Soft acids table

The Hard-Soft-Add-Base (HSAB) theory was developed by Pearson in 1963. According to this theory, Lewis acids and Lewis bases are divided into two groups on one hand hard acids and bases, which are usually small, weakly polarizable species with highly localised charges, and on the other hand soft acids and bases which are large, polarizable species with delocalised charges. A selection of Lewis acids, ordered according to their hardness in aqueous solution is presented in Table 1.3. 

Bases of low polarizabiUty such as fluoride and the oxygen donors are termed hard bases. The corresponding class a cations are called hard acids the class b acids and the polarizable bases are termed soft acids and soft bases, respectively. The general rule that hard prefers hard and soft prefers soft prevails. A classification is given in Table 3. Whereas the divisions are arbitrary, the trends are important. Attempts to provide quantitative gradations of "hardness and softness" have appeared (14). Another generaUty is the usual increase in stabiUty constants for divalent 3t5 ions that occurs across the row of the Periodic Table through copper and then decreases for zinc (15). 

STRUCTURE-REACTIVITY RELATIONSHIPS Table 7-15. Examples of Hard-Soft Acid-Bases... 

As already noted in section 1.4.3, geochemical features of ore fluids responsible for base-metal and gold-silver types of deposits are distinct. They are summarized in Table 1.22. The differences in metals concentrated to the deposits and geochemical fectures of ore fluids responsible for both types of deposits are interpreted in terms of HSAB (hard, soft, acids and bases) principle by Pearson (1963, 1968) below. 

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... 

Ligand preference and possible coordination geometries of the metal center are important bioinorganic principles. Metal ligand preference is closely related to the hard-soft acid-base nature of metals and their preferred ligands. These are listed in Table 1.7.6... 

Table 1.7 Hard-Soft Acid-Base Classification of Metal Ions and Ligands...

As was pointed out in the previous chapter, biologically important metal ions and their ligands can be classified according to the hard-soft theory of acids and bases (Table 2.1). While there are exceptions, most metal ions bind to donor ligands as a function of preferences based on this concept, with hard acids (metal ions) binding preferentially to hard bases (ligands) and soft acids to soft bases. 

Using these properties, a number of species have been placed in the hard, soft, or borderhne categories in Table 3.2. This table can be used to predict, at least qualitatively, the strength of complexation as measured by the stability constants. For example, Pu is a hard acid, F, a hard base, and T, a soft base. This leads to the prediction that log PdPuF O would be larger than log PdPuFOi the experimental log (3i values are 6.8 and <1.0, respectively. By contrast, since Cd is a soft acid, log Pi(CdF ) could be expected to be less than log (3i(CdT) the respective values are 0.46 and 1.89. However, many metals of interest such as... 

Table 3.2 Partial List of Hard and Soft Acids and Bases... 

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. ... 

As predicted by the soft acid behaviour36 of copper(I), very few examples of the fluoride ion coordinated to copper(I) are known (Table 16). It occurs on the fluoride ion in [Cu(PPh3)3F],37 and on the coordinated tetrafluoroborate anion in [Cu(PPh3)3(FBF3)] (4) 67 in both cases a tetrahedral CuP3F chromophore is present. 

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