Lewis acid softness parameter

A detailed discussion and comparison of all these and further solvent softness scales can be found in references [173, 238, 239]. For other Lewis acid/base parameters of EPD and EPA solvents, derived from calorimetric measurements [e.g. Gutmann s donor and acceptor numbers), see reference [65] and Section 2.2.6. 

Ahrland et al. (1958) classified a number of Lewis acids as of (a) or (b) type based on the relative affinities for various ions of the ligand atoms. The sequence of stability of complexes is different for classes (a) and (b). With acceptor metal ions of class (a), the affinities of the halide ions lie in the sequence F > Cl > Br > I , whereas with class (b), the sequence is F < Cl" < Br < I . Pearson (1963, 1968) classified acids and bases as hard (class (a)), soft (class (b)) and borderline (Table 1.23). Class (a) acids prefer to link with hard bases, whereas class (b) acids prefer soft bases. Yamada and Tanaka (1975) proposed a softness parameter of metal ions, on the basis of the parameters En (electron donor constant) and H (basicity constant) given by Edwards (1954) (Table 1.24). The softness parameter a is given by a/ a - - P), where a and p are constants characteristic of metal ions. 

Cu2+(aq) to 0.9 in [Cu(tetb)]2+. There is also a modest increase in softness due to changes in EA and CA parameters, which can be interpreted in terms of the symbiosis proposed by Jorgensen (33). The low Da parameter for [Cu(tetb)]2+ suggests that the main effect in transforming Cu(II) to a soft Lewis acid in [Cu(tetb)]2+ is the altered steric situation in coordination of bulky donor atoms to the metal ion. 

Drago and co-workers Introduced an empirical correlation to calculate the enthalpy of adduct formation of Lewis acids and bases ( 5). In 1971, he and his co-workers expanded the concept to a computer-fitted set of parameters that accurately correlated over 200 enthalpies of adduct formation ( ). These parameters were then used to predict over 1200 enthalpies of interaction. The parameters E and C are loosely Interpreted to relate to the degree of electrostatic and covalent nature of the Interaction between the acids and bases. This model was used to generalize the observations involved in the Pearson hard-soft acid-base model and render it more quantitatively accurate. 

These qualitative explanations, whether they be hard-soft or ionic-covalent or Class A-Class B, all suffer from the arbitrary way in which they can be employed. All Lewis acid-base type interactions are composed of some electrostatic and some covalent properties, i.e., hardness and softness are not mutually exclusive properties. Predictions are straightforward when dealing with the extremes, but with other more ambiguous systems, one can be very arbitrary in explaining results and the predictive value is impaired. What is needed is a quantitative assessment of the essential factors which can contribute to donor strength and acceptor strength. Proper combination of these parameters should produce the enthalpy of adduct formation. Until this can be accomplished, one could even question the often made assumption that the strength of the donor-acceptor interaction is a function of the individual properties of a donor or acceptor. 

The ratio, C/E, gives a quantitative order of relative hardness or "softness for the various Lewis acids and agrees fairly well with the qualitative classification of Pearson (2). The adds which do not follow the qualitative classification are BF3 and SO2. As mentioned above, the parameters for BF3 were determined from data limited to oxygen donors. The qualitative ordering of SO 2 is incorrect, Emd as will be shown shortly when strong interactions are compared with weak ones, the procedures... 

M. Misono, E. Ochiai, Y. Saito, and Y. Yoneda, A new dual parameter scale for the strength of Lewis acids and bases with evaluation of their softness, J. Inorg. Nucl. Chem. 29 2685 (1967). 

They are relatively large for aromatic solvents i.e. soft EPD solvents) compared to other empirical Lewis basicity parameters that have been determined by employing rather hard Lewis acid probes, e.g. SbCb-derived DN values [cf. Eq. (7-10)], CH3OD-derived B values [cf. Eq. (7-34)], and 4-nitrophenol-derived ji values [cf. Eq. (7-51)]. The... 

The data collected by Gritzner [79] and his analysis led him to the conclusion that different interactions of hard and soft cations with donors cannot be accounted for by using only one parameter. Gritzner [79] also tried other correlations. No correlation was found with the acceptor number and the other Lewis acidity parameter, x, introduced by Dimroth and Reichardt [19, 83]. Only those parameters which represent donor properties of solvents are correlated with the change of E /2 potentials for the electroreduction of cations. 

For an introduction to Lewis acids and the Misono softness parameter, see, e.g.. Chap. 3 in G. Sposito, The Thermodynamics of Soil Solutions. Clarendon Press, Oxford, 1981. 

Several individuals have worked to quantify these trends, making scales of Lewis acidity and basicity. Gutmann has created a series of donor numbers (DN) and acceptor numbers (AN) for various solvents, while Drago and Wayland have assigned parameters E and C, which measure electrostatic interactions and covalent bonding potential, respectively. Lastly, Pearson treats each Lewis acid and base with two parameters, relating what is called the strength of the acid/base and the softness/hardness of the acid/base. In fact, the HSAB trend discussed above is primarily a concept developed by Pearson. 

While it is well known how the acid strength of Bronsted sites depends on the physicochemical parameters of a zeolite, the situation in the case of Lewis centers is much less established. In particular the influence of the chemical composition and crystalline structure on the more tenuous concept of softness and hardness of the metal sites is a subject of recent interest [1]. 

The second class contains dual parameters, which occur in pairs of complementary attribntes cationic and anionic charge, Lewis or Brpnsted acidity and basicity (and refinements such as hard or soft acidity and basicity), electrophi-licity and nucleophilicity, and hydrogen-bonding tendency as donor and as acceptor (Table A.2b). A number of the entries in the table are incomplete in that only one of a potential pair of complementary parameters has been investigated. A table of values of most of the listed parameters for selected solvents forms Table A. 3. 

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