Solvent Exchange with Metal Ions

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). 

Kinetic parameters are shown in Table 4.1 for the exchange of the first-row (and one second-row ) divalent transition metal ions in water. Since AV for a D mechanism is K, 

The exchange of the trivalent ions of the metals Cr, Fe, Ru and Ga in water is governed by the rate law 

---

From the ratio of the areas of nmr peaks due to coordinated and free solvent, or from simple isotopic analyses, the value of n can be determined. It may be necessary to slow the exchange process (4.4) by lowering the temperature of the solution. A variety of solvation numbers n is observed, with four and six being the most prevalent. As we have noted already, there is a wide range of labilities associated with the solvent exchanges of metal ions (Fig. 4.1). 

Most metal ion-exchanged TSMs swell with water or polar organic solvents. The resultant intercalate stores up both the solvent and the metal ion in the interlayer spaces, and is regarded as a mass of the microvessels filled with the solution of the metal ion. For liquid-phase homogeneous catalyses... 

Rates of Solvent Exchange with Solvation Spheres of Metal Ions... 

Gutmann s DN values are based on reaction enthalpies, so a correlation with SH rather than with AG is not unreasonable. This tentatively emerging correlation with DN values is, however, easily upset by steric factors, either in the form of ligand bulk 2 or of solvent bulk. Activation enthalpies for solvent exchange at metal(ii) centres, for instance Fe +, do not correlate satisfactorily with values, nor do activation enthalpies for Na+ release from the 2,2,2-cryptate (1). The noncorrelation of activation enthalpies with DN values in many systems need not cause too much surprise, as these DN values are based on enthalpies of reaction of the respective solvents with antimony pentachloride, a process significantly different from that involved in metal ion solvation. 

Gadolinium is found in several other minerals, including monazite and bastnasite, both of which are commercially important. With the development of ion-exchange and solvent extraction techniques, the availability and prices of gadolinium and the other rare-earth metals have greatly improved. The metal can be prepared by the reduction of the anhydrous fluoride with metallic calcium. 

---