Hard cation

The most effective Lewis-acid catalysts for the Diels-Alder reaction are hard cations. Not surprisingly, they coordinate to hard nuclei on the reacting system, typically oxygen atoms. Consequently, hard solvents are likely to affect these interactions significantly. Table 1.4 shows a selection of some solvents ranked according to their softness. Note that water is one of the hardest... 

Compound 5 was analyzed by NMR spectroscopy to gain information relative to conformation and complexation preferences. When complexation with potassium cations was attempted, the N—CHj signals were affected more than others. When the cation present was Ag , the protons adjacent to sulfur were more strongly affected. This observation may indicate the relative binding sites for soft versus hard cations in this system. ... 

Fig. 6.10 Examples of the directional effect of oxide ligand arrangement on the location of hard cations and orientation of M-O linkages.

Structure-determining Factors in the Absence of Hard Cations... 

D and 5 0 data on fluid inclusions and minerals, 8 C of carbonates, salinity of inclusion fluids together with the kind of host rocks indicate that the interaction of meteoric water and evolved seawater with volcanic and sedimentary rocks are important causes for the formation of ore fluids responsible for the base-metal vein-type deposits. High salinity-hydrothermal solution tends to leach hard cations (base metals, Fe, Mn) from the country rocks. Boiling may be also the cause of high salinity of base-metal ore fluids. However, this alone cannot cause very high salinity. Probably the other processes such as ion filtration by clay minerals and dissolution of halite have to be considered, but no detailed studies on these processes have been carried out. 

The difference in the kinds of metals enriched in Kuroko, base metal vein-type and precious metal vein-type deposits could be explained in terms of the HSAB (hard, soft, acids and bases) principle (Pearson, 1963). According to this principle, relatively hard cations (base metal (Cu, Pb, Zn, Fe, Mn, Ag) ions) tend to combine preferentially with chloride ion in hydrothermal solution, while soft cations (Au, Ag, Tl, Hg ions etc.) combine with H2S and HS . The differences in salinity of ore fluids in base-metal-rich deposits (base metal vein-type deposits and Kuroko deposits) and base-metal-poor deposits (precious metal vein-type deposits) is also in accordance with the HSAB principle. 

Smaller, hard cations, such as alkyltrimethylammonium salts and crown ether-cation complexes, will promote the elimination of dichlorocarbene to form a hard ion pair of type Q+C1 . 

The thennodynainics of complexation between hard cations and hard (O, N donor) hgands often are characterized by positive values of both the enthalpy and entropy changes. A positive AH value indicates that the products are more stable than the reactants, i.e., destabilizes the reaction, while a positive entropy favors it. If TAS > AH°, AG° will be negative and thus log(3 positive, i.e., the reaction occurs spontaneously. Such reactions are termed entropy driven since the favorable entropy overcomes the unfavorable enthalpy. 

The two 5B oxides (Nb205, Ta20s) are insoluble in HOH, dilute acids, and dilute bases, but Nb205 dissolves in concentrated bases whereas Ta205 does not. All the elements in their highest oxidation state are hard cations and therefore will be particularly attracted to the hard atoms F and O. 

A convenient way for comparing the complexation of different hard cations with a specific... 

Fig. 3. Correlations between ionic index (z2//) and complexation constants log10 /3f for (a) the monohydroxo, (b) the monofluorido, and (c) the monosulphato complexes of hard cations and the borderline cation Ni(II).

The HSAB (hard and soft acids and base) principle is that hard acids prefer to interact with a hard base, and soft acids with soft bases. Hard bases are not polarizable, and inclnde those with 0-donor atoms. Soft bases are more polarizable, and inclnde S-donor bases. Solvent hardness/softness can be assessed by comparing the Gibbs free energy of transfer of a soft cation like Ag from hard water to the solvent with the Gibbs free energy of transfer of similarly sized hard cations like Na and K. Table 3.9 shows some solvents listed in increasing softness. ... 

In Section 4.5 it was shown that the bonding strengths of soft cations are less well defined than those of hard cations since they are able to form stable compounds with anions having a wider range of anion bonding strengths. Consequently they also display a wider range of coordination numbers. Thus the soft Zn + cation is found in four and six coordination in contrast to the similar, but hard, cation which is usually found only in 6-coordination. Most hard... 

The primary constraint on the coordination number is the anion-anion repulsion. Clearly this determines the maximum possible coordination number and, for hard cations, this is normally the coordination number that is observed. However, if the cation is soft or if the maximum possible coordination number gives rise to very small bonding strengths, other coordination numbers may be found as discussed in Section 6.3. A number of examples will illustrate how these various factors work together. 

In Scheme 1-71, the reversible reaction is shifted to the right when the anion, X, is larger and the cation, M+, is smaller. For example, this shift to the right is 100% in the presence of Na+, PI v, and only 35% in the presence of Na+, F (Hamon Astruc 1988). The equilibrium takes place as an exchange reaction between the two ion pairs. Reactions of this type are based on the symbiotic-effect premise The interaction between a hard cation and a hard anion or between two soft ions is stronger than that between two ions of different types. 

As for carbanions, the reactivity of anionic non-carbon nucleophiles depends on the cation. The nucleophilicity and basicity of a given anionic nucleophile will usually be enhanced if it does not form strong bonds either with the cation or with the solvent. Hard cations, for example Li+ or Ti4+, will significantly reduce the reactivity of hard anions (RO-, R2N , F ), whereas soft cations (Cs+, Cu+, Pd2+) will form strong bonds with soft anions (RS , I , CN , H , R ) and thereby reduce their reactivity. 

The complex binding can be electrostatic, covalent, or a combination of both. Electrostatically bound complexes, where the metal atom and the ligand are separated by one or more hydrogen molecules, are called outer-sphere complexes. They are less stable and are formed when hard cations come into contact with hard ligands (Table 10). 

Hg2+, Cd2+, Cu+). Since lanthanide ions are hard cations, their complexation therefore requires hard donor atoms such as oxygen or nitrogen. 

Determine the amount of water to be treated per cycle and the amount of hardness to be removed. Softening of water requires use of a cation-exchange resin operated in sodium form to exchange divalent hardness cations for sodium regenerated with aqueous sodium chloride solution. Total amount of water to be treated is (100 gal/min)(60 min/h)(8 h/cycle) = 48,000 gal/cycle (182 m3/ cycle). 

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