Substitution reactions interchange

In inert systems such as technetium and rhenium, ligand substitution reactions-including solvolysis-proceed under virtually irreversible conditions. Thus, the nature of the reaction center, the nature of the leaving group, and the nature and position of the other ligands in the complex affect the rates and activation parameters in a complicated manner. Most substitution reactions take place via interchange mechanisms. This is not too surprising when the solvent is water - or water-like - and where, in order to compete with the solvent,... 

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 ... 

Pressure-decelerated water exchange reactions and evidence for dissociative interchange in corresponding net substitution reactions... 

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... 

Langford and Gray proposed in 1965 (13) a mechanistic classification for ligand substitution reactions, which is now generally accepted and summarized here for convenience. In their classification they divided ligand substitution reactions into three categories of stoichiometric mechanisms associative (A) where an intermediate of increased coordination number can be detected, dissociative (D) where an intermediate of reduced coordination number can be detected, and interchange (I) where there is no kinetically detectable intermediate [Eqs. (2)-(4)]. In Eqs. (2)-(4), MX -i and... 

It was found that AV for both the acid-independent and acid-catalyzed pathways are approximately zero (109). Thus it was interpreted that Eq. (17) can be considered a substitution reaction of the Cr(III) central cation which proceeds through an interchange, I, mechanism... 

When an associative mode of activation is indicated it is instructive to examine the ways in which reactivity, as measured by the second-order rate constant k2, depends upon the nature of the nucleophile and if a large number of substrates show the same pattern of preference it is useful to consider scales of nucleophilicity. A systematic study of the substitution reactions of fra/is-[Pt(py)2Cl2] (it appears that pyridine and piperidine were used interchangeably) in methanol at 30 °C (displacement of chloride) led to the establishment of an n scale, nm d= log,o(k2/kI) for the standard reaction,447 and later the more dimensionally correct Up,0 scale448-449 ( p,° = log,0(li2/ki)[MeOH]) so that npt° = npt+1-41 (unfortunately this distinction has not been strictly adhered to in the literature).449 A collection of n 0 values will be found in Table 14. 

The soluble derivatives of s- and t- alcohols, Ln(OR)3 , prepared by alcohol interchange from Ln(OPr )3 (method 6, which was the most popular one at the first step ofthe studies [1084, 1159, 1397, 1122]), are not the homoleptic derivatives either, presumably. In some cases the substitution reactions are even incomplete. Thus it has been established that the refluxing of Sc5(p.5-pO)(p3-OPti)4(p-OPri)4(OPri)5] with even a huge excess of BuOH leads only to substitution ofthe 5 terminal OPr -groups by OBu [1607]. The same note can be made on the alcohol interchange products of Ce(OPr )4 . 

Double decomposition is similar in concept to the substitution reaction, except that both anion-exchange and cation-exchange membranes are employed. Simultaneous interchange of anion-cation pairing takes place to form products that would otherwise require multistep procedures to prepare and purify. Pure materials can be produced from crude raw materials by means of double decomposition, and reactions otherwise impractical by conventional reaction methods can be performed. An example application is the reaction between potassium chloride and sodium nitrate to produce potassium nitrate and sodium chloride. 

Substitution of several metal-carbonyl complexes Cr(CO)6 and Mn(CO)5 (amine) show a small dependence on the nature and concentration of the entering hgand. Under pseudo-first-order conditions, the rate laws for these substitutions have two terms, as shown for Cr(CO)6 (as for some substitution reactions with 16e complexes, see equation 5). The second-order term was always much smaller than the first-order term. A mechanism that ascribes the second-order term to dissociative interchange (U) has been suggested for the Mo(CO)5Am system (Am = amine) and involves a solvent-encased substrate and a species occupying a favorable site for exchange. Thus, the body of evidence for the simple metal carbonyls indicates that CO dissociation and is the mechanism of ligand substitution reactions. 

For the second reaction step, the observed rate constant of this reaction (kobsi) was independent of the hydrogen peroxide concentration. As described above, the temperature dependence of kobsi was used to construct a linear Eyring plot from which AH and A5 were obtained. The volume of activation was derived from the slope of the hnear plot of In kobsi versus pressure at 25 °C in the pressure range 10 -170 MPa. The reported volumes of activation can be used to construct a volume profile for the overall reaction. The positive activation volume, AV = -F6.8 0.4cm mol, suggests a dissociative interchange Id) mechanism for the hgand substitution reaction on [Fe(edta)OH] with hydrogen peroxide. 

H. M. Marques, J. C. Bradley, and L. A. Campbell, J. Chem. Soc., Dalton Trans., 2019 (1992). Ligand Substitution Reactions of Aquacobalamin Evidence for a Dissociative Interchange Mechanism. 

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