Chemistry Reference
In-Depth Information
Table 7.6
Comparison of ligand field splitting energy (
o
) for different ligands
bound to d
3
electronic configuration octahedral metal complexes [ML
6
]
n
+
.
o
(cm
−
1
)
Ligand
V
2
+
Cr
3
+
Mn
4
+
Br
−
—
12 700
—
Cl
−
7 200
13 200
17 900
F
−
—
14 900
21 800
OH
2
12 300
17 400
∼
24 000
NH
3
—
21 600
—
CN
−
—
26 700
—
Whereas this latter effect can be understood in part on the basic of electrostatic arguments
(ion charge and size), the variation with ligand type poses more of a problem. That there is
an apparent correlation between the donor atom position in the Periodic Table and splitting
energy (C
Br) is very difficult to reconcile using a
simple electrostatic view, such as met in the crystal field theory. Moreover, there is a clear
trend with position in the Periodic Table for metals, with the splitting for 4d and 5d metal
ions significantly larger than those for 3d. This is exemplified for the cobalt triad, where
for [M(NH
3
)
6
]
3
+
N
O
F and also F
Cl
o
varies from Co (22 900 cm
−
1
) to Rh (34 000 cm
−
1
)to
Ir (41 200 cm
−
1
). Again, the crystal field model struggles to interpret this observation, and
we are drawn into the different ligand field model to provide the better interpretation.
Apart from the position of absorption bands, it is notable that the intensity of bands
varies with coordination geometry. For perhaps the two most common shapes, octahedral
and tetrahedral, it is noted that the intensity of absorbance bands for tetrahedral complexes
are invariable greater (up to 50-fold) than those for octahedral complexes (Figure 7.5),
although both are significantly smaller than absorbances of organic chromophores. This is
the result of the transitions involving d electrons moving between d orbitals (called d-d
transitions) being partly forbidden under the theory that governs our understanding of their
behaviour, with the 'forbiddenness' relaxed by different effects. For tetrahedral shape, there
is greater relaxation of the rules. As a general guide, for a particular coordination number,
the lower the symmetry the more relaxation of the rules applies and hence the larger the
absorbance bands.
complexes,
tetrahedral
MA
5
C
MA
5
B
octahedral
wavelength
wavelength
Figure 7.5
The electronic spectra of (at left) a tetrahedral versus an octahedral complex, and (at right) the shift
in the spectrum of an octahedral complex on replacement of just one donor group by another with a
different position in the spectrochemical series.
