HSAB classification

Arranged in order of increasing /A, or ionicity on the M-L band. Also shown are ionic radii and HSAB classification (5) of the Lewis acids. 

Some comments about the choice of the conditions The use of acetonitrile as solvent and the selected temperature have been already discussed. Iron (III) and copper (II) were selected for a couple of reasons. First of all, they are ubiquitous ions and typical autoxidation catalysts. Iron (III) is a hard acid and copper (II) a borderline acid according to the HSAB classification, so it is reasonable to expect they will react differently, with a different complexing power. Manganese (II) has also been proposed as a widespread catalyst of autoxidation (49). 

Hard-Soft Classification The hard and soft acid-base (HSAB) classification is used for rationalisation of differences, when two acids (Aj and A2) are competing for two bases (B and B2). 

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... 

Lanthanides, e.g. La(III), are considered as hard acids in the HSAB classification of Pearson (7), located between Sr (II) and Ti(IV). Divalent lanthanides are Lewis acids, as deduced from their chemical properties.Uranium ions (U(VI), U(IV)) are also hard acids. 

The numerical values of r) are generally satisfactory when compared with the qualitative HSAB classification of Table 1.16. For instance, the cations classified hard acids, such as Li+, Mg + and Al +, have large r] values, those classified borderline, such as Ni + and Cu +, have intermediate values, and those classified soft, such as I+, Pd + and Ag+, have small values. Moreover, the expected increased hardness with increased oxidation state is shown by the r] values of Fe, Fe + and Fe +. 

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]. 

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