The classification of acids and bases

Key points: Hard and soft acids and bases are identified empirically by the trends in stabilities of the complexes that they form: hard acids tend to bind to hard bases and soft acids tend to bind to soft bases. It proves helpful when considering the interactions of Lewis acids and bases containing elements drawn from throughout the periodic table to consider at least two main classes of substance. The classification of substances as ‘hard’ and ‘soft’ acids and bases was in troduced by R.G. Pearson; it is a generalization—and a more evocative renaming—of the distinction between two types of behaviour that were originally named simply ‘class a’ and ‘class b’ respectively, by S. Ahrland, J. Chatt, and N.R. Davies. The two classes are identified empirically by the opposite order of strengths (as measured by the equilibrium constant, K_f, for the formation of the complex) with which they form complexes with halide ion bases:

Hard acids bond in the order: I^– < Br^– < Cl^– < F^–.

Soft acids bond in the order: F^– < Cl^– < Br^– < I^–

Figure 4.12 shows the trends in Kf for complex formation with a variety of halide ion bases. The equilibrium constants increase steeply from F^– to I^– when the acid is Hg₂ , in dicating that Hg2 is a soft acid. The trend is less steep but in the same direction for Pb₂, which indicates that this ion is a borderline soft acid. The trend is in the opposite direction for Zn₂, so this ion is a borderline hard acid. The steep downward slope for Al₃ indicates that it is a hard acid. A useful rule of thumb is that small cations, which are not easily polarized, are hard and form complexes with small anions. Large cations are more polarizable and are soft.

For Al³, the binding strength increases as the electrostatic parameter (£=z²/r) of the anion increases, which is consistent with an ionic model of the bonding. For Hg², the binding strength increases with increasing polarizability of the anion. These two correlations suggest that hard acid cations form complexes in which simple Coulombic, or ionic, interactions are dominant, and that soft acid cations form more complexes in which covalent bonding is important.

A similar classification can be applied to neutral molecular acids and bases. For example, the Lewis acid phenol forms a more stable complex by hydrogen bonding to (C₂H₅)₂O: than to (C₂H₅ )2S: This behaviour is analogous to the preference of Al3 for F^– over Cl^–. By contrast, the Lewis acid I2 forms a more stable complex with (C₂H₅ )2S: We can conclude that phenol is hard whereas I2 is soft. In general, acids are identified as hard or soft by the thermodynamic stability of the complexes they form, as set out for the halide ions above and for other species as follows:

Hard acids bond in the order: R₃P << R₃N, R₂S << R₂O. Soft acids bond in the order: R₂O << R₂S, R₃N << R₃P.

Bases can also be defined as soft or hard. Bases such as halides and oxoanions are classified as hard because ionic bonding will be predominant in most of the complexes they form. Many soft bases bond through a carbon atom, such as CO or CN^–. In addition to donating electron density to the metal through a interaction, these small multiply bonded ligands are able to accept electron density through the low-lying empty π orbitals (the LUMO) present on the base. The bonding is, consequently, predominantly covalent in character. As these soft bases are able to accept electron density into orbitals they are known as π acids. It follows from the definition of hardness that:

Hard acids tend to bind to hard bases.

Soft acids tend to bind to soft bases.

When species are analysed with these rules in mind, it is possible to identify the classification summarized in Table 4.5.

Figure 4.12 The trends in stability constants for complex formation with a variety of halide ion bases. Hard ions are indicated by the blue lines, soft ions by the red line. Borderline hard or borderline soft ions are indicated by green lines.

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مواضيع ذات صلة

Halogens as Lewis acids

Group 14 Lewis acids

Group 13 Lewis acids

Lewis acids and bases of the s-block elements

Lewis acidity

Anhydrous sulfuric acid

Hydrogen fluoride

Liquid ammonia

Polyoxo compound formation

Amphoterism

Acidic and basic oxides

Periodic trends in aqua acid strength