Lewis acids relative strength

The boron tnhahdes are strong Lewis acids, however, the order of relative acid strengths, BI > > BCl > BF, is contrary to that expected... 

Apart from the hardness and softness, two reactivity-related features need to be pointed out. First, iron salts (like most transition metal salts) can operate as bifunctional Lewis acids activating either (or both) carbon-carbon multiple bonds via 71-binding or (and) heteroatoms via a-complexes. However, a lower oxidation state of the catalyst increases the relative strength of coordination to the carbon-carbon multiple bonds (Scheme 1). 

Elanany, M., Koyama, M., Kubo, M. el al. (2005) Periodic density functional investigation of Lewis acid sites in zeolites Relative strength order as revealed from NH3 adsorption, Appl. Surf. Sci., 246, 96. 

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. 

In principle, the A 0(H) function is of limited interest for kinetic applications because the indicators are chemically very different from the organic substrates generally used. On the other hand, as the measurements are based on pH determination, the length of the acidity scale is limited by the pA" value of the solvents. However, very interesting electrochemical acidity studies have been performed in HF by Tremillon and co-workers, such as the acidity measurement in anhydrous HF solvent and the determination of the relative strength of various Lewis acids in the same solvent. By studying the variation of the potential of alkane redox couples as a function of acidity, the authors provide a rational explanation of hydrocarbon behavior in the superacid media.48... 

Table 1.2. Relative Strength of MF -Type Lewis Acids... 

Figure 1.9. Relative strength of some strong Lewis acids as measured in HF on the pH scale by electrochemical titration.48...

From all these observations, we conclude that CO is an excellent probe for exploring the electric fields at cationic sites and, indirectly, for investigating the relative Lewis acid strengths, which follow the sequence Mg2+ > Mg + > Mg2+. Also, flat faces of MgO are unreactive and reactive sites with distinct acidic and basic character can be found only at defective positions such as corners, reconstructed (111) faces, edges, and steps. 

One issue is that most metal complexes formed in ionic liquids are anionic and these will have a significant effect on viscosity and mass transport. The effect of metal ion concentration on reduction current will therefore not be linear. Relative Lewis acidity will affect mass transport, ionic strength and speciation and accordingly the nucleation and growth mechanism of metals would be expected to be concentration dependent. 

Many physical chemists have embraced the concepts of donicity (donor numbers, DN) and acceptor numbers (AN) as developed by Gutmann and his co-workers [12], The DN is measured by the heat of reaction of the donor solvent and antimony pentachloride in a 1 1 ratio as a dilute solution in 1,2-dichloro-ethane. It is taken to be a measure of the strength of the Lewis base. The AN is measured as the relative shift of the 31P NMR peak in triethylphosphine oxide dissolved in the sample solvent. Hexane is given the value of zero on the scale, and antimony pentachloride is given the value of 100. The AN is taken to be a measure of the strength of the Lewis acid. The applications of the concepts have... 

By comparison with Lewis bases in superacids, there is a greater diversity of Lewis acids and of relative strengths of the acids in the three fluoro-acids under consideration. Each of the three acids will be presented separately in the remainder of this section. HF will be considered first because higher acidities occur in HF than in HSO3F and CF3SO3H and easier control of acidity can be achieved. 

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