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HSAB Principle – Definition, Interactions in HSAB, Characteristics, Applications, Practice Problems and FAQ

HSAB Principle – Definition, Interactions in HSAB, Characteristics, Applications, Practice Problems and FAQ

It’s experiment time!

Take two beakers containing equal amounts of water. To one beaker, add a teaspoon of washing soda (Na2CO3) and a teaspoon of baking soda (NaHCO3) to the other.

Which salt dissolves easily?

You can see that washing soda dissolves easily in water compared to baking soda.

Y’all must be wondering “what is the chemistry behind this difference?”

The answer to your question is the Hard and Soft Acids and Bases or the HSAB principle.

Let’s get to know more about HSAB on this concept page.

TABLE OF CONTENTS

  • HSAB – Introduction
  • HSAB – Definition
  • HSAB – Interactions
  • HSAB – Characteristics
  • HSAB – Applications
  • HSAB – Limitations
  • Practice Problems
  • Frequently Asked Questions – FAQ

HSAB – Introduction

Ralph Pearson developed the Hard and Soft Acids and Bases (HSAB) Principle to understand metal complexes’ stability and their respective mechanisms. This section outlines Pearson's HSAB theory's principle, definition, examples, conceptual framework, applications, and limitations.

HSAB – Definition

Lewis acids and bases are classified as hard or soft based on the HSAB concept.

The hardness and softness of acids and bases are determined by how tightly any species holds the electron cloud.

A tightly bound electron cloud with low polarisability hardens a species, whereas a loosely bound electron cloud with high polarisability softens it. In general, species with high electronegativities are hard, while those with low electronegativities are soft. The third category, which is an intermediate category, appears on the borderline, however.

  • Hard Lewis Acids: They are strongly solvated, have small ionic radii, a high positive charge, vacant orbitals in the valence shell, and high-energy LUMOs.
  • Soft Lewis Acids: Their atomic orbitals are completely filled, they have a low positive charge, large ionic radii, and low energy LUMOs.
  • Hard Lewis Bases: They are very electronegative, have small ionic radii, are strongly solvated, weakly polarisable, and have high energy HOMOs.
  • Soft Lewis Bases: They have low energy HOMOs, a large ionic radius, moderate electronegativity, and strong polarizability.
  • Borderline: The properties of borderline Lewis acids and bases are intermediate.

Remember that a Lewis acid or base does not have to have all of the properties to be classified as hard, soft, or borderline. In summary, Hard acids and bases are small and non-polar, whereas Soft acids and bases are bigger in size and more polar.

HSAB – Interactions

Hard acids have the strongest interactions with hard bases, while soft acids have the strongest interactions with soft bases, indicating that hardness prefers hard-hard interactions and softness prefers soft-soft interactions.

Combining hard acid and soft base, or hard base and soft acid will result in a less stable acid-base complex.

  1. Strong ionic interaction results from a large electronegativity difference between hard acid and base, whereas soft acid and base interaction results in strong covalent interaction.
  1. Interactions between hard acid and soft base or soft acid and hard base, on the other hand, result in polar covalent interactions that are more reactive and less stable.

This is why, in order to form more stable complexes, hard acid prefers to combine with hard acid and soft acid prefers to combine with a soft base.

HSAB – Characteristics

  1. Hard Acids
  1. They have ionic radii less than 90 pm.
  2. They are strongly solvated.
  3. They have a high positive charge.
  4. They have low electronegativity.
  5. They have low electron affinity.
  6. They have vacant orbitals present in their valence shells.
  7. They have high-energy LUMOs.

Examples: H+,Na+,Li+,Be2+,K+,Sr2+,Mg2+, Al3+,In3+,Ga3+,Fe3+,Ti4+,U4+, BeX2(X=F, Cl), B(OR)3 

  1. Soft Acids
  1. They have ionic radii greater than 90 pm.
  2. They have low-energy LUMOs.
  3. They have a low or partial positive charge.
  4. They have intermediate electronegativity
  5. They have intermediate electron affinity.
  6. They have completely filled orbitals present in their valence shells.

