Complex ions kinetic considerations

The effects of concentration, velocity and temperature are complex and it will become evident that these factors can frequently outweigh the thermodynamic and kinetic considerations detailed in Section 1.4. Thus it has been demonstrated in Chapter 1 that an increase in hydrogen ion concentration will raise the redox potential of the aqueous solution with a consequent increase in rate. On the other hand, an increase in the rate of the cathodic process may cause a decrease in rate when the metal shows an active/passive transition. However, in complex environmental situations these considerations do not always apply, particularly when the metals are subjected to certain conditions of high velocity and temperature. 

The complexity of the system consisting of the diazonium ion and the four reaction products shown in Scheme 5-14 is evident. In contrast to the two-step reaction sequence diazonium ion <= (Z)-diazohydroxide <= (Z)-diazoate (Scheme 5-1 in Sec. 5.1), equilibrium measurements alone cannot give unambiguous evidence for the elucidation of the mechanistic pathway from, for example, diazonium ion to ( )-diazoate. Indeed, kinetic considerations show that, depending on the reaction conditions (pH etc.) and the reactivity of a given diazonium ion (substituents, aromatic or heteroaromatic ring), different pathways become dominant. 

From the results presented it follows that the driving force behind the growth of technetium clusters in the process of their reduction is a decrease in the total electron energy of the ions due to the formation of M-M bonds. In fact, as is shown in Fig. 6, if the M-M bonds were absent the total electron energy of technetium complexes would be considerably higher and the complex would be unstable. However, besides purely thermodynamic reasons leading to the cluster formation, there should also be kinetic possibilities for these processes to take place. This aspect of technetium cluster formation is partially considered below. 

These measurements have been carried out in collaboration with de Maeyek[4]). The rate constant was found to be (1 3 0-2)-10n litres/ mol-sec thus the neutralization reaction is the fastest known bi-molecular reaction in aqueous solution. Molecular-kinetic considerations show that the velocity of recombination is solely determined by the collision frequency of the ions. Furthermore, the effective cross section of the proton is so large that the reaction already proceeds spontaneously when ions approach each other within a distance of two to three H-bonds. This means that the motion of the proton within the hydration complex (the diameter of which corresponds to about two to three H-bonds) proceeds rapidly compared to the actual movement of the ions towards each other. 

The kinetics of formation of complex ions is the subject of considerable interest in inorganic chemistry [9]. If the discussion is limited to octahedral complexes, then the type of reaction being considered is... 

The nitroprusside ion, [Fe(CN)sNO] has been widely used pharmacologically for the provision of NO. The related iron(II) complex ion is also of considerable interest but kinetic studies involving formation or dissociation of the latter can be fraught with the experimental uncertainty of knowing whether NO solutions are free from (oxidising) impurities. It has been shown recently from ambient and high pressure kinetic studies that the formation of [Fe(CN)5NO] from the pentacyanoaqua complex ion and NO is characterised by a markedly positive AV" value (-H7.4 0.3 cm mol ) and a positive AS value (-1-34 4 J mol K" ). 

The splitting of inorganic pyrophosphafe (PPj) into two inorganic phosphate ions is catalyzed by pyrophosphatases (p. 636) that apparently occur universally. Their function appears to be simply to remove the product PPj from reactions that produce it, shifting the equilibrium toward formation of a desired compound. An example is the formation of aminoacyl-tRNA molecules needed for protein synthesis. As shown in Eq. 17-36, the process requires the use of two ATP molecules to activate one amino acid. While the "spending" of two ATPs for the addition of one monomer imif to a polymer does not appear necessary from a thermodynamic viewpoint, it is frequently observed, and there is no doubt that hydrolysis of PP ensures thaf the reaction will go virtually to completion. Transfer RNAs fend to become saturated with amino acids according to Eq. 17-36 even if the concentration of free amino acid in the cytoplasm is low. On the other hand, kinetic considerations may be involved. Perhaps the biosynthetic sequence would move too slowly if if were nof for the extra boost given by the removal of PPj. Part of the explanation for the complexity may depend on control mechanisms which are only incompletely understood. 

Color and the Colors of Complexes 24-8 Aspects of Complex-Ion Equilibria 24-9 Acid-Base Reactions of Complex Ions 24-10 Some Kinetic Considerations 24-11 Applications of Coordination Chemistry... 

Some Kinetic Considerations—Also important in determining properties of a complex ion is the rate at which the ion exchanges ligands between its coordination sphere and the solution. Exchange is rapid in a labile complex and slow in an inert complex. 

The rate (kinetics) and the completeness (fraction dissolved) of oxide fuel dissolution is an inverse function of fuel bum-up (16—18). This phenomenon becomes a significant concern in the dissolution of high bum-up MO fuels (19). The insoluble soHds are removed from the dissolver solution by either filtration or centrifugation prior to solvent extraction. Both financial considerations and the need for safeguards make accounting for the fissile content of the insoluble soHds an important challenge for the commercial reprocessor. If hydrofluoric acid is required to assist in the dissolution, the excess fluoride ion must be complexed with aluminum nitrate to minimize corrosion to the stainless steel used throughout the facility. Also, uranium fluoride complexes are inextractable and formation of them needs to be prevented. 

A further factor which must also be taken into consideration from the point of view of the analytical applications of complexes and of complex-formation reactions is the rate of reaction to be analytically useful it is usually required that the reaction be rapid. An important classification of complexes is based upon the rate at which they undergo substitution reactions, and leads to the two groups of labile and inert complexes. The term labile complex is applied to those cases where nucleophilic substitution is complete within the time required for mixing the reagents. Thus, for example, when excess of aqueous ammonia is added to an aqueous solution of copper(II) sulphate, the change in colour from pale to deep blue is instantaneous the rapid replacement of water molecules by ammonia indicates that the Cu(II) ion forms kinetically labile complexes. The term inert is applied to those complexes which undergo slow substitution reactions, i.e. reactions with half-times of the order of hours or even days at room temperature. Thus the Cr(III) ion forms kinetically inert complexes, so that the replacement of water molecules coordinated to Cr(III) by other ligands is a very slow process at room temperature. 

The reaction was followed by means of the strong absorption of the Os(II) complex at 480 m/i. Unlike the Tl(riI) + Fe(II) system, there is a slight increase in rate as the hydrogen-ion concentration is increased. The kinetic data were interpreted on the basis that both Tl and TIOH react with Os(bipy)3 (with rate coefficients and respectively). At 24.5 °C and ju = 2.99 M, kj = 36.0 l.mole. see and= 14.7 l.mole sec corresponding activation energies are 6.90 and 11.5 kcal.mole" The latter values are considerably smaller than those for the T1(III) + T1(I) exchange and for the Tl(III)- -Fe(II) reaction . On the other hand, all three reactions are subject to retardation by Cl ions. 

Reduction of ketones to triphenylsilyl ethers is effected by the unique Lewis acid perfluorotriphenylborane. Mechanistic and kinetic studies have provided considerable insight into the mechanism of this reaction.186 The salient conclusion is that the hydride is delivered from a borohydride ion, not directly from the silane. Although the borane forms a Lewis acid-base complex with the ketone, its key function is in delivery of the hydride. 

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