Hard and Soft Acids and Bases (HSAB) Theory

 

Hard Soft Acid Base Theory

 

The concept of hard and soft acids and bases (HSAB) is a fundamental framework in the field of chemistry, particularly in the context of Lewis acid-base theory. It was introduced by Ralph Pearson in 1963 as a way to classify and understand the interactions between Lewis acids (electron-pair acceptors) and Lewis bases (electron-pair donors) based on their "hardness" or "softness." According to this concept, hard acids prefer to bond with hard bases and soft acids bond with soft bases. Hard acids and bases are not easily polarizable and the interactions between them are predominantly ionic. Soft acids and bases are usually polarizable. Therefore their interactions are covalent.

 

Here's a brief overview of hard and soft acids and bases: 

Hard hard (h-h) interactions: Generally, in the hard-hard interactions, because of their nonpolarizable characters, the ionic interaction is predominant. In fact both the hard acid and hard base are tiny in size which in turn favours the ionic interaction. In this regard the possibility of covalent interaction cannot be completely ruled out. The covalent bond energy is also inversely proportional to the bond length.

Soft soft (s-s) interactions: In the soft-soft interactions, due to the participation of large polarizable soft basis and acids with large number of the d-electrons, the covalent interaction is highly favoured. It should be mentioned that in the shop adduct, the mutual polarization is important to introduce the covalent interaction. Besides the covalent interaction, because of the presence of polarizable moieties, the London dispersion force may also contribute significantly.

In addition to these, the possibility of π back bonding (metal→ligand) may also stabilize the soft-soft interaction. Generally, the soft acids contain a large number of d-electrons. The π back bonding and σ bonding are mutually interdependent in a synergistic fashion. The π acid ligands such as CO, CN^-, SH^-, C₂H₄ etc. are the representative examples of soft bases. For the halides, the ease of accessibility of vacant d orbital for π acceptance is:

I^- > Br^- > Cl^-

Hard (h-s) soft interaction and Pauling Pearson Paradox: From the stand point of covalent interaction and covalent ionic resonance energy, the hard-soft (h-s) interactions are expected to be stable than soft soft (s-s) interaction. This Apparent contradiction may be avoided by considering the overall equilibrium, i.e.,

h-s + s-h ⇋ h-h + s-s

If the total stabilizing contributions from h-h and s-s interactions in the products exceed those of h-s and s-h interactions in the reactants, then the equilibrium will move towards the right hand direction. Hence to determine the position of equilibrium, we are two considered the overall contribution rather than the individual contribution of the adducts. In many cases h-h interaction may reside on the driver seat and s-s interaction in the product appears as a consequence only. It is being illustrated in the following examples

LiI (h-s) + CsF (s-h) ⇋ LiF (h-h) + CsI (s-s), ΔH = -63 KJ mol^-1

HgF₂ (s-h) + BeI₂ (h-s) ⇋ BeF₂ (h-h) + HgI₂ (s-s), ΔH = -397 KJ mol^-1

The enthalpy of formation in kJ mol^-1 for the compounds are LiI (-347), CsF (-502), LiF (-573), CsI (-335), HgF₂ (-536), BeI₂ (-577), BeF₂ (-1264), HgI₂ (-293). Thus, here the stability order of the interaction is

h-h > h-s and h-s > s-s

Thus in the above examples, it is evident that though the hard soft (h-s) interaction stabilize more than the soft soft interactions the highest stabilization brought about by the hard hard interaction in LiF and BeF₂, drives the above equilibrium to the right hand direction. Thus the principal of Pearson's HSAB works quantity the Pauling's concept of covalent ionic resonance. This phenomenon is described as Pauling Pearson Paradox. But this is not really a paradox in terms of thermodynamics of the overall process as discussed above.

 

Hard Acids (HA):

Hard acids are typically small, highly charged, and have a relatively low polarizability (ability to deform their electron cloud). They often have high electronegativity values. These include alkali and alkaline earth metal ions, lighter transition elements.

Hard acids include metal cations such as H⁺, Li⁺, Na⁺, Mg²⁺, Al³⁺, and first row transition metals Ti^4+, Cr³⁺, Fe³⁺, Co²⁺ etc.

Ø  1) Cations contain smaller ionic radii.

Ø  2) Cations have higher positive charge density or higher positive charge.

Ø  3) These cations are not polarizable.

Ø  4) These cations have lesser number of valence electrons.

 

Soft Acids (SA):

Soft acids are usually larger, less charged, and have a higher polarizability. They are more likely to be found in lower oxidation states and have lower electronegativity values. These includes heavy transition metal ions i.e. 2^nd and 3^rd row transition elements or 4d and 5d metal ions such as Hg²⁺, Pd²⁺, Pt²⁺, Cd²⁺ and lower oxidation state ions such as Cu⁺, Ag⁺, Hg⁺.

