Mechanisms sphere, interchange, associative

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). 

Ij mechanism. This is the dissociative interchange mechanism and is similar to the previons one in the sense that dissociation is stiU the major ratecontrolling factor. Therefore, we are stiU dealing with an SnI process. Nevertheless, differently from the D mechanism, no experimental proof exists that an intermediate of lower coordination is formed. The mechanism involves a fast onter sphere association between the initial complex and the... 

The reaction of Mg + with pyrophosphate is about twice as slow in D2O as in H2O (at 15 °C). This difference is attributed to a change in the outer-sphere association constant rather than to a change in the interchange rate constant. Kinetics of solvolysis of [Fe(bipy)3] + in D2O lend support to the mechanism of dissociation via a unidentate-bipyridyl transient intermediate postulated, for aqueous solution, many years ago. ... 

The dependence of rate constant on ionic strength is still widely used, often in conjunction with the dependence of rate constant on dielectric constant, as an indicator of substitution mechanism. Recent instances of this classical approach include the reaction of /ra j-[Co(dmgH)2(SCN)(tu)] with thiourea (tu), aquation of trans-[Rh(dmgH)2Cl(tu)], aquation of the [Co(02CCHaCl)(NH3)6]"+ cation, and substitution at the [Fe(CN)6(OH2)] anion by nitrite, thiocyanate, sulphite, or nitrosobenzene. Salt effects on observed rate constants for the reaction of nickel(n) with pyrophosphate operate via the outer-sphere association constant rather than via the interchange rate constant. ... 

Activation parameters for reaction of [Co(NH3)5(OH2)] with salicylate are = 129 kJ mol" and = +69 J K mol" These values refer to reaction of the aquo complex with the salicylate monoanion (salH ). The [Co(NH3)5(OH2)] cation reacts with all three forms of succinate, succ, succH , and SUCCH2. Rate constants have been determined in the temperature range 323.2-338.6 K. The mechanism involves outer-sphere association followed by rate-determining interchange and water loss, in other words the classical... 

As indicated in Section 5.7.4.3, the effects of ion pairing on rates of dimethyl sulfoxide exchange with the [Co(NH3)5(dmso)] cation have been established.The small accelerating effects of ion pairing are listed in Table 5.14. Activation enthalpies and activation entropies for dimethylformamide exchange with [Co(NH3)5(dmf)] both decrease as the proportion of dimethylformamide increases in water + dimethylformamide solvent mixtures. These trends are discussed in terms of an interchange mechanism and the effects of interactions between free and coordinated dimethylformamide molecules on the outer-sphere association constant. 

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