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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.
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.
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 ...
Ralph Gottfrid Pearson (January 12, 1919 – October 12, 2022) was an American physical inorganic chemist best known for the development of the concept of hard and soft acids and bases (HSAB). He received his Ph.D. in physical chemistry in 1943 from Northwestern University , and taught chemistry at Northwestern faculty from 1946 until 1976 ...
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 ...
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°.