Chemistry Reference
In-Depth Information
O
L
C
O
L
C
M
L
L
C
M
C
O
C
C
O
O
O
L
mer
-isomer,
C
2v
L
fac
-isomer,
C
3v
σ
v
O
σ
v
b
1
L
C
b
2
O
L
L
L
C
b
1
b
3
M
M
σ
v
'
b
3
b
2
C
C
O
O
C
C
O
O
σ
v
L
C
2
L
σ
v
C
3
axis into page
Figure 6.26
The facial (fac) isomer and meridian (mer) isomer of the general complex
ML
3
(CO)
3
. In each case the upper diagram shows a sketch in the normal orientation for
'octahedral' complexes and the lower pictures use a view that should make the symmetry
elements easier to see. The basis arrows along carbonyl bonds that are used in the vibrational
analysis of carbonyl stretching modes are drawn slightly to the side of each ligand for clarity.
Note that, for the C
2v
mer-isomer, the basis vectors b
1
and b
2
are symmetry related to each
other, but not to b
3
.
The analysis of the
fac
-isomer is identical to the ammonia N H stretching modes
example of Section 6.6.2, so that the three basis vectors give rise to three vibrational modes
with irreducible representations:
=
A
1
+
E
(6.66)
Vibrational modes following the
A
1
and
E
representations will be IR active; however,
because the two vibrations in the
E
representation are degenerate, only two IR bands would
be expected. The pattern of CO bond stretches in these modes is analogous to the N
H
stretching modes shown in Figure 6.20.
For the
C
2
v
mer
-isomer the
σ
v
plane will be taken to be the one containing all three
CO bonds, as drawn in Figure 6.26. The reducible representation for this isomer generated
with this setting is shown at the top of Table 6.19. This table also shows that the application
of the reduction formula gives
=
2
A
1
+
B
2
(6.67)
For the
C
2v
case we have found three irreducible representations, all of which are IR
active.
