Rate Constants For Complex Formation Research Articles

Metal Flux and Dynamic Speciation at (Bio)interfaces. Part II: Evaluation and Compilation of Physicochemical Parameters for Complexes with Particles and Aggregates

The computation of metal flux in aquatic systems at consuming surfaces like organism membranes must consider the diffusion processes of metal ions, ligands, and complex species, as well as the kinetic and thermodynamic aspects of their chemical interactions. Many natural ligands, however, have complicated properties (formation of successive complexes for simple ligands, polyelectrolytic properties and chemical heterogeneity for macromolecular ligands, large size distribution and fractal structure for suspended aggregates). These properties should be properly modeled to get the correct values of the chemical rate constants and diffusion coefficients required for flux computations. The selection of the most appropriate models and parameter values is far from straightforward. In this series of papers, models and compilations of parameters for application to the three most important types of complexants found in aquatic systems, the small, simple ligands, the fulvic and humic compounds, and the colloidal "particles" or aggregates, are discussed. In particular, new approaches are presented to compute the rate constants of metal complex formation for both fulvics/humics and particles/aggregates. A method to include the site distribution of fulvics/ humics and the size distribution of particles/aggregates in metal flux computation at consuming interfaces is also discussed in detail. These models and parameters are discussed critically and presented in a single consistent framework, applicable to the computation of metal flux in presence of any of the above complexants ortheir mixtures. Part I of the series focuses on simple ligands and fulvic/ humic compounds. Part II deals with particulate and aggregate complexants.

Metal Flux and Dynamic Speciation at (Bio)interfaces. Part I: Critical Evaluation and Compilation of Physicochemical Parameters for Complexes with Simple Ligands and Fulvic/Humic Substances

In the computation of metal flux in aquatic systems, at consuming surfaces like organism membranes, diffusion processes of metal ions, ligands, and complex species, as well as the kinetic and thermodynamic aspects of their chemical interactions, must be considered. The properties of many natural ligands, however, are complicated (formation of successive complexes for simple ligands, polyelectrolytic properties and chemical heterogeneity for macromolecular ligands, large size distribution and fractal structure for suspended aggregates). These properties should be properly modeled to get the correct values of the chemical rate constants and diffusion coefficients required for flux computations. The selection of the most appropriate models and parameter values is far from straightforward. This series of papers discusses the various models and compiles the parameters needed for the three most important types of complexants found in aquatic systems: the small, simple ligands, the fulvic and humic compounds, and the colloidal "particles" or aggregates. In particular, new approaches are presented to compute the rate constants of metal complex formation, with both fulvics/humics and particles/aggregates. The method to include the site distribution of fulvics/humics and the size distribution of particles/aggregates in metal flux computation at consuming interfaces is also discussed in detail. These models and parameters are discussed critically and presented in the same framework, forthe computation of metal flux in presence of any of the above complexants or mixtures. Such parameters, largely spread in the literature, are gathered here and selected specifically for environmental applications. The focus in Part I of the series is on simple ligands and fulvic/humic compounds. Part II deals with particulate and aggregate complexants.

Electrochemical study of catechol–boric acid complexes

The electrochemical behavior of catechol–boric acid complexes in aqueous solutions has been studied using cyclic voltammetry and steady-state voltammetry with a rotating disk electrode. Different patterns of reactivity and various kinetic pathways have been examined. Based on a CE mechanism, the dissociation constant and homogeneous rate constants of complex formation and dissociation were estimated for each pathway by comparing the experimental cyclic voltammograms with the digitally simulated results. Also, the effects of pH and substituted groups on dissociation constants and reaction kinetics were studied.

Investigation on extraction rate of lanthanides with extractant-impregnated microcapsule

Abstract Extraction of lanthanum, samarium and erbium into a microcapsule containing 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (EHPNA) is studied. The extraction kinetic model used to predict uptake curves is based on the interfacial reaction model accompanied by intraparticle diffusion expressed by Fick's law. The uptakes are analyzed using the kinetic model with the separately determined complex formation rate constants and the intraparticle diffusion coefficient. The rate constants used in the calculation are determined from the analysis of the kinetics data for the liquid–liquid extraction system under a low-extractant concentration. The effects of pH and metal concentration on the metal uptakes into the microcapsule are successfully predicted with the determined rate constants. From the results, it is confirmed that the extraction mechanism of metal ions into the microcapsule is same to that for a liquid–liquid extraction system.

