Kinetics of complexation in aqueous solutions

A detailed kinetic study of the water exchange of the dioxo complex of rhenium(V) (206), however, gave very valuable information about the relative reactivities of the aqua and hydroxo complexes toward substitution The ratio of the water exchange rate of the aqua and hydroxo complexes of rhenium(V) was determined to be about 50. The 0 NMR studies of these complexes demonstrate its effectiveness for the characterization of these type of complexes in aqueous solutions (207). 

Water plays a crucial role in the inclusion process. Although cyclodextrin does form inclusion complexes in such nonaqueous solvents as dimethyl sulfoxide, the binding is very weak compared with that in water 13 Recently, it has been shown that the thermodynamic stabilities of some inclusion complexes in aqueous solutions decrease markedly with the addition of dimethyl sulfoxide to the solutions 14,15>. Kinetic parameters determined for inclusion reactions also revealed that the rate-determining step of the reactions is the breakdown of the water structure around a substrate molecule and/or within the cyclodextrin cavity 16,17). 

Stability of the bidentate and multidentate complexes in aqueous solution [16] compared with monodentate complexes. Kinetic studies of gold(III) reactions with ethylenediamine and related ligands show that the initial displacement of one end of the chelate is most often followed by rapid reclosure of the ring, rather than displacement of the second bond to the metal ion [15]. 

This is the most common and stable state of chromium in aqueous solution. The Cr3+ ion, with 3d3 electrons, forms mainly octahedral complexes [CrX6], which are usually coloured, and are kinetically inert, i.e. the rate of substitution of X by another ligand is very slow consequently a large number of such complexes have been isolated (see below, under chromium(III) chloride). 

Dangles, O., Elhabiri, M., and Brouillard, R., Kinetic and thermodynamic investigation of the aluminium-anthocyanin complexation in aqueous solution, J. Chem. Soc., Perkin Trans. 2, 2587, 1994. 

Arteel GE, Briviba K, Sies H (1999) Protection against peroxynitrite. FEBS Lett 445 226-230 Asaumi A, Ogino T, Akiyama T, Kawabata T, Okada S (1996) Oxidative damages by iron-chelate complexes depend on the interaction with the target molecules. Biochem Mol Biol Int 39 77-86 Awad HH, Stanbury DM (1993) Autoxidation of NO in aqueous solution. Int J Chem Kinet 25 375-381... 

Rate data for the iodination of pyrazole in aqueous solution showed the reaction to be first-order in both iodine and heterocycle and an inverse first-order [H+] dependence was found over the pH range 5.96-6.74 (64JA2857). A kinetic study of the aqueous iodination of pyrazole coordinated to Ni2+ showed the coordinated ligand to react more rapidly, and a [H+] dependence that differed from that of the free ligand (82JA2460). However, the results of this study should be viewed with caution, as the presence of several nickel-pyrazole complexes in solution necessarily leads to uncertainties about the exact nature of the reactive species. 

Evidence for the existence of a true mononuclear Au(II) complex was first provided in 1954 by Rich and Taube (79), who, on the basis of kinetic evidence, proved the transient existence of [AuClJ2- in aqueous solution, as a reaction intermediate in the Fe(II)-catalyzed exchange of a radioactive Cl with [AuCLj]- [Eq. (2)]. 

As a preliminary to a discussion of kinetics of reactions in aqueous mixtures, it is interesting to review briefly the behaviour of equilibrium quantities as a function of co-solvent mole fraction. Interpretation of the data is necessarily complex because, for example, in the case of acid dissociation constants, the quantity 5mAXie represents the result of the individual variations of the partial molar quantities for acid, conjugate base and hydrogen ion. Nevertheless patterns of behaviour are observed which demonstrate the impact of co-solvent on water structure and on solute properties along the lines discussed in the previous section. 

There are literally thousands of chromium(III) complexes that, with a few exceptions, are hexacoordinate and octahedral. An important characteristic of these complexes in aqueous solutions is their relative kinetic inertness. Ligand displacement reactions of Cr111 complexes are only 10 times faster than those of Co111, with half-times in the range of several hours. It is largely because of this kinetic inertness that so many complex species can be isolated as solids and that they persist for relatively long periods of time in solution, even under conditions of marked thermodynamic instability. 

