I Hard and Soft Acids and Bases Ralph G Pearson University Evanston Illinois 60201 I HSAB Part I I Fundamental principles According to G N Lewis a base is ion which has ID: 228676
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I Hard and Soft Acids and Bases, Ralph G. Pearson Northwestern University Evanston, Illinois 60201 I HSAB, Part I I Fundamental principles According to G. N. Lewis a base is ion which has are not already shared covalent bond. acid is similarly unit in vacant orbital electrons can be accommodated. base reaction is may be adduct, acid-base complex. acid-base complexes exist under dif- ferent names. The is usually acid, avoid confusion with Brfinsted identical with Brfinsted base. Sidgwick suggested the terms electron acceptor, electron donor, These terms used, of weak generalized acid- interactions almost always discussed under the heading of donor-acceptor complexes. electron donor, is substances called bases. sometimes for special categories. use a metal in speak- rates is usual electro- phile and to call B a nucleophile.' Probably the most important class of chemical reac- tion is the generalized acid-base reaction of eqn. (1). The easiest way to appreciate this is different kinds acid-base complexes, A:R, formed. For example, all or ions several bases or ligands simultaneously they are m+y be electrically case have a complex ion combination may be electrically case normal inorganic as SnCli is formed. Most cations are T,ewis most anions salts are MgClz in the solid state met,hod here is to ment,ally dissect, t,he organic molecule into two fragments, one of which is a 1,ewis other a base. For example, ethyl alcohol can be thought of as composed of t,he carbonium ion, C2Hs+, hydroxide ion, OH-. can be as acylium ion, CH3CO+, which is an acid, and t.he ethoxide ion, CZH50-. Even a hydrocar- bon can be broken down (conceptually) into an acid such as H+ and a carbanion, Thus methane Lewis acid because that it electrons from CH3C1, and a is a purely conceptual process do with stabil- ity of CH3f. point is most reactions can be classified as being exchanges of the chloride ion by other bases, or of the methyl cat- ion by other Lewis acids. Just as the proton does not exist free under ordinary circumstances, so it is likely that CH3+ does not ordinarily exist species. Volume 45, ~umb;r 9, September 1968 / 581