Plug−Plug Kinetic Capillary Electrophoresis: Method for Direct Determination of Rate Constants of Complex Formation and Dissociation

We present a method for direct determination of rate constants of complex formation, k(on), and dissociation, k(off). The method is termed plug-plug kinetic capillary electrophoresis (ppKCE). To explain the concept of the method, we consider the formation of a noncovalent complex C between molecules A and B; A is assumed to migrate slower in electrophoresis than B. In ppKCE, a short plug of A is injected into a capillary, followed by a short plug of B. When a high voltage is applied, the electrophoretic zone of B moves through that of A, allowing for the formation of C. When the zones of A and B are separated, C starts dissociating. The features of the resulting electropherogram are defined by both binding and dissociation. We developed a unique mathematical approach that allows finding k(on) and k(off) from a single electropherogram without nonlinear regression analysis. The approach uses algebraic functions with the only input parameters from electropherograms being areas and migration times of electrophoretic peaks. In this work, we explain theoretical bases of ppKCE and prove the principle of the method by finding k(on) and k(off) for a protein-ligand complex. The unique capability of the method to directly determine both k(on) and k(off) along with its simplicity make ppKCE highly attractive to a broad community of molecular scientists.

Kinetics of Ligand Binding to Nucleic Acids

Ligand binding to nucleic acids (NA) is considered as a stationaiy Markov process. It is shown that the probabilistic description of ligand-NA binding allows one to describe not only the kinetics of the change of number of bound ligands at arbitrary fillings but also to calculate stationaiy values of the number of bound ligands and its dispersion. The general analysis of absoiption isotherms and kinetics of ligand binding to NA make it possible to determine of rate constants of ligand-NA complex formation and dissociation.

Metal Speciation Dynamics in Colloidal Ligand Dispersions

In this work we propose a dynamic metal speciation theory for colloidal systems in which the complexing ligands are localized on the surface of the particles; i.e., there is spatial heterogeneity of binding sites within the sample volume. The differences between the complex formation and dissociation rate constants of complexes in colloidal dispersions and those in homogeneous solutions originate from the differences in kinetic and mass transport conditions. In colloidal systems, when the effective rate of dissociation of the surface complexes becomes fully diffusion controlled, its value is defined via the geometrical parameters of the particle. We assess the extent to which the conventional approach of assuming a homogeneously smeared-out ligand distribution overestimates the lability of surface complexes in colloidal ligand dispersions. The validity of the theory is illustrated by application to binding of lead and cadmium by carboxyl modified latex particles: our approach correctly predicts the formation/dissociation rate constants, which differ by several orders of magnitude from their homogeneous solution counterparts.

Kinetic studies on the first dihydrogen aquacomplex, [Ru(H 2)(H 2O) 5] 2+: Formation under H 2 pressure and catalytic H/D isotope exchange in water

The ruthenium(II) hexaaqua complex [Ru(H 2O) 6] 2+ reacts with dihydrogen under pressure to give the η 2-dihydrogen ruthenium(II) pentaaqua complex [Ru(H 2)(H 2O) 5] 2+.The complex was characterized by 1H, 2H and 17O NMR: δ H = −7.65 ppm, J HD = 31.2 Hz, δ O = −80.4 ppm ( trans to H 2) and δ O = −177.4 ppm ( cis to H 2).The H–H distance in coordinated dihydrogen was estimated to 0.889 Å from J HD, which is close to the value obtained from DFT calculations (0.940 Å).Kinetic studies were performed by 1H and 2H NMR as well as by UV–Vis spectroscopy, yielding the complex formation rate and equilibrium constants: k f = (1.7 ± 0.2) × 10 −3 kg mol −1 s −1 and K eq = 4.0 ± 0.5 mol kg −1.The complex formation rate with dihydrogen is close to values reported for other ligands and thus it is assumed that the reaction with dihydrogen follows the same mechanisn ( I d).In deuterated water, one can observe that [Ru(H 2)(H 2O) 5] 2+ catalyses the hydrogen exchange between the solvent and the dissolved dihydrogen.A hydride is proposed as the intermediate for this exchange.Using isotope labeling, the rate constant for the hydrogen exchange on the η 2-dihydrogen ligand was determined as k 1 = (0.24 ± 0.04) × 10 −3 s −1.The upper and lower limits of the p K a of the coordinated dihydrogen ligand have been estimated:3 < p K a < 14.

