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HSAB theory

The HSAB concept is an acronym for 'hard and soft acids and bases'. Also known as the Pearson acid base concept, HSAB is widely used in chemistry for explaining stability of compounds, reaction mechanisms and pathways. It assigns the terms 'hard' or 'soft', and 'acid' or 'base' to chemical species. 'Hard' applies to species which are small, have high charge states (the charge criterion applies mainly to acids, to a lesser extent to bases), and are weakly polarizable. 'Soft' applies to species which are big, have low charge states and are strongly polarizable.[1]

The theory is used in contexts where a qualitative, rather than quantitative description would help in understanding the predominant factors which drive chemical properties and reactions. This is especially so in transition metal chemistry, where numerous experiments have been done to determine the relative ordering of ligands and transition metal ions in terms of their hardness and softness.

HSAB theory is also useful in predicting the products of metathesis reactions. Quite recently it has been shown that even the sensitivity and performance of explosive materials can be explained on basis of HSAB theory [2]

Ralph Pearson introduced the HSAB principle in the early 1960s[3][4] as an attempt to unify inorganic and organic reaction chemistry.[5].



The gist of this theory is that soft acids react faster and form stronger bonds with soft bases, whereas hard acids react faster and form stronger bonds with hard bases, all other factors being equal.[6] The classification in the original work was mostly based on equilibrium constants for reaction of two Lewis bases competing for a Lewis acid.

Hard acids and hard bases tend to have:

Examples of hard acids are: H+, alkali ions, Ti4+, Cr3+, Cr6+, BF3. Examples of hard bases are: OH, F, Cl, NH3, CH3COO, CO32–. The affinity of hard acids and hard bases for each other is mainly ionic in nature.

Soft acids and soft bases tend to have:

  • large size
  • low or zero oxidation state
  • high polarizability
  • low electronegativity
  • energy high-lying HOMO (bases) and energy-low lying LUMO (acids).[6]

Examples of soft acids are: CH3Hg+, Pt4+, Pd2+, Ag+, Au+, Hg2+, Hg22+, Cd2+, BH3. Examples of soft bases are: H, R3P, SCN, I. The affinity of soft acids and bases for each other is mainly covalent in nature.

HydroniumH+MercuryCH3Hg+, Hg2+, Hg22+HydroxylOH-HydrideH-
Alkali metalsLi+,Na+,K+ PlatinumPt4+AlkoxideRO-ThiolateRS-
Boron trifluorideBF3 boraneBH3CarboxylateCH3COO-ThiocyanateSCN-
CarbocationR3C+P-chloranilCarbonateCO32-carbon monoxideCO
bulk MetalsM0HydrazineN2H4BenzeneC6H6
Table 1. Hard and soft acids and bases

Borderline cases are also identified: borderline acids are trimethylborane, sulfur dioxide and ferrous Fe2+, cobalt Co2+ and lead Pb2+ cations. Borderline bases are: aniline, pyridine, nitrogen N2 and the azide, bromine, nitrate and sulphate anions.

Generally speaking, acids and bases interact and the most stable interactions are hard-hard (ionogenic character) and soft-soft (covalent character).

An attempt to quantify the 'softness' of a base consists in determining the equilibrium constant for the following equilibrium:

BH + CH3Hg+ ↔ H+ + CH3HgB

Where CH3Hg+ (methylmercury ion) is a very soft acid and H+ (proton) is a hard acid, which compete for B (the base to be classified).

Some examples illustrating the effectiveness of the theory:

  • Bulk metals are soft acids and are poisoned by soft bases such as phosphines and sulfides.
  • Hard solvents such as hydrogen fluoride, water and the protic solvents tend to solvatate strong solute bases such as the fluorine anion and the oxygen anions. On the other hand dipolar aprotic solvents such as dimethyl sulfoxide and acetone are soft solvents with a preference for solvatating large anions and soft bases.
  • In coordination chemistry soft-soft and hard-hard interactions exist between ligands and metal centers.

