B Carries Two Inequivalent n-Pairs

Sulfur dioxide is an example of a simple Lewis base that carries two sets of inequivalent n-pairs, one set on each O atom. The n-pair model (in which the n bonding pairs are not drawn and are ignored here) is shown in Fig. 10. The geometries of SO2 HF [126,127], SO2 HCl [28,126] and SO2 GIF [70] have all been determined from investigations of their rotational spectra. Each molecule is planar and belongs to the Cs point group. Scale drawings for S02- HCl and S02- GIF are displayed in Fig. 10. 

We note that the HCl and GIF molecules attach, approximately at least, along the axis of the cis n-pair, as required by rule 1, with angles 0 of 143.0( 1)° and 131.9(6)°, respectively, although the former value may be influenced by 

The experimentally determined geometries of the complexes of the simplest n electron donor, ethene, with HCl [128] and CIF [65] are displayed in Fig. 11. 

Ethyne has two jt bonding orbitals at right angles to each other and a resultant jr electron density that is cylindrically symmetric with respect to the internuclear axis. Complexes of ethyne with HF [133], HCl [134], HBr [135], ClF [66], CI2 [47], BrCl [50], Br2 [92] and ICl [95] have been characterised by 

Rule 2 can also be tested when the Lewis base B carries no n-pairs but two or more jt-electron pairs, either conjugated or cumulative. Strictly, cyclopropane might be considered in this category but has been discussed in Sect. 3.2.2 as the prototype of a pseudo-n donor for convenience. 

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