Ralph Pearson

{{Short description|American chemist (1919–2022)}}

{{more footnotes needed|date=February 2013}}

{{Infobox scientist

|name = Ralph Pearson

|birth_name = Ralph Gottfrid Pearson

|birth_date = {{birth date|1919|1|12}}

|birth_place = Chicago, Illinois, U.S.

|death_date = {{death date and age|2022|10|12|1919|1|12}}

|death_place =

|workplaces = Northwestern University
University of California at Santa Barbara

|alma_mater = Northwestern University

|field = Inorganic chemistry

|known_for = Development of hard and soft acid base theory (HSAB)

}}

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, when he moved to University of California at Santa Barbara (UCSB). He retired in 1989 but remained active in research in theoretical inorganic chemistry until his death.{{cite journal

|author = Ford, P. C.

|year =1999

|journal = Coordination Chemistry Reviews

|volume = 187

|title = Interview with Ralph G. Pearson

|doi = 10.1016/S0010-8545(98)00223-9

|pages = 3–15

|issue = 1}}

In 1958 Pearson and Fred Basolo, his colleague at Northwestern wrote the influential monograph "Mechanisms of Inorganic Reactions",{{cite journal|author = Murmann, R. K.

|year =1968

|journal = J. Chem. Educ.

|volume = 45

|title = Book review of Mechanisms of inorganic reactions - A study of metal complexes in solution (Basolo, Fred; Pearson, Ralph G.)

|doi = 10.1021/ed045pA146

|pages = A146–A148

|issue = 2

|doi-access =

}} 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}}). A subsequent edition was with John W. Moore as co-author ({{ISBN|978-0471035589}}).

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.{{cite journal|title=Hard and Soft Acids and Bases|author=Pearson, Ralph G.|journal= J. Am. Chem. Soc. |year=1963| volume= 85| issue=22| pages=3533–3539| doi=10.1021/ja00905a001|bibcode=1963JAChS..85.3533P }} 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 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. This concept of "absolute hardness" was later connected with the concept of (absolute) electronegativity.{{cite journal

|author = Pearson, R. G.

|year =1988

|journal = Inorganic Chemistry

|volume = 27

|title = Absolute electronegativity and hardness: application to inorganic chemistry

|doi = 10.1021/ic00277a030

|pages = 734–740

|issue = 4|s2cid =55897650

}} Pearson died on October 12, 2022, at the age of 103.{{cite web|title=Ralph G. Pearson - Department of Chemistry - UC Santa Barbara|url=https://www.chem.ucsb.edu/people/ralph-g-pearson|publisher=University of California Santa Barbara Chemistry and Biochemistry|access-date=October 17, 2022}}