) complex with a non-innocent ligand ([Ni(L3-L3)] (3)). While complex 3 is analogous to 2, its electrical conductivity is much higher than that of 2. The molecular structures of 1b, 1c, 2, and 3 were determined by X-ray crystallography.     

Effects of various competing ligands on the kinetics of trace metal complexes of Laurentian Fulvic Acid in model solutions and natural waters

The objective of this work was to study the effects of the following Ligands: Chelex-100, Dowex MAC-3 and Dowex 50WX-8 using Competing Ligand Exchange Method. This objective was achieved by investigating complex dissociation kinetics of trace metals: Co(II), Ni(II), Cu(II), Zn(II), Cd(II), Mn(II) and Pb(II) of a well-characterized Laurentian Fulvic Acid (LFA) in model solutions and in a natural waters of Lake Heva (Québec, Canada). The effects of variation in the competing ligands (including their quantities) on the complex dissociation kinetics were quantitatively characterized by their first-order dissociation rate coefficients. The kinetic lability of the metal complexes varied with the metal-to-LFA ratio, as expected from the theory of metal complexes of the chemically and physically heterogeneous complexants, LFA. The general trend in the metal-binding by the above competing ligands was: Dowex 50WX-8 > Chelex-100 > Dowex MAC-3. However, no difference was found between the Dowex 50WX-8 and Chelex-100 for Cd(II), Zn(II), and Co(II). The results revealed the importance of the quantity of Chelex-100 as a competing ligand in the metal(II)-LFA complexation, on the dissociation kinetics of these complexes in model solutions. By developing Competing Ligand Exchange Method as an analytical technique, for studying the relative affinities of the above competing ligands for metals complexation in natural waters this work has made a substantial contribution to analytical chemistry.     

Protolytic dissociation of copper(II) and nickel(II) dipyrrolylmethenates in benzene solutions of acetic acid

Protolytic dissociation of copper(II) and nickel(II) dipyrrolylmethenates in benzene solutions of acetic acid has been studied. The results have completed the knowledge of kinetics of dipyrrolylmethene complexes dissociation in acidic medium. The effect of the nature of complex forming atom, ligand, and other factors on the complexes kinetic stability has been analyzed.     

Kinetics and mechanism of complex formation of nickel(II) with tetra-N-alkylated cyclam in N,N-dimethylformamide (DMF): comparative study on the reactivity and solvent exchange of the species Ni(DMF)6(2+) and Ni(DMF)5Cl+

13C NMR was used to study the rate of DMF exchange in the nickel(II) cation Ni(DMF)6(2+) and in the monochloro species Ni(DMF)5Cl+ with 13C-labeled DMF in the temperature range of 193-395 K in DMF (DMF = N,N-dimethylformamide). The kinetic parameters for solvent exchange are kex = (3.7 +/- 0.4) x 10(3) s-1, delta H++ = 59.3 +/- 5 kJ mol-1, and delta S++ = +22.3 +/- 14 J mol-1 K-1 for Ni(DMF)6(2+) and kex = (5.3 +/- 1) x 10(5) s-1, delta H++ = 42.4 +/- 4 kJ mol-1, and delta S++ = +6.7 +/- 15 J mol-1 K-1 for Ni(DMF)5Cl+. Multiwavelength stopped-flow spectrophotometry was used to study the kinetics of complex formation of the cation Ni(DMF)6(2+) and of the 100-fold more labile cation Ni(DMF)5Cl+ with TMC (1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) and TEC (1,4,8,11-tetraethyl-1,4,8,11-tetraazacyclotetradecane) in DMF at 298 K and I = 0.6 M (tetra-n-butylammoniumperchlorate). Equilibrium constants K for the addition of the nucleophiles DMF, Cl-, and Br- to the complexes Ni(TMC)2+ and Ni(TEC)2+ were determined by spectrophotometric titration. Formation of the complexes Ni(TMC)2+ and Ni(TEC)2+ was found to occur in two stages. In the initial stage, fast, second-order nickel incorporation with rate constants k1(TMC) = 99 +/- 5 M-1 s-1 and k1 (TEC) = 235 +/- 12 M-1 s-1 leads to the intermediates Ni(TMC)int2+ and Ni(TEC)int2+, which have N4-coordinated nickel. In the second stage, these intermediates rearrange slowly to form the stereochemically most stable configuration. First-order rate constants for the one-step rearrangement of Ni(TMC)int2+ and the two-step rearrangment of Ni(TEC)int2+ are presented. Because of the rapid formation of Ni(DMF)5Cl+, the reactions of Ni(DMF)6(2+) with TMC and TEC are accelerated upon the addition of tetra-n-butylammoniumchloride (TBACl) and lead to the complexes Ni(TMC)Cl+ and Ni(TEC)Cl+, respectively. For initial concentrations such that [TBACl]o/[nickel]o > or = 20, intermediate formation is 230 times (TMC) and 47 times (TEC) faster than in the absence of chloride. The mechanism of complex formation is discussed.     

Homogeneous isotopic exchange between nickel(II) and bis(resacetophenone oxime) nickel(II) complex

Isotopic exchange behaviour of bis(resacetophenone oxime) nickel(II) complex with nickel(II) in tri-n-butyl phosphate (TBP) and ethanol medium has been studied. The studies were carried out at different temperatures by varying the concentrations of both metal ion and the complex. Experimental observations showed that the complex is kinetically labile. Increase in temperature increases the isotopic exchange rate. Increase in concentration of either metal ion or complex results in significant increase of the reaction rate.     

High-spin metal complexes containing a ferromagnetically coupled tris(semiquinone) ligand

The tris-bidentate ligand 1,3,5-tris(5'-tert-butyl-3',4'-dihydroxyphenyl)benzene ((TBCat)(3)Ph) was synthesized. The reaction of this molecule in basic solution with two paramagnetic acceptors, i.e., a nickel(II)minus signtetraazamacrocyclic ligand complex (Ni(CTH)) (CTH = dl-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane) and manganese(II)-hydrotris[3-(4'-cumenyl)-5-methylpyrazolyl]borate (Mn(Tp(Cum,Me))), yielded two complexes whose analytical formulas are consistent with those of trinuclear complexes. Spectroscopic and magnetic measurements suggest that these derivatives contain divalent metal ions coordinated to the tris(semiquinone) form of the ligand. Analysis of the magnetic data shows that the pi-connectivity of the ligand enforces ferromagnetic coupling between the three semiquinone units of the molecule, giving rise to complexes with S = 9/2 (M = Ni(II)) and S = 6 (M = Mn(II)) ground states. The coupling within the tris(semiquinone) unit is quite large (J = -26 cm(-1) for the nickel(II) derivative and J = -40 cm(-1) for the manganese(II) one, using the general exchange Hamiltonian H = sigma J(ij)S(i)S(j)), and it is of the same order of magnitude as that observed in an analogous series of bis(semiquinone) complexes that we recently reported.     

Formation and dissociation kinetics and crystal structures of nickel(II)-macrocyclic tetrathiaether complexes in acetonitrile. Comparison to nickel(II)-macrocyclic tetramines

Complex formation and dissociation rate constants have been independently determined for solvated nickel(II) ion reacting with eight macrocyclic tetrathiaether ligands and one acyclic analogue in acetonitrile at 25 degrees C, mu = 0.15 M. The macrocyclic ligands include 1,4,8,11-tetrathiacyclotetradecane ([14]aneS4) and seven derivatives in which one or both ethylene bridges have been substituted by cis- or trans-1,2-cyclohexane, while the acyclic ligand is 2,5,9,12-tetrathiatridecane (Me2-2,3,2-S4). In contrast to similar complex formation kinetic studies on Ni(II) reacting with corresponding macrocyclic tetramines in acetonitrile and N,N-dimethylformamide (DMF), the kinetics of complex formation with the macrocyclic tetrathiaethers show no evidence of slow conformational changes following the initial coordination process. The differing behavior is ascribed to the fact that such conformational changes require donor atom inversion, which is readily accommodated by thiaether sulfurs but requires abstraction of a hydrogen from a nitrogen (to form a temporary amide). The latter process is not facilitated in solvents of low protophilicity. The rate-determining step in the formation reactions appears to be at the point of first-bond formation for the acyclic tetrathiaether but shifts to the point of chelate ring closure (i.e., second-bond formation) for the macrocyclic tetrathiaether complexes. The formation rate constants for Ni(II) with the macrocyclic tetrathiaethers parallel those previously obtained for Cu(II) reacting with the same ligands in 80% methanol-20% water (w/w). By contrast, the Ni(II) dissociation rate constants show significant variations from the trends in the Cu(II) behavior. Crystal structures are reported for the Ni(II) complexes formed with all five dicyclohexanediyl-substituted macrocyclic tetrathiaethers. All but one are low-spin species.     

The thermodynamic characteristics of formation of nickel(II) complexes with L-tyrosine in aqueous-ethanolic solvents

Acid dissociation constants of L-tyrosine (H₂Tyr) and the constants of complex formation between L-tyrosine and nickel(II) ions in water-ethanol mixtures were determined potentiometrically. The Gibbs energies of transfer of tyrosine, HTyr⁻ tyrosinate anion, and [NiHTyr]⁺ complex from water into binary solvents were calculated. An increase in the stability constants of the [NiHTyr]⁺ and [Ni(HTyr)₂] complexes in solvents with a high content of ethanol was caused by weakening of the solvation of amino ligand donor groups.