The Hard/Soft Acid/Base Rule: A Perspective from Conceptual Density‐Functional Theory

Pearson's hard/soft acid/base (HSAB) rule indicates that, all other things being equal, hard acids prefer reacting with hard bases and weak acids with weak bases . While venerable, ubiquitous, and indubitably useful, the HSAB rule is not a physical law, nor are the hardness/softness and acidity/basicity physical observables. The goal of this chapter is to discuss the attendant ambiguities in the applicability and reliability of the HSAB rule and promulgate some opinions on fruitful directions for further research.