HSAB theory: Hard and Soft, Acids and Bases

The Hard and Soft, Acids and Bases (HSAB) concept was introduced by R. G. Pearson in 1963. It is a widely used technique in chemistry to explain the reaction mechanisms and pathways of several compounds. The terms ‘soft’ and ‘hard’ are paired up with the terms ‘acid’ and ‘base’, one by one to create four categories of chemical species that gives rise to a new view of the concept of electrophiles and nucleophiles.

According to HSAB theory, soft acids prefer to combine with a soft base, and similarly, hard acids prefer a hard base. They react in this order to give stable complex compounds. The high polarizability of chemical species belongs to the softness of acids and bases.

According to HSAB theory, hard and soft, acids and bases are explained as:

Soft Bases

A soft base has the following characteristics:

• High polarisability is the main identification of a soft species.
• Their low electronegativity tells about their low reactivity.
• Their larger ionic radii correspond to the big sizes.
• They prefer to combine with soft acids.
• Usually, anions and neutral molecules are soft bases.

Examples of some soft bases are given in the table given below:

Hard bases

A hard base has the following characteristics:

• Low polarisability is the main identification of any hard specie.
• Their high electronegativity reflects their high reactivity.
• Their small ionic radii correspond to small sizes.
• They prefer to combine with hard acids.
• Hard acids are usually anions and neutral molecules, just like soft bases.

Examples of some hard bases are given in the table below:

Soft acids

A soft acid has the following characteristics:

• High polarizability, as in all soft species.
• Their larger size corresponds to their high polarizability.
• Low charge densities and oxidation states are one of their indications.
• Soft acids show zero or low positive oxidation states.
• Their completely filled atomic orbitals prove that they have large sizes.
• Soft acids always prefer to combine with soft bases.
• Mostly heavy metal ions are soft acids.

Examples of some soft acids are given in the table below:

Hard acids

A hard acid has the following characteristics:

• Low polarizability, as in all hard species.
• Their smaller size corresponds to their low polarizability.
• High charge densities and oxidation states are one of their indications.
• Hard acids show high positive oxidation states.
• Their empty atomic orbitals prove that they have small sizes.
• Hard acids always prefer to combine with hard bases.
• Mostly, light metal ions are hard acids.

Examples of some hard acids are given in the table below:

Acid-base strength (hardness and softness)

The hardness and softness of an acid or a base have no relationship with the strength of their acidity or basicity because they are both good on their own terms.

For example:

• OH^– and F^–, both are hard bases but OH^– is a 10³ times stronger base than F^–.
• (CH₃CH₂)₃P and SO₃^2-, both are soft bases but (CH₃CH₂)₃P is a 10⁷ times stronger base than SO₃^2-.

There are many reactions that cannot be explained on the basis of this HSAB principle. These reactions can be explained by the relative basic strength of bases.

For example:

• Soft base SO₃^2- can displace hard base F^–.

In this case, the strength of the base is SO₃^2-﹥ F^–.

• Hard base OH^– can displace soft base SO₃^2-.

In this case, the strength of the base is OH^–﹥ SO₃^2-.

These reactions prove that reactions proceed irrespective of the hardness and softness of an acid or a base.

Applications of the HSAB concept

• The relative stability of complexes

HSAB concept explains the relative stability of complexes very well.

For example:

[Cd(CN)₄⦎^2- is a more stable complex than [Cd(NH₃)₄⦎²⁺

This is because, Cd²⁺ is a soft acid and CN^– is a soft base, while NH₃ is a hard base.

• Prediction of rates and direction of a reaction

The reactions which involve the hard-hard or soft-soft combinations are faster than those that involve a hard-soft chemical species combination. The reaction proceeds in that direction which involves a hard-hard or soft-soft combination.

For example:

Reaction (i) will go to the right because it prefers the combination of hard acid (H⁺) with a hard base (OH^–).

Reaction (ii) will proceed to the left because it prefers the combination of soft base (SH^–) with a soft acid (CH₃Hg⁺).

• The relative strength of halogen acids

The hard base F^– is strongly bonded to the hard acid H⁺ so HF shall be a highly stable compound. Hence, the increasing order of acidic strength of halogen acids will be:

• Biological applications

Chemical-induced toxicity in biological systems has successfully been applied, by soft and hard acids and bases.

Most soft acids and soft bases are harmful to alive organisms. This is because they can combine with soft acids and soft bases present inside the body.

For example:

CO is a soft base that combines with soft acid iron in the hemoglobin and reduces its oxygen-carrying capacity.

• Poisonous metal catalysts

This application of soft and hard, acids and bases is used to improve the selectivity of reactions. It can also be used to separate or isolate the desired product or intermediate from a chemical reaction.

When an impurity binds with the activation site of a catalyst, the catalyst is deactivated and this is known as poisoning a metal catalyst.

For example:

The soft bases (P, As, and Sb) poison the soft metal catalysts.

Limitations of the HSAB concept

Although very useful, the HSAB theory also has some shortcomings:

• It has no direct quantitative scale of acid-base strength, as it is a widely general technique in this perspective.
• HSAB principle fails to explain the reaction which happens due to the combination of hard acid and soft base or vice versa.

Additional resources

• Lewis acid vs. Lewis Base
• Hydrochloric vs. Sulphuric acid
• Buffer Solution

Concepts Berg

What is soft, hard acid?

Hard acids usually have a small size and a high positive oxidation state. For example, Cr³⁺, BF₃, etc. On the other hand, soft acids usually have a larger size and a low positive oxidation state. For example, BH₃, Au⁺, etc.

What do you mean by hard and soft bases?

Soft bases have high polarizability and low electronegativity. For example, CO, H^–, etc. On the contrary, hard bases have low polarizability and high electronegativity. For example, NH₃ and RO^–, etc.

What are borderline acids and bases?

Borderline acids and bases have characteristics intermediate between hard and soft. For example, trimethyl borane, pyridine, aniline, etc.

Why do soft acids bases tend to have high polarizability?

The soft acid and bases have a large size, as a result, they have a small degree of charge density. Therefore, their electronic cloud can easily be distorted under the electric field.

Is sodium acetate classified as an acid or a base?

Sodium acetate is a basic salt. It is formed by the reaction of acetic acid and sodium bicarbonate.

Why hydride ion is a soft base?

The hydride ions have a large size and low electronegativity. They also have high polarizability. Hence, soft base.

References

• Hard and Soft Acids and Bases Principle in Organic Chemistry By Tse-Lok Ho (Case Western Reserve University, Ohio, US)
• HSAB_theory (Wikipedia)

• Application of the Hard and Soft, Acids and Bases (HSAB) Theory to Toxicant–Target Interactions (pubs.acs.org)