What are hard nucleophiles and soft electrophiles

The HSAB concept (also Pearson concept) is the acronym for Hard-S.often-A.cid-B.ase, i.e. hard-soft-acid-base concept and is one of the acid-base concepts. The HSAB concept is used in many areas of chemistry to explain the stability of complex compounds as well as reaction mechanisms. It is based on the Lewis acid-base concept, i.e. the reactivity of electron pair donors (Lewis bases) and electron pair acceptors (Lewis acids).


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 positive charge density, ie a high positive charge and a small radius (large “charge / radius ratio”). “Soft”, on the other hand, describes particles with a low positive charge density, i.e. those with a low positive charge (or higher negative charge) and a large radius (small “charge / radius ratio”). In addition, hard particles are weakly polarizable (but strongly polarizing) and soft particles are easily polarizable (but weakly polarizing).
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.

The concept mostly serves the qualitative rather than the quantitative description of chemical reactions. Although some successful quantitative models have also been established that allow the dissociation energies of Lewis acid-base adducts to be determined.
To put it simply: hard particles tend to form stable bonds with hard particles, soft particles tend to form stable bonds with soft particles. If hard particles react with soft particles, neither a strong covalent nor a strong electrostatic bond is to be expected (however, the sum of both can still lead to high binding energies in individual cases). The HSAB concept is used in practice, for example, in qualitative analysis (separation walk). Although the separation passage is much older than the HSAB concept, the way it works can largely be understood with the HSAB concept. It is also very helpful in understanding the structures and reactions of complexes.


Ralph Pearson published his HSAB concept in the 1960s as an approach to combine organic and inorganic reaction chemistry. Since then, new areas of application have appeared again and again, which makes the HSAB concept an important explanatory concept, especially in inorganic chemistry.


H+; N / A+; K+; Tl3+; Cr3+; Cr6+; BF3; Mn2+; Al3+; Co3+; Ga3+; Fe3+; Mg2+; Approx2+;

Pt2+; Pt4+; Pd2+; Au+; Ed2+; Ed22+; CD2+; CD2+; Cu+; Pb2+; bra3

H2O; OH-; F.-; Cl-; NH3; RNH2; CH3COO-; CO32-; N2H4; PO43-

SCN-; R.2S; RSH; CN-; R.3P

Category: acid-base reaction