Chemistry Reference
In-Depth Information
Problem 1.2:
Square planar complexes have five planes of symmetry; using the struc-
ture of [PtCl
4
]
2
−
from Figure 1.19a, find these planes and sketch out the result of each
associated operation on the positions of the Cl atoms. Hence, confirm that no mirror
plane gives the arrangement shown after the inversion operation in Figure 1.19a.
1.3 Examples of the Impact of Geometric Symmetry on Chemistry
So far, we have only considered some of the geometric factors involved with symmetry.
Even so, the use of symmetry to identify equivalent atoms or groups in molecules already
allows some insight to be gained into the way symmetry can be used to interpret the chem-
ical behaviour of molecules possessing, or lacking, the symmetry elements introduced so
far. We can also start to explore how symmetry helps deduce chemical structure from
experimental data.
1.3.1 Oxygen Transfer via Metal Porphyrins
Rotation axes and mirror planes are very common symmetry elements in molecules and
can lend important properties to the structure. For example, in biology, the porphyrin ring
(Figure 1.20) is the basic structure of an important class of tetra-dentate ligands. The ide-
alized unsubstituted ring has a
C
4
principal axis at its centre perpendicular to the plane of
the ring. This will give rise to two
C
4
(
C
4
and
C
4
3
) and one
C
2
(
C
4
2
) operations. Figure 1.20
also shows that there are four
C
2
axes in the plane of the ring: two
C
2
and two
C
2
. Each
of these axes can be used to turn the planar molecule over so that the upper and lower
faces are interchanged. Since this is achieved through symmetry operations, it implies
C
2
′
NH
HN
C
2
′′
HN
NH
C
2
′
C
2
′′
Figure 1.20
The porphyrin ring structure. A C
4
axis passes through the centre of the ligand
perpendicular to the molecular plane; this is also a C
2
axis, since C
2
1
C
4
2
. The four C
2
axes
shown in the molecular plane form two sets of two, labelled C
2
and C
2
.
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