Chemical hardness

Chemical hardness
Hydrogen H+infiniteFluoride F-7
Aluminum Al3+45.8Ammonia NH36.8
Lithium Li+35.1hydride H-6.8
Scandium Sc3+24.6carbon monoxide CO 6.0
Sodium Na+21.1hydroxyl OH-5.6
Lanthanum La3+15.4cyanide CN-5.3
Zinc Zn2+10.8phosphane PH35.0
Carbon dioxide CO210.8nitrite NO2-4.5
Sulfur dioxide SO25.6Hydrosulfide SH-4.1
Iodine I23.4Methane CH3-4.0
Table 2. Chemical hardness data

In 1983 Pearson together with Robert Parr extended the qualitative HSAB theory with quantitative chemical hardness (η) defined as [7]:

\eta = 0.5(I - A) \,

with I\, the ionization potential and A\, the electron affinity.

When the electronegativity (χ) as the Mulliken scale:

\chi = 0.5(I + A) \,

is the first derivative in a plot of energy E\, versus the amount of electrons N\, with fixed nuclear charge Z\, in an atom or molecule:

\chi = \left(\frac{\partial E}{\partial N}\right)_Z \,

then the chemical hardness is simply the second derivative:

\eta = 0.5\left(\frac{\partial^2 E}{\partial N^2}\right)_Z \,

Hardness and electronegativity are related as:

2\eta = -\left(\frac{\partial \chi}{\partial N}\right)_Z \,

and in this sense hardness is a measure for resistance to deformation or change. Likewise a value of zero denotes maximum softness.

In a compilation of hardness values only that of the hydride anion deviates. Another discrepancy noted in the original 1983 article are the apparent higher hardness of Tl3+ compared to Tl+.

Kornblum's rule

An application of HSAB theory is the so-called Kornblum's rule which states that in reactions with ambident nucleophiles, the more electronegative atom reacts when the reaction mechanism is SN1 and the less electronegative one in a SN2 reaction. This rule (established in 1954) [8] actually predates HSAB theory but in HSAB terms its explanation is that in a SN1 reaction the carbocation (a hard acid) reacts with a hard base (high electronegativity) and that in a SN2 reaction tetravalent carbon (a soft acid) reacts with ditto soft bases.


  1. ^ Jolly, W. L.. Modern Inorganic Chemistry. ISBN 0070327602. 
  2. ^ [1]E.-C. Koch, Acid-Base Interactions in Energetic Materials: I. The Hard and Soft Acids and Bases (HSAB) Principle-Insights to Reactivity and Sensitivity of Energetic Materials, Prop.,Expl.,Pyrotech. 30 2005, 5
  3. ^ Pearson, Ralph G. (1963). "Hard and Soft Acids and Bases". J. Am. Chem. Soc. 85 (22): 3533 - 3539. doi:10.1021/ja00905a001.
  4. ^ Pearson, Ralph G.. "Hard and soft acids and bases, HSAB" (subscriber access). J. Chem. Educ. 1968 (45): 581643.
  5. ^ [2]R. G. Pearson, Chemical Hardness - Applications From Molecules to Solids, Wiley-VCH, Weinheim, 1997, 198 pp
  6. ^ a b c IUPAC, Glossary of terms used in theoretical organic chemistry, accessed 16 Dec 2006.
  7. ^ Robert G. Parr and Ralph G. Pearson (1983). "Absolute hardness: companion parameter to absolute electronegativity". J. Am. Chem. Soc. 105 (26): 7512 - 7516. doi:10.1021/ja00364a005.
  8. ^ The Mechanism of the Reaction of Silver Nitrite with Alkyl Halides. The Contrasting Reactions of Silver and Alkali Metal Salts with Alkyl Halides. The Alkylation of Ambident Anions Nathan Kornblum, Robert A. Smiley, Robert K. Blackwood, Don C. Iffland J. Am. Chem. Soc.; 1955; 77(23); 6269-6280. doi:10.1021/ja01628a064

See also

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "HSAB_theory". A list of authors is available in Wikipedia.
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