Chemistry Reference
In-Depth Information
z
z
z
x
x
x
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y
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dxz
dyz
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dx
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-y
2
dz
2
FIGURE 3.7
Orbitals
d
in an octahedral surrounding.
from e
g
must balance the energy decrease of the six electrons from t
2g
according to
the energetic gravity center rule. Through preferential occupying of t
2g
orbitals with
electrons, the system's energy decreases with 2/5 Δ
o
multiples, and by completion of
e
g
orbitals with electrons, the energy increases with 2/5 Δ
o
multiples. In other terms,
each electron from t
2g
contributes −4Dq to the crystal field stabilization energy,
while each electron from e
g
contributes with +6Dq to the CFSE.
In the case of
d
1
,
d
2
, and
d
3
configurations, the first three electrons successively
occupy the orbitals from the t
2g
group, according to the Hund rule. The values of
CSFE are −4Dq, −8Dq, and −12Dq, respectively. The
d
4
,
d
5
,
d
6
,
d
7
configurations of
the associated electrons involve two possible position modes: either in the e
g
orbitals,
with uncoupled spin, or in the t
2g
orbitals with coupled electronic spins.
From these different arrangements, two types of complexes can be formed:
high-spin complexes with uncoupled
d
electrons, and low-spin complexes with cou-
pled
d
electrons. The crucial factor that occurs in these situations is the spin-pairing
energy (P) as follows:
a. When Δ
o
< P, a low field is generated. For a
d
4
configuration, for example,
the fourth electron is placed in an e
g
orbital resulting in the overall configu-
ration t
2g
3
e
g
1
(and t
2g
3
e
g
2
, t
2g
4
e
g
2
, t
2g
5
e
g
2
, respectively).
b. When Δ
o
> P, a strong field is generated. In a
d
4
configuration, the fourth
electron is placed in a t
2g
orbital resulting in the overall configuration t
2g
4
(and t
2g
5
, t
2g
6
, t
2g
6
e
g
1
, respectively).
