Question

What is Hard Acid Soft Base Theory?

Answer 1

Hint: Hard Acid Soft Base Theory (HSAB) is also known as Pearson acid-base concept. It helps in explaining the stability of compounds, mechanisms of reactions and their pathways. This theory indicates a chemical compound by ‘hard’ or ‘soft’ , ‘acid’ or ‘base’ on the basis of their properties and polarizability.

Complete answer:
Hard Acid Soft Base Theory is a qualitative concept which was proposed by Ralph Pearson. It explains the stability of compounds (metal complexes) , mechanisms and pathways of their chemical reactions. According to HSAB theory, lewis acids and bases are classified as hard or soft on the basis of some properties.
We will now see the detailed properties of ‘hard acid’, ‘hard base’, ‘soft acid’ and ‘ soft base’.
Hard Acids-
They have small atomic radii $( < 90pm)$ .
They possess a high positive charge.
Have empty orbitals in the valence shell to accept electrons.
Have low electronegativity and electron affinity.
High energy LUMO (where, LUMO is lowest unoccupied molecular orbital and which accepts the electrons)
Strongly solvated
Examples- ${H^ + },{\text{ }}L{i^ + },{\text{ }}N{a^ + },{\text{ }}{K^ + },{\text{ }}B{e^{2 + }},{\text{ }}M{g^{2 + }},{\text{ }}C{a^{2 + }},{\text{ }}S{r^{2 + }},{\text{ }}S{n^{2 + }},A{l^{3 + }},{\text{ }}G{a^{3 + }},{\text{ }}I{n^{3 + }},{\text{ }}C{r^{3\;}}$ ,etc.

Soft Acids-
They have large radii $( > 90pm)$ .
They possess low positive charge.
They have completely filled orbitals in valence shell.
Possess intermediate electronegativity.
Low energy LUMO.
Examples- $C{u^ + },{\text{ }}A{g^ + },{\text{ }}A{u^ + },{\text{ }}H{g^ + }\;,{\text{ }}C{s^ + }\;,{\text{ }}T{l^ + }\;,{\text{ }}H{g^{2 + }}\;,{\text{ }}P{d^{2 + }},{\text{ }}C{d^{2 + }}\;,{\text{ }}P{t^{2 + }}\;,B{H_{3,}}$ etc.

Hard Bases-
Highly solvated.
They have small radii $( \sim 120pm)$ .
Have high electronegativities.
They are weakly polarizable.
They do not easily get oxidised.
They have high energy HOMO (where HOMO is the highest occupied molecular orbital, from where the electrons get removed).
Example- ${H_2}O,{\text{ }}O{H^ - },{\text{ }}{F^ - },{\text{ }}C{l^ - },{\text{ }}C{H_3}C{O_2} - ,{\text{ }}P{O_4}^{3 - },{\text{ }}S{O_4}^{2 - },{\text{ }}C{O_3}^{2 - },{\text{ }}N{O_3}^ - ,{\text{ }}Cl{O_4}^ - ,{\text{ }}ROH$ ,etc.
Soft Bases-
They have large atomic radii $( > 170pm)$ .
They have high polarizability.
They can easily be oxidised.
Low energy HOMO.
Examples- ${S^{2 - }},{\text{ }}RSH,{\text{ }}R{S^ - },\;{I^ - },{\text{ }}C{N^ - },{\text{ }}SC{N^ - },{\text{ }}{S_2}{O_3}^ - ,{\text{ }}{R_3}P$ ,etc.
There are some borderline acids $\left( {F{e^{2 + }}\;,{\text{ }}N{i^{2 + }}\;,{\text{ }}C{u^{2 + }}\;,{\text{ }}Z{n^{2 + }}\;,{\text{ }}P{b^{2 + }}\;,{\text{ }}B{{\left( {C{H_3}} \right)}_3},{\text{ }}S{O_2},{\text{ }}N{O^ + }} \right)$ and bases (Aniline, pyridine, ${N_3}^ - ,{\text{ }}B{r^ - },{\text{ }}N{O_2}^ - ,{\text{ }}S{O_3}^{2 - },{\text{ }}{N_2}$ ) which have intermediate properties.
Therefore, we can say that according to HSAB theory,
Hard acids prefer to bond with hard bases and due to the large electronegativity difference between them, they form ionic interactions.
Soft acids prefer to bond with soft bases and due to very low electronegativity difference between them, they form covalent bonds.

Note:
There are many applications of HSAB theory such as, it can help in predicting the product of a reaction and whether the reaction will move in forward or backward direction. It also helps in determining the mechanism of a reaction and the strength of the product formed during a chemical reaction.