Ø      1)These cations have larger ionic radii.

Ø      2) These cations lower positive charge density.

Ø    3) These cations are easily polarizable.

Ø    4) Cations have higher number of valence electrons.

 

Hard Bases (HB):

Hard bases typically have lone pairs of electrons that are localized, and they prefer to interact with hard acids. They have a strong tendency to form covalent bonds. In general molecules and donor atoms of high electronegativity are considered as hard bases such as H₂O, NH₃, F^-, ROH etc.

Ø     1) These bases have high electronegativity.

Ø   2) They have low polarizability.

Ø    3) These bases have smaller size.

Ø  4) Accumulation of non-polarizable moieties.

 

Soft Bases (SB):

Soft bases often have diffuse, easily polarizable electron clouds, and they prefer to interact with soft acids. They tend to form more ionic or dative bonds. These bases include molecules and ions where donor atoms are P, As, S, Se etc. which have extended π electron systems. Example: R₃P, R₂S, I^-, SCN^-.

Ø      1) Donor atoms are comparatively larger in size.

Ø    2) They have lower electronegativity

Ø    3) Contain higher polarizability.

Ø    4) Accumulation of polarizable moieties.

   Understanding the concept of hard and soft acids and bases is essential in various areas of chemistry, including coordination chemistry, organometallic chemistry, and in predicting the outcomes of chemical reactions and complex formation.

Table 1. Classification of acids

Hard Acids	Soft acids	Border line
H+, Li+, Na+, K+, Be2+, Mg2+, Ca2+, Al3+, Ga3+, In3+, Ti4+, Zr4+, BF3	Cu+, Ag+, Au+, Pd2+, Cd2+, Hg2+, Hg22+, Tl+, Ga3+, BH3, Pt2+	Fe2+, Co2+, Ni2+, Rh3+, Ru3+, Sb3+, Bi3+, Sn3+, Pb2+, GaH3, Zn2+, Cu2+

 

Table 2. Classification of bases

Hard Base	Soft Base	Border line
NH3, N2H4, RNH2, ROH, R2O,R2CO, H2O, OH-, F-, Cl-, NO3-,SO42-, PO43-, CH3COO-	RNC, CO, C2H4, C6H6, RSH, R2S, RS-, NCS-, I-, H-, R-, R3P, R3As	C6H5NH2, C6H5N, N2, NO2-, SO32-, Br-

 

The Hard and Soft Acid-Base (HSAB) principle is a concept in chemistry that helps us understand the behavior of acids and bases in various chemical reactions. It has broad applications in several areas of chemistry. Here are some specific applications of the HSAB principle:

 

Stability of Complexes:

Coordination Chemistry: HSAB theory is widely used to predict the stability of coordination complexes. It suggests that "hard" Lewis acids (e.g., metal ions with high charge and small size) prefer to bind with "hard" Lewis bases (e.g., small, highly electronegative atoms like oxygen and nitrogen), while "soft" acids (e.g., larger, less electronegative metal ions) prefer to bind with "soft" bases (e.g., larger, less electronegative atoms like sulfur and phosphorus). This knowledge helps chemists design and predicts the stability of coordination compounds.

Ag⁺ (Soft acid) + 2I^- (Soft base) → AgI₂^- (Stable)

Ag⁺ (soft acid) + 2F^- (Hard base) → AgF₂^- (unstable)

 

Course of Reactions:

HSAB principles can guide the prediction of reaction pathways. For example, understanding whether a reaction proceeds via nucleophilic or electrophilic attack can be explained using the concept of hard and soft acids and bases. Like reaction between LiI and CsF will always produce LiF and CsI.

LiI (Hard-Soft) + CsF (Soft-Hard) → LiF (Hard-Hard) + CsI (Soft-Soft)

CaS (Hard-Soft) + H₂O (Hard) → CaO (Hard-Hard) + H₂S (Soft)

 

Classification of Acids and Bases:

HSAB theory provides a framework for classifying acids and bases as "hard" or "soft" based on their properties. This classification helps chemists make predictions about how different compounds will react with each other. For instance, knowing that a particular reaction involves a hard acid and a hard base can inform us about the expected reaction outcome.

 

Occurrence of Ores and Minerals:

In the field of geology and mineralogy, the HSAB principle can help explain the formation and stability of certain minerals and ores. For example, understanding the chemical interactions between metal ions (acidic) and ligands (basic) in geological processes can provide insights into ore formation and mineral stability.

