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HSAB is an acronym for "hard and soft (Lewis) acids and bases". HSAB is widely used in chemistry for explaining the stability of compounds , reaction mechanisms and pathways. It assigns the terms 'hard' or 'soft', and 'acid' or 'base' to chemical species .
A qualitative theory of hard and soft acids and bases (HSAB) was proposed in 1963 in an attempt to unify the theories of reactivity in inorganic and organic chemistry. [4] In this theory 'Hard' applies to species that are small, have high charge states, and are weakly polarizable. 'Soft' applies to species that are large, have low charge states ...
Hard acids form stronger complexes with hard bases than with soft bases. In general terms hard–hard interactions are predominantly electrostatic in nature whereas soft–soft interactions are predominantly covalent in nature. The HSAB theory, though useful, is only semi-quantitative. [28] The hardness of a metal ion increases with oxidation ...
The classification into hard and soft acids and bases (HSAB theory) followed in 1963. The strength of Lewis acid-base interactions, as measured by the standard enthalpy of formation of an adduct can be predicted by the Drago–Wayland two-parameter equation.
Hard metal cations, as classified by HSAB theory, tend to form N-bonded complexes (isothiocyanates), whereas class B or soft metal cations tend to form S-bonded thiocyanate complexes. For the isothiocyanates, the M-N-C angle is usually close to 180°. For the thiocyanates, the M-S-C angle is usually close to 100°.
In the theory of chemical reactivity, the Klopman–Salem equation describes the energetic change that occurs when two species approach each other in the course of a reaction and begin to interact, as their associated molecular orbitals begin to overlap with each other and atoms bearing partial charges begin to experience attractive or repulsive electrostatic forces.
Reed, James L. Electronegativity: Chemical Hardness II. Journal of Physical Chemistry A (1997), 101(40), 7401-7407. ACS abstract extract: "It has been suggested that the abs. hardness of d. functional theory be identified with the chem. hardness of Pearson's principle of hard and soft acids and bases. It is unclear whether these two hardnesses ...
Mohs hardness of materials (data page) Vickers hardness test; Brinell scale This page was last edited on ...