Solvent exchange activation volumes

Measuring the pressure dependence of the exchange rate constant leads to activation volumes, AV and this technique has become a major tool for the mechanistic identification of solvent exchange mechanisms (8,16,17). In the last 25 years high-pressure, high-resolution NMR probes were developed which allow the application of all NMR techniques described to pressures up to several hundreds of mega Pascals (18). 

The increase in ionic radius from Be2+ to Mg2+, which is accompanied by an increase in coordination number from 4 to 6, is responsible for a substantial increase in lability (Table III, (37-43)). The two activation volumes measured are positive as well as all the activation entropies. The rate laws determined for non-aqueous solvents in inert diluent are first order, showing a limiting D mechanism for all solvent exchange reactions on [MgS6]2+. 

Mechanistic interpretation of activation volumes on square-planar complexes is complicated by the geometry. The sterically less crowded complexes may have loosely bound solvent molecules occupying the axial sites above and below the plane. Replacing them in the formation of a five-coordinate transition state or intermediate may result by compensation in relatively small volume effects. It is therefore difficult to distinguish between Ia and A mechanisms from the value of the activation volume. Nevertheless, the AV values are negative and together with the second-order rate laws observed, point to an a-activation for those solvent exchange reactions. 

Solvent exchange processes are the most fundamental substitution reactions that characterize the lability and reaction mechanism of a metal center within a coordination geometry. For these reactions the interpretation of the volume of activation becomes rather straightforward since these reactions do not involve major changes in solvation due to changes in dipole moments and electrostriction, which can in... 

An attempt is made to account quantitatively for the volumes of activation, AV, of ligand substitution processes. Causes of the pressure-dependence of AV include solvational change, for which a versatile analysis is developed. The pressure-independent AV values of solvent exchange reactions are good measures of the non-sol-vational components of AV for related net reactions. For water exchange, one can predict... 

DR. SWADDLE We are measuring the volume of activation for solvent exchange in a region where it is in the NMR time frame in other words, where the NMR line broadening is exchange controlled, i.e., k = 1(T s. ... 

Valuable information on mechanisms has been obtained from data on solvent exchange (4.4).The rate law, one of the most used mechanistic tools, is not useful in this instance, unfortunately, since the concentration of one of the reactants, the solvent, is invariant. Sometimes the exchange can be examined in a neutral solvent, although this is difficult to find. The reactants and products are however identical in (4.4), there is no free energy of reaction to overcome, and the activation parameters have been used exclusively, with great effect, to assign mechanism. This applies particularly to volumes of activation, since solvation differences are approximately zero and the observed volume of activation can be equated with the intrinsic one (Sec. 2.3.3). 

Is this the rds There is striking correlation of the exchange rate constants for MS. and the values of k in (4.81). In addition the volumes of activation are positive for solvent exchange and interaction of M(dmf).+ (M = Mn, Co, Ni, Zn and Cd) with N-Metpp in dmf (compare Table 4.4). In spite of these two facts however, it is considered that one of two further steps, probably the first, controls the overall rate. A sitting-atop (SAT) complex is formed in which metal is attached by two bonds to the porphyrin and two N - H bonds remain intact. 

The pressure dependence of the exchange rate constant leads to the activation volume, AV, which has become the major tool for the experimental determination of solvent exchange mechanisms (15,17,18). This is mainly due to the direct connection between the sign of AV and the intimate mechanism for solvent exchange. 

The interpretation of the activation volume for solvent exchange reactions on metal ions is based on the transition state theory (19). The experimentally... 

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