Hard soft acids-bases HSAB principle
Hard soft acid-base (HSAB) principles provide the stability of the complex between lewis acid and bases. This principle was proposed in 1963 by Ralph G. Pearson.
According to the HSAB principle, the complex most stable when participating acid and base are either both soft or both hard.
Thus to study the donner properties of different bases and preferences of a particular base to bind acid. Hydrogen ion and methyl mercury (II) ion used for this comparison study.
Hydrogen ion and methyl mercury ion have only one coordination position to form only one coordinate bond but these two cations are different from their preferences to bases.
But the preference was estimated from the experimental determination of equilibrium constants for the exchange reactions.
Thus the experimental results indicate that
- The bases in which nitrogen, oxygen or fluorine is donor atom, prefer to coordinate with the hydrogen ion.
- But the bases in which phosphorus, sulfur, iodine, bromine, chlorine or carbon is donor atom, prefer to coordinate with mercury.
Lewis acid-base reaction mechanism
|A||+||:B||⇄||A : B|
Thus the species A is called Lewis acid or a generalized acid but B is called Lewis base or a generalized base. And a strong acid and a strong base will form a stable complex A : B.
Definition of hard and soft acids
The preferences of a given Lewis acid towards ligands of different donor atoms are usually determined from the stability constant values of the respective complexes.
When this is done, metal complexes with different donor atoms can be classified into two sets based on their complex stability.
- Hard acids prefer to combine hard bases
- Soft acids prefer to combine soft bases
What are the hard acids?
Hard acids have small acceptor atoms, are of high positive charge and do not contain unshared pair of electrons in their valence shell, but all these properties may not appear in one and the same acid.
Thus these properties lead to high electronegativity and low polarizability and hard to oxidize.
Among these elements electronegativity of nitrogen greater than phosphorus thus hardness of nitrogen greater than phosphorus.
|List of hard acids|
|H+, Li+, Na+, K+, Be+2, Mg+2, Ca+2,
Sr+2, Mn+2, Al+3, Ga+3, In+3, La+3,
Lu+3, Cr+3, Co+3 Fe+3, As+3, Si+3,
Ti+4, U+4, Ce+3, Sn+4, VO+2, UO2+2,
What are soft acids?
Soft acids have large acceptor atoms, low positive charge and contain ushered pairs of electrons in their valence shell but all these properties may not appear in one and the same acid.
These properties lead to high polarizability and low electronegative.
|List of Soft acids|
|Cu⁺, Ag⁺, Au⁺, Tl⁺, Hg⁺, Pd⁺, Cd⁺, Pt⁺, CH₃Hg⁺, Tl⁺³, BH₃, GaCl₃, InCl₃, I⁺, Br⁺, I₂, Br₂, and zerovalent metal atom|
|List of borderline acids|
|Fe⁺, Co⁺², Ni⁺², Cu⁺², Zn⁺², Pb⁺², Sn⁺², Sb⁺², Bi⁺², Rh⁺², B(CH₃)₃, SO₂, NO⁺, GaH₃|
Definition of hard and soft bases
The donor atoms of low electronegativity, high polarizability, and easy to oxidize are called soft bases by Pearson since they holding on to their valence electrons rather loosely.
The donor atoms of high electronegativity, low polarisability and hard to oxidize are called hard bases by Pearson since they hold on to their electrons strongly.
Classification of hard and soft acid-base
In simple terms, hardness associated with a tightly held electron shell with little tendency to polarize.
But On the other hand, softness associated with a loosely bound polarizable 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 top to bottom in the periodic table.
Classification the following as soft and hard acids and bases.
- Hydride ion.
- Nickel (IV) ion.
- Iodine (+1) ion.
- Hydrogen ion.
- The hydride ion has a negative charge and too large in size compared to the hydrogen atom.
Hence the electronegativity of hydride ion quite low and polarizability very high.
So the valence electron in hydride ion loosely bound and it is a soft base.
- The nickel (IV) has quite a high positive charge and small size compared to the nickel (II).
Hence electronegativity of nickel (IV) will be very high and polarisability will low. So it is hard acid.
- Mono-positive iodine has a low positive charge and has a large size.
Thus it has low electronegativity and high polarizability and hard to oxidize. So it is a soft base.
- Hydrogen ion has the smallest size with a high positive charge density and absence of unshared pair of electrons in its valence shell.
All these will give a high electronegativity and very low polarizability. Hence hydrogen ion is a hard acid.
The hard soft acid base principle in chemistry
According to the HSAB principle, when a chemical reaction occurs between acids and bases
- A hard acid will prefer to combine with a hard base.
- A soft acid will prefer to combine with a soft base.
Thus a more stable product will be obtained when a hard acid reacts with hard bases or a soft acid reacts with a soft base.
Why hard acids react with hard bases?
This is because hard acid – hard base may interact with strong ionic forces of attraction.
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, low positive charge and contain ushered pairs of electrons in their valence shell.
Applications of HSAB principle
The HSAB concept is extremely useful in elucidating many properties of acids and bases.
Why the behavior of boron in boron trifluoride and boron trihydride is different?
Due to the presence of hard fluoride ions in boron trifluoride easy to add hard bases.
But the presence of soft hydride ions in boron trihydride easy to add soft bases.
Why [CoF6]-3 more stable than [CoI6]-3?
In [CoF6]-3 and [CoI6]-3 both complexes cobalt is +3 oxidation state and act as a hard acid. But fluoride ion is a hard base and iodide ion is a soft base.
Thus in [CoF6]-3, hard acid, and hard base form a stable complex than the [CoI6]-3 form by a hard acid and soft base.
Why mercury hydroxide more readily dissolved in acidic solution than the mercury sulfide?
In the case, mercury is a soft acid and hydroxide and sulfide are a hard base and soft base respectively.
Thus mercury sulfide formed by soft acid and soft base will be more stable than mercury hydroxide formed by soft acid and hard base.
So due to lower stability mercury hydroxide dissolved readily in acidic solution but mercury sulfide does not readily dissolve in acidic solution.
Mode of occurrence of ore
The existence of certain ore can be rationalized by applying the HSAB principle.
Hard acids such as magnesium, calcium, and aluminum occur in nature as magnesium carbonate, calcium carbonate, and aluminum oxide but magnesium calcium and aluminum sulfides did not occur in nature.
Since the anion CO3-2 and S-2 are hard and soft base respectively. Thus CO3-2 prefers to combine with hard acids such as magnesium, calcium, and aluminum.
On the other hand, soft acids such as Cu+, Ag+, and Hg+2, occur in nature as sulfides because it prefers to combine with soft base S-2.
The borderline acids such as Ni+2, Cu+2, and Pb+2 occur in nature both as carbonates and sulfides.
Because the combination of hard acids and hard bases occurs mainly through ionic bonding and soft acids and soft bases occur mainly by covalent bonding.
Silver fluoride and silver iodide
We know that mono positive silver ion is a soft acid but fluoride ion and iodide ion is a hard base and soft base respectively.
Hence soft acid silver will prefer to combine with the soft base iodide ion. Thus silver iodide is stable but silver fluoride does not exist.