What are hard and soft acids and bases?
Hard-soft acid-base (HSAB) Principle
According to Lewis Concept, acids and bases neutralization reaction involve an interaction of a vacant orbital of an acid (A) and a filled or unshared orbital of a base (B).
A +: B ⇄ A: B
The species A is called Lewis acid or a generalized acid and B is called Lewis base or a generalized base. Strong acid and a strong base B will form the stable complex A: B. A concept known as the Principle of hard-soft acid-base (HSAB) Principle is very helpful in making the stability of the complex A: B. According to Soft and hard acids and base principle the complex A: B is most stable when A and B are either both soft or both hard. The complex is least stable when one of the reactants (namely A and B) is very hard and the other one is very soft.
In order to arrive at a comparative estimate of the donor properties of bases, the preferences of a particular base to bind a hydrogen ion and methyl mercury (II) ion, [CH₃HgB]⁺ was determined. Both the proton and methyl mercury cation can accommodate only one coordinate chemical bonding but the two cations vary widely in their preferences to bases. This preference was estimated from the experimental determination of dynamic equilibrium constants for the exchange reactions:
BH⁺ + [CH₃Hg(H₂O)]⁺ ⇄ [CH₃HgB]⁺ + H₃O⁺
The results indicate that bases in which the donor atom is nitrogen, oxygen, and fluorine prefer to coordinate with the proton. Bases in which the donor atoms are P, S, I, Br, Cl, or C prefer to coordinate with chemical element mercury.
What are the hard bases?
The donor atoms in the first group have high electronegativity, low electric polarization, and hard to oxidize. Such donors have been named ‘hard bases’ by Pearson since they hold on to their electron particles strongly.
What are the soft bases?
The donor atoms of the second category are of low electronegativity, high polarization, and are easy to oxidize. Such donors have been called ‘soft bases’ since they are holding on to their valence electrons rather loosely.
In simple terms, hardness is associated with a tightly held electron shell with little tendency to polarise. On the other hand, softness is associated with a loosely bound polarisable electron energy levels.
It will be seen that within a group of the periodic table softness of the Lewis bases increases with the increase in the size of the donor atoms. Thus, among the halide ions softness increases in the order: F⁻ㄑCl⁻ㄑBr⁻ㄑI⁻. Hence F — is the hardest and I — is the softest base.
What are the hard acids?
After having gone through a classification of bases, a classification of Lewis acids is necessary. The preferences of a given Lewis acid towards ligands of different donor atoms are usually determined from the stability constant values of the respective reaction complexes or from some other useful equilibrium constant measurements in learning chemistry. When this is done, crystalline solid metal complexes with different donor atoms can be classified into two sets based on the sequences of their stabilities.
Hard acids have small acceptor atoms, are of high positive charge, and do not contain unshared pair of electrons in their valence shell, although all these properties may not appear in one and the same acid. These properties lead to high electronegativity and low polarizability. In keeping with the naming of the bases, such acids are termed as ‘hard acids’. N≫P; O≫S; F〉Cl〉Br〉I
What are the soft acids?
Soft acids have large acceptor atoms, are of low positive charge, and contain ushered pairs of electrons in their valence shell. These properties lead to high polarizability and low electronegativity. Again in keeping with the naming of the bases, such acids are termed ‘Soft acids’. N≫P; O≫S; F〉Cl〉Br〉I
Soft and hard acids and bases principle
This principle also means that if there is a choice of reaction between an acid and two bases and two acids and a base, A hard acid will prefer to combine with a hard base and a soft acid will prefer to combine with soft base and thus a more stable product will be obtained in chemistry.
The hard acid — hard base may interact with polarity. Hard acids have small acceptor atoms and positive charge while the hard bases have small-donor atoms but often with a negative charge. Hence a strong ionic interaction will lead to the hard acid-base combination.
On the other hand, a soft acid — soft base combination mainly a covalent interaction. Soft acids have large acceptor atoms, are of low positive charge, and contain ushered pairs of electrons in their valence shell.
Application of SHAB principle
The SHAB concept is extremely useful in elucidating many properties of chemical elements and will be often referred to at appropriate places.
1. BF₃ and BH₃ molecule the boron is trivalent but quite different in behavior is noted. The presence of hard fluoride ions in BF₃ makes it easy to add other hard bases. The presence of soft hydrides ions BH₃ makes it easy to add other soft bases.
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