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February 17, 2003
Volume 81, Number 07
CENEAR 81 07 p. 50
ISSN 0009-2347


JACS AT 125
HARD AND SOFT ACIDS AND BASES
Ralph Pearson's qualitative principle provides a useful way to predict chemical reactivity

STEVE RITTER

The idea of hard and soft acids and bases is now such a basic part of chemistry that perhaps many chemists don't know where the concept originated. In 1963, Ralph G. Pearson, then an inorganic chemistry professor at Northwestern University, first used the adjectives to describe sets of Lewis acids and bases that had been segregated according to their characteristics [J. Am. Chem. Soc., 85, 3533 (1963)]. In that paper, the 13th most cited in the 125-year history of JACS, Pearson proposed a general rule: Hard acids prefer to associate with hard bases, and soft acids prefer soft bases.

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PRINCIPLED Pearson, in 1969. NORTHWESTERN PHOTO
The chemistry community immediately realized the utility of the HSAB principle, as it has come to be called, which allows qualitative predictions of the outcome of chemical reactions and the relative stabilities of the products. "The concept certainly filled a gap in the chemist's vocabulary," Pearson tells C&EN, "and became a useful way of describing the properties of chemical systems."

Soft acids are defined as electron-pair acceptors--Lewis acids--in which the acceptor atom has a zero or low positive charge and a relatively large size. These characteristics give rise to low electronegativity and high polarization of valence electrons, meaning soft acids are easily oxidized. Hard acids have the opposite characteristics that result in low polarization of valence electrons. Soft bases are electron-pair donors--Lewis bases--in which the donor atom, similar to a soft acid, has a high polarizability. Hard bases have a donor atom with the opposite characteristics.

For example, a metal in a low oxidation state, such as Ni(0), is a soft acid that is stabilized when surrounded by soft bases, such as CO in Ni(CO)4. But in a high oxidation state, such as Ni(V), the metal is a hard acid that is stabilized by hard bases, such as oxide ions in NiO43–. "The HSAB rule must not be taken to mean more than it says," Pearson pointed out in a paper in Science [151, 172 (1966)]. "For example, it certainly does not say that soft acids do not ever complex with hard bases, or that hard acids do not form stable complexes with any soft bases."

Working with chemistry professor Jon Songstad of the University of Bergen, in Norway, Pearson followed up on his initial paper with another highly cited JACS paper, ranked 124th, that focused on application of the HSAB principle to organic chemistry [J. Am. Chem. Soc., 89, 1827 (1967)]. In a nucleophilic substitution reaction in which one Lewis base replaces another, for example, if the acid site is hard, then softness (polarizability) in the nucleophile will not provide a high rate of reaction. If the acid is soft, then a soft nucleophile will react more quickly.

Over time, many aspects of chemistry have been identified that can be rationalized by using the HSAB principle. Examples include coordination compounds, charge-transfer complexes, hydrogen bonding, free-radical complexes, solvent-solute interactions, solid-state compounds, and catalysis. Indeed, most inorganic and organic molecules can be thought of as acid-base complexes, Pearson notes.

One of the pitfalls of the HSAB principle is that the adjectives hard and soft don't mean the same as strong and weak, he adds. That makes it difficult in general to quantify the strength of hard and soft acids and bases. For example, OH is a stronger base than H2O, yet both are hard bases. Similarly, Mg2+ is a stronger acid than Na+, yet both are hard acids.

"There has been some activity by various groups to find empirical scales of hardness and softness that would be useful to predict reaction rates, equilibrium constants, and activation energies," Pearson says. "But it turns out that there is no one set of unifying numbers that describes everything, so the concept has largely remained a qualitative tool."

Pearson, now 84, moved in 1976 to the University of California, Santa Barbara. He retired in 1989, but he has remained active as an emeritus professor, focusing on theoretical chemistry. His latest work is revisiting quantum mechanics as he learned it during graduate school and seeing how it might be presented differently in light of newer developments, such as density functional theory.

CLASSIFIED
Hard and soft acids and bases are segregated by polarizability
HARD ACIDS SOFT ACIDS
H+, Na+, Ca2+, Mn2+, Al3+, N3+, Cl3+, Gd3+, Cr3+, Co3+, Fe3+, BF3, B(OR)3, AlCl3, SO3, CO2, RCO+, RPO2+, NC+ M0 (metal atoms), Cu+, Ag+, Hg+, Pd2+, Pt2+, Co(CN)52–, InCl3, BH3, RS+, Br2, RO(dot), RO2(dot), carbenes
HARD BASES SOFT BASES
H2O, OH, F, CH3CO2, SO42–, CO32–, NO3, PO43–,ClO4, NH3, RNH2, ROH, R2O, RO R2S, RSH, I, SCN, S2O32–, R3P, (RO)3P, CN, RNC, CO, C2H4, C6H6, H, R
BORDERLINE ACIDS BORDERLINE BASES
Fe2+, Co2+, Ni2+, Sn2+, Ru2+ Rh3+, Ir3+, SO2, B(CH3)3, R3C+, C6H5+ C6H5NH2, C6H5N, N2, N3, Br, NO2, SO32–


C&EN is celebrating the 125th volume of the Journal of the American Chemical Society by featuring selected papers from among its 125 most cited. These papers were ranked 13th and 124th.



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Related People
Additional Reading on the History and Development of the HSAB Principle
J. Am. Chem. Soc., 84, 16 (1962)

C&EN, May 31, 1965, pages 90-103

Chem. Br., 3, 103 (1967)

J. Chem. Ed., 45, 581 and 643 (1968)

J. Am. Chem. Soc., 105, 7512 (1983)

Chem. Rev., 75, 1 (1975)

Coord. Chem. Rev., 100, 403 (1990)

J. Chem. Ed., 76, 267 (1999)

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