Chemistry Reference
In-Depth Information
(a)
C
2
(
Z
)
σ
(
XZ
)
D
2h
σ
(
XY
)
C
2
(
Y
)
C
2
(
X
)
σ
(
YZ
)
(b)
D
4h
+
σ
h
NH
3
Cl
H
3
N
Cl
Co
NH
3
C
4
NH
3
Figure 3.22
Example molecules belonging to D
n
h
point groups: (a) ethene, D
2h
; (b) the
trans-isomer of [Co(NH
3
)
4
Cl
2
]
+
,D
4h
.
The
trans
-isomer of [Co(NH
3
)
4
Cl
2
]
+
has a
C
4
axis defined by the Cl Co Cl line,
as shown in Figure 3.22b. The four amine ligands are in the horizontal mirror plane,
which also contains four
C
2
axes (two,
C
2
, along Co NH
3
bonds and two,
C
2
, bisect-
ing H
3
N Co NH
3
angles). The complex is shown with the principal axis running from
front left to back right of the diagram, but remember that this axis sets the vertical ref-
erence direction used in discussing the geometry. This complex is an example of the
D
4h
point group. Note that in this assignment we have ignored the amine H atoms. The amine
ligands are able to rotate freely around the Co NH
3
bonds, and so the orientations of
the H atoms will be dynamically averaged out and will not influence the properties of the
complex.
Problem 3.3:
Elemental sulfur can form molecular S
8
ring structures which belong to
the
D
4d
point group. Sketch out the conformation of S
8
this implies.
Problem 3.4:
Benzene (C
6
H
6
) is a planar molecule with a principal
C
6
axis. Assign its
point group and illustrate the positions of the symmetry elements on sketches of the