Examples: Ag+,Cu+,Hg+,Au+,Hg2+,Metal in zero oxidation state, BH3

  1. Borderline Acids

Example: Fe2+,Co2+,Ni2+, Cu2+,Zn2+, Pb2+ , B(CH3)3, SO2, NO+

  1. Hard Bases
  1. They have small radii of less than 120 pm.
  2. They have high-energy HOMOs.
  3. They have a high negative charge.
  4. They are electronegative species.
  5. They are weakly polarisable.
  6. They are difficult to oxidise.
  7. They are highly solvated.

Examples: NH3,  CO32-,PO43-, NO3-, ROH, NH3, RNH2,H2O, F-,Cl-

  1. Soft Bases
  1. They have large radii of greater than 170 pm.
  2. They have low-energy HOMOs.
  3. They have a low negative charge.
  4. They have high polarizability.
  5. They can easily undergo oxidation.

Examples: R3P,R3As,  S2O3-, S2-,RS-, R-,SCN-, RNC, H-, CO, C2H4, C6H6

  1. Borderline Bases

Example: Aniline, pyridine, N3-, N2, Br-, SO32-

Important Note: Soft bases such as alkenes and aromatic rings prefer to form complexes with soft acids. Thus, complexes of Ag+,Pt2+, Hg2+ are common, but complexes of Na+,Mg2+, Al3+.

HSAB – Applications

  1. Solubility in water: The compound formed by the combination of soft acid and the soft base is more covalent and less soluble in polar solvents such as water. Silver iodide, AgI, for example, is insoluble in water due to its covalent nature as a combination of soft acid, Ag+, and soft base, I-.

Lithium iodide, LiI, on the other hand, is the result of a reaction between Li+ (hard acid) and I- (soft base). As a result, it is polar covalent and thus water-soluble.

  1. Using HSAB to predict the direction of inorganic reactions: The HSAB principle helps to predict the outcome of a few reactions. Based on soft or hard acid/base interactions, we can predict whether a reaction will proceed to the right or left.
  1. Because As3+ is softer than P3+ and I- is softer than F-, the reaction between AsF3and PI3 happens as.

AsF3+PI3AsI3+PF3

Remember that both As3+ and P3+ are soft, but As3+ is slightly softer due to their larger size.

  1. Because Mg2+ is a harder acid than Ba2+ and O2- is a harder base than S2-, the reaction shown below is possible. Mg2+, a harder acid, combines with a harder oxide ion.

MgS+BaOMgO+BaS

  1. In hydrogen bonding: Strong hydrogen bonds are possible in NH3, H2O and HF because the donor atoms (F, O, and N) are hard Lewis bases. They interact with H, which is a hard acid and is partially positively charged. Therefore, the interactions are stronger.

HSAB – Limitations

Pearson's HSAB theory is directly opposed to Fajan's rules.

  1. Fajan’s Rule predicts that beryllium salts will be more covalent. However, according to the HSAB principle, the Be2+ ion is a hard acid and is expected to exhibit charge controlled bonding, resulting in beryllium compounds with a more ionic nature. But this is not the case.
  1. The HSAB principle states that because the hydrogen ion, H+, is a hard acid and the hydride ion, H-, is a soft base, its interactions must be polar covalent and H2 must be unstable. In reality, H2 is a pure covalent stable molecule.

Practice Problems

1. Suggest the correct statement when PF2I is made to react with mercury.

  1. PF2Iwillreactwithmercury(Hg)toformHg2I2becausemercury(Hg)isasoftacidandI-isasoftbase.
  2. PF2Iwillnotreactwithmercury(Hg)toformHg2F2becausemercury(Hg)isahardacidandF-isasoftbase.
  3. PF2Iwillreactwithmercury(Hg)toformHg2F2becausemercury(Hg)isahardacidandF-isahardbase.
  4. PF2Iwillnotreactwithmercury(Hg)toformHg2I2becausemercury(Hg)isahardacidandI-isasoftbase.

Answer: A

Solution: PF2I will react with mercury (Hg) to form Hg2I2 because it is iodine rather than fluorine that is removed from PF2I.

2PF2I+HgHg2I2+P2F4

The reason behind this reaction is that iodide ion is a soft base whereas fluoride ion is a hard base. So, Hg being a soft acid reacts with a soft base, I-, to form Hg2I2.

