Catalytic processes ligand substitution

As a complex functions as a catalyst, it is often necessary for one ligand to enter the coordination sphere of the metal and another to leave (before or as the other ligand enters). These processes are substitution reactions, which were discussed in some detail in Chapter 20. As the catalytic processes are illustrated, it will be seen that some of the elementary steps are substitution reactions. A substitution reaction can be shown by the general equation... 

In the various homogeneous catalytic schemes, the solvent may be coordinated to the metal or may simply be present as bulk solvent. When a ligand leaves the coordination sphere of a metal, it may be replaced by a molecule of solvent in a process that is either associative or dissociative. There is no general way to predict which type of mechanism is operative, so in some cases the substitution reactions will be described as they relate to specific processes. Because substitution reactions have been described in Chapter 20, several other types of reactions that constitute the steps in catalytic processes will be described in greater detail. 

Moreover, looking for more effective ligands, Sharpless and his group prepared and tested a number of cinchona alkaloid derivatives, first in the stoichiometric ADH process [33] and then in the catalytic process. They found that aryl ethers of dihydroquinidine, as 4a and 4b, are excellent ligands for ADH of dialkyl substituted olefins (Table 10.3). 

A unique aspect of the catalytic activity of CA is the fact that the hydroxo form of the enzyme catalyzes the hydration of CO2 through the direct binding of CO2 to the hydroxo ligand, whereas the aqua form of the enzyme catalyzes the dehydration of hydrogen carbonate through a ligand substitution process. This difference in mechanism is nicely demonstrated by the overall volume profile shown in Figure 23, which was constructed on the basis of the effect of pressure on the catalytic hydration and dehydration processes. Both these catalytic processes show characteristic pH dependencies that center around the pXa value of the coordinated water molecule. Many model Zn(II) and... 

The PTA ligand has recently been employed as a water-soluble ligand in a variety of studies, including catalytic biphasic hydrogenation reactions,4,5 ligand-substitution reactions in metal clusters,6 and enzyme-mediated oxo-transfer processes.7... 

The possibility of the practical application of the catalytic photode-composition of water based on the reactivity of the excited states of tris(2,2 -bipyridine) complexes of ruthenium(III) and ruthenium(II) has attracted considerable interest, but it is now clear that the efficiency of this process is limited not only by the lack of efficient catalysts, particularly for the dioxygen-evolving path, but also by both thermal and photochemical ligand oxidation 1,2) and ligand substitution reactions (3) of the 2,2 -bipyridine complexes. The stoichiometrically analogous tris(2,2 -bipyridine) and tris(l,10-phenanthroline) complexes of both... 

Mechanistic Pathways for Ligand Substitution Processes in Metal Carbonyls, 21, 113 Mechanistic Pathways in the Catalytic Carbonylation of Methanol by Rhodium and Iridium Complexes, 17, 255... 

In the Shell process (SHOP) phosphine-modified cobalt-catalyzed hydrofor-mylation is one of the steps in the synthesis of linear alcohols with 12-15 carbon atoms (see Section 7.4.1). Two important characteristics of this reaction should be noted. First, the phosphine-modified precatalyst HCo(CO)3(PBu3) is less active for hydroformylation than HCo(CO)4 but more active for the subsequent hydrogenation of the aldehyde. In this catalytic system both hydroformylation and hydrogenation of the aldehyde are catalyzed by the same catalytic species. Second, the phosphorus ligand-substituted derivatives are more stable than their carbonyl analogues at higher temperatures and lower pressures (see Table 5.1). 

The value for the rate constant of formation of Equation (19) (k ) was obtained to be 90 M s . The key processes for the catalytic extraction of Ni(II)-pan with PADA are the fast aqueous phase formation of NKpada) " " and the adsorption of Ni(pan)(pada)+, followed by the ligand substitution of pada with pan [50]. This scheme is generally of importance as a guideline for the acceleration of the extraction rate using the interfacial reaction. 

Ligand substitution reactions (see Ligand Substitution) are at the heart of many catalytic transformations, since creation of vacant coordination sites oftentimes requires liberation of one or more ligands from the coordination sphere of the metal. Important and/or industrially relevant processes using organoruthenium catalysts include the... 

The reactivity of Cr complexes is marked by very slow Ligand Substitution reactions, resulting in unusual configurational stability. Many chiral complexes have been resolved. There has been a recent resurgence of interest in organochrominm(III) complexes, owing to the importance of chromium in catalytic processes see Chromium Organometallic Chemistry). 

The analogous uranium(III) compounds also show unusual reactivity patterns. For example, addition of COT to (Cp )3U yields a mixed metallocene dimer (18) bridged by a COT ligand in this reaction, the (Cp )3U complex has effectively acted as a formal three-electron reductant (equation 3)." A variety of substituted (Cp )3U complexes form adducts with CO and CNR (isocyanides) - these are rare examples of actinide metals with r-acidic ligands." Paramagnetic uranium alkyl complexes are active in a range of catalytic processes." ... 

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