Hard-soft, acid-base concept

The strength of the complexation is a function of both the donor atom and the metal ion. The solvent medium is also an important factor because solvent molecules that are potential electron donors can compete for the Lewis acid. Qualitative predictions about the strength of donor-acceptor complexation can be made on the basis of the hard-soft-acid-base concept (see Section 1.2.3). The better matched the donor and acceptor, the stronger is the complexation. Scheme 4.3 gives an ordering of hardness and softness for some neutral and ionic Lewis acids and bases. 

The HSAB concept is the hard-soft acid-base concept. 

Classification of biologically important metal ions and ligands according to the hard-soft acid-base concept and their general characteristics... 

A quantitative scale of reactivity for aromatic substrates (fused, heterocyclic, and substituted rings) has been devised, based on the hard-soft acid-base concept (p. 375). From molecular-orbital theory, a quantity called activation hardness can be calculated for each position of an aromatic ring. The smaller the activation hardness, the faster the attachment at that position hence the treatment predicts the most likely orientations for incoming groups. 

TABLE 2.1 Classification of Biologically Important Metal Ions and Ligands According to the Hard—Soft Acid—Base Concept and their General Characteristics... 

An enolate formed from an unsymmetric ketone can be of two types the less electronegative (usually more substituted) side is deprotonated preferentially by many homogeneous transition metal complexes with N-donor or halide ligands, " in agreement with local hard-soft acid-base concepts. But steric hindrance plays a greater role with heterogeneous catalysts, to such an extent that the less substituted side is often deprotonated preferentially. ... 

Drago, R. S., Kabler, R. A. (1972). Quantitative evaluation of the HSAB (hard-soft acid-base) concept. 7 o>g. Chem. 11, 3144-3145. 

Recent work has made the connection between electronegativity and the hard-soft/acid-base concept clearer and has served to give the latter... 

The most useful qualitative approach for making predictions in this regard is the hard-soft-acid-base concept. This concept suggests that reactions will occur most readily between species that are matched with respect to hardness and softness. 

---