HSAB theory

The HSAB concept (also called Pearson's concept ) is the acronym for " Hard and Soft Acids and Bases ", so the concept of "hard and soft acids and bases ", and is one of the acid-base concepts.

The American chemist Ralph G. Pearson published the concept in 1963. The HSAB concept is used in many areas of chemistry to estimate the stability of compounds and their reactivity.

It is based on the Lewis acid - base concept, ie, on the reactivity of electron pair donors (Lewis bases) and electron-pair acceptors ( Lewis acids).

  • 2.1 Examples

Tenets of the model and estimation of, hardness ' and ' softness '

Within this concept, a distinction is made between hard and soft bases, and between hard and soft acids.

  • " Hard " describes particles (atoms, ions and molecules) that have a high charge density, ie a high charge and a small radius (large " charge / radius ratio ").
  • " Soft " particles, however, referred with low charge density, ie, those with low charge and large radius ( small " charge / radius ratio ").

In addition, hard particles hardly polarizable (but strongly polarized ) and soft particles easily polarizable (but weakly polarized ) are.

A rule of thumb for the anions is the consideration of the stability of the halide: the fluoride ion should therefore form a hard base with hard metal ions in stable compounds, while, conversely, soft acids should prefer the soft iodide.


  • A classic hard acid is a proton, a hard base, the fluoride ion. An aqueous solution of hydrogen fluoride thus has a lower dissociation higher than the hydrogen halides.
  • , However, are soft, the cadmium ( II) ion as an acid and the sulphide - ion as the base. Cadmium sulfide should therefore be a rather bad releasable in hydrogen.

The bond between the Lewis acid and Lewis base adducts in which is formed of soft species, has a more covalent character, the binding of two species is hard to describe rather electrostatic ( ionic).


The concept usually is more qualitative than quantitative description of chemical reactions. And there were also established several successful quantitative models to determine the dissociation energies of Lewis acid - base adducts.

Practical application finds the HSAB concept, for example, in qualitative analysis ( separation process ). Although the classical separation process is far older than the HSAB concept, its operation can be largely understood with the HSAB concept. It is also very helpful in understanding the structures and modes of reaction of complexes. Also in the Goldschmidt classification in geology to confirm the predictions of the Pearson concept.


  • The silver cation is a soft acid; Accordingly, the stability of the silver halides fluoride should increase by iodine. In fact, this is confirmed: While silver fluoride is readily soluble form of chloride, bromide and iodide increasingly stable (increasing schwerlöslichere ) compounds with high covalent bonding components.
  • HSAB by hydrofluoric acid HF should be a weak acid because the bond between H and F has a strong ionic components. In fact, the strength of the hydrogen halides HF with pKa of about 3 rises to the HI with a pKa of -10; Accordingly, hydroiodic acid is 10 trillion times more acidic than HF.


In spite of many correct predictions the HSAB concept can fail if other effects predominate:

  • After HSAB sodium ions should form stable salts with hydroxide or fluoride; but they are easily soluble in water, because it is for hydration by the solvent.
  • When perchloric acid should be find a low acid strength as in HF is similar; the perchloric acid but is actually one of the strongest acids that are out there. Here outweighs the great stability of the highly symmetrical perchlorate anion.

Even for qualitative statements so competitive effects must be brought into the discussion.

Classification of some acids and bases

  • Hard acids: H ; Na ; K ; Cr3 ; Cr6 ; BF3; Mn2 ; Al3 ; Co3 ; Ga3 ; Fe3 ; Mg2 ; Ca2
  • Soft acids: Pt2 ; Pt4 ; Pd2 ; Au ; Hg2 ; Hg22 ; Cd2 ; Cu ; Pb2 ; BH3; Ag ; Tl
  • Hard bases: H2O; OH; F; Cl -; NH3; R3N; CH3COO -; CO32 -; N2H4; PO43 -
  • Soft bases: SCN; R2S; R- SH; CN; R3P; I-