Transition metal chemistry notes by Azabo Kennedy

1. Strong and weak field ligands
and Jahn Teller effect
Group 5

2. Recap of crystal field theory by Hans
and Vleck
• Bonds between ligand and metal ion were
completely ionic
In Metal ion surrounded by other atoms/
ligands
d-orbitals are at a higher energy level than
in an isolated metal ion …..why???

3. • 5. (a) Distinguish between strong field ligands
and weak field ligands (use appropriate
examples in each case).
• b) Explain clearly, using a relevant example,
the meaning of Jahn-Teller distortion.

4. Consider an Octahedral complex
Cordination no. 6
Ligands approach the metal ion from different
directions and hence affect d-orbitals in
different ways

6. • dx2-y2 and dz2 orbitals are set mutually
parpendicular to x y and z axes ( eg orbitals )

7. • dxy dzy and dxz are lie between the axes and
are collectively called t2g orbitals

8. Ligand donor groups approach the metal ion
along the axes to form octahedral complexes
e- on the ligands repel strongly e- in eg d-
orbitals on the metal ion than those in t2g
orbitals

9. • This removes the degeneracy of d orbitals and
splits them into two sets, the eg set at
higher energy and the t2g set at lower energy
This energy separation is called crytal field
spliting / octahedral energy ( Δoct)
Crystal field splitting energy …..difference in
energy between t2g and eg

11. • . It is proportional to the crystal field strength
of the ligands ( how strongly the ligand electrons
repel the electrons on the metal ion)
• The d-electrons on the metal ion occupy the t2g set
in preference to the high energy eg set
• Electrons that occupy the eg orbitals are strongly
repelled by the relatively close approach of ligands
This distablise octahedral complexes

12. Electrons occupy orbitals in the arrangement
that result in lowest energy
• Energy expenditure that is necessary to pair
electrons by bring two negatively charged
Particles into the same region of space
Electron pairing energy, P

13. Compared to Δoct
• If Δoct < P
The electrons occupy all the 5 non
degenerate orbitals singly before
pairing
Such a complex would have the same
number of unpaired electrons on the
metal ion as when the metal is
uncompleted
This is called a high spin complex.

14. IF Δoct > P
Electrons will be at lower energy if they pair in
the t2g orbital,
before any electrons occupy the higher energy
eg orbitals
 complex could have fewer unpaired electrons
on the metal atom than when the metal is
uncomplexed,
called a low spin complex.

15. weak field ligands
Cause only small crystal field splitting
energy
Δoct < P making electron pairing
unfavorable

16. • A ligand that exerts a weak crystal
or ligand field and generally
forms high spin complexes with
metals.

17. Strong field ligand
• These cause a larger value of crystal field
splitting energy
Δoct > P making electron pairing more
favorable

18. • A ligand that exerts a strong crystal
or ligand electric field and generally
forms low spin complexes with metal
ions when possible.

19. Weak field ligand
• Low CFSE
• High P.E
• High spin complex
• Mostly
paramagnetic
Strong field ligand
• High CFSE
• Low P.E
• Low spin complex
• Mostly
diamagnetic

20. Example
• [CoF6]3- Is paramagnetic whereas [Co(CN)6]
is diamagnetic.

23. JAHN TELLER DISTORTION
Outline of the discussion;
In this discussion we shall look at but not restricted to the following
aspect:
 What is jahn teller distortion
 Jahn teller distortion in octahedral complexes
 Splitting of t2g and eg due to tetragonal distortion
 Relationship between d-electron configuration and type of jahn
teller distortion
 Problems based on jahn teller distortion and their solutions.

24. Jahn-Teller distortion is the geometric distortion of anon linear
molecule to reduce its symmetry and energy.
This distortion is typically observed in octahedral complexes and can also
be seen in tetrahedral complex; but in this lecture we shall look at the
distortion in majorly octahedral complexes where the distortion is more
pronounced.
NB :Jahn teller distortion depends upon the electronic state of the
system.
For a given octahedral complex, the five d atomic orbitals are split into
two degenerate sets when constructing a molecular orbital diagram.
These are represented by the sets' symmetry labels: t2g
(dxz, dyz, dxy) and eg (dz2 and dx2−y2). When a molecule possesses a
degenerate electronic ground state, it will distort to remove the
degeneracy and form a lower energy (and by consequence, lower
symmetry) system. The octahedral complex will either elongate or
compress the z ligand bonds

25. Lets use the hypothetical molecule below to explain Z-in and Z-out
phenomena

26. There are two types of metal ligand bonds; axial and equatorial. An
octahedral complex has two axial bonds and four equatorial bonds. The z
axis is located along the axial bonds.
When an octahedral complex exhibits elongation, the axial bonds are
longer than the equatorial bonds. For a compression, it is the reverse; the
equatorial bonds are longer than the axial bonds.
Elongation and compