HSAB concept

The HSAB (also Pearson concept is called) is the acronym for " H ard and S often A cids and B ases", so the concept of "hard and soft acids and bases," and 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 assess the stability of compounds and their reactivity .

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

Basic statements of the model and estimation of 'hardness' and 'softness'

The basic statement of the HSAB concept is: Soft acids react preferentially with soft bases and hard acids with hard bases.

Hard	Soft
Small ion radii	Large ionic radii
High loads	Low loads
Low polarizability	Great polarizability

Hard and soft bases

It is therefore essential for the concept to distinguish 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”, on the other hand, refers to particles with a low charge density, i.e. those with a low charge and a large radius (small “charge / radius ratio”).

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

A rule of thumb for anions is to consider the stability of the halides: As a hard base, the fluoride ion should form stable compounds with hard metal ions, while, conversely, soft acids should prefer the soft iodide.

Examples

• A classic hard acid is the proton , a hard base is the fluoride ion. An aqueous solution of hydrogen fluoride accordingly has a lower dissociation than the higher hydrogen halides .

• On the other hand, the cadmium (II) ion as an acid and the sulfide ion as a base are soft . Cadmium sulfide should therefore be a rather poorly soluble substance in water.

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

application

The concept mostly serves the qualitative rather than the quantitative description of chemical reactions. However, some successful quantitative models have also been established to determine the dissociation energies of Lewis acid-base adducts.

The HSAB concept is used in practice, for example, in qualitative analysis ( separation walk ). Although the classic separation process is much older than the HSAB concept, its functionality can largely be understood with the HSAB concept. It is also very useful in understanding the structures and reactions of complexes . The predictions of the Pearson concept are also confirmed in the Goldschmidt classification in geology.

Examples

• The silver cation is a soft acid; accordingly, the stability of the silver halides should increase from fluorine to iodine. In fact, this is confirmed: while silver fluoride is easily soluble, chloride, bromide and iodide form increasingly stable (increasingly poorly soluble) compounds with high covalent bond proportions.

• According to HSAB, hydrofluoric acid HF should be a weak acid because the bond between H and F has strong ionic components. Indeed, the strength of the hydrohalic acids increases from HF with pKa of around +3 to HI with a pKa of −10; thus hydriodic acid is 10 trillion times stronger acid than HF.

Limits

Despite many correct predictions, the HSAB concept can fail if other effects predominate:

• According to HSAB, sodium ions should form stable salts with hydroxide or fluoride; however, these are easily soluble in water because the solvent causes hydration .

• In the case of perchloric acid , similar to HF, a low acid strength should be determined; Perchloric acid is actually one of the strongest acids that exist. The high stability of the highly symmetrical perchlorate anion predominates here.

Even for qualitative statements, competing effects must be included in the considerations.

Classification of some acids and bases

Acids

• Hard: H ⁺ ; Li ⁺ ; Na ⁺ ; K ⁺ ; Be ²⁺ ; Mg ²⁺ ; Ca ²⁺ ; BF ₃ ; BCl ₃ ; Al ³⁺ ; AlCl ₃ ; AlH ₃ ; Cr ³⁺ ; Fe ³⁺ ; Co ³⁺ ; Ti ⁴⁺ ; Cr ⁶⁺ ; Mn ²⁺ ; Ga ³⁺
• Border area: B (CH ₃ ) ₃ ; Zn ²⁺ ; Ni ²⁺ ; Co ²⁺ ; Fe ²⁺ ; Cu ²⁺ ; Pb ²⁺ ; Rh ³⁺ ; Ir ³⁺ ; Ru ³⁺

• Soft: Pt ²⁺ ; Pt ⁴⁺ ; Pd ²⁺ ; Au ⁺ ; Hg ²⁺ ; Hg ₂ ²⁺ ; Cd ²⁺ ; Cu ⁺ ; Bra ₃ ; Ag ⁺ ; Tl ⁺ ; Ge ²⁺ ; Rh ⁺ ; Ir ⁺ ; Ru ²⁺ ; Br ₂ ; I ₂

Bases

• Hard: H ₂ O ; OH ^- ; F ^- ; Cl ^- ; NH ₃ ; R ₃ N ; CH ₃ COO ^- ; CO ₃ ²⁻ ; N ₂ H ₄ ; PO ₄ ³⁻ ; O ^2- ; ClO ₄ ^- ; SO ₄ ^2- ; NO ₃ ^- ; RO ^- ; RNH ₂
• Limit range: Br ^- ; NO ₂ ^- ; SO ₃ ^2- ; N ₃ ^- ; N ₂ ; C ₅ H ₅ N

• Soft: SCN ^- ; R ₂ S ; R-SH ; CN ^- ; R ₃ P ; I ^- ; S ²⁻ ; H ^- ; H ₂ S ; HS ^- ; CO ; S ₂ O ₃ ^2- ; C ₂ H ₄ ; C ₆ H ₆

literature

• Ralph G. Pearson: The HSAB Principle - more quantitative aspects. In: Inorganica Chimica Acta . Vol. 240, no. 1/2, 1995, ISSN  0020-1693 , pp. 93-98.
• Duward F. Shriver, Peter W. Atkins , Cooper H. Langford: Inorganic Chemistry. 2nd Edition. Wiley-VCH, Weinheim 1997, ISBN 3-52729250-0 .

Individual evidence

1. ^ RG Pearson: Hard and Soft Acids and Bases . In: Journal of the American Chemical Society . Vol. 85, No. 22, 1963, pp. 3533-3539, doi : 10.1021 / ja00905a001 .
2. ↑ Entry on hard acid . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.H02740 Version: 2.3.2.
3. ↑ Entry on soft base . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.H02742 Version: 2.3.2.