Catalytic Poisoning:

In catalysis, the HSAB concept can be applied to explain catalytic poisoning phenomena. If a foreign substance (a "poison") can strongly coordinate with a catalyst (acting as a Lewis acid or base) due to hard-soft interactions, it can inhibit or reduce the catalytic activity by occupying active sites or altering the catalyst's reactivity.

Reaction Pathway

 Sometimes, the HSAB principle can explain the right mechanistic pathway this is being illustrated in the following reactions

(a) CH₃-Cl + KCN → CH₃-CN

(b) CH₃-Cl + AgCN → CH₃-NC

The reaction (a) passes through and interchanges (l) process in which carbon bears a partial positive charge. And such a carbon centre being relatively softer then a carbocation centre generated in SN1 reactions. Such carbon centre prefers the nucleophilic attack by the soft carbon and of the ambidentate thyroid nucleophile. This is why, it produces CH₃-CN. On the other hand, Ag+ induces the process to pass through a D process, i.e., S_N1 or dissociative process. As Ag+ snatches the chloride ion the carbocation centre which is hard in character is generated. The heart carbocation and Centre prefers the nucleophilic attack from the hard inside of the ambidentate cyanide nucleophile.

Heavy metal sulphides and group analysis in analytical chemistry

 In the classical group analysis the sulphides of relatively heavy metals (soft centre) are recommended to be treated with HNO₃ to bring the cations in solution. Actually, due to the soft-soft interactions, the heavy metal sulphides are so stable that the heavy metal ions cannot be separated from the S^2- ions as long as the sulphide ions (S^2-) are present in the system. To separate these metal ions, the sulphide ions are to be destroyed first by using some oxidizing agent like HNO₃. This is why, the simple attacking by the acids like HCl and H₂SO₄ will not be able to separate the heavy metal ions from the sulphide ions.

HgO is soluble in HCl but HgS is not: This can also be explained on the basis of HSAB principle, i.e.

HgO (s-h) + 2HCl (h-s) → HgCl₂(s-s) + H₂O (h-h)

On the other hand, HgS being a combination of soft and soft centres is highly stable and it does not dissolve in HCl. In fact, the combination in HgS is better than in HgCl₂.

Separation of Cu²⁺ and Cd²⁺

 Cu²⁺ and Cd²⁺ are separated by passing H₂S in the presence of KCN in which CN^- being a soft (carbon end) base prefers a soft acid. It reacts as follows

Cd²⁺ + CN^- → [Cd(CN)₄]^2-

Cu²⁺ +CN^- → [Cd(CN)₄]^2-

Here, reduction of Cu²⁺ followed by complexation is occurring. Cu²⁺ is a border line acid while Cd²⁺ is a soft acid. But in the reaction, Cu²⁺ is reduced to Cu⁺ and it becomes highly soft and matches properly with CN^-. This is why Cu^I(CN)₄^3- is stable than Cd^II(CN)₄^2-. So, on passing H₂S, only CdS is precipitated. The observation suggest that Cu(I) matches better with CN^- then with S^2- while Cd(II) matches better with S^2- then with CN^-.

Because of the tremendous stability of [Cu(CN)₄]^2- the formal reduction potential of the Cu²⁺/Cu⁺ couple increases sufficiently to oxidize CN^- to (CN)₂ (cyanogen gas) and Cu²⁺ is reduced to the said Cu(I)-cyano complex. The higher stability of the [Cu(CN)₄]^3- complex than that of the [Cd(CN)]^2- complex can be explained by considering the π acidic character of the CN- ligands. The π acceptance from the metal Centre to the ligand is favored when the metal Centre is in low Oxidation State. Cd²⁺ is less ready then Cu⁺ to push back the electron cloud towards the π acid ligand. It may be mentioned that because of the same ground, Ni(0) can form the carbonyl complex Ni(CO)₄ but Zn²⁺ fails through both Ni(0) and Zn(II) are isoelectronic.

Hydrolysis of LiI and LiF

 LiI being a combination of hard and soft centre, is unstable with respect to hydrolysis leading to LiOH which is the hard-hard combination. Thus, as soon as, the OH^- centre is available, Li⁺ leaves I^- and combines with OH^-. On the other hand LiF being a combination of hard and hard centres is stable with respect to hydrolysis. It suggests that Li⁺ matches better with F^- than with OH^-.

Overall, the HSAB principle is a valuable tool in understanding and predicting the behavior of acids, bases, and other chemical species in various chemical and biochemical processes. Its applications extend beyond the examples mentioned here, making it a fundamental concept in chemistry.

Reference

1) Concise inorganic chemistry by J. D. Lee.

2) Inorganic Chemistry by James E. Huheey, Ellen A Keither, Richard L. Keither, Okhil K. Medhi.