The Interaction of Dihalogens and Hydrogen Halides with Lewis Bases in the Gas Phase: An Experimental Comparison of the Halogen Bond and the Hydrogen Bond - Halogen Bonding: Fundamentals and Applications - page 42

Fig. 16 The experimental geometries of methylenecyclopropane ··· HCl and methylene-

cyclopropane -electron model for the Lewis base is also

shown. The angles C- ∗· · · HandC- ∗··· Cl, respectively, where ∗ is the centre of the C - C
double bond, are both close to 90 ◦ , as required by rule 2. The halogen bond again exhibits
a smaller non-linearity

··· ClF, drawn to scale. The π

θ than the hydrogen bond. See Fig. 1 for key to the colour coding

of atoms

symmetry that the hydrogen bond is significantly non-linear while the corres-
ponding halogen bond is not. We shall return later (Sect. 6) to this important
difference between the two types of intermolecular bond. Other complexes of
methylenecyclopropane with HX (X = F [144] and Br [145]) have geometries
similar to that for X = Cl.

3.3
Angular Geometries of B···ClF and B···HCl
in Which B is a Mixed n-Pair/π-Pair Donor

According to rule 3, if a Lewis base B carries both non-bonding and -

bonding electron pairs, the n-pairs are definitive of the angular geometry.
There are many examples of simple Lewis bases B that can in principle act
as either n- or

π

-electron pair donors. These include CO, HCN, H 2 CO, furan,
thiophene, pyridine, etc. We note that, for convenience, we considered H 2 CO
earlier as an example of a Lewis base carrying a pair of equivalent n-pairs
and ignored the

π

ClF [79] are
examples that obey rule 3. The complexes HX with carbon monoxide when
X = F [146], Cl [147], Br [148], and I [149] have all been investigated through
their rotational spectra. Each is linear, with the order of the atoms OC

π pair. In fact, H 2 CO ··· HCl [121] and H 2 CO ···

HX
in the lowest energy conformer, so that the n-pair on the C atom takes prece-
dence over the

···

π pairs (and indeed the n-pair on O), as predicted by rule 3.