Ralph Pearson

Ralph G. Pearson (born January 12, 1919, Chicago) is a 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, when he moved to University of California at Santa Barbara (UCSB). He retired in 1989 but remains active in research in theoretical inorganic chemistry.

In 1963 he proposed the qualitative theory of hard and soft acids and bases (HSAB) in an attempt to unify the theories of reactivity in inorganic and organic chemistry. 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 and are strongly polarizable. Acids and bases interact, and the most stable interactions are hard-hard and soft-soft.

In 1967 Pearson and Fred Basolo, his colleague at Northwestern wrote the influential monograph "Mechanisms of Inorganic Reactions", which integrated concepts from ligand field theory and physical organic chemistry and signaled a shift from descriptive coordination chemistry to a more quantitative science.

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.

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