Solvent exchange reaction

Relative Energies (kcalmol x) of the Stationary Points within the Solvent Exchange Reaction (177)... 

Both solvent exchange reactions show an interchange (I) mechanism (Figs. 25 and 26). Therefore, the quadratic pyramidal ground state with a second solvent molecule in the second... 

Quantum chemical methods are well established, accepted and of high potential for investigation of inorganic reaction mechanisms, especially if they can be applied as a fruitful interplay between theory and experiment. In the case of solvent exchange reactions their major deficiency is the limited possibility of including solvent effects. We demonstrated that with recent DFT-and ab initio methods, reaction mechanisms can be successfully explored. To obtain an idea about solvent effects, implicit solvent models can be used in the calculations, when their limitations are kept in mind. In future, more powerful computers will be available and will allow more sophisticated calculations to be performed. This will enable scientists to treat solvent molecules explicitly by ab initio molecular dynamics (e.g., Car-Parrinello simulations). The application of such methods will in turn complement the quantum chemical toolbox for the exploration of solvent and ligand exchange reactions. 

Labile species are usually main group metal ions with the exception of Cr2+ and Cu2+, whose lability can be ascribed to Jahn-Teller effects. Transition metals of classes II and III are species with small ligand field stabilization energies, whereas the inert species have high ligand field stabilization energies (LFSE). Examples include Cr3+ (3d3) and Co3+ (3d6). Jahn-Teller effects and LFSE are discussed in Section 1.6. Table 1.9 reports rate constant values for some aqueous solvent exchange reactions.8... 

Solvent exchange reactions on metal cations are among the most simple chemical reactions a solvent molecule situated in the first coordination shell of the ion is replaced by another one, normally entering from the second shell. They are generally considered as fundamental reactions for metal ions in solution, since they constitute an important step in complex-formation reactions on metal cations. The reaction is... 

Solvent exchange reactions have been reviewed several times in the last 10 years. A comprehensive review by Lincoln and Merbach was published in this series in 1995 (6). More recent reviews focused more on high pressure techniques for the assignment of reaction mechanisms (7-9) or on water exchange (10). This review is a follow up of the exhaustive Lincoln and Merbach review (6). The main features of solvent exchange on metal ions will be pointed out, taking into account developments and new results from the last 10 years. 

The relation between the pressure induced changes of the observed exchange rates and the underlying solvent exchange reaction mechanisms is visualized in Fig. 2. Applying pressure to a d-activated exchange... 

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+. 

Square-planar stereochemistry is mostly confined to the d8 transition metal ions. The most investigated solvent exchange reactions are those on Pd2+ and Pt2+ metal centers and the mechanistic picture is well established (Table XIV (194-203)). The vast majority of solvent exchange reactions on square-planar complexes undergo an a-activated mechanism. This is most probably a consequence of the coordinatively unsaturated four-coordinate 16 outer-shell electron complex achieving noble gas... 

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. 

Reports on studies of solvent exchange reactions on actinide compounds are very scarce. The most studied cation is U02+. The commonly observed solvated species have five (H20, DMSO, DMF, TMP) (262,263)... 

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... 

Figure 1. Constancy of AV for solvent exchange reactions where AV = —6.5 cm6/mol for Cr(DMF)63+-DMF solvent exchange at 338 K (a) and pressure dependence of AV for reactions involving charge development and, hence, solvational change where AV0 = —18.5 err /mol for Co(NH3)sSO/ direct aquation at 298 K (b).

An Alternative Approach to Accounting for AV Values for Solvent Exchange Reactions... 

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

---