Metal ions softness

Metal ions Softness parameter Metal ions Softness parameter... 

Similarly, relevant electrophiles (Lewis acids) including A-type metal cations (hard), bivalent transition metal ions (borderline), and B-type metal ions (soft) can be categorized (see Stumm and Morgan 1996). Note that in organic molecules, the atom where a nucleophile attacks (i.e., the electrophilic site) may possess harder (e.g., C=0, P=0) or softer (e.g., CH3-X) character. 

Effect of donor atom type on selectivity. Changing the type of donor atom at the binding site alters the nature of the ligand-cation interactions and can lead to quite subtle changes in complexation selectivity (15,26). For example, the replacement of oxygen (hard base) as the donor atom by sulphur (soft base) in 18-crown-6 enhances the complexation of transition metal ions (soft acids), for example Ag, and reduces that of alkali metals (hard acids), for example (Table 2.2) (17, 27). With nitrogen as a donor atom, however, the complexation of transition metal ions (Ag" ) is also promoted without substantially diminishing that of alkali metals (K ) (17). 

FIGURE 1.4 The percentage of injected metal bound to gastropod hemolymph protein as related to the metal ion softness. 

FIGURE 1.5 Elimination of metal ions from crayfish hemolymph is related to metal ion softness. Strong covalent bonding slows elimination. In this illustration, the iron datum was omitted because it was derived from the citrate salt, whereas the other metals were prepared from chloride salts. Based on trends shown here and in Figure 1.3, one could incorrectly assume that metal softness might always be the best descriptor for predicting trends. However, as Ahrland explains, soft and polarizable are not synonymous a soft acceptor is certainly always polarizable, but a highly polarizable acceptor need not necessarily be soft, i.e., have (b) properties. For metal ion acceptors, the outer d-electrons are as essential as the polarizability (Ahrland, S. 1968. Thermodynamics of complex formation between hard and soft acceptors and donors. Struct. Bond. 5 118-149). 

Kinraide, T.B. 2009. Improved scales for metal ion softness and toxicity. Environ. Toxicol. Chem. 28 525-533. 

The relationship between metal ionic characteristics and the maximum biosorption capacity was estabUshed using QSAR models based on the classification of metal ions (soft, hard, and borderhne ions). Ten kinds of metal were selected and the waste biomass of Saccharomyces cerevisiae obtained from a local brewery was used as biosorbent. Eighteen parameters of physiochemical characteristics of metal ions were selected and correlated with Ths suggestion was made that classification of metal ions could improve the QSAR models and different characteristics were significant in correlating with ax, such as polarizing power Z /r or the first hydrolysis constant logRo or ionization potential IP. 

A strong correlation was observed between a newly developed scale of metal ion softness and a scale of toxicity. The scale of metal ion softness was developed by normalizing and averaging the values from 8 previously developed scales of metal ion softness. The scale of toxicity was obtained by normalizing and averaging toxicity values from 10 previously published toxicity studies that used many different taxa. An equation was developed that predicted toxicity based on water softness and ion charge (R =0.923). 

The strength of the complexation is a function of both the donor atom and the metal ion. The solvent medium is also an important factor because solvent molecules that are potential electron donors can compete for the Lewis acid. Qualitative predictions about the strength of donor-acceptor complexation can be made on the basis of the hard-soft-acid-base concept (see Section 1.2.3). The better matched the donor and acceptor, the stronger is the complexation. Scheme 4.3 gives an ordering of hardness and softness for some neutral and ionic Lewis acids and bases. 

The nature of the donor atoms in the chelating agent. Ligands which contain donor atoms of the soft-base type form their most stable complexes with the relatively small group of Class B metal ions (i.e. soft acids) and are thus more selective reagents. This is illustrated by the reagent diphenylthiocarbazone (dithizone) used for the solvent extraction of metal ions such as Pd2+, Ag+, Hg2+, Cu2+, Bi3+, Pb2+, and Zn2 +. ... 

Many metal ions parallel the behaviour of either zinc(ii) or mercury(ii), and Pearson described them as being Class a) (hard) or Class b) (soft) metals respectively. For example, iron(iii) exhibits hard behaviour, whilst lead(ii) is soft (Table 9-3). 

Table 9-4. Hard and soft transition-metal ions and ligands.

Another feature of the metal ions that are typically involved in cementitious bonding in AB cements is that most of them fall into the category of hard in Pearson s Hard and Soft Acids and Bases scheme (Pearson, 1963). The underlying principle of this classification is that bases may be divided into two categories, namely those that are polarizable or soft, and those that are non-polarizable or hard. Lewis acids too may be essentially divided into hard and soft, depending on polarizability. From these classifications emerges the useful generalization that hard acids prefer to associate with hayd bases and soft acids prefer to associate with soft bases (see Section 2.3.7). 

Ahrland et al. (1958) classified a number of Lewis acids as of (a) or (b) type based on the relative affinities for various ions of the ligand atoms. The sequence of stability of complexes is different for classes (a) and (b). With acceptor metal ions of class (a), the affinities of the halide ions lie in the sequence F > Cl > Br > I , whereas with class (b), the sequence is F < Cl" < Br < I . Pearson (1963, 1968) classified acids and bases as hard (class (a)), soft (class (b)) and borderline (Table 1.23). Class (a) acids prefer to link with hard bases, whereas class (b) acids prefer soft bases. Yamada and Tanaka (1975) proposed a softness parameter of metal ions, on the basis of the parameters En (electron donor constant) and H (basicity constant) given by Edwards (1954) (Table 1.24). The softness parameter a is given by a/ a - - P), where a and p are constants characteristic of metal ions. 

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... 

Yamada, S. and Tanaka, M. (1975) Softness of some metal ions. Inorg. Nucl. Chem., 37, 587-589. 

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