Interchange mechanism substitution

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... 

It is not surprising that the substitution processes on seven-coordinate 3d metal ions follow an interchange, rather than a limiting dissociative mechanism. In the case of seven-coordinate Fe(III) complex we even found an associative interchange mechanism for the substitution of solvent molecules as a result of the high n-acceptor ability of the fully conjugated pentadentate ligand system present in its equatorial plane 44). 

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 ... 

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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