Interchange mechanisms

Reaction 6 might conceivably proceed through either an ion-atom interchange mechanism (Reaction 7) or a charge transfer mechanism (Reaction 8),... 

Realizing that the last four reactions of the ion-atom interchange mechanism listed each have only one-half the statistical probability of occurring as do the first four and assuming no isotope effect on the rate constants, we can write the following set of rate equations ... 

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,... 

Scheme 3. Anhydride interchange mechanism proposed for polymerization. The same mechanism may be responsible for cyclization.

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 IPCM calculations, the molecule is contained inside a cavity within the polarizable continuum, the size of which is determined by a suitable computed isodensity surface. The size of this cavity corresponds to the molecular volume allowing a simple, yet effective evaluation of the molecular activation volume, which is not based on semi-empirical models, but also does not allow a direct comparison with experimental data as the second solvation sphere is almost completely absent. The volume difference between the precursor complex Be(H20)4(H20)]2+ and the transition structure [Be(H20)5]2+, viz., —4.5A3, represents the activation volume of the reaction. This value can be compared with the value of —6.1 A3 calculated for the corresponding water exchange reaction around Li+, for which we concluded the operation of a limiting associative mechanism. In the present case, both the nature of [Be(H20)5]2+ and the activation volume clearly indicate the operation of an associative interchange mechanism (156). 

All employed methods to include solvent effects corroborate the nature of the transition state for water exchange via an interchange mechanism and show very satisfactorily the mode... 

The ammonia exchange proceeds as in the case of water exchange through an associative interchange mechanism, showing a trigonal bipyramidal transition state structure [Be(NH3).5r1,... 

Experimentally Merbach et al. (67) found for the DMSO exchange at [Be(DMSO)4]2+ (144) an associative interchange or an associative mechanism by NMR studies. Up to now no adequate computational study investigated the mechanism in detail, but exploratory calculations suggest an associative interchange mechanism (161). 

Interchange mechanisms (IA or ID) in a preformed OS complex will generate the following observed rate laws (which cannot distinguish IA from ID) with the equilibrium constant =Kos (equation 1.15) and k = k, (equation 1.16) ... 

All A mechanisms must be associatively and all D mechanisms must be dissociatively activated. The interchange mechanisms (I) include a continuous spectrum of transition states where the degree of bondmaking between the entering ligand and the complex ranges from very substantial (Ia mechanism) to negligible (Id mechanism) and inversely... 

Ru(edta)(H20)] reacts very rapidly with nitric oxide (171). Reaction is much more rapid at pH 5 than at low and high pHs. The pH/rate profile for this reaction is very similar to those established earlier for reaction of this ruthenium(III) complex with azide and with dimethylthiourea. Such behavior may be interpreted in terms of the protonation equilibria between [Ru(edtaH)(H20)], [Ru(edta)(H20)], and [Ru(edta)(OH)]2- the [Ru(edta)(H20)] species is always the most reactive. The apparent relative slowness of the reaction of [Ru(edta)(H20)] with nitric oxide in acetate buffer is attributable to rapid formation of less reactive [Ru(edta)(OAc)] [Ru(edta)(H20)] also reacts relatively slowly with nitrite. Laser flash photolysis studies of [Ru(edta)(NO)]-show a complicated kinetic pattern, from which it is possible to extract activation parameters both for dissociation of this complex and for its formation from [Ru(edta)(H20)] . Values of AS = —76 J K-1 mol-1 and A V = —12.8 cm3 mol-1 for the latter are compatible with AS values between —76 and —107 J K-1mol-1 and AV values between —7 and —12 cm3 mol-1 for other complex-formation reactions of [Ru(edta) (H20)]- (168) and with an associative mechanism. In contrast, activation parameters for dissociation of [Ru(edta)(NO)] (AS = —4JK-1mol-1 A V = +10 cm3 mol-1) suggest a dissociative interchange mechanism (172). 

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