By metal ions and complexes

Hydrolysis of coordinated ligands is a special case of nucleophilic attack. Two examples involving inorganic ligands have already been given in Section II. A on aquation of cobalt(III) complexes. Many further examples will be found in the following Section VII.B on catalysis of hydrolysis of organic substrates by metal ions and complexes. 

Lewis acid catalysis of the hydrolysis of peptide bonds by metal ions and complexes has been investigated S with regard to the hydrolytic stability of proteins. An amide linkage in a protein has a half-life of about seven years at neutral pH and 25 C. In model systems, the hydrolysis rate can be increased by ca. 10 through coordination of the amide carbonyl group to a cationic metal complex. The coordination to cobaltflll) and subsequent Lewis acid assisted hydrolysis of the amide group in the simple amide, 4-formylmorpholine is illustrated in Scheme 1.19 55... 

Titanium tetrachloride and tin tetrachloride can form complexes that are related in character to both those formed by metal ions and those formed by neutral Lewis acids. Complexation can occur with an increase in the coordination number at the Lewis acid or with displacement of a chloride from the metal coordination sphere. 

Zeng, J., and Jewsbury, R. A. (1993). Enhanced bioluminescence of bacterial luciferase induced by metal ions and their complexes. In Szalay, A. A., et al. (eds.), Biolumin. Chemilumin., Proc. Int. Symp., 7th, pp. 173-177. Wiley, Chichester, UK. 

According to R. Brdicka and K. Vesely the carbonyl form of formaldehyde is reduced and the limiting kinetic current is given by the rate of the chemical volume reaction of dehydration. An analogous situation occurs for the equilibria among complexes, metal ions and complexing agents if the rates of complex formation and decomposition are insufficient to preserve the equilibrium. A simple example is the deposition of cadmium at a mercury electrode from its complex with nitrilotriacetic COO"... 

Catalysis by Transition Metal Ions and Complexes in Hydrocarbon Oxidation by Dioxygen... 

CATALYSIS BY TRANSITION METAL IONS AND COMPLEXES IN HYDROCARBON OXIDATION BY DIOXYGEN... 

Ketones are resistant to oxidation by dioxygen in aqueous solutions at T= 300-350 K. Transition metal ions and complexes catalyze their oxidation under mild conditions. The detailed kinetic study of butanone-2 oxidation catalyzed by ferric, cupric, and manganese complexes proved the important role of ketone enolization and one-electron transfer reactions with metal ions in the catalytic oxidation of ketones [190-194],... 

The Schiff base can undergo a variety of reactions in addition to transamination, shown in Fig. 6.4 for example, racemization of the amino acid via the a-deprotonated intermediate. Many of these reactions are catalyzed by metal ions and each has its equivalent nonmetallic enzyme reaction, each enzyme containing pyridoxal phosphate as a coenzyme. Many ideas of the mechanism of the action of these enzymes are based on the behavior of the model metal complexes. 

Catalytic Decomposition of Hydrogen Peroxide by Ferrous Ions Catalysis by Transition Metal Ions and Complexes in Liquid-Phase Oxidation of Hydrocarbons and Aldehydes by Dioxygen... 

In the first step, a substrate coordinates to a metal catalyst and forms an intermediate mixed complex (LMS in Scheme 13). The substrate is then activated by metal ions and dissociates from the catalyst. The complex catalyst, having accomplished its purpose, is regenerated to the original complex. The catalytic action of a metal ion depends substantially on the nature of the ligands in the intermediate mixed complex. Certain ligands induce an increase in catalytic activity, while others, e.g. multidentate ligands such as ethylenediaminetetracetic acid, inhibit the catalytic action of a metal ion. Therefore, if a polymer ligand is used as one component of a metal-complex catalyst, its properties may affect the catalytic action of the metal ion. 

The resultant hydroxyl radicals are effective in initiating many chain reactions. The number of metal ions and complexes which are capable of activating hydrogen peroxide in this manner is quite large and is determined in part by the redox potentials of the activator. Related systems in which free radicals are generated by the intervention of suitable metallic catalysts include many in which oxygen is consumed in autoxidations. Cobalt(H) compounds which act as oxygen carriers can often activate radicals in such systems by reactions of the type ... 

Thermal decompositions of peroxycarboxylic acids and dieir salts can proceed by free-radical and nonradical padrs. Often the decomposition products and the rate are affected by the nature of the solvent. Peroxycarboxylic acids undergo photodecomposition and radical-induced decomposition. They also are decomposed by a variety of metals, metal ions, and complexes. 

The hydrolysis of 8-acetoxyquinoline (159) is subject to catalysis by metal ions and detailed kinetic studies of the reaction have been reported.509 The metal ion could be bound to the carbonyl oxygen (160) or the ether oxygen (161) and the actual structure of the catalytically active complex was not unequivocally defined. 

The trans cis isomerization of [Cr(C204)(H20)2] is also catalyzed by metal ions and a detailed study of catalysis by Mg11 has been published.540 Dissociation of oxalato complexes of Crin of the type [Cr(ox)n(OH2)6 2n](3 2n)+ (n = l, 3) is also promoted by a series of transition metal ions.541... 

It is found that the hydrolysis of fluorophosphate esters is also accelerated by transition metal ions and complexes. This would be an observation of little general interest, except for the fact that fluorophosphate esters form one of the more commonly encountered types of nerve gases (Fig. 4-51). The hydrolysis of fluorophosphate esters is increased dramatically in the presence of copper(n) and other transition metal complexes, and this sug-... 

These concepts are important for an understanding of the roles played by metal ions and their complexes in the catalysis of oxidation reactions via homo-lytic mechanisms. Thus, metal complexes may function as catalysts by interfering with any of the various initiation, propagation, and termination steps outlined earlier. 

The ability to catalyze certain reactions of molecular hydrogen homogeneously in solution has been demonstrated for many transition metal ions and complexes (34)—among them complexes of Cu Cu Ag Hg Hgi, Col, Coll, pdii, Ptii, Rhi, Rh i, Ru i Ruiii, and Ir. In each case it appears that H2 is split by the catalyst with the formation of a reactive transition metal hydride complex (which may or may not be detected) as an intermediate. Three distinct mechanisms by which this can occur have been recognized (34), which are exemplified by the following reactions. 

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