Ligand substitution reactions model mechanisms

The number of studies of inorganic reaction mechanisms by theoretical methods has increased drastically in the last decade. The studies cover ligand substitution reactions, insertion reactions oxidative addition, nucleophilic and electrophilic attack as well as metallacycle formation and surface chemistry, in addition to homogeneous and heterogeneous catalysis as well as metalloenzymes. We can expect the modeling to increase further both in volume and in sophistication [173],... 

Neither AG° nor A0 depends upon the mechanism of the reaction. But even if AG° is strongly negative, the yield of thermodynamically favored products may be negligible if the reaction proceeds too slowly on the human timescale or if the slowness of a critical step in the reaction sequence relative to some alternative steers the reaction to other (metastable) products. Thus, as Taube1 emphasized, we need to understand what makes some ligand substitution reactions fast and others slow. The mechanism of reaction is a simplified hypothetical model, an approximation to reality that purports to trace the progress of the system from reactants to products, and is significant only insofar as it helps us understand the kinetics and stereochemistry of the reaction (rather than vice versa as some workers tend to believe). 

Model Mechanisms and Other Considerations. Ligand substitution [Eq. (20)] is a fundamental reaction of metal complexes commonly observed as the result of photoexcitation. 

For a vibronically relaxed bound ES, ligand substitution mechanisms can be discussed in terms of models developed for analogous thermal reactions [36. The limiting mechanisms would be the dissociative (D) and associative (A) pathways, where the rate-limiting steps are, respectively, dissociation of the M-X bond or formation of the M-Y bond to form distinct intermediates (Eqs 6.16 and 6.17). The electronic nature of such intermediates is ambiguous, since these species may also be electronic excited states. For example, the cis to trans isomerization concomitant with the photoaquation of Cl from the Rh(lII) complex cis-Rh(NH3)4Cl2 was successfully explained by a model where Cl dissociation gave a pentacoordinate intermediate in a triplet LF excited state [37, 38]. 

Despite its success as an anticancer drug, the mechanism by which the drug targets DNA in the body is not fully understood, although it is known that the nucleobase guanine (see Fig. 10.13) binds more readily to Pt(ll) than the other nucleobases in DNA. Among model studies reported is that of the reactions of cisplatin and three related complexes with L-histidine and 1,2,4-triazole (Nu). The ligand substitutions occur in two, reversible steps ... 

The kinetics and mechanism of a range of slow metal-ligand substitution processes have been investigated, and a generalized theory was proposed that was very similar to the Berezin model for the interpretation of micelle kinetics in aqueous solutionsJ Ligands of different hydrophobicity were studied in terms of their reaction with Ni +(aq), and it was found that for p3rridine-2-azo-p-phenol (PAP), the rate constants kf for reaction in SDS micellar solution and water/Na-AOT/heptane systems were comparable, as shown in Table 10.2. kf is expressed as a first-order rate constant in the microemulsion.)... 

Before long, the results of individual studies were being consolidated into models, many of which traced their origins to the better-established field of mechanistic organic chemistry. For a time this sufficed, but major revisions and new assignments of mechanism became necessary for both ligand substitution and oxidation-reduction reactions. Mechanistic inorganic chemistry thus took on a shape of its own. 

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