Soft ions

Soft ions These have high polarizability. They usually have a large number of d electrons in the outer electronic shells. As a result, they form covalent bonds with the water molecules in the solvent to form hydrates, and with the ligand to form complexes. Beyond the formation of covalently bonded hydrates, they do not have a large effect on the structure of a polar solvent. 

Using these criteria, Ahrland classifies the alkali and alkaline earth metal ions as hard ions, with the hardness decreasing as the radius of the ion increases. The transition metal ions, Ag+, Pd2+, and Hg2+, on the other hand, are soft, with the softness increasing in that order, so that the Hg2+ ion is an especially soft ion. 

The complexation represented by reaction (15.37) can be thought of as consisting of two parts  

Formation of a chemical bond between the metal ion and the macrocyclic ligand. We will designate AH and AS) as the enthalpy and entropy changes for this process. 

The AH and AS for these steps, and the combination to give ArH° and ArS° for the overall reaction, depends upon the hardness and softness of the ions and ligands involved in the process. To see how the concept of hard and soft ions and ligands helps to explain the observed values for ArH° and Ar5°, let us consider two examples  

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They indicated that the softness parameter may reasonably be considered as a quantitative measure of the softness of metal ions and is consistent with the HSAB principle by Pearson (1963, 1968). Wood et al. (1987) have shown experimentally that the relative solubilities of the metals in H20-NaCl-C02 solutions from 200°C to 350°C are consistent with the HSAB principle in chloride-poor solutions, the soft ions Au" " and Ag+ prefer to combine with the soft bisulfide ligand the borderline ions Fe +, Zn +, Pb +, Sb + and Bi- + prefer water, hydroxyl, carbonate or bicarbonate ligands, and the extremely hard Mo + bonds only to the hard anions OH and. Tables 1.23 and 1.24 show the classification of metals and ligands according to the HSAB principle of Ahrland et al. (1958), Pearson (1963, 1968) (Table 1.23) and softness parameter of Yamada and Tanaka (1975) (Table 1.24). Compari.son of Table 1.22 with Tables 1.23 and 1.24 makes it evident that the metals associated with the gold-silver deposits have a relatively soft character, whereas those associated with the base-metal deposits have a relatively hard (or borderline) character. For example, metals that tend to form hard acids (Mn +, Ga +, In- +, Fe +, Sn " ", MoO +, WO " ", CO2) and borderline acids (Fe +, Zn +, Pb +, Sb +) are enriched in the base-metal deposits, whereas metals that tend to form soft acids... 

If, on the other hand, loose ion pairs (between soft ions) are involved, microwave acceleration is limited, because ionic interactions are only slightly modified from GS to TS. 

The reactive species is the acylium ion resulting from abstraction of a chloride anion from benzoyl chloride (Eq. 50). This reagent comprises an ion pair formed between two large (soft) ions which are therefore associated as loose ion pairs. According to these assumptions, the absence of a microwave effect should be expected as the polarity evolution is very weak between the GS and TS (two loose ion pairs of similar polarities). 

The asymmetry found in the electron density of these soft ions can be modelled by adding one or more point multipoles to the point charge used to... 

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. 

A solvent softness scale, dependent on the thermodynamics of transfer of ions from water to the target solvent, has been proposed (Marcus 1987). Since soft ions prefer soft solvents and hard ions hard solvents, and since silver ions are soft, whereas sodium and potassium ions are hard, the difference ... 

Fig. 11.5 (right). The influence of hard and soft ions on the TPR patterns (10 K/min) of Co-containing crystalline compounds, (a) C03O4 (b) C0AI2O4 (c) C0M0O4. Only the TCD signals... 

Since water is a hard solvent, the Gibbs energy of transfer of ions from water as a reference solvent to other solvents should depend on the softness of these solvents in a different manner for hard and soft ions. For ions of equal charge and size, hard ions should prefer water and soft ions the softer solvents. The definition of p by Eq. (7-12e) has been given because the size of the soft Ag ion is intermediate between those of hard Na and K . The p values for 34 organic solvents have been determined e.g. p = —0.12 for 2,2,2-trifluoroethanol, 0.00 for water (by definition), 0.64 for pyridine,... 

Nitrogen and sulfur atoms are softer (they have more charge delocalization) than oxygen atom. Therefore, macrocyclic hosts containing nitrogen atoms or sulfur atoms preferentially recognize soft ions. Thioether-type crown com-... 

DN Depletion/no effect Cs, Ag, Mo Heterogeneous group soft ions... 

R. Vuilleumier and M. Sprik (2001) Electronic properties of hard and soft ions in solution Aqueous Na+ and Ag+ compared. J. Chem. Phys. 115, p. 3454... 

A pA a chart can be used as a reference for nucleophilicity only if the difference in softness is considered. A partially plus carbon atom is a much softer electrophile than a proton. Soft ions are more nucleophilic in protic solvents because tighter solvation greatly decreases the nucleophilicity of the hard ions. 

Conversely, very non-poisonous or inert metals are needed when artificial prostheses have to be introduced during surgery and so metals which, if dissolved, would give soft ions are chosen, e.g. gold, silver, tantalum, and platinum or their alloys. Because they give soft ions there is negligible tendency for those metals to give up electrons to form soft metal ion—hard solvent bonds (water is hard). 

Inert electrolytes show ion specificity, as discussed in Chapter 2 and illustrated in Figures 2.4, 2.5, 2.9, and 2.10. The anions affect the positive branches of the charging and electrokinetic curves, and the cations affect the negative branches. The affinity to particular monovalent anions and cations depends on the character of the surface, and follows the hard- soft acid-base principle that is, hard surfaces prefer to adsorb hard ions, and soft surfaces prefer to adsorb soft ions. The effects of ion specihcity on the charging and electrokinetic curves are usually minor. The affinity series observed in coagulation behavior are termed Hoffmeister series. 

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