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Hard Soft Acid Base

Acid Chemistry

Hard soft acid base theory

Hard soft acid base theory or HSAB-principle proposed by Ralph Pearson (1963) are very helpful for predicting the stability and acids bases properties in the chemical complex. Lewis acid base theory explains the neutralization reaction of hard soft acid base in terms of electronic configuration with the formation of the complex by a coordinate covalent chemical bond between vacant orbital of acids and filled orbital of the bases. Therefore, the hard soft acid base (HSAB) principle explains the stability of these acid base complexes. Soft acids form stable complexes with bases that are high electric polarization and good reducing agents.

Experimental chemical reaction and acid and bases also indicate that the hard acid prefers to combine with hard bases and soft acid prefers to bind soft bases. Therefore, the hard acid and the hard base will equilibrium to form a stable complex like A: B in chemical science.

Definitions, Examples and Chemical Properties of Hard, Soft and Borderline Acids

Definition of Hard Acid

Hard acids have small acceptor atoms, are of high positive charge, and do not contain unshared pairs of electrons in their valence shell, but all these definitions may not appear in one and the same acids. Therefore, these basic properties lead to high electronegativity and low polarization and hard to oxidize. For example, N > P, O > S, F > Cl > Br > I. Among these chemical elements electronegativity and ionization energy of nitrogen greater than phosphorus thus hardness of nitrogen greater than phosphorus.

Definition of Soft Acid

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. This definition of acid leads to high polarity and low electronegativity and electron affinity. Therefore, the softness of periodic table chemical elements N < P, O < S, F < Cl < Br < I. Among these chemical elements electronegativity and electron affinity of oxygen greater than phosphorus thus softness of oxygen lower than sulfur.

Definition of Hard and Soft Base

  • 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.
  • 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.

Stable Hard Soft Acid Base list in Chemistry

Hard soft acid base examples

In simple terms, hardness is associated with a tightly held electron with little tendency to polarity. On the other hand, softness is associated with a loosely bound polarizable electron. Within the group of the periodic table with the increasing atomic number, the softness of the Lewis bases increases. Therefore, among the halide ions, softness increases top to bottom in the periodic table like FㄑClㄑBrㄑI.

Question: Which of the following chemical compounds as soft and hard acids and bases, Hydride ion, Nickel (IV) ion, Iodine (+1) ion, the 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 is loosely bound by the nucleus and it is a soft base.
  • The nickel (IV) has quite a high nuclear charge and a small size compared to the nickel (II). Hence electronegativity of nickel (IV) will be very high and polarisability will low. Therefore nickel(IV) is a hard acid.
  • Mono-positive iodine has a low positive charge and has a large size. Therefore the low electronegativity, high polarizability form a soft base.
  • Hydrogen ion has the smallest dimensions with a high positive charge density and absence of the unshared pair of electrons in its valence shell. All these will give a high electronegativity and very low polarizability. Hence ionic hydrogen bonding easy to a hard base and acts as hard acid.

HSAB theory

This principle was proposed in 1963 by Ralph G. Pearson, according to the hard soft acid base or HSAB principle, the chemical complex most stable when participating acids and bases are either both soft or both hard. Therefore to study the donner properties of different soft or hard bases preferences of a particular base to bind acid. Hydrogen ion and methyl mercury (II) ion used for this comparison in inorganic chemistry.

HSAB theory examples

Hard or soft acid like Hydrogen ion and methyl mercury ion has only one coordination position to form only one coordinate chemical bond. But these two cations are different from their preferences to bases. But the preference was estimated from the experimental determination of chemical equilibrium constants for the redox reaction.

BH+ + [CH3Hg(H2O)]+ ⇄ [CH3HgB]+ + H3O+

Thus the experimental results indicate that the bases in which nitrogen, oxygen, or fluorine is the 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.

Difference between hard acid and soft acid

Hard acids have small acceptor atoms and a positive charge while the 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-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.

Application of HSAB principle

The hard soft acid base or HSAB principle is extremely useful in elucidating many useful properties to learning chemistry. For example, 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.

Metal Ore and Hard Soft Acids Bases

The concentration of certain ore on the earth’s surface can be rationalized by applying the hard soft acid base or HSAB-principle.

  • Therefore according to the hard soft acid base principle, magnesium, calcium, and aluminum occur on the earth’s surface as magnesium carbonate, calcium carbonate, and aluminum oxide but magnesium calcium and aluminum sulfides did not occur in the earth’s environment.
  • On the other hand, soft acids such as Cu+, Ag+, and Hg+2, occur in nature as sulfides. The borderline acids such as Ni+2, Cu+2, and Pb+2 occur in nature both as carbonates and sulfides.

Therefore, the definition and chemical properties of hard-soft acid-base combination list provide the occurrence of the complex ore in our environment, hard acids and bases stable through ionic bonding and soft acids and bases stable by covalent bonding. For example, the crystalline solid molecule of Mg(OH)2 forms through ionic bonding while HgI2 molecule through covalent chemical bonding by Hard soft acid base reaction.

Hard soft acid base problems

Problem: Why [CoF6]-3 more stable than [CoI6]-3?

Solution: In [CoF6]-3 and [CoI6]-3 both complexes cobalt is +3 oxidation number or state and act as a hard acid. But fluoride ion is a hard base and iodide ion is a soft base. Therefore, in [CoF6]-3, the hard acid and base form a stable complex than the [CoI6]-3 form by a hard acid and soft base.

Problem: Why silver iodide is stable but silver fluoride does not exist?

Solution: 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. Therefore, from the definition, soft acid silver will prefer to combine with the soft base iodide ion. Hence silver iodide is stable but silver fluoride does not exist.

Problem: Why hard soft acid base complex like mercury hydroxide more readily dissolved in low pH solution than the mercury sulfide?

Solution: Mercury is a soft acid, and hydroxide and sulfide are a hard base and soft base respectively. Thus mercury sulfide formed by soft acid-bases will be more stable than mercury hydroxide formed by soft acid-base. So due to lower stability mercury hydroxide dissolved readily in low pH solution. But mercury sulfide does not readily dissolve in low pH or acidic solution.