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).
[1][2] In 1958 Pearson and Fred Basolo, his colleague at Northwestern wrote the influential monograph "Mechanisms of Inorganic Reactions",[3] which integrated concepts from ligand field theory and physical organic chemistry and signaled a shift from descriptive coordination chemistry to a more quantitative science.
With another Northwestern colleague, Arthur Atwater Frost, Pearson wrote in 1961 another classic text, Kinetics and Mechanism: A Study of Homogeneous Chemical Reactions (ISBN 9780471283478).
[4] In this theory 'Hard' applies to species that are small, have high charge states, and are weakly polarizable.
In 1983 in collaboration with Robert Parr, he refined the HSAB theory into a quantitative method by calculating values of “absolute hardness” using density functional theory, an approximate method in molecular quantum mechanics.