Polycomb group protein complexes exchange rapidly in livingDrosophila

Fluorescence recovery after photobleaching (FRAP) microscopy was used to determine the kinetic properties of Polycomb group (PcG) proteins in whole living Drosophila organisms (embryos) and tissues (wing imaginal discs and salivary glands). PcG genes are essential genes in higher eukaryotes responsible for the maintenance of the spatially distinct repression of developmentally important regulators such as the homeotic genes. Their absence, as well as overexpression, causes transformations in the axial organization of the body. Although protein complexes have been isolated in vitro, little is known about their stability or exact mechanism of repression in vivo. We determined the translational diffusion constants of PcG proteins, dissociation constants and residence times for complexes in vivo at different developmental stages. In polytene nuclei, the rate constants suggest heterogeneity of the complexes. Computer simulations with new models for spatially distributed protein complexes were performed in systems showing both diffusion and binding equilibria, and the results compared with our experimental data. We were able to determine forward and reverse rate constants for complex formation. Complexes exchanged within a period of 1-10 minutes, more than an order of magnitude faster than the cell cycle time, ruling out models of repression in which access of transcription activators to the chromatin is limited and demonstrating that long-term repression primarily reflects mass-action chemical equilibria.

Thermochemistry of Protein−DNA Interaction Studied with Temperature-Controlled Nonequilibrium Capillary Electrophoresis of Equilibrium Mixtures

We introduce temperature-controlled nonequilibrium capillary electrophoresis of equilibrium mixtures (NECEEM) and demonstrate its use to study thermochemistry of protein-DNA interactions. Being a homogeneous kinetic method, temperature-controlled NECEEM uniquely allows finding temperature dependencies of equilibrium and kinetic parameters of complex formation without the immobilization of the interacting molecules on the surface of a solid substrate. In this work, we applied temperature-controlled NECEEM to study the thermochemistry of two protein-DNA pairs: (i) Taq DNA polymerase with its DNA aptamer and (ii) E. coli single-stranded DNA binding protein with a 20-base-long single-stranded DNA. We determined temperature dependencies of three parameters: the equilibrium binding constant (Kb), the rate constant of complex dissociation (k(off)), and the rate constant of complex formation (k(on)). The Kb(T) functions for both protein-DNA pairs had phase-transition-like points suggesting temperature-dependent conformational changes in structures of the interacting macromolecules. Temperature dependencies of k(on) and k(off) provided insights into how the conformational changes affected two opposite processes: binding and dissociation. Finally, thermodynamic parameters, DeltaH and DeltaS, for complex formation were found for different conformations. With its unique features and potential applicability to other macromolecular interactions, temperature-controlled NECEEM establishes a valuable addition to the arsenal of analytical methods used to study dynamic molecular complexes.

Equilibrium and kinetics of the dinuclear complex formation between N, N′-ethylenebis(salicylideneiminato)copper(II) and metal(II,I) ions in acetonitrile

The equilibrium constants and rate constants of the reactions between N, N′-ethylenebis(salicylideneiminato)copper(II) ([Cu(salen)]) and metal(II,I) ions in acetonitrile have been spectrophotometrically determined. [Cu(salen)] acts as a didentate ligand to form a dinuclear complex. The rate constants for the very labile Mn(II), Fe(II), and Zn(II) ions were directly evaluated using a variable flow-rate instrument that was newly constructed for this study. The rate constants of the dinuclear complex formation for a series of metal(II) ions vary in parallel with those of the acetonitrile solvent exchange on the corresponding metal(II) ions. This finding indicates that the dinuclear complex formation reaction of the metal(II) ions proceeds via almost the same reaction mechanism as for the acetonitrile solvent exchange reaction.

Metal ion extraction in surfactant solution: Ni 2+(aq) and Cd 2+(aq) with the ligands PADA and PAR in SDS micellar systems

There is much interest in industrial chemistry in the selective extraction and concentration of toxic metals from aqueous process streams and liquid wastes. In this paper, the use and scope of two simple ligands pyridine-2-azo- p-dimethylaniline (PADA) and 4-(2-Pyridylazo) resorcinol (PAR) in micellar solution are explored in relation to nickel and cadmium ion extraction. The thermodynamics and kinetics of the complexation reactions of Ni 2+(aq) and Cd 2+ (aq) with PADA and PAR are discussed; also the stripping reaction, to re-form the metal ion, which is achieved on addition of H +(aq) is investigated. A dramatic enhancement of metal ion extraction ability by the ligands is observed for both systems, the increase in the apparent rate and equilibrium constants for complex formation being of the order of 100. The stripping reaction is rapid for both metal ions. The mechanisms of the various reactions are discussed.

