Lewis affinity measurement

Lewis Basicity and Affinity Measurement Definitions and Context... 

A few scales of Lewis affinity and some spectroscopic scales of Lewis basicity (see below) have been constructed by carrying out the reaction A - - B AB on a dilute solution of the acid in pure, liquid base as solvent. This pure base method will be studied in Chapter 4. It gives solvent basicity scales which are not strictly equivalent to solute basicity scales measured on a dilute solution of the acid and the base in an inert solvent. 

Lewis Basicity and Affinity Measurement Definitions and Context 43 Table 1.15 Lewis basicity of halide ions in aqueous solution. ... 

The mathematics of the Lewis acid/base concept is that of a data matrix of m rows and n columns. Data are complexation constants, as logA" or AG. Each row corresponds to a Lewis acidity scale towards a reference base B° (/ = 1 to m) and each column corresponds to a Lewis basicity scale towards a reference acid A°j (J = I to n). For a rigorous treatment, the data measured in different media cannot be mixed in the same data matrix. In the matrix measuring Lewis affinity, the data are complexation enthalpies. There are extrathermodynamic relationships (isoequilibrium relationships or enthalpy-entropy compensation law) which allow transformations between blocks of the affinity and basicity matrices. In the principal component analysis of Lewis basicity, this justifies, somewhat, the mixing of affinity columns and basicity columns in a unified basicity-affinity matrix. 

The first calorimetric measurements on the reaction of SbCb with Lewis bases in solution seem to have been made in 1963 by Olofsson [1], Later, this author published a series of papers between 1963 and 1973 on the enthalpy of complexation of SbCls with numerous carbonyl bases [2-11], and also a few ethers [11, 12], methanol [12], water [13] and nitrobenzene [11], Gutmann extended this series to a host of different Lewis bases [14,15] and proposed, in 1966, the concept of donor number (DN) [16-18] to express quantitatively the Lewis basicity of soivents. Although the DN scale has been proposed and extensively used [19-21] as a solvent parameter, it relies on measurements made on dilute solutions of bases. It is, therefore, a solute scale and not a solvent scale of Lewis affinity. 

In the case of standardized BF3 affinity measurements (Chapter 3), the gaseous Lewis acid is introduced by small increments (5-10 additions, as in a stepwise titration) in a dichloromethane solution of the Lewis base, until saturation. This saturation corresponds to the formation of the 1 1 complex for a monofunctional base. If the adduct is not dissociated, the values of the ratio Q/n for the consecutive 5-10 injections are practically constant, and their mean value can be taken as the molar enthalpy of complexation. 

A significant difference of 4.4 1.0 kJ mol (that is, a 21% relative differenee) is found between the enthalpies measured in CCU and in cyclohexane. It appears that the diiodine affinities measured in alkanes must not be mixed with those measured in CCI4, as was unfortunately done in the Drago EC analysis of Lewis affinity. Moreover, the experimental value to be compared with the diiodine affinity of DMSO computed in vacuo by quantum chemical methods is the value measured in cyclohexane. Taking into account the solvent effect of cyclohexane, a good calculated value should be at least —25.3 ( 0.8) kJ mor. ... 

Metals that are soft Lewis acids, for example cadmium, mercury, and lead, are extremely hazardous to living organisms. Tin, in contrast, is not. One reason is that tin oxide is highly insoluble, so tin seldom is found at measurable levels in aqueous solution. Perhaps more important, the toxic metals generally act by binding to sulfur in essential enz Tnes. Tin is a harder Lewis acid than the other heavy metals, so it has a lower affinity for sulfur, a relatively soft Lewis base. 

A measure of the Lewis acidity of a metal ion is determined by its affinity for a pair of electrons, and the greater this affinity, the more stable the complexes formed by the metal ion will be. However, removing electrons from a metal to produce an ion is also related to the attraction the metal atom has for electrons. Therefore, it seems reasonable to seek a correlation between the stability constants for complexes of several metals with a given ligand and the total energy necessary for ionization to produce the metal ions. The first-row transition metal ions react in solution with ethylenediamine, en, to form stable complexes. We will consider only the first two steps in complex formation, which can be shown as follows ... 

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