We have Investigated the kinetics of base hydrolysis reactions of the cobalt acldate complexes In aqueous solution and In several mlcroemulslon solutions In which detergent concentrations are at least twice the respective cmc. The results are complicated by the onset of a slow secondary reaction which Is presumably formation of Insoluble, polymeric hydroxo, or hydrated hydroxo compounds. 

The Kolbe-Schmitt reaction[l] has long history related with aspirin and has been a name reaction used for the longest period in an industrial process. While the demand for the manufacturing aromatic hydroxycarboxyhc acids is still successively coming out today with a number of patents, the mechanism of the reaction has remained unsolved. The present nmr spectroscopic studies have proved a [substrate CO2] complex or an intermediate prior to the formation of carboxyhc acids. Another puzzling question about the unstable complex even to moisture, is why the carboxylation of polyhydroxybenzenes, such as resorcinol, should proceed in aqueous solutions. Herein also reported are kinetic studies on the carboxylation of resorcinol in aqueous solutions of alkali hydrogencarbonates. 

III) forms very strong and kinetically stable cyanide complexes in aqueous solution (97). The study, performed by means of quantitative 205T1NMR and NMR spectroscopy in combination with pH measurements, resulted in the determination of the stability constants (Table... 

As far as the excimer decay kinetics of PAA in aqueous media is concerned, de Melo and coworkers [122,130,131] have investigated the time-resolved fluorescence from a series of samples modified with various amounts of pyrene and naphthalene, respectively. Even when the aromatic content was as low as 2mol%, excimer formation was evident in the steady-state spectra. The fluorescence decays were complex irrespective of the label and were best modeled by a triple-exponential function (as in Eq. 2.8) both when emission was sampled in the monomer and excimer regions. In contrast to the distribution of rate constants in the blob model [133,134], the authors favored a scheme that describes the decay kinetics in terms of discrete rate constants. The data were also consistent with previous schemes [124-127] that account for the presence of two distinct types of monomer in addition to that of excimer in macromolecular systems one monomer enjoys kinetic isolation and is unable to form excimers, whereas the second is able to participate in excimer formation within its fluorescence lifetime. The authors [130] concluded from both steady-state and time-resolved data that PAA undergoes a conformational change from a compact form in acidic solution to an open coil at high pH. Furthermore, as the... 

It is perhaps surprising that no p f measurements in water for even the most prototypical of all Fischer carbene complexes such as 68 had been reported before 1989/ 9 The most likely explanation for this state of affairs is that in aqueous solution the conjugate anions of carbene complexes hydrolyze rapidly which precludes equilibrium determinations by classical photometric methods. In fact, most p f values of Fischer complexes in aqueous solution were obtained by kinetic rather than equilibrium methods. For 68 the p f was determined in KOH solution by applying the relationship where is the rate... 

So, NMR spectra of 4,4 -bipyridyl, pyrazine, and ethylenediamine coordinated to undecatungstocobalto(III)silicate and -borate anions. Identification of 1 1 and dumbbell-shaped 12 complexes, Bull. Korean Chem. Soc, 14 759 (1993) (c) J.L. Samonte and M.T. Pope, Derivatization of polyoxotungstates in aqueous solution. Exploration of the kinetic stability ofcobalt(II)- and cobalt(III) derivatives of lacunary anions with pyridine and pyridine-type ligands. Can. J. Chem.. In press... 

Y. S. Fu, T. W. Swaddle, Electrochemical kinetics of cyanometalate complexes in aqueous-solution at... 

Chromium(iH) Complexes.15 There are literally thousands of chro-mium(m) complexes which, with a few exceptions, are all hexacoordinate. The principal characteristic of these complexes in aqueous solutions is their relative kinetic inertness. 

BAN/GLA] Banyai, I., Glaser, J., Micskei, K., Toth, I., Zekany, L., Kinetic behavior of carbonate ligands with different coordination modes Equilibrium dynamics for uranyl(2+) carbonato complexes in aqueous solution. A C and 0 NMR study, Inorg. Chem., 34, (1995), 3785-3796. Cited on page 349. 

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