Sweeping capillary electrophoresis: a non-stopped-flow method for measuring bimolecular rate constant of complex formation between protein and DNA.

We introduce sweeping capillary electrophoresis (SweepCE), a non-stopped-flow method for directly measuring the bimolecular rate constant of complex formation, and demonstrate its use for studying protein-DNA interaction. The capillary is prefilled with a solution of DNA, and electrophoresis is then carried out from a solution of the protein in a continuous mode. Because the electrophoretic mobility of the protein is greater than that of DNA, the protein continuously mixes with DNA and forms the protein-DNA complex. The complex migrates with a velocity higher than that of DNA and causes sweeping of DNA, which gave the name to the method. The bimolecular rate constant, kon, of complex formation can be determined from the time profile of DNA concentration using a simple mathematical model of the sweeping process. In this proof-of-principle work, we used SweepCE to directly measure kon = (3.4 +/- 0.6) x 106 M-1 s-1 for the interaction between single-stranded DNA-binding protein and a 15-mer DNA oligonucleotide. Along with nonequilibrium capillary electrophoresis of equilibrium mixtures (NECEEM), SweepCE establishes a universal and comprehensive platform for studying kinetic and equilibrium parameters of complex formation between biopolymers.

Kinetics of iron complexation by dissolved natural organic matter in coastal waters

In coastal waters, where conditions are variable and can change rapidly, the kinetics of iron complexation by NOM is a major factor affecting its speciation. In this study we have examined the kinetics of complexation of ferrous and ferric iron by terrigenous NOM in terms of formation rate constants and dissociation rate constants by employing competitive ligand methods with visible spectrophotometry for determination of the complexed iron. Rate constants for organic ferrous complex formation ranged from 500 to 7.5×10 4 M −1 s −1, and rate constants for complex dissociation ranged from 1×10 −6 to 3.6×10 −3 s −1. Formation rate constants for organic ferric complexes were comparable to those determined for strong iron binding ligands found in the open oceans, and from 2.1×10 5 to 9.6×10 7 M −1 s −1. Ferric complex dissociation data were fitted by a two ligand model, with rate constants for both ligand classes typically higher than those for the strong ligands in the open ocean. Rate constants varied from 2×10 −4 to 4×10 −3 s −1 for the ‘weak’ ligand class, and from 1.0×10 −6 to 1.3×10 −4 s −1 for the ‘strong’ class. All rate constants varied by several orders of magnitude between NOM samples of different origin, reflecting the highly variable composition of these substances. Calculated conditional stability constants for organic complexes with ferric species in seawater, Fe′, were generally similar to those measured in the open oceans. Conditional stability constants for the ferrous complexes were less than those for the ferric complexes by two to four orders of magnitude. There was a weak positive correlation between the conditional stability constants for ferrous and ferric complexes, suggesting that similar functional groups are involved in binding each of the two forms of iron. One of the samples of NOM had a conditional stability constant with Fe′ that was comparable with those for siderophores and similar strong iron binding compounds. These results suggest that in the absence of oxidants, complexes between ferrous iron and NOM may be relatively long-lived. Our work also suggests that organic complexes between iron and terrigenous NOM may be quite strong, and will have a major effect on iron solubility in coastal waters.

Theory of capillary formation in alginate gels

The formation of capillaries in alginate gel is a dissipative process coupled with hydrodynamic flow in the immediate neighborhood of the front of gel formation. The hydrodynamic flow is due to the contraction of the alginate chains resulting from the crosslinking reaction. As shown earlier, capillary formation only occurs above a critical value of the contraction velocity. In this paper a complete theoretical model of the ratio of the actual to the critical contraction velocity is presented. The theoretical predictions are compared with experimental data for the formation of copper alginate gel from solutions of sodium alginate and copper dichloride. The ratio of the actual to the critical contraction velocity is described as a function of the bulk concentrations, the diffusion coefficients, the properties of the alginate molecules and the rate constant of copper alginate complex formation. For capillary formation to occur, the rate constant must be neither too small nor too large. In agreement with experimental data the model predicts that capillary formation is restricted to a finite time interval and will not take place if the copper concentration is too low or the alginate concentration is too high.

Non-equilibrium capillary electrophoresis of equilibrium mixtures--appreciation of kinetics in capillary electrophoresis.

We describe a new electrophoretic method (patent pending), Non-Equilibrium Capillary Electrophoresis of Equilibrium Mixtures (NECEEM), and demonstrate its application to the study of protein-DNA interactions. A single NECEEM experiment allows for the determination of equilibrium and kinetic parameters of protein-DNA complex formation. The equilibrium mixture is prepared by mixing protein and DNA; it contains three components: free protein, free DNA, and the protein-DNA complex. A small plug of such a mixture is injected onto a capillary and the three components are separated under non-equilibrium conditions using a run buffer that does not contain the components of the equilibrium mixture. The protein-DNA complex decays during the NECEEM separation; the resulting electropherograms contain characteristic peaks and exponential curves. A simple analysis of a single electropherogram reveals two parameters: the equilibrium dissociation constant of the protein-DNA complex and the monomolecular rate constant of complex decay. The bimolecular rate constant of complex formation can then be calculated as the ratio of the two experimentally-determined constants. NECEEM was applied to find the equilibrium and kinetic parameters of interaction between an E. coli single-stranded DNA binding protein and a fluorescently-labeled oligonucleotide. The constants determined by NECEEM are in good agreement with those obtained by other methods. The new method is simple, fast, and accurate. It can be equally applied to other non-covalent molecular complexes.

Nonequilibrium capillary electrophoresis of equilibrium mixtures--a single experiment reveals equilibrium and kinetic parameters of protein-DNA interactions.

We introduce a novel electrophoretic method, nonequilibrium capillary electrophoresis of equilibrium mixtures (NECEEM), and demonstrate its use for studying protein-DNA interactions. The equilibrium mixture of protein and DNA contains three components: free protein, free DNA, and the protein-DNA complex. A short plug of such a mixture is injected into the capillary, and the three components are separated under nonequilibrium conditions. The resulting electropherograms are composed of characteristic peaks and exponential curves. An easy nonnumerical analysis of a single electropherogram reveals two parameters: the equilibrium binding constant and the monomolecular rate constant of complex decay. The bimolecular rate constant of complex formation can then be calculated as the product of the two experimentally determined constants. NECEEM was applied to study the interaction between single-stranded DNA binding protein and a fluorescently labeled 15-mer oligonucleotide. It allowed us to measure for the first time the rate constant of complex decay for this important protein-DNA pair, k-1 = 0.03 s-1. The value of the equilibrium binding constant, Kb = 3.6 x 10-6 M-1, was in good agreement with those measured by other methods. As low as 10-18 mol of the protein was sufficient for the measurements. Thus, the new method is simple, informative, and highly sensitive. Moreover, it can be equally applied to other noncovalent protein-ligand complexes. These features of NECEEM make this method an indispensable tool in studies of macromolecular interactions. They also emphasize the potential role of NECEEM in the development of extremely sensitive protein assays using nucleotide aptamers.

Kinetics and mechanism of the formation of nitroprusside from aquapentacyanoferrate(III) and NO: complex formation controlled by outer-sphere electron transfer.

The kinetics and mechanism of the reaction between nitric oxide and aquapentacyanoferrate(III) were studied in detail. Pentacyanonitrosylferrate (nitroprusside, NP) was produced quantitatively in a pseudo-first-order process. The complex-formation rate constant was found to be 0.252 +/- 0.004 M(-1) s(-1) at 25.5 degrees C, pH 3.0 (HClO(4)), and I = 0.1 M (NaClO(4)), for which the activation parameters are DeltaH++ = 52 +/- 1 kJ mol(-1), DeltaS++ = -82 +/- 4 J K(-1) mol(-1), and DeltaV++ = -13.9 + 0.5 cm(3) mol(-1). These data disagree with earlier studies on complex-formation reactions of aquapentacyanoferrate(III), for which a dissociative interchange (I(d)) mechanism was suggested. The aquapentacyanoferrate(II) ion was detected as a reactive intermediate in the reaction of aquapentacyanoferrate(III) with NO, by using pyrazine and thiocyanate as scavengers for this intermediate. In addition, the reactions of other [Fe(III)(CN)(5)L](n-) complexes (L = NCS(-), py, NO(2)(-), and CN(-)) with NO were studied. These experiments also pointed to the formation of Fe(II) species as intermediates. It is proposed that aquapentacyanoferrate(III) is reduced by NO to the corresponding Fe(II) complex through a rate-determining outer-sphere electron-transfer reaction controlling the overall processes. The Fe(II) complex rapidly reacts with nitrite producing [Fe(II)(CN)(5)NO(2)](4)(-), followed by the fast and irreversible conversion to NP.

A detailed mechanistic study of the substitution behavior of an unusual seven-coordinate iron(III) complex in aqueous solution.

A detailed mechanistic study of the substitution behavior of a 3d metal heptacoordinate complex, with a rare pentagonal-bipyramidal structure, was undertaken to resolve the solution chemistry of this system. The kinetics of the complex-formation reaction of [Fe(dapsox)(H(2)O)(2)]ClO(4) (H(2)dapsox = 2,6-diacetylpyridine-bis(semioxamazide)) with thiocyanate was studied as a function of thiocyanate concentration, pH, temperature, and pressure. The reaction proceeds in two steps, which are both base-catalyzed due to the formation of an aqua-hydroxo complex (pK(a1) = 5.78 +/- 0.04 and pK(a2) = 9.45 +/- 0.06 at 25 degrees C). Thiocyanate ions displace the first coordinated water molecule in a fast step, followed by a slower reaction in which the second thiocyanate ion coordinates trans to the N-bonded thiocyanate. At 25 degrees C and pH <4.5, only the first reaction step can be observed, and the kinetic parameters (pH 2.5: k(f(I)) = 2.6 +/- 0.1 M(-1) s(-1), DeltaH(#)(f(I)) = 62 +/- 3 kJ mol(-1), DeltaS(#)(f(I)) = -30 +/- 10 J K(-1) mol(-1), and DeltaV(#)(f(I)) = -2.5 +/- 0.2 cm(3) mol(-1)) suggest the operation of an I(a) mechanism. In the pH range 2.5 to 5.2 this reaction step involves the participation of both the diaqua and aqua-hydroxo complexes, for which the complex-formation rate constants were found to be 2.19 +/- 0.06 and 1172 +/- 22 M(-1) s(-1) at 25 degrees C, respectively. The more labile aqua-hydroxo complex is suggested to follow an I(d) or D substitution mechanism on the basis of the reported kinetic data. At pH > or =4.5, the second substitution step also can be monitored (pH 5.5 and 25 degrees C: k(f(II)) = 21.1 +/- 0.5 M(-1) s(-1), DeltaH(#)(f(II)) = 60 +/- 2 kJ mol(-1), DeltaS(#)(f(II)) = -19 +/- 6 J K(-1) mol(-1), and DeltaV(#)(f(II)) = +8.8 +/- 0.3 cm(3) mol(-1)), for which an I(d) or D mechanism is suggested. The results are discussed in terms of known structural parameters and in comparison to relevant structural and kinetic data from the literature.

Electron-transfer kinetics and equilibria of copper(ii/i) complexes with 1,4,7-trithiacyclononane. A square scheme mechanism involving ligand additionBased on the presentation given at Dalton Discussion No. 4, 10–13th January 2002, Kloster Banz, Germany.

The electron-transfer kinetics of copper(II/I) complexes formed with the macrocyclic terdentate ligand 1,4,7-trithiacyclononane ([9]aneS3 = TTCN = L) have been investigated under a variety of conditions. The relevant equilibrium constants, complex formation and dissociation rate constants, and redox potentials in both water and acetonitrile have also been determined. The predominant oxidized species in both solvents is CuIIL2, although the 1 ∶ 1 complex, CuIIL(H2O)3, can become dominant in water at high Cu(II) concentrations. The predominant reduced species is the 1 ∶ 1 complex, CuIL (i.e., CuIL(H2O) or CuIL(CH3CN)), as confirmed by electrospray mass spectrometry, pulsed square-wave voltammetry, cyclic voltammetry and the ligand dependence of the oxidation kinetics. Electron transfer occurs almost exclusively through the bis redox couple, CuII/IL2, even for solutions containing predominantly CuIIL(H2O)3. In the latter case, reduction involves a three-step sequence in which (i) CuIIL(H2O)3 reacts with L to produce CuIIL2, (ii) electron transfer occurs and (iii) L dissociates again to yield CuIL(H2O). The sluggishness of direct electron transfer in the 1 ∶ 1 complex is attributed to the unfavorable energetics of forming or dissociating strong copper–solvent bonds combined with the accompanying re-structuring of the surrounding solvent.