Metal hydrolysis term

The stoichiometry of the proton in Eqs. (10.7) and (10.8) (denoted by j) is a collective term that represents three possible configurations for the M Hy cit +- - (6Z ) complex. The value of j is 1 when Hc iP occurs in the complex, as in MHcif (6Z ). In this instance, a single carboxyl and the hydroxyl are protonated on the citrate molecule. Such complexes are significant only when solution pH values are less than 4. More commonly, j is zero or negative, the latter representing either the occurrence of H iciP in the complex (all citrate moieties ionized) [as in MH icit (ag)], or the occurrence of H ciP and a metal hydrolysis product in the complex [as in MOH(H icit)2 (ag)]. Chemical models derived from potentiometric acid-base titration studies cannot distinguish between the two potential proton sources (citrate hydroxyl or metal-bound water), as titrations... 

Manning (2) Precipitation of metal oxides and hydroxides by hydrolysis includes interpretation in terms of potential -pH diagrams. 

The ability of metal ions to accelerate the hydrolysis of a variety of linkages has been a subject of sustained interest. If the hydrolyzed substrate remains attached to the metal, the reaction becomes stoichiometric and is termed metal-ion promoted. If the hydrolyzed product does not bind to the metal ion, the latter is free to continue its action and play a catalytic role. The modus operandi of these effects is undoubtedly as a result of metal-complex formation, and this has been demonstrated for both labile and inert metal systems. Reactions of nucleophiles other than HjO and OH will also be considered. 

If measurements are to be carried out at low activities (for example in studying complexation equilibria), standard solutions cannot be prepared by simple dilution to the required value because the activities would irreproducibly vary as a result of adsorption effects, hydrolysis and other side reactions. Then it is useful to use well-defined complexation reactions to maintain the required metal activity value [14, 50, 132]. EDTA and related compounds are very well suited for this purpose, because they form stable 1 1 complexes with metal ions, whose dissociation can be controlled by varying the pH of the solution. Such systems are often termed metal-ion buffers [50] (cf. also p. 77) and permit adjustment of metal ion activities down to about 10 ° m. (Strictly speaking, these systems are defined in terms of the concentration, but from the point of view of the experimental precision, the difference between the concentration and activity at this level is unimportant.)... 

Acid hydrolysis of an octahedral metal ion complex is typically a dissociative or SNl-type reaction. In the case of base hydrolysis, reactions tend to display SN2-type reaction mechanisms, although others take place by what is termed an SnI-conjugate base mechanism. The latter involves attack by an electrophile to abstract a proton... 

The observation that the pH of salt solutions, especially of those containing polyvalent metal ions, decreases on heating has lead to a different, yet very elegant, technique to generate well-defined metal (hydrous) oxides. Obviously, the increased acidity of such solutions must be due to the release of protons from the hydrated cations, which in turn change to hydroxide complexes. This process was termed by the senior author as forced hydrolysis (7,9-11). At appropriate temperatures and... 

Very acidic (high valent) cations will readily hydrolyse in aqueous solution, often even at low pH. These cations tend to form the polymeric metal oxide chains mentioned previously. This hydrolysis can be controlled by addition of boric acid (see Sec. 3.2.4.4) and forms the basis of a technique referred to as liquid phase deposition. This method can be reasonably included in the more general term of chemical solution deposition, and is treated, although not comprehensively, in this book. Ref 5 deals more thoroughly with this technique and describes many cases of SiOi as well as some examples of several other oxides not covered in this chapter. 

The above ideas are not limited to species with central metal ions. They apply to the higher oxidation states of non-metallic elements. Many simple anions do exist with primary hydration spheres in which the positive ends of dipoles are attracted to the central negative charge. Table 3.8 gives examples of ions that may be thought about in terms of the hydrolysis of parent hypothetical hydrated ions. 

In the same year Jacob Berzelius introduced the concept of catalysis, which he developed as a result of studies of the effects of acids and bases in promoting the hydrolysis of starch and of the effects of metals on the decomposition of hydrogen peroxide. Berzelius proposed the term catalyst from the Greek "katalysis," meaning "dissolution." Although he had been concerned primarily with inorganic catalysts, Berzelius recognized that a natural catalyst,... 

This reaction process is depicted in general terms in equation (34), and includes the category generalized in equation (35). The earliest example of this type of reaction appears to be the ready transesterification of uncomplexed ester groups shown in Scheme 39,133,134 which intramolecular participation forms a new chelate ring in the transition state. Since this discovery, numerous studies have been made on the intramolecular catalysis of ester hydrolysis by metal-complexed hydroxide... 

It was shown in Section III,B that a divalent cation was essential for enzymic hydrolysis of PP. The optimal concentrations of cation were of approximately the same level as that of PP, (Table IV), suggesting that metal ion was necessary for stoichiometric combination with PPi anion. Considering the case of Mg2 and PP- at pH 9.1, the situation can be analyzed in terms of specific Mg-PP, complexes. 

Essentially three different routes can be considered for the base hydrolysis of an amino acid ester in the presence of a metal ion (equations 8-10). In general terms hydrolysis of the monodentate N-coordinated ester (equation 8) would be expected to be somewhat similar to base hydrolysis... 

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