Chemistry Reference
In-Depth Information
For the
d
8
,
d
9
,
d
10
configurations, there is just one possibility for the arrangement
of electrons, namely: t
2g
6
e
g
2
, t
2g
6
e
g
3
, t
2g
6
e
g
4
.
In the general case of an electronic configuration represented as t
2g
m
e
g
n
with x
electronic pairs, CFSE can be calculated with the general formula:
CFSE = −4mDq + 6nDq + xP = −(4m − 6n)Dq + xP.
(3.33)
CFSE can be spectroscopically measured: it is
inversely
proportional
with the
light absorbed when an excited electron promotes from the t
2g
level in e
g
level:
10Dq = hc/λ.
For octahedral transition metal complexes, Δ
o
varies depending on the nature
of the ligands, which can be arranged in increasing order of field strength form-
ing a series called the
Fajans-Tsushida series
or
spectrochemical series
(so called,
because the series can be determined using UV-visible absorption spectroscopy):
I
−
< Br
−
< Cl
−
< SCN
−
< Ni2+
3
−
< (EtO)
2
PS
2
−
< F
−
< (NH)
2
CO
< OH
−
< C
2
O
4
2−
< H
2
O < NCS
−
< H
−
< C
N
−
< NH
2
CH
2
CO
2
−
< NH
3
< C
5
H
5
N < en < SO
3
2−
< NH
2
OH < NO
2
−
< phen < CH
3
−
<
C
N
−
In general, the predicted order of decreasing tendency of donor ligands to cause
spin pairing is C > N > O > S > F > Cl > Br. The actual order found is S > O and
Cl > Br > F.
The Δ
o
depends not only on the nature of the ligands, but also on the metal and
its oxidation state. The approximate spectrochemical series for metal ions is shown
here:
Mn
2+
< Ni
2+
< Co
2+
< Fe
2+
< V
2+
< Co
3+
< Mn
4+
< Mo
3+
<
Rh
3+
< Ru
3+
< Pd
4+
< Ir
3+
< Pt
4+
This series is not quite as regular as the spectrochemical series of ligands, but some
general regularities are seen:
1. For a given metal and ligand set, Δ
o
increases with increasing oxidation
state (Co
2+
< Co
3+
, etc.).
2. For a given oxidation state and ligand set, Δ
o
increases down a group
(Co
3+
<< Rh
3+
< Ir
3+
).
3. For a given oxidation state and ligand set, Δ
o
varies irregularly across the
transition metals (groups 3 to 12).
Relative to an octahedral field some general conclusions can be drawn:
•
CFSE has higher values in a strong field than in a weak field.
•
CFSE is zero for the
d
0
,
d
5
(high spin) and
d
10
configurations.
•
The
d
3
and
d
8
configurations are the most stable configurations in a weak
field, while the
d
6
low-spin configuration is the most stable in a strong field.