So, option A is the correct answer.

2. Why is PbS (Lead sulphide) insoluble in water?

  1. Pb2+ is a hard acid and S2- is a hard base and hence polar in nature.
  2. Pb2+ is a soft acid and S2- is a soft base and hence less polar in nature.
  3. Pb2+ is a hard acid and S2- is a soft base and hence less polar in nature.
  4. Pb2+ is a soft acid and S2- is a hard base and hence polar in nature.

Answer: B

Solution: The compound formed by combining soft acid and the soft base has a higher covalent bond and is less soluble in polar solvents such as water. Because of its covalent nature as a combination of soft acid, Pb2+,, and soft base, S2-, lead sulphide, PbS, is insoluble in water.

So, option B is the correct answer.

3. Which of the following characteristics belong to soft bases?

  1. They have low-energy HOMOs.
  2. They have a high negative charge.
  3. They have low polarizability.
  4. They are difficult to oxidise.

Answer: A

Solution: Soft bases have the following characteristics.

  1. They have large radii of greater than 170 pm.
  2. They have low-energy HOMOs.
  3. They have a low negative charge.
  4. They have high polarizability.
  5. They can easily undergo oxidation.

Therefore, among the given options, only option A is a characteristic of soft bases.

So, option A is the correct answer.

4. Which of the given ions form a stable complex?

  1. F-,Sb3+
  2. S2-,Hg2+
  3. O2-,Al3+
  4. CO32-,H+
     
  1. i, ii and iii
  2. i, iii and iv
  3. ii, iii and iv
  4. i, ii and iv

Answer: C

Solution: Hard acid prefers to combine with hard acid to form more stable complexes, whereas soft acid prefers to combine with a soft base.

  1. F- being small in size and less polarisable is a hard base, and Sb3+ being large in size and more polarisable is a soft acid. So, Soft acid + Hard base forms an unstable complex.
  1. S2- being large in size and more polarisable is a soft base, and Hg2+ being large in size and more polarisable is a soft acid. So, Soft acid + Soft base forms a stable complex.
  1. O2- being small in size and less polarisable is a hard base and Al3+ being small in size and less polarisable is a hard acid. So, Hard acid + Hard base forms a stable complex.
  1. CO32- being small in size and less polarisable is a hard base and Hbeing small in size and less polarisable is a hard acid. So, Hard acid + Hard base forms a stable complex.

Therefore, from the above discussion, stable complexes will be formed by the combination of ions given in ii, iii and iv.

So, option C is the correct answer.

Frequently Asked Questions – FAQ

1. How does the HSAB principle explain metal catalyst poisoning?
Answer:
Soft acid-soft base interactions between soft metal ions and soft ligands lead to the poisoning of metal catalysts. Strongly adsorbed on the metal surface, these ligands obstruct the active sites. Hard bases or ligands including N, O, or F have no effect on these soft acid catalysts.

2. What role does HSAB play in biological systems?
Answer:
 Numerous chemical toxins and/or active metabolites, which establish covalent bonds with nucleophilic targets on biological macromolecules, are electrophiles that harm cells. Predicting the result of such reactions has shown to be a useful use of the Hard and Soft Acids and Bases (HSAB) theory. This idea uses the polarizability of electrons as an inherent electronic feature to categorise responding nucleophils and electrophiles as either hard or soft. These HSAB definitions have been used to successfully address chemical-induced toxicity in biological systems.

3. What exactly is Kornblum's rule?
Answer: 
Kornblum's rule is an application of HSAB theory, which states that in a SN1 reaction, a carbocation (a hard acid) reacts with a hard base (high electronegativity), whereas in a SN2 reaction, tetravalent carbon (a soft acid) reacts with soft bases.

4. How does the HSAB principle control where minerals are found?
Answer: 
According to the HSAB idea, soft acids prefer to bind to soft bases to form covalent complexes of metal while hard acids prefer to bind to hard bases to form ionic complexes of metal.

Related Topics

Acids Bases and Salt

Lowry Bronsted theory

Arrhenius theory

Lewis concept

Electrolytes and Non-Electrolytes

Strong acids

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