Dissociative mechanism substitution reactions

This pathway is energetically favored by the gain in aromaticity of the six-membered ring in the -intermediate, a stabilization not available to Cp. It should be noted, however, that this indenyl effect is specific to an associative mechanism substitution reactions of RuCl(rf-ind)(PPh3)2, which proceed via a dissociative pathway, are only one order of magnitude faster than those of RuCl(Cp)(PPh3)2.6... 

Mechanisms and Rates of Lead-Chelate Dissociation and Substitution Reactions... 

In the foregoing examples of nucleophilic substitution at phosphorus(v), an associative mechanism appears to operate. In contrast, both solvent effects and product stereochemistry in solvolysis of methyl A -cyclohexyl-phosphoramidothioic chloride can be interpreted much more readily in terms of a dissociative mechanism of reaction via the three-co-ordinated transition state (16). Stereochemical studies of reactions of cyclic phos-phorus(v) esters also suggest a dissociative mechanism. ... 

In the true dissociative mechanism the reaction should involve an intermediate of reduced coordination number. The rate of substitution should be almost independent of nature of entering ligand. The rate of exchange of solvent molecules from the inner coordination sphere to the bulk should ideally equal the rate of formation of a liganded complex. (The rate of reaction should be independent of the ligand concentration. The latter statement need not always be true however. Consider for example the process... 

Octahedral substitution reactions (e.g. those involving cobalt(III) complexes) may proceed by both Sf l or 8 2 reactions. In the S l case a slow dissociative mechanism (bond breaking) may take place. Reaction with the substituting... 

Studies of the stereochemical course of rmcleophilic substitution reactions are a powerful tool for investigation of the mechanisms of these reactions. Bimolecular direct displacement reactions by the limSj.j2 meohanism are expected to result in 100% inversion of configuration. The stereochemical outcome of the lirnSj l ionization mechanism is less predictable because it depends on whether reaction occurs via one of the ion-pair intermediates or through a completely dissociated ion. Borderline mechanisms may also show variable stereochemistry, depending upon the lifetime of the intermediates and the extent of internal return. It is important to dissect the overall stereochemical outcome into the various steps of such reactions. 

The results of kinetic studies suggest that alkane substitution reactions typically proceed by a radical chain mechanism (Section 13.9). The initiation step in the chlorination of methane is the dissociation of chlorine ... 

It will not have escaped the reader s attention that the kinetically inert complexes are those of (chromium(iii)) or low-spin d (cobalt(iii), rhodium(iii) or iridium(iii)). Attempts to rationalize this have been made in terms of ligand-field effects, as we now discuss. Note, however, that remarkably little is known about the nature of the transition state for most substitution reactions. Fortunately, the outcome of the approach we summarize is unchanged whether the mechanism is associative or dissociative. 

Kinetic analysis of the substitution reactions indicate that they follow a dissociative mechanism. It has also been shown that two water molecules in [Cr(H20)5I]2+ undergo exchange with labeled water. It is interesting that one exchange is rapid and occurs before I- leaves. However, this is not true of the chloride compound. Therefore, it appears that the iodide ion labilizes the water trans to it, but the chloride does not. 

How would the volume of activation and the entropy of activation be useful when deciding whether a substitution reaction follows a dissociative or interchange mechanism ... 

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. 

Bob s research interests and knowledge across chemistry were great. Throughout his career he retained an interest in biomimetic chemistry, specifically the study of metal ion-promoted reactions and reactions of molecules activated by metal ion coordination. His early interests in carbohydrate chemistry inspired him to study metal ion catalysis of both peptide formation and hydrolysis as well as studies in inorganic reaction mechanisms. He was particularly interested in the mechanisms of base-catalyzed hydrolysis within metal complexes and the development of the so-called dissociative conjugate-base (DCB) mechanism for base-catalyzed substitution reactions at inert d6 metal ions such as Co(III). 

So the tertiary halide reacts by a different mechanism, which we call SnI- It s still a nucleophilic substitution reaction (hence the S and the N ) but this time it is a unimolecular reaction, hence the 1 . The rate-determining step during reaction is the slow unimolecular dissociation of the alkyl halide to form a bromide ion and a carbocation that is planar around the reacting carbon. 

The kinetics and mechanisms of substitution reactions of metal complexes are discussed with emphasis on factors affecting the reactions of chelates and multidentate ligands. Evidence for associative mechanisms is reviewed. The substitution behavior of copper(III) and nickel(III) complexes is presented. Factors affecting the formation and dissociation rates of chelates are considered along with proton-transfer and nucleophilic substitution reactions of metal peptide complexes. The rate constants for the replacement of tripeptides from copper(II) by triethylene-... 

In recent years there has been a tendency to assume that the mechanisms of substitution reactions of metal complexes are well understood. In fact, there are many fundamental questions about substitution reactions which remain to be answered and many aspects which have not been explored. The question of associative versus dissociative mechanisms is still unresolved and is important both for a fundamental understanding and for the predicted behavior of the reactions. The type of experiments planned can be affected by the expectation that reactions are predominantly dissociative or associative. The substitution behavior of newly characterized oxidation states such as copper-(III) and nickel (III) are just beginning to be available. Acid catalysis of metal complex dissociation provides important pathways for substitution reactions. Proton-transfer reactions to coordinated groups can accelerate substitutions. The main... 

As we have seen, an area of major importance and of considerable interest is that of substitution reactions of metal complexes in aqueous, nonaqueous and organized assemblies (particularly micellar systems). The accumulation of a great deal of data on substitution in nickel(II) and cobalt(II) in solution (9) has failed to shake the dissociative mechanism for substitution and for these the statement "The mechanisms of formation reactions of solvated metal cations have also been settled, the majority taking place by the Eigen-Wilkins interchange mechanism or by understandable variants of it" (10) seems appropriate. Required, however, are more data for substitution in the other... 

Another way of looking at the question of creation of a vacant site and coordination of the substrate is the classical way by which substitution reactions are described (Figure 2.1). Two extreme mechanisms are distinguished, an associative and a dissociative one. In the dissociative mechanism the ratecontrolling step is the breaking of the bond between the metal and the leaving ligand. A solvent molecule occupies the open site, which is a phenomenon that does not appear in the rate equation. Subsequently the solvent is replaced by the... 

The associative mechanism resembles a conventional radical (hydrogen atom) substitution reaction where the 7T-bonded benzene molecule is attacked by a hydrogen atom formed by the dissociative adsorption of water or hydrogen gas. The activation energy in this process is essentially due to the partial localization of one tt electron in the transition complex 21, 31). The transition state differs, however, from conventional substitution reactions by being 77-bonded to the catalyst surface ... 

In the dissociative mechanism, the tt complex adsorbed aromatic reacts with a metal radical (active site) by a substitution process. During this reaction [Eq. (9)] the molecule rotates through 90°, and changes from its horizontally tt complex adsorbed position to a vertically cr-bonded chemisorbed state ... 

It has been shown that the interpretation of catalytic reactions involving group VIII transition metals in terms of n complex adsorption possesses considerable advantages over classical theories by providing a link between theoretical parameters and chemical properties of aromatic reagents and catalysts. The concept has led to the formulation of a number of reaction mechanisms. In heavy water exchange the dissociative tt complex substitution mechanism appears to predominate it could also play a major role when deuterium gas is used as the second reagent. The dissociative mechanism resolves the main difficulties of the classical associative and dissociative theories, in particular the occurrence... 

Data for water exchange with Fe(H20)j+ are shown in Table 4.1. " The value for AV indicates an interchange dissociative mechanism, which is also reflected in data for the reaction of Fe + with tpy AV = -1-3.5 cm mol , ) and other ligands. " One of the earliest studies of substitution in a labile metal ion was of the reaetion of Fe + with bpy and phen in acid solution (Sec. 2.1.4). 

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