JACS AT 125
HARD AND SOFT ACIDS AND BASES Ralph Pearson's qualitative principle provides useful way to predict chemical reactivity
reaction. If the acid is soft, then a soft nu cleophile will react more quickly. Over time, many aspects of chemistry have been identified that can be rational ized by using the HSAB principle. Exam ples include coordination compounds, charge-transfer complexes, hydrogen bonding, free-radical complexes, solventsolute 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 prin ciple is that the adjectives hard and soft don't mean the same as strong and weak, he adds. That makes it difficult in gener al to quantify the strength of hard and soft acids and bases. For example, O H " is a
lence electrons. Soft bases are electron-pair donors—Lewis bases—in which the donor atom, similar to a soft acid, has a high po larizability 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 sta bilized when surrounded by soft bases, such as C O 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 N i 0 4 3 " . "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." PRINCI PLED Pearson, in 1969. "Working with chemistry professor Jon Songstad of the University of stronger base than H 2 0 , yet both are hard Bergen, in Norway, Pearson followed up bases. Similarly, Mg 2+ is a stronger acid on his initial paper with another highly citthan Na+, yet both are hard acids. ed^4CS paper, ranked 124th, that focused "There has been some activity by vari on application of the HSAB principle to ous groups to find empirical scales of hard organic chemistry [J Am. Chem. Soc, 89, ness and softness that would be useful to 1827 (1967)]. In a nucleophilic substitu predict reaction rates, equilibrium con tion reaction in which one Lewis base re stants, and activation energies," Pearson places another, for example, if the acid site says. "But it turns out that there is no one is hard, then softness (polarizability) in the set of unifying numbers that describes nucleophile will not provide a high rate of everything, so the concept has largely re mained a qualitative tool." Pearson, now 84, moved in 1976 to the CLASSIFIED University of California, Santa Barbara. He Hard and soft acids and bases are segregated by polarizability retired in 1989, but he has remained ac tive as an emeritus professor, focusing HARD ACIDS SOFT ACIDS + 2+ 2+ 3 3+ 3+ 3+ + + + 2+ 2+ on theoretical chemistry. His latest work H \ Na , Ca , Mn , A l \ N , Cl , Gd , M° (metal atoms), Cu , Ag , Hg , Pd , Pt ( is revisiting quantum mechanics as he Co(CN) 5 2 -, lnCl 3 , BH 3 , R S \ Br 2 , R 0 \ R0 2 ' , Cr 3 *, Co 3+ , Fe 3+ , BF 3 , B(0R) 3 , AlCl 3> S0 3 C0 2 , RC0 + , RP0 2 + , NC + carbenes learned it during graduate school and see ing how it might be presented differently HARD BASES SOFT BASES in light of newer developments, such as H 2 0, OH", F", CH 3 C0 2 -, SO,-2", CO, 2 ", NO,", R2S, RSH, I", SON", S 2 0 3 2 ', R3P, (R0] 3 P, density functional theory.-STEVE RITTER 3
T
HE IDEA OF HARD AND SOFT
acids and bases is now such a ba sic part of chemistry that per haps many chemists don't know where the concept originated. In 1963, Ralph G. Pearson, then an inor ganic chemistry professor at Northwestern University, first used the adjectives to de scribe 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. The chemistry community immediate ly realized the utility of the HSAB princi ple, as it has come to be called, which al lows qualitative predictions of the outcome of chemical reactions and the relative sta bilities of the products. "The concept cer tainly filled a gap in the chemist's vocabu lary," 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 electroneg ativity and high polarization ofvalence elec trons, meaning soft acids are easily oxidized. Hard acids have the opposite characteris tics that result in low polarization of va
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P0 4 -,C10 4 -, NH 3 , RNH 2 , ROH, R 2 0, RQ-
CN-, RNC, CO, C 2 H A , C 6 H 6 , Η~, R~
BORDERLINE ACIDS Fe 2+ , C o 2 \ Ni 2 + , S n 2 \ Ru 2+ Rh 3+ , Ir 3 *, S0 2 , B(CH 3 ) 3 , R3C+, C 6 H 5 +
BORDERLINE BASES C 6 H 5 NH 2 , C 6 H 5 N, N 2 , N 3 ", B r , N0 2 ", S0 3 2
C & E N / F E B R U A R Y 17. 2 0 0 2
C&EN is celebrating the 125th volume of the Journal of the American Chemical Society^ featuringselected papersfrom among its 125 most cited. These papers were ranked 13th and 124th. HTTP://WWW.CEN-ONLINE.ORG
