Metals HSAB classification

In acidic solutions, organic amines protonate to form cations with hydrophobic tails. These ions will seek out and blanket cathodic surfaces, much as carboxylate anions seek out and cover anodic spots in neutral or basic media. The usual choices include amylamine (C5H11NH2), cyclohexyl-amine (C6H11NH2), pyridine (C5H5N), and morpholine [0(CH2CH2)2NH]. Metallic iron behaves as a soft acid in terms of the HSAB classification (Section 2.9), despite the indubitable hard behavior of its trivaJent ion, and so molecules with soft donor atoms adsorb more strongly than hard bases (S > N > 0). This principle can be applied to the design of inhibitors. 

Donor atoms Metals removed HSAB classification of acid and base... 

Transition metal complexes are often very good nucleophiles and qualify as being supersoft under Pear son s HSAB classification for example, reaction with soft methyl iodide can be as much as 3 X 105 times faster than the reaction with hard methyl tosylate. Because soft nucleophiles are those with large a values in the Edwards equation, that rates for the transition metal nucleophiles are effectively correlated with oxidation potentials is not surprising. In the last chapter in this section, Chapter 16, Pearson uses recently obtained values of pKa for transition metal complexes to test the full Edwards... 

Their tendency to complex with type B metals is the opposite sequence. This led to a classification of ligands into type A ligands (e.g. F"), which tend to complex with type A metals, and type B ligands (e.g. I ), which tend to complex with type B metals. The classification was introduced by Ahrland in 1958. See also HSAB principle. 

In spite of a very large amount of data, listed in two books by Sillen and Marten (1964, second edition [148], and 1971, supplement [149]), the stability constants of metal-ion complexes with organic and inorganic ligands, measured mainly in aqueous solution, could not furnish any useful basicity scale. They did, however, help to establish the important HSAB classification and principle (Pearson, 1963-1969) [150-155]. 

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... 

Classification of metals and ligands according to the HSAB principle... 

The classification of metals, their salts, and organometallic compounds, based on the Principle of Hard-Soft Acids and Bases (HSAB) of Pearson [112,116,156-158], is discussed in detail in Sec. 1.2.2. 

Later workers have correlated the classification of elements in class (a) and class (b) with Pearson s principle of hard and soft acids and bases (HSAB principle) (see Hard Soft Acids and Bases) on the basis that class (a) metal ions are hard acids and class (b) are soft acids. Borderline elements in the Ahrland-Chart-Davies classification tend to be harder in the higher oxidation states and softer in their lower oxidation states. 

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