Lanthanides solvent exchange reactions

Within solution inorganic chemistry, there would be no apparent reason to obtain NMR spectra at high pressures in structural characterization studies. It prevails that most applications of hp NMR spectroscopy relate to solvent exchange reactions on solvated metal ions their mechanisms often have direct bearing upon the kinetics and mechanisms of substitution of one or more solvent molecules from a metal center by other ligands. The first part of the results section provides ample illustration of the value of high-pressure measurements on transition metal and lanthanide ions, fully... 

A large number of nuclei such as H, N, 0, Cl have been employed to probe lanthanide coordination properties (e.g. Brucher et al. 1985, Fratiello et al. 1991). Special interest was taken in the DMF solutions, where solvent exchange reactions have been characterized at variable temperature and pressure (Cossy and Merbach 1988). Information about the coordination environment of R(III) ions can also be gained by directly observing the R(III) nuclei, which is possible for (Fratiello et al. 1989a), and La (Bunzli et al. 1987). 

The complications which result from the hydrolysis of alkali metal cyanides in aqueous media may be avoided by the use of non-aqueous solvents. The one most often employed is liquid ammonia, in which derivatives of some of the lanthanides and of titanium(III) may be obtained from the metal halides and cyanide.13 By addition of potassium as reductant, complexes of cobalt(O), nickel(O), titanium(II) and titanium(III) may be prepared and a complex of zirconium(0) has been obtained in a remarkable disproportion of zirconium(III) into zirconium(IV) and zirconium(0).14 Other solvents which have been shown to be suitable for halide-cyanide exchange reactions include ethanol, methanol, tetrahydrofuran, dimethyl sulfoxide and dimethylformamide. With their aid, species of different stoichiometry from those isolated from aqueous media can sometimes be made [Hg(CN)3], for example, is obtained as its cesium salt form CsF, KCN and Hg(CN)2 in ethanol.15... 

The interpretation of conductance data is complicated by the labile nature of the lanthanide complexes in solution which results in ligand exchange and dissociation reactions. It is difficult to understand the nature of the complex species present in solution. A combination of conductance data and molecular weight determination may be useful in determining the coordination number and structure of the complexes in solution. However, due to the poor solubility of lanthanide complexes in suitable solvents, molecular weight data have been obtained for only a few complexes. The dissociative reactions of lanthanide complexes in solution are well illustrated by the TPPO complexes of lanthanide isothiocyanates (202). In chloroform solution, the dissociation... 

In chapter 102, Drs. K.L. Nash and J.C. Sullivan explore the chemical kinetics of solvent and ligand exchange in aqueous lanthanide solutions. These authors deal with redox reactions readily available only from the Ce(IV)/Ce(III) and the Eu(II)/Eu(III) couples among the lanthanides. A wealth of tabulated information on rate and equilibrium constants is provided in textual and tabular form. 

The redox reaction has been utilized in the separation of light actinide elements (U, Np, and Pu) with both ion-exchange process and solvent extraction process. For trivalent heavy actinides with Z> 94 (except No), separation of these actinide ions from lanthanide ions is required for safe storage of long-lived nuclear waste and transmutation of these nuclides. Fundamental researches have widely been carried out by several groups for the purpose of quantitative separation of transuranium elements. Recent topics on the development and application of solvent extraction for the separation of transuranium elements are briefly summarized below. 

The separation process in both ion exchange and solvent extraction consists, in its most elementary form, of the transfer of a (typically) charged metal ion (or complex), from a polar aqueous phase to an immiscible phase (with different solvating properties) with concomitant charged naturalization. The effectiveness of any separation process is a function of the ability of the reactions to accompUsh phase transfer (because without phase transfer, there can be no separation), and the relative affinity of the counterphase for the species to be separated. In the case of the trivalent lanthanides and actinides, the latter aspect must exploit the slight differences in ionic radii and covalency of the metal ions. It is conceivable that differences in rate of reaction could be utilized, but such data are much more difficult to obtain, and few examples of kinetic-based separations are extant. 

But whether the result of metal-ligand covalent bonding or a more subtle polarizability effect, extractants and complexants containing soft donor atoms are central to most ion exchange and solvent extraction separations of lanthanides from actinides. To generalize, those materials with the greatest potential for increased covalent interactions provide the most significant opportunity for successful lanthanide/ actinide separations. As discussed below, the sheer multiplicity of reactions involved in separations processes offer many opportunities to exploit this difference in soft donor interactions. 

Basic to any studies of the lanthanides and actinides is the ability to separate the members of these difficultly separable series. Kenneth L. Nash reviews the methods and effectiveness of separation by solvent extraction, ion exchange and necessary accompanying reactions in chapter 121. 

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