Ligands hard and soft

In 1923, the American chemist G.N. Lewis provided a broad definition of acids and bases which covered acid—base reactions not involving the traditional proton transfer an acid is an electron pair acceptor (Lewis acid) and a base is an electron pair donor (Lewis base). The concept was extended to metal—hgand interactions with the ligand acting as donor, or Lewis base, and the metal ion as acceptor, or Lewis acid. 

The general characteristics of each group are summarised in Table 2.1 along with a classification of metal ions and ligands of importance in biological inorganic chemistry. In general, hard acids prefer hard  

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

High charge density Small ionic radius 

Low polarisability High electronegativity Vacant, high-energy orbitals Hard to oxidise 

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Mixed N and S donor ligands (Figure 13) combine hard and soft ligand characteristics, while typically the M-N distance... 

The water molecules are substitutionally labile and the soft M(I)(C0)3+ species reacts with ligands containing both hard and soft ligands, for example, thioethers, histidine, and carboxylates, to form kinetically stable octahedral complexes. In fact, histidine in a peptide is such a strong binding ligand that it often competes with the intended bifunctional ligand. 

The constants used in these computer plots are listed in Table 9-1. Further discussion and examples are available in the references in Chapter 8. Linear relations among the formation constants of a large number of hard and soft ligands when plotted as free... 

Ferbinteanu, M., Roesky, H.W., Gimpoesu, F., Atanasov, M., Kopke, S. and Herbst-Irmer, R., New synthetic and structural aspects in the chemistry of allq laluminum fluorides. The mutual influence of hard and soft ligands and the hybridization as rigorous structural criterion, Inorg. Chem. 40 (19), 4947-4955 (2001). 

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