The HSAB (Hard Soft Acid Base) Theory

HSAB is an extremely useful qualitative theory that enables predictions of what adducts will form in a complex mixture of potential Lewis acids and bases. Although there have been numerous attempts to make the theory quantitative by assigning numbers representing "hardness" and "softness" to acids and bases, these have not been particularly successful. Even if only qualitative, the theory is so useful that it is essential to know something about it. Fundamentals. The basic premise of Hard/Soft Acid/Base Theory is very simple: Hard acids prefer hard bases; soft acids prefer soft bases. We must now define these terms.

Hard acids (in context, HA) are characterized by (s,f blocks, left side of d block in higher OS's)

Low electronegativity of the acidic atom.

A value in the range 0.7-1.6 is typical of hard acids;

Relatively small size; Relatively high charge (> 3+).

High charge often results in small size, because the remaining electrons are contracted toward the nucleus by the substantial excess positive charge. Specific examples of hard acids are the metal cations from the s and f blocks, and the higher-charged ions from the left side of the d block. Na⁺, Mg²⁺, Fe³⁺, and Al³⁺ are examples of hard acids.

Hard bases (in context, HB) are characterized by

Very high electronegativity of the donor atom (in the range 3.4-4);

Relatively small size of the donor atom.

The combination of high electronegativity and small size results in a nonpolarizable electron cloud surrounding the donor atom. The only 2 donor atoms with electronegativities in the specified range are oxygen and fluorine. So the hard bases are those in which the donor atom is either O or F. Specific examples are O^2-, F^-, OH₂, CO₃^2-, and PO₄^3-.

Soft acids (in context, SA) are characterized by an acceptor atom of

intermediate to high (1.9-2.5);

large size;

low charge (1+, 2+)

Species of large size generally have many electrons, some of which can be quite far from the nucleus. The low charge of the species results in a polarizable (distortable) electron cloud. Specific examples of soft acids include Cu⁺, Hg²⁺, Au⁺, Ag⁺, and Pb²⁺. Note that these metals are all clustered in the same region of the periodic table.

Soft bases (in context, SB) are characterized by donor atom of

intermediate to high electronegativity (2.1-3.0)

large size, leading to polarizability

Specific examples of soft bases are S^2-, RSe^-, I^-, and Br^-. Note that these fall in groups 15-17 in periods with n > 3.

In addition to the fundamental "hard" and "soft" categories, two additional categories are useful. Borderline acids (in context, BA) are intermediate between hard and soft acids. Thus they tend to have lower charge and somewhat larger size than hard acids, and higher charge and somewhat smaller size than soft acids. The 2+ ions of the d block, such as Fe²⁺, Cu²⁺, Ni²⁺, and Zn²⁺, are borderline acids. Borderline bases (in context, BB) are intermediate between hard and soft bases. They tend to be larger and less electronegative than hard bases, smaller and more electronegative than soft bases. Bases in which the donor atom is N or Cl fall in this category. Thus NH₃, Cl^-, RCl, and pyridine are borderline bases.

Example: Classify each species as Lewis acid or base; as hard, soft, or borderline. Which of the Lewis bases would prefer to form adducts with each of the acids?

Fe³⁺ : this has high positive charge, so is expected to be a hard acid.

I^- : This is a large anion with low electronegativity and low charge; it is expected to be a soft base.

CH₃^- : This is an anion, so is probably a base. The donor atom has low electronegativity and relatively low charge. Even though the donor atom is fairly small, this behaves as a soft base.

CO₃^2-: This is an anion with oxygen atoms as potential donors. It is a hard base.

Cu⁺ : This is a transition metal cation with a low charge. It is expected to be a soft acid.

Cl^- : This is an anion with a chlorine donor atom. It should be a borderline base.

Se(CH₃)₂F^- : This is an anion with two potential donor atoms: F, which is a hard donor; and Se, with two electron pairs, which is expected to be a soft base. This species is definitely a base, but can be soft or hard depending on circumstances.

We predict that Fe³⁺ should prefer to form adducts with the carbonate anion and the F donor of Se(CH₃)₂F^-. Cu⁺ should prefer adducts with the soft bases, I^- and CH₃^-. Cl^- will probably bind to either the hard or soft acid, but would prefer a borderline acid such as Fe²⁺.

Because hard acids and bases tend to be highly charged and nonpolarizable, the interaction between them is largely ionic. Their small sizes allow the acid and base to get close enough together so that the ionic interaction is quite strong (remember Coulomb's Law: force of attraction increases as the attracting species get closer together). A good image to keep in mind when thinking of a hard acid/hard base adduct is the juxtaposition of a golf ball (the "acid") with a baseball (the base). Both of these items are small, hard, and undistortable (try pushing the golf ball and baseball together--what happens?). In contrast, soft acids and soft bases have covalent interactions because their electron clouds are polarizable. A good image for soft-soft interactions is the juxtaposition of a large nerf ball (the base) with a smaller nerf ball (the acid). Here the electron clouds are squishy, and can be easily distored during the interaction.

Pearson, 1960