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Lecture 30
Electronic Spectra of Coordination Compounds
1) Jahn-Teller effect
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Tetragonal distortions
Octahedral complexes can be a subject to tetragonal or
trigonal distortions leading to less symmetrical but
more stable structures.
According to the Jahn-Teller theorem, a non-linear
molecule with a not-completely filled degenerate
electronic levels can undergo a (vibrational) distortion
lowering its symmetry and energy.

b1g
(dx2-y2)

In the case of octahedral complexes the tetragonal
distortion reduces Oh symmetry of a complex to D4h
producing either elongated or compressed tetragonal
bipyramid and reduces degeneracy of eg and t2g orbitals.
The most pronounced stabilization due to the tetragonal
distortion is expected for the following configurations: d1
(compression), d4, d7, d9 (various types), d2 (elongation).
No stabilization and thus no distortion is expected for d3,
d5 (high spin), d6 (low spin) and d8 configurations.
a1g(dz2)
eg
b1g (dx2-y2)
eg

(dz2)
(dx2-y2)
a1g (dz2)

t2g
b2g(dxy)
b2g (dxy)

eg
z
D4h
compression
Oh
D4h
elongation
 >>  > 
e-e repulsions >>  > 
2) Static and dynamic Jahn-Teller effect
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The Jahn-Teller effect is more pronounced when the
former eg level is not completely filled (1 > 2), that
is for configurations d4 (high spin), d7 (low spin)
and d9. In these cases a static Jahn-Teller effect
can be observed and the species of D4h symmetry
can exist in a solid phase.
elongation:
3-
F
MnIII
F
F
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F
H3N
F
1.79
H3N
eg (dz2, dx2-y2)
(dx2-y2) b1g

eg
t2g
2+
2.62
2.09
(dz2) a1g
(dxy) b2g
NH3
F
Tetragonal distortion of d1 species
NH3
CuII
z
D4h
Oh
NH
2.07 3
NH3
d4
d9
Much weaker stabilization corresponds to the case
of the asymmetrically filled former t2g level such as
in d1 species.
In such cases the tetragonal distortion is reflected
mainly in electron absorption. We observe
appearance of new bands like in the case of
Ti(H2O)63+ (two new bands appears as shoulders).
compression
elongation
d1
Ti(H2O)63+
3) Charge transfer bands
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Similar to d-d transitions, charge-transfer (CT) transitions also involve the metal dorbitals. CT bands are observed if the energies of empty and filled ligand- and metalcentered orbitals are similar.
The direction of the electron transfer is determined by the relative energy levels of these
orbitals: i) ligand-to-metal charge transfer (LMCT) like in MnO4-, CrO42- etc. or ii) metal-to
ligand charge transfer (MLCT) like in [Fe(bpy)3]2+. The simplified diagrams below are the
modified versions of what we had in Lecture 26. Bold arrows show possible CT transitions.
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MnVII
4 O2-
d
3 bpy -GO's
FeII
d6
3t2
0
bpy =
2a1
N
(p) t2
3t2g
(s) a1
2t2
29500
e
(d) e
t2
t1
17700
30300
1t2
1a1
eg
44400
t1(n)
t2()
a1
N
t2g*)
eg

t2g
2t2g
1t2g
t2g)