Soft and hard acids and bases

According to Lewis Concept, an acid-base reaction involves an interaction of a vacant orbital of an acid (A) and a filled or unshared orbital of a base (B).
A + : B A: B 
Lewis Acid
(Acceptor)
Lewis Base
(Donor)
Adduct or Complex
    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 Soft and Hard Acids and Bases(SHAB) 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 different bases, the preferences of a particular base to bind a proton H⁺ and methyl mercury (II) ion, [CH₃HgB]⁺ was determined.
    Both the proton and methyl mercury cation can accommodate only one coordinate bond but the two cations vary widely in their preferences to bases. This preference was estimated from the experimental determination of equilibrium constants for the exchange reactions:
    BH⁺ + [CH₃Hg(H₂O)]⁺ ⇄ [CH₃HgB]⁺ + H₃O⁺ 
    The results indicate that bases in which the donor atom is N, O or F prefer to coordinate to the proton. Bases in which the donor atoms are P, S, I, Br, Cl or C prefer to coordinate to mercury.

Hard bases

    The donor atoms in the first group have high electronegativity, low polarisability and hard to oxidize. Such donors have been named ‘hard bases' by Pearson since they hold on to their electrons strongly.

Soft bases

    The donor atoms of the second category are of low electronegativity, high polarizability, 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 shell.
    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⁻
    Thus F - is the hardest and I - is the softest base.
Definition of hard acid and soft acids and bases
Soft and hard bases

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 is usually determined from the stability constant values of the respective complexes or from some other useful equilibrium constant measurements.
    When this is done, 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

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
Difference between hard and soft acid
Soft and hard acids

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.
    The hard acid - hard base may interact with strong ionic forces. 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 hands, a soft acid - soft base combination mainly a covalent interaction. Soft acids have large acceptor atoms, are of low positive charge and contain ushered pair 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 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.
  2. [CoF₆]⁻³ is more stable than [CoI₆]⁻³. It will be seen that Co⁺³ is a hard acid, F⁻ is a hard base and I⁻ is a soft base.
    Hence [CoF₆]⁻³ (hard acid + hard base) is more stable than [CoI₆]⁻³ (hard acid + soft base).
  3. The existence of certain metal ores can also be rationalized by applying SHAB principle. Thus hard acids such as Mg⁺², Ca⁺² and Al⁺³ occur in nature as MgCO₃, CaCO₃, and Al2O₃ and not as sulfides (MgS, CaS, and Al2S3) since the anion CO₃⁻² and O⁻² are hard bases and S⁻² is a soft base. Soft acids such as Cu⁺, Ag⁺ and Hg⁺², on the other hand, occurs in nature as sulfides. 
  4. The borderline acids such as Ni⁺², Cu⁺², and Pb⁺² occur in nature both as carbonates and sulfides. The combination of hard acids and hard bases occurs mainly through ionic bonding as in Mg(OH)2 and that of soft acids and soft bases occurs mainly by covalent bonding as in HgI2.

Questions answers

    Question
    Classify the following as soft and hard acids and bases. (i) H- (ii) Ni+4 (iii) I+ (iv) H+
    Answer
  1. The hydride ion has a negative charge and is far too large in size compared to the hydrogen atom. Its electronegativity is quite low and it will be highly polarisable by virtue of its large size. Hence it is a soft base.
  2. Quadrivalent nickel has quite a high positive charge. Compared to bivalent nickel its size will be much smaller. Its electronegativity will be very high and polarisability will low. Hence it is hard acid.
  3. Mono-positive iodine has a low positive charge and has a large size. It has a low electronegativity and high polarizability. Hence it is soft acids.
  4. H⁺ has the smallest size with a high positive charge density. It has no unshared pair of electrons in its valence shell. All these will give a high electronegativity and very low polarizability. Hence H⁺ is a hard acid.
    Question
    AgI2- is stable, but AgF2- does not exist. Explain.
    Answer
    We know that Ag⁺ is a soft acid, F⁻ is hard to base and I⁻ is the soft base.
    Hence AgI₂⁻ (soft acid + soft base) is a stable complex and AgF₂⁻ (soft acid + hard base) does not exist.
    Question
    Explains why Hg(OH)₂ dissolved readily in acidic aqueous solution but HgS does not?
    Answer
    In the case of Hg(OH)₂ and HgS, Hg is a soft acid and OH⁻ and S⁻² is hard to base and soft base respectively. Evidently, HgS (Soft acid + Soft base) will be more stable than Hg(OH)₂ (Soft Acid + Hard base).
    More stability of HgS than that of Hg(OH)₂ explain why Hg(OH)₂ dissolved readily in acidic aqueous solution but HgS does not.

Soft and hard acids and bases, definition, examples and applications of soft and hard acids and bases and SHAB principles with problems and its solutions

[Chemical kinetics] [column1]

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