Principle of hard and soft acids

The principle of hard and soft acids and bases and the problem of competitive coordination in complex compounds. A, D. Garnovskii, D. A. Osipov and S. B. Bulgarevich, Russ. Chem. Rev. (Engl. Transl), 1972, 41, 341-359 (441). 

The Lewis acid/base complex is formed via an overlap between a doubly occupied orbital of the donor D and vacant orbital of the acceptor A. This acid/base approach was extended by Pearson who divided Lewis acids and bases into two groups, hard and soft, according to their electronegativity and polarizability (principle of hard and soft acids and bases (HSAB concept). Hard acids (e.g., H, Lf, Na, BF3, AICI3, hydrogen-bond donors HX) and hard bases (e.g., F", CL, HO, RO, H2O, ROH, R2O,... 

Pearson RG, Songsted J. Application of the principle of hard and soft acids and bases to organic chemistry. J Am Chem Soc 1967 89 1827-1836. 

The Principle of Hard and Soft Acids and Bases states that hard acids form more stable complexes with hard bases and soft bases form more stable complexes with soft acids. In orbital terms hard molecules have a large gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), and soft molecules have a small HOMO-LUMO gap. In recent years it has been possible to correlate the hardness with the electronic properties of the atoms involved. Thus, if the enthalpy of ionisation (I) and the electron affinity (A) are known the so-called absolute hardness (t ) and absolute electronegativity (%) can be found from r = (I - A) / 2 and % = (I + A) / 2. For example, the first and second ionisation enthalpies of sodium are 5.14 and 47.29 eV. Thus for Na+, I = + 47.29 and A = + 5.14, so r = (47.29 - 5.14) / 2 = 21.1. Similarly for silver the first and second ionisation enthalpies are 7.58 and 21.49eV, so for Ag+ we have, n = (21.49 - 7.58) 12 = 6.9. 

One of the most useful tools for predicting the outcome of chemical reactions is the principle of hard and soft acids and bases (HSAB), formulated by Pearson in 1963 [13-15]. This prindple states that hard acids will react preferentially with hard bases, and soft acids with soft bases, hard and soft referring to sparsely or highly polarizable reactants. A selection of hard and soft Lewis acids and bases is given in Table 1.1. 

Reactions of carbocations with free CN- occur preferentially at carbon, and not nitrogen as predicted by the principle of hard and soft acids and bases.69 Isocyano compounds (N-attack) are only formed with highly reactive carbocations where the reaction with cyanide occurs without an activation barrier because the diffusion limit has been reached. A study with the nitrite nucleophile led to a similar observation.70 This led to a conclusion that the ambident reactivity of nitrite in terms of charge control versus orbital control needs revision. In particular, it is proposed that SNl-type reactions of carbocations with nitrite only give kinetically controlled products when these reactions proceed without activation energy (i.e. are diffusion controlled). Activation controlled combinations are reversible and result in the thermodynamically more stable product. The kinetics of the reactions of benzhydrylium ions with alkoxides dissolved in the corresponding alcohols were determined.71 The order of nucleophilicities (OH- MeO- < EtO- < n-PrCT < / -PrO ) shows that alkoxides differ in reactivity only moderately, but are considerably more nucleophilic than hydroxide. 

Principle of Hard and Soft Acids and Bases (HSAB)... 

Regioselective synthesis is examined mainly on the basis of the conception of competitive coordination and the principle of hard and soft acids and bases [14]. A description of polyhedron-programmed synthesis is given, taking into consideration thin structure compounds to be used as ligands (number, nature, and mutual situation of donor centers and the presence and character of organic fragments, annealed to a metal-cycle). 

Both metals and non-metals can be either (a) or (b) type of acids depending on their charge and size. Since the features which promote class (a) behavior are those which lead to low polarizability, and those which create type (b) behavior lead to high polarizability, it is convenient to call class (a) acids "hard" acids and class (b) acids "soft" acids. We then have the useful generalization that "hard acids prefer to associate with hard bases, and soft acids prefer soft bases" (Pearson, 1997). This is the Principle of Hard and Soft Acids and Bases, or the... 

Sullivan, P. J. 1977. The principle of hard and soft acids and bases as applied to exchangeable cation selectivity in soils. Soil Sci. 124 117-121. 

The principle of hard and soft acids and bases ought to apply. The thiocyanate ion will be softer on the sulfur atom and harder on the nitrogen atom, the cyanide ion will be softer on the carbon atom and harder on the nitrogen atom, and the nitrite ion will be harder on the oxygen atom and softer on the nitrogen atom. We might expect that harder electrophiles will give the isothiocyanates 4.25, the isonitriles 4.26 and the nitrites 4.28. However, other factors are at work, and this pattern is unreliable. Earlier attempts to use these expectations to explain the patterns of reactivity in this area have been overtaken by more recent work by Mayr.16... 

Cyanide ions react with the soft alkyl halides in SN2 reactions and with the hard carbocations in SnI reactions to give, almost always, the nitrile 4.27, which is thermodynamically preferred. Isonitrile products are formed along with the nitrile products when the cation is so reactive that the rate has reached the diffusion-controlled limit, and the reversible reaction that would equilibrate the products is too slow. One consequence when reactions are as fast as this is that there is a barrierless combination of ions, and selectivity is not then controlled by the kinetic factors associated with the principle of hard and soft acids and bases. 

For all their faults and limitations, frontier orbital theory and the principle of hard and soft acids and bases remain the most accessible approaches to understanding many aspects of reactivity. Since they fill a gap between the chemist s experimental results and a state of the art theoretical description of his or her observations, they will continue to be used until something better comes along. 

Studying acid-base properties of acids and bases, Pearson put forward the principle of hard and soft acids and bases according to which hard acids prefer to react with hard bases, while soft acids prefer to react with soft bases [12,13]. This principle has been widely used in chemistry, however, no quantitative characteristics of the hardness of acids and bases were known. 

This dispersed element occurs with lead in galena, PbS, where T1+ and Bp+ (or Sb +j replace two Pb +. T1+ replaces the similarly sized K+ and Rb+ (Table 2) occurring in some feldspars and micas. Thallium usually appears in the 1+ oxidation state, but appears as 3+ in the rare mineral Avicennite, TI2O3, found in Central Asia. If Tl(ni) were to appear in the ocean, it would be present as Tl(OH)3(aq). Contrary to the usual principle of hard and soft acids and bases see Hard Soft Acids and Bases), Tl(III) is softer than T1(I).21... 

Later workers have correlated the classification of elements in class (a) and class (b) with Pearson s principle of hard and soft acids and bases (HSAB principle) (see Hard Soft Acids and Bases) on the basis that class (a) metal ions are hard acids and class (b) are soft acids. Borderline elements in the Ahrland-Chart-Davies classification tend to be harder in the higher oxidation states and softer in their lower oxidation states. 

The solvation of ionic species involved in a reaction has to be considered. Furthermore, many reactions involve metal-ion catalysis in which the principles of hard and soft acids and bases come into play. Thus a large soft metal may facilitate the reaction of the softer iodides, allowing a harder base to react at the carbon centre. Finally, there are radical reactions based on the alkyl halides, particularly bromides and iodides, involving homolytic fission of the C-X bond. 

Hardness and softness refer to special stability of hard-hard and soft-soft interactions and should be carefully distinguished from inherent acid or base strength. For example. both OH and F are hard bases yet the basicity of the hydroxide ion is about lO times that of the fluoride ion. Similariy, both S( and EtjP may be considered soft bases however, the latter is 10 times as strong (toward CHjHg ). It is possible for a strong acid or base to displace a weaker one, even though this appears to violate the principle of hard and soft acids and bases. For example, the stronger, softer base, the sulfite ion, can displace the weak, hard base, fluoride ion, from the hard add. the proton, H -... 

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