The Donor Number or SbCls Affinity Scale

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. 

The SbCls molecule shows symmetry in its trigonal bipyramidal configuration. By complexation with one molecule of Lewis base, B, it distorts to an irregular octahedron as shown by the X-ray stmctures of the solid complexes Usted in Table 2.1. 

Interestingly, SbCls coordinates to the nitrogen and not to the sulfur atom of the polyfunctional bases S2N2 (CSD reference code GIRXEK) and S4N4 [23]. According to the HSAB rule that hard acids prefer hard bases, and since nitrogen bases are classified as hard or borderline whereas sulfur bases are soft, this site preference enables SbCls to be 

Lewis Basicity and Affinity Scales Data and Measurement Christian Laurence and Jean-Franpois Gal 2010 John Wiley Sons, Ltd 

C17H21NO2 [(4-fert-butyl-1 -pyridinio)(4-methoxyphenyl)methoxy] FOCFIM 1.945 (Sb-O) 

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