Interchange mechanism substitution reactions

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 ... 

Chloride substitution kinetics of [NiniL(H20)2]3+, and its protonated form [NiniL(H20)(H30)]4+, where L = 14 -oxa-1,4,8,11 -tetraazabicy-clo[9.5.3]nonadecane, yield fyn20)2 = 1400 M 1s 1 and (h2o)(H3o+) = 142M 1s V The reverse, chloride dissociation, reactions have (h2o)ci = 2.7 s 1 (h3o+)ci = 0.22 s All four reactions occur through dissociative interchange mechanisms, like earlier-studied substitutions at nickel(III) (359). 

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... 

One mechanistic study7S worth describing here concerns the photoreactivity of [Pt(diethylenetriamine)Br]+. Photolysis in the presence of N02 accelerates the substitution of Br to yield [Pt(diethylenetriamine)N02 ]+. The reaction was shown to proceed via [Pt(diethylenetriamine)OH2 ]2+ which is rapidly anated by either Br or N02 The essential evidence rests in the fact that photolysis in basic solution yields only [Pt(diethylenetriamine)OH]+ even in the presence of N02 This result prompts the postulate that a dissociative interchange mechanism obtains as proposed for Co(CN) -. 6S ... 

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. 

FIGURE 12-11 The Interchange Mechanism in Square-Planar Reactions, (a) Direct substitution by Y. (b) Solvent-assisted substitution. 

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