Copper complexes formation rate constants

When the model could be fitted to experimental data values for the rate constant kR could be estimated from the adjustment of 61 value (eq 3.55) and measurements or estimations of the parameters Dhr, K and K2 These ks values were regarded as representative of the complex formation rate constants for water replacement on the ion by water soluble extractant anions. So, values of 26 m (kmol and 0.00468 [ti (kmol s)" have been found [58] for the complexation of cobalt and nickel by HEHEHP, and (2-9) x 10 m (kmol s) for the reaction of copper with... 

Figure 4. Copper complexation by a pond fulvic acid at pH 8 as a function of the logarithm of [Cu2+]. On the x-axis, complex stability constants and kinetic formation rate constants are given by assuming that the Eigen-Wilkens mechanism is valid at all [M]b/[L]t. The shaded zone represents the range of concentrations that are most often found in natural waters. The + represent experimental data for the complexation of Cu by a soil-derived fulvic acid at various metakligand ratios. An average line, based on equations (26) and (30) is employed to fit the experimental data. Data are from Shuman et al. [2,184]...

The kinetics and mechanisms of substitution reactions of metal complexes are discussed with emphasis on factors affecting the reactions of chelates and multidentate ligands. Evidence for associative mechanisms is reviewed. The substitution behavior of copper(III) and nickel(III) complexes is presented. Factors affecting the formation and dissociation rates of chelates are considered along with proton-transfer and nucleophilic substitution reactions of metal peptide complexes. The rate constants for the replacement of tripeptides from copper(II) by triethylene-... 

We can now make sensible guesses as to the order of rate constant for water replacement from coordination complexes of the metals tabulated. (With the formation of fused rings these relationships may no longer apply. Consider, for example, the slow reactions of metal ions with porphyrine derivatives (20) or with tetrasulfonated phthalocyanine, where the rate determining step in the incorporation of metal ion is the dissociation of the pyrrole N-H bond (164).) The reason for many earlier (mostly qualitative) observations on the behavior of complex ions can now be understood. The relative reaction rates of cations with the anion of thenoyltrifluoroacetone (113) and metal-aqua water exchange data from NMR studies (69) are much as expected. The rapid exchange of CN " with Hg(CN)4 2 or Zn(CN)4-2 or the very slow Hg(CN)+, Hg+2 isotopic exchange can be understood, when the dissociative rate constants are estimated. Reactions of the type M+a + L b = ML+(a "b) can be justifiably assumed rapid in the proposed mechanisms for the redox reactions of iron(III) with iodide (47) or thiosulfate (93) ions or when copper(II) reacts with cyanide ions (9). Finally relations between kinetic and thermodynamic parameters are shown by a variety of complex ions since the dissociation rate constant dominates the thermodynamic stability constant of the complex (127). A recently observed linear relation between the rate constant for dissociation of nickel complexes with a variety of pyridine bases and the acidity constant of the base arises from the constancy of the formation rate constant for these complexes (87). 

The glycine complex of cobalt(n)-bipy, like that of copper(n>-bipy, has a comparatively high stability constant relative to the bis-glydnate of the metal. It has been shown that this can be attributed to a lower dissociation rate constant for the former (55 s at 25 °C) than for the latter (330 s ), the formation rate constants being rather similar (1.6 x 10 and 2.0 x 10 1 mol s respectively). 

Formation kinetics for eight tetraaza macrocycles of the cyclam type reacting with copper(II) have been analyzed in terms of rate constants for reaction with [Cu(OH)3] and with [Cu(OH)4]2. There is a detailed discussion of mechanism and of specific steric effects (292). Complex formation from cyclam derivatives containing -NH2 groups on the ring -CH2CH2CH2- units proceeds by formation followed by kinetically-distinct isomerization. The dramatic reactivity decreases consequent on... 

Eichhom and his co-workers have thoroughly studied the kinetics of the formation and hydrolysis of polydentate Schiff bases in the presence of various cations (9, 10, 25). The reactions are complicated by a factor not found in the absence of metal ions, i.e, the formation of metal chelate complexes stabilizes the Schiff bases thermodynamically but this factor is determined by, and varies with, the central metal ion involved. In the case of bis(2-thiophenyl)-ethylenediamine, both copper (II) and nickel(II) catalyze the hydrolytic decomposition via complex formation. The nickel (I I) is the more effective catalyst from the viewpoint of the actual rate constants. However, it requires an activation energy cf 12.5 kcal., while the corresponding reaction in the copper(II) case requires only 11.3 kcal. The values for the entropies of activation were found to be —30.0 e.u. for the nickel(II) system and — 34.7 e.u. for the copper(II) system. Studies of the rate of formation of the Schiff bases and their metal complexes (25) showed that prior coordination of one of the reactants slowed down the rate of formation of the Schiff base when the other reactant was added. Although copper (more than nickel) favored the production of the Schiff bases from the viewpoint of the thermodynamics of the overall reaction, the formation reactions were slower with copper than with nickel. The rate of hydrolysis of Schiff bases with or/Zw-aminophenols is so fast that the corresponding metal complexes cannot be isolated from solutions containing water (4). 

A large number of 7i-complexes of Cu(I) are known. Recently a detailed study of the kinetics of formation (123) and the stability constants of copper(I) complexes with the three acid-base forms of fumaric and maleic acid were carried out (153). The results demonstrate that the rate constants of the reaction... 

For the decarboxylation of oxaloacetic acid the reactions shown in Scheme 26 can be considered. Early measurements of rate constants (k) and formation constants (XMA) are summarized in Table 24. The copper complex CuA decarboxylates some 9.4 x 102 times faster than the dianion A2-. 

Kinetically slow steps in the formation of melanin from DOPA are the formation of dopaquinone from DOPA (step 1, kD), the reaction of dopachrome to dihydroxyindole (step 2), and the polymerization to form melanin (step 3, kM). Step 1 and step 2 proceed with about the same rate in the oxidative coupling polymerization catalyzed by tyrosinase. However, step 1 becomes remarkably slow when a macromolecule-metal complex is used as a catalyst. The copper complex in poly(l-vinylimidazole-co-vinylpyrrolidone) has been found [38] to act as an excellent catalyst and to exhibit the highest activity for melanin formation. The ratio of the rate constants ( m/ d) is approximately 3 (tyrosinase... 

Reaction of Cytochrome cIinn with Bis(ferrozine)copper(II) Knowledge of the redox properties of cytochrome c was an encouragement to initiate a kinetics investigation of the reduction of an unusual copper(II) complex species by cyt c11. Ferrozine (5,6-bis(4-sulphonatophenyl)-3-(2-pyridyl)-1,2.4-triazine)286 (see Scheme 7.1), a ligand that had come to prominence as a sensitive spectrophotometric probe for the presence of aqua-Fe(II),19c,287 forms a bis complex with Cu(II) that is square pyramidal, with a water molecule in a fifth axial position, whereas the bis-ferrozine complex of Cu(I) is tetrahedral.286 These geometries are based primarily upon analysis of the UV/visible spectrum. Both complexes are anionic, as for the tris-oxalato complex of cobalt in reaction with cytochrome c (Section 7.3.3.4), the expectation is that the two partners will bind sufficiently strongly in the precursor complex to allow separation of the precursor formation constant from the electron transfer rate constant, from the empirical kinetic data. 

The modulation of the coordination to the transition metal has not necessarily positive implications on the reactivity. For instance, we observed [50] that the copper(II) complex (8) of tetramethyl-l,2-diaminoethane catalyzes the hydrolysis of the phosphoric acid triester PNPDPP via an electrophilic mechanism which involves the pseudointramolecular attack of deprotonated water, as illustrated in (9). The electrophilic mechanism contribution to the hydrolytic process totally disappears in micellar aggregates made of the amphiphilic complex (10). Clearly, micellization does not allow the P O group of the substrate to interact with the metal ion. This could be a result of steric constraint of the substrate when bound to the micelle and/or the formation of binuclear dihydroxy complexes, like (11), in the aggregate. So, in spite of the quite large rate accelerations observed [51] in the cleavage of PNPDPP in metallomicelles made of the amphiphilic complex (10), the second-order rate constant [allowing for the difference in pXa of the H2O molecules bound to copper(II) in micelles and monomers] is higher for (8) than for (10) (k > 250). 

The intermediate produced in the flash photolysis of copper(ii) oxalato complexes in deaerated aqueous solution has been identified as CUCO2. This species, which is also generated by pulse radiolysis of the Cu -oxalate-formate system, decays by first-order kinetics. A dependence of the rate constant on pH and on the concentrations of Cu and oxalate ions is established and this is interpreted in terms of competing reactions of CUCO2 [equations (7) and (8)]. 

Metal ions have quite marked effects on the hydrolysis of methyl 8-hydroxyquinoline-2-carboxylafe (50) and ethyl l,10-phenanthroline-2-carboxylate (51). Base hydrolysis of the 8-hydroxyquinoline derivative (50) was studied over the pH range 9.2-12.1 and values of fcoH determined for HA and the anion A". Formation constants were determined at 25 C for the equilibrium + A iMA", as were the rate constants fcon for the base hydrolysis of the MA" complexes (Table 19). Quite large rate accelerations are observed (10 -10 ) when comparisons are made with the base hydrolysis of A". The charge carried by the complex does not appear to be a major factor in determining base hydrolysis rates. Thus at 25 °C the complex CuA undergoes base hydrolysis (fcoH = 6.3xl0 M s ) at a very similar rate to the corresponding copper(II) complex of ethyl l,10-phenanthroline-2-carboxylate (51) which carries a dipositive... 

The maximum catalytic activity for IVI-AA copolymer has been observed at a polymer copper(I I) ratio of 17 1. Activation energies and effective rate constants, Ar ff, have been determined from results of the measurement of the rate of quinone formation over a 288-313 K range of temperature. Thus, at pH 5.25 and [Cu " ] = 1.2-10 mole/1, these values are 111.4kJ/mole and 1.32-10 min , and 79.5 kJ/mole and 8.55 10 min for low-molecular weight and polymeric catalysts respectively. Because the limiting step of the process is the oxidation from Cu(I) to Cu(Il), the incorporation of polyampholytes which form more stable complexes with Cu(II) than with Cu(I) is expected to increase the catalytic activity of the Cu(II) ions. 

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