Solvent Exchange Mechanism

Our B3LYP/6-311+G calculations supported a limiting associative exchange mechanism on the aquated lithium cation as shown in Fig. 6 (90). 

Water exchange proceeds through a trigonal bipyramidal reactive intermediate [Li(H20)5]+, reached via a late transition state. In accordance with the above-mentioned experimental observation of a very fast exchange process, the activation barrier 

Our computations revealed unequivocally that an A mechanism does not operate (156). The trigonal bipyramidal pentaaqua 

To test the reliability of the B3LYP hybrid density functional, we computed the MP2(full)/6-311+G energies based on the structures obtained at the B3LYP/6-311+G level. The activation barrier at this level is only 12.6 kcal mol-1. This is not 

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). 

---

Solvent exchange on the first-row transition metal [M(solvent)6]2+ species has been subjected to extensive study, as is exemplified by Table III, which contains data for four solvent systems which have been particularly intensively studied (46, 47, 97, 99, 103, 110-117). The solvent exchange mechanism progressively changes from Ia to Id as the number of d-electrons increases and rM decreases for H20, MeOH, and MeCN solvents, as judged from the changes in sign of AV. It is also seen that lability decreases with increase in AHi substantially, as... 

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). 

Density functional theory has also been applied successfully to describe the solvent exchange mechanism for aquated Pd(II), Pt(II), and Zn(II) cations (1849 ). Our own work on aquated Zn(II) (19) was stimulated by our interest in the catalytic activity of such metal ions and by the absence of any solvent (water) exchange data for this cation. The optimized transition state structure clearly demonstrated the dissociative nature of the process in no way could a seventh water molecule be forced to enter the coordination sphere without the simultaneous dissociation of one of the six coordinated water molecules. More... 

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. 

Helm L, Merbach AE (2005) Inorganic and Bioinorganic solvent exchange mechanism. Chem Rev 105 1923... 

Figure 6 Volume profiles for solvent exchange mechanisms...

Helm L, Merbach AE. Inorganic and bioinorganic solvent exchange mechanisms. Chem Rev. 2005 105 1923-1959. 

---