Scales of Hardness or Softness

Gritzner (1997) has recalled Ahrland s caution that a soft-soft interaction is not as simple as the hard-hard interaction as described by Lewis, that is, donation of share of an electron pair of the base to a vacant orbital on the acid to form a o bond, but it may also involve n back-donation from the acid to the base. There is a difference in kind as well as strength of interaction. Nevertheless, many authors have found it convenient to consider all Lewis-type acid-base reactions as lying on a spectrum from fully hard, exemplified by hydrogen bond formation, to very soft, typified by mercury-sulfur bonding, and have endeavored to construct scales of hardness or softness. They are considered in the next section. 

Edwards (1954, 1956), in a study of Lewis add-base reactions involving metal cations and anionic ligands, showed that the equilibrium constants can be correlated by Equation 3.21  

The ligand parameters are derived from independent properties of the ions Y from the standard oxidation potential for the reaction 

Reactions of the type Ah- B - AB where A and B are a Lewis acid and base have been studied by calorimetry (in a poorly coordinating solvent) by Drago and Wayland (1965). They showed that the enthalpy of reaction between uncharged species can be expressed by Equation 3.23  

FIGURE 3.11 Edwards s parameters, versus a, for 17 cations, with a proposed softness scale superposed. Circles represent hard ions, crosses soft, and squares borderline. Note the apparently anomalous position of Zn. Data from Yingst and McDaniel (1967). 

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