Chemistry Reference
In-Depth Information
Fig. 11
The experimental geometries of ethene
···
HCl and ethene
···
ClF (drawn to scale)
π
and the
-electron model of ethene. See Fig. 1 for key to the colour coding of atoms
These two molecules are clearly isostructural and of C
2
v
symmetry, with the
XY or HX molecule lying along the C
2
axis of ethene that is perpendicu-
lar to the plane containing the C
2
H
4
nuclei. Other complexes ethene
XY,
where XY
=
Cl
2
[46], BrCl [51], Br
2
[89] and ICl [96], and other complexes
ethene
···
HX, where X is F [129] or Br [130], have also been shown to have
the form illustrated in Fig. 11. It is of interest to note that C
2
H
4
···
···
Cl
2
was
detected through its UV spectrum many years ago [131] and that the pre-
reactive complex C
2
H
4
Br
2
has recently been shown to be important on the
overall reaction coordinate for bromination through autocatalytic action of
bromine [132].
Each angular geometry can be rationalised on the basis of rule 2 (see
earlier) with the aid of the familiar
···
bonding electron density distribution
of ethene, which is included in Fig. 11. In all cases, the electron acceptor
molecule XY or HX lies along the symmetry axis of the
π
orbital of the Lewis
base. The electrophilic end,
δ
+
XofXYor
δ
+
H of HX, as appropriate, interacts
with the
π
-electron density. There is no evidence that the hydrogen bonds
or the halogen bonds in these complexes are not strictly linear in the equi-
librium geometry (i.e. that the arrangements
π
∗···
H-X or
∗···
X-Y are not
collinear, where
is the midpoint of the C - Cbond).Inviewofthesymmetry
of ethene, non-linear hydrogen or halogen bonds are not expected.
Ethyne has two
∗
π
bonding orbitals at right angles to each other and a re-
sultant
electron density that is cylindrically symmetric with respect to the
internuclear axis. Complexes of ethyne with HF [133], HCl [134], HBr [135],
ClF [66], Cl
2
[47], BrCl [50], Br
2
[92] and ICl [95] have been characterised by
π
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