Reactions ligand exchange mechanisms

Transmetalation, though, requires enhanced electrophilicity of the Pd. Additionally a free coordination site may be required, which may be freed by dissociation of either a neutral or an anionic ligand. The involvement of five-coordinate species and association-dissociation ligand-exchange mechanisms in the individual steps of Pd-catalyzed reactions also cannot be neglected (Scheme 3).384... 

Chemical analysis reveals that the ratio of acac ligands to V centers on the surface is 1, which means that the reaction has followed a ligand exchange mechanism ... 

Figure 7 presents the overall, idealized reaction mechanism. The surface of MCM-48 contains 0.9 OH / nmJ, which react completely with DMDCS in the liquid phase, if NEt3 is used as a catalyst. The majority of the silanols react monofunctionally but a small fraction also reacts further, according to reaction (3) to yield inert, bidentate species. All chlorine functions on the surface are converted towards hydroxyls upon hydrolysis. The VO(acac)2 is reacted in a gas-phase reactor with this silylated, hydrolyzed surface. All recreated silanols react with the VO(acac)2 in a 1 1 stoichiometry, following a ligand-exchange mechanism. Upon calcination at 450°C, the acac ligands are decomposed but the methylsilyl functions remain intact. Most of the V-species are converted into isolated, tetrahedral VvOx species, as indicated in Figure 4. However, a small fraction clusters to form surface oligomers, hereby recreating a fraction of the silanols.

Stability of Metal Complex. - The stability of the metal complex towards ligand exchange reaction plays a major role in determining the mechanism by which the metal complex becomes anchored to the surface. The ligand exchange mechanism for anchoring the metal complex to the surface proceeds as... 

To confirm fhat a dissociative ligand exchange mechanism is in operation, the kinetics of fhe rapid reaction of fhese complexes wifh efhyl vinyl ether was studied by NMR and UV/VIS spectroscopies. The results were in complete agreement with a dissociative hgand exchange mechanism for aU complexes in this series, with the possible exception of fhe diiodo derivatives [101]. 

Strongly binding anions, such as molybdate, sorb on oxides by a ligand-exchange mechanism that involves the exchange of an oxide-surface hydroxyl group (SOH) for an aqueous anion (A ) (Stumm, Kummert, and Sigg, 1980 Sposito, 1984 Stumm, 1992). This reaction results in the formation of an inner sphere complex and is illustrated as follows ... 

ZsR — Zp.) Equation 5.25b describes the inner-sphere complexation of an anionic species through the ligand exchange mechanism. In this reaction, actually a special case of Eq. 5.25a, SR comprises a permanent structural moiety S and an exchangeable ligand R the entity R in the general surface complex SR C does not exist, and so the symbol for the complex is shortened to SC, where C More generally, one could... 

Fig. 1.14. Reaction diagram for the dissociative ligand exchange mechanism for d-l cations.

Lithiation at C2 can also be the starting point for 2-arylatioii or vinylation. The lithiated indoles can be converted to stannanes or zinc reagents which can undergo Pd-catalysed coupling with aryl, vinyl, benzyl and allyl halides or sulfonates. The mechanism of the coupling reaction involves formation of a disubstituted palladium intermediate by a combination of ligand exchange and oxidative addition. Phosphine catalysts and salts are often important reaction components. 

The mechanism of action of the cyanation reaction is considered to progress as follows an oxidative addition reaction occurs between the aryl halide and a palladium(O) species to form an arylpalladium halide complex which then undergoes a ligand exchange reaction with CuCN thus transforming to an arylpalladium cyanide. Reductive elimination of the arylpalladium cyanide then gives the aryl cyanide. 

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