Associative interchange

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). 

If a NH3 molecule was included in the exchange process, we obtained an interchange type of mechanism. Therefore, we conclude that the larger molar volume of ammonia compared to water is responsible for the mechanistic changeover from an associative (A) to an associative interchange (Ia) type of mechanism. 

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). 

In the case of the beryllium dication, we again found an associative interchange (Ia) mechanism for HCN exchange at... 

NO was proposed to occur through an associative interchange mechanism (Ia). A recent study of the formation of [Fe(H20)5(N0)]2+ from aquated ferrous ion (30) resulted in activation parameters similar to those for chelated ferrous ion (Table II). The small and positive activation volumes were used to assign the reaction mechanism as dissociative interchange in character. 

Pressure-accelerated water exchange reactions and evidence for associative interchange in corresponding net substitution reactions ... 

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). 

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