Interchange dissociative substitution reactions

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). 

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... 

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. 

On the basis of the reported activation parameters, a dissociative mechanism for the first substitution step and a dissociative or dissociative interchange (Id) mechanism for the second substitution step are suggested for the reaction in MeOH and EtOH. 

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. 

The dissociative nature of the reaction is evident from the magnitudes of the volumes of activation (+6.4cm3mol-1 for the substitution of ImH by H20). As in another report above the certainty of Id character is clouded by the fact that the values ofAV are composites of a precursor complex component and a contribution from the interchange step itself. A full D mechanism could not be categorically ruled out based on the properties of the system, and since the precursor formation step could contribute a negative volume term. Nevertheless a dissociative interchange mechanism was proposed as the more likely and a thorough discussion of mechanistic features, available at the time, of related systems was included. 

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