Metal ion selectivity

Important, though not generally appreciated, problems of the application of molecular mechanics to the prediction of metal ion selectivities are the neglect of solvation and entropic effects. These have been discussed in Chapter 2, Sections 2.7 and 2.8. The sum of all the other terms mentioned above may be related to the difference in total strain energy between the metal-free and the coordinated ligand. 

The estimate of relative stabilities via the comparison of total strain energies is in general limited to a series of conformers and isomers (see for instance Chapter 7 and the relevant chapters in Parts I and III). The determination by molecular mechanics calculations of relative stabilities of a series of complexes with metal ions having differing geometric preferences (electronic effects) and preferences in terms of donor atoms is therefore a questionable approach. A comparative study is only useful if the structural preferences of the different metal ions are similar and/or if the electronic effects may be separated from steric effects. 

The well-known Irving-Williams series1661 and the hard and soft acids and bases (HSAB) principle1671 are established empirical concepts related to the stability of 

The well known Irving-Williams series1253 2561 and the HSAB principle1257-1 are established empirical concepts related to the stability of metal complexes, and the question of whether these are accounted for in a general molecular mechanics approach have not always been appreciated when metal ion selectivities have been modeled via empirical force field calculations. The assumption that the position of the minima and the steepnesses of the potential energy functions for the metal-ligand bonds for varying metal ions and constant donor sets accurately reflect var- 

---

Macrocyclic ligands such as crown ethers have been widely used for metal ion extraction, the basis for metal ion selectivity being the structure and cavity size of the crown ether. The hydrophobicity of the ligand can be adjusted by attachment of alkyl or aromatic ligands to the crown. Impressive results have been obtained with dicyclohexano-18-crown-6 as an extractant for Sr in [RMIM][(CF3S02)2N] IL/aque-... 

Tabushi, I. Yamamura, K. Water Soluble Cyclophanes as Hosts and Catalysts, 113,145-182 (1983). Takagi, M., and Ueno, K. Crown Compounds as Alkali and Alkaline Earth Metal Ion Selective Chromogenic Reagents. 121, 39-65 (1984). 

Crown compounds as alkali and alkaline earth metal ion selective chromogenic reagents. M. Takagi and K. Ueno, Top. Curr. Chem., 1984,121, 39-65 (37),... 

When chlorination is carried out with N-haloamines and sulfuric acid (catalyzed by either UV light or metal ions), selectivity is much greater than with other reagents. In particular, alkyl chains are chlorinated with high regioselectivity at the position next to the end of the chain (the to -1 position). Some typical selectivity values are ... 

The ideas presented here merely scratch the surface of factors that control metal ion selectivity in biological systems. It is hoped that in future the picture will become even clearer, enabling us to learn much more about ligand design and selective metal ion complexation. 

Molecular mechanics calculations have become a well established tool in the area of coordination chemistry, including the coordination chemistry of nickel375-379 where they are often applied for the analysis or the prediction of structures,380 the computation of isomer or conformer ratios and metal ion selectivities,381,382 and for simulating spectroscopic properties in combination with AOM calculations or by hybrid quantum mechanics/molecular mechanics (QMMM) methods.383,384 Details of the various approaches, e.g., the incorporation of d-electron stabilization energy... 

Metal-ion selectivity by cryptands may be markedly affected by solvent... 

In summary, although the metal-ion selectivity of the cryptands is normally largely enthalpy-controlled, entropic terms may also be quite important. Once again, the factors underlying these respective terms may be quite variable and, as a consequence, a criterion for preferred complexation based solely on a match of the cavity for the cation radius may not always be appropriate. 

Bajc, A.F. 1998. A comparative analysis of enzyme leach and mobile metal ion selective extractions case studies from glaciated terrain, northern Ontario. Journal of Geochemical Exploration, 61, 113-148. 

Of interest is the metal ion selectivity of bifunctional exchangers having both sulfonic and phosphonic acid groups, since phosphonic acid resins prefer Pb(II) to Ba(II) but sulfonic acid resins show the reversed metal ion selectivity. 

As conclusion, bifunctional fibers having both phosphonic acid and sulfonic acid groups exhibit the characteristic metal ion selectivity and high breakthrough capacities in addition to the extremely fast adsorption rates. Studies on behavior of FPS-f in adsorption of other heavy metals like Fe(III) are now in progress. The bifunctional fiber developed in this work is attractive to application to the protection of the environment because of its extremely rapid adsorption rates and characteristic metal ion selectivity. 

Jyo A., Yamabe K., Egawa, H., Metal ion selectivity of macroreticular styrene divinylbenzene copolymer-based methylenephosphonic acid resin. Sep. Sci. Technol. 32, 1997,1099-1105. 

The continuous flow method is still necessary when one must use probe methods which respond only relatively slowly to concentration changes. These include pH, Oj-sensitive electrodes, metal-ion selective electrodes,thermistors and thermocouples, " epr and nmr detection. Resonance Raman and absorption spectra have been recorded in a flowing sample a few seconds after mixing horseradish peroxidase and oxidants. In this way spectra of transients (eompounds I and II) can be recorded, and the effext of any photoreduction by the laser minimized. ... 

Lewis acid-base interactions, 42 89-143 change of HSAB behavior, 42 104 chelate ring size and metal ion selectivity, 42 109-118... 

---