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The HSAB concept is an initialism for "hard and soft (Lewis) acids and bases". Also known as the Pearson acid base concept, HSAB is widely used in chemistry for explaining stability of compounds, reaction mechanisms and pathways. It assigns the terms 'hard' or 'soft', and 'acid' or 'base' to chemical species. 'Hard' applies to species which are small, have high charge states (the charge criterion applies mainly to acids, to a lesser extent to bases), and are weakly polarizable. 'Soft' applies to species which are big, have low charge states and are strongly polarizable. The concept is a way of applying the notion of orbital overlap to specific chemical cases. The theory is used in contexts where a qualitative, rather than quantitative, description would help in understanding the predominant factors which drive chemical properties and reactions. This is especially so in transition metal chemistry, where numerous experiments have been done to determine the relative ordering of ligands and transition metal ions in terms of their hardness and softness. HSAB theory is also useful in predicting the products of metathesis reactions. In 2005 it was shown that even the sensitivity and performance of explosive materials can be explained on basis of HSAB theory.〔() E.-C. Koch, Acid-Base Interactions in Energetic Materials: I. The Hard and Soft Acids and Bases (HSAB) Principle-Insights to Reactivity and Sensitivity of Energetic Materials, ''Prop.,Expl.,Pyrotech. 30'' 2005, 5 〕 Ralph Pearson introduced the HSAB principle in the early 1960s as an attempt to unify inorganic and organic reaction chemistry.〔() R. G. Pearson, Chemical Hardness - Applications From Molecules to Solids, Wiley-VCH, Weinheim, 1997, 198 pp〕 ==Theory== The gist of this theory is that ''soft'' acids react faster and form stronger bonds with ''soft'' bases, whereas ''hard'' acids react faster and form stronger bonds with ''hard'' bases, all other factors being equal.〔IUPAC, (Glossary of terms used in theoretical organic chemistry ), accessed 16 Dec 2006.〕 The classification in the original work was mostly based on equilibrium constants for reaction of two Lewis bases competing for a Lewis acid. Hard acids and hard bases tend to have the following characteristics: * small atomic/ionic radius * high oxidation state * low polarizability * high electronegativity (bases) * hard bases have highest-occupied molecular orbitals (HOMO) of low energy, and hard acids have lowest-unoccupied molecular orbitals (LUMO) of high energy.〔〔Miessler G.L. and Tarr D.A. "Inorganic Chemistry" 2nd ed. Prentice-Hall 1999, p.181-5〕 Examples of hard acids are: H+, light alkali ions (Li through K all have small ionic radius), Ti4+, Cr3+, Cr6+, BF3. Examples of hard bases are: OH–, F–, Cl–, NH3, CH3COO–, CO32–. The affinity of hard acids and hard bases for each other is mainly ionic in nature. Soft acids and soft bases tend to have the following characteristics: * large atomic/ionic radius * low or zero oxidation state bonding * high polarizability * low electronegativity * soft bases have HOMO of higher energy than hard bases, and soft acids have LUMO of lower energy than hard acids. (However the soft-base HOMO energies are still lower than the soft-acid LUMO energies.)〔〔 Examples of soft acids are: CH3Hg+, Pt2+, Pd2+, Ag+, Au+, Hg2+, Hg22+, Cd2+, BH3. Examples of soft bases are: H–, R3P, SCN–, I–. The affinity of soft acids and bases for each other is mainly covalent in nature. Borderline cases are also identified: borderline acids are trimethylborane, sulfur dioxide and ferrous Fe2+, cobalt Co2+ caesium Cs+ and lead Pb2+ cations. Borderline bases are: aniline, pyridine, nitrogen N2 and the azide, bromide, nitrate and sulfate anions. Generally speaking, acids and bases interact and the most stable interactions are hard-hard (ionogenic character) and soft-soft (covalent character). An attempt to quantify the 'softness' of a base consists in determining the equilibrium constant for the following equilibrium: :BH + CH3Hg+ ↔ H+ + CH3HgB Where CH3Hg+ (methylmercury ion) is a very soft acid and H+ (proton) is a hard acid, which compete for B (the base to be classified). Some examples illustrating the effectiveness of the theory: * Bulk metals are soft acids and are poisoned by soft bases such as phosphines and sulfides. * Hard solvents such as hydrogen fluoride, water and the protic solvents tend to solvate strong solute bases such as the fluorine anion and the oxygen anions. On the other hand dipolar aprotic solvents such as dimethyl sulfoxide and acetone are soft solvents with a preference for solvating large anions and soft bases. * In coordination chemistry soft-soft and hard-hard interactions exist between ligands and metal centers. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「HSAB theory」の詳細全文を読む スポンサード リンク
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