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Hard Soft Acid Base List

Hard Soft Acid Base and HSAB-Principle

Hard soft acid base definition and HSAB-principle proposed by Ralph Pearson (1963) are very helpful for predicting the stability and acids bases properties in the chemical complex. Lewis theory explains the neutralization reaction 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. But the hard soft acid base (HSAB) principle explains the stability of these acid base complexes. Therefore, soft acids form stable complexes with bases that are high 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 equlibrium 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 pair of the electron particles in their valence shell, but all these definitions may not appear in one and the same acids. Thus these basic properties lead to high electronegativity and low polarization and hard to oxidize.

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.

N < P, O < S, F < Cl < Br < I

Among these elements electronegativity and electron affinity of oxygen greater than phosphorus thus softness of oxygen lower than sulfur.

Definition of Hard and Soft Base

  1. 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.
  2. 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.
Stable Hard Soft Acid Base list in Chemistry

Properties of Hard and Soft Acid-Base

In simple terms, hardness associated with a tightly held electron with little tendency to polarity. On the other hand, softness 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. Thus among the halide ions, softness increases top to bottom in the periodic table.


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.

  1. Answer: 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 by the nucleus and it is a soft base.
  2. 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. So it is hard acid.
  3. Mono-positive iodine has a low positive charge and has a large size. Therefore the low electronegativity, high polarizability form a soft base.
  4. 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 hard base and acts as hard acid.

Hard-Soft Acid-Base (SHAB Principle)

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

Hydrogen ion and methyl mercury ion have 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

  1. The bases in which nitrogen, oxygen, or fluorine is the donor atom, prefer to coordinate with the hydrogen ion.
  2. But the bases in which phosphorus, sulfur, iodine, bromine, chlorine, or carbon is donor atom, prefer to coordinate with mercury.

Why hard acids react with hard bases?

  • Hard acids have small acceptor atoms and 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 electron configuration in their valence shell.

Applications of HSAB principle

The HSAB principle is extremely useful in elucidating many acid-base properties to learning chemistry. For example, why the behavior of boron in boron trifluoride and boron trihydride is different?

  1. Due to the presence of hard fluoride ions in boron trifluoride easy to add hard bases.
  2. But the presence of soft hydride ions in boron trihydride easy to add soft bases.

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

Answer: 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. Thus in [CoF6]-3, the hard acid and base form a stable complex than the [CoI6]-3 form by a hard acid and soft base.

Soft and Hard Acid Bases of Silver

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.

Question: Why mercury hydroxide more readily dissolved in low pH solution than the mercury sulfide?

Answer: 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-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.

Metal Ore and Hard Soft Acids and Bases

The concentration of certain ore in the earth’s surface can be rationalized by applying the HSAB-principle. Magnesium, calcium, and aluminum occur in 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 molecule of Mg(OH)2 forms through ionic bonding while HgI2 molecule through covalent bonding by acid-base reaction.