Lewis acids and bases strengths

Fig. 10.9. Known silicate compounds as a function of the Lewis acid and base strengths of their ions. Points are from Reference [42]. The thick diagonal line corresponds to a perfect valence match...

However, donor-acceptor interactions are affected not only by the Lewis acid and base strengths, but also by other, steric and electron structural, factors. Thus, even in systems where either solely the donor or the acceptor property of the solvent is manifested, solvents with different space requirements may interact to different extents because of the steric properties of the reference solute and a reference acceptor with a tendency for dative 7c-bonding (back-coordination) will interact more strongly with jr-acceptor solvent molecules (e.g., acetonitrile) than would be expected from their basicity. The solvent donicity investigations by Burger et al [Bu 71, 74] with transition metal complex reference acceptor model systems have clearly shown the great extent to which such secondary effects may distort the solvent scale. 

Keywords Bond Bond valence Bond valence theory Bonding geometry Bonding strength Lewis acids and base strengths Lone pairs Steric effects Structure prediction... 

Cations are Lewis acids, and anions are Lewis bases, and their respective bonding strengths are also their Lewis acid and Lewis base strengths. So far this discussion has focused on simple ions, that is, ions that consist of a single atom, in which the Lewis acid and base strengths can be calculated using Eq. (7). Complex ions are those composed of more than one atom. They differ from simple ions in that the different atoms in the complex may each have their own Lewis acid or base strength. 

Table 6 Residual valences, Fr, and Lewis acid and base strengths of the second row hydrides in valence units...

It is interesting to compare the properties of the hydrides formed by the anions of the second period H4C, H3N, H2O, and HF. Their formulas are written to emphasize that in each case hydrogen is the cation, since it has the lower electronegativity. Because these are all neutral molecules, the sum of the valences of any external bonds they form must be zero. Table 6 shows the estimated Lewis acid and base strengths of these compounds. 

Lewis Acid and Base Strengths of Complex Ions. 245... 

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. 

An inner-sphere complex is formed between Lewis acids and bases, while an outer-sphere complex involves a water molecule interposed between the acid and the base. A hard Lewis acid is a molecular unit of small size, high oxidation state, high electronegativity, and low polarizability whereas a soft Lewis acid is a molecular unit of relatively large size, characterized by low oxidation state, low electronegativity, and high polarizability. Based on this characterization, hard bases prefer to complex hard acids, and soft bases prefer to complex soft acids, under similar conditions of acid-base strength. 

The evaluation and correlation of strengths of Lewis acids and bases have attracted the interest of many inorganic chemists. Recently gas-phase data have become available. but before that many systems were studied in aprotic, nonpolar solvents. Even today, such solvents allow the collection of large amounts of data by various methods. Solvation effects will be small and, it is hoped, approximately equal for reactants and products such that their neglect will not cause serious errors. 

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). 

The topic of interactions between Lewis acids and bases could benefit from systematic ab initio quantum chemical calculations of gas phase (two molecule) studies, for which there is a substantial body of experimental data available for comparison. Similar computations could be carried out in the presence of a dielectric medium. In addition, assemblages of molecules, for example a test acid in the presence of many solvent molecules, could be carried out with semiempirical quantum mechanics using, for example, a commercial package. This type of neutral molecule interaction study could then be enlarged in scope to determine the effects of ion-molecule interactions by way of quantum mechanical computations in a dielectric medium in solutions of low ionic strength. This approach could bring considerable order and a more convincing picture of Lewis acid base theory than the mixed spectroscopic (molecular) parameters in interactive media and the purely macroscopic (thermodynamic and kinetic) parameters in different and varied media or perturbation theory applied to the semiempirical molecular orbital or valence bond approach [11 and references therein]. 

Laurence, C., Gal, J. (2010). Lewis Basicity and Affinity Scales Data and Measurement. New York John Wiley Sons. Includes a discussion of the various acid-base theories and quantitative scales for judging the strengths of Lewis acids and bases. 

Recently, the C-NMR method was also applied to these studies and the following order of the relative strengths of Lewis acids and bases, rdated to equilibrium (47) was given ... 

In the Bond Valence Model, atoms are characterized by their atomic valence (oxidation state) and by their Lewis acid or Lewis base strength, the latter two quantities correlating with electronegativity and respectively. The acid and base strengths are both estimates of the valence of the bonds formed by the atoms. Hence, for a... 

In this chapter, we explain how the scientific understanding of acids and bases has changed through time. We start by explaining how to measure their strengths using the pH scale. We also explain the multiple theories of acid-base chemistry, from Lavoisier s first attempt to classify these materials, to the modern understanding of Lewis acids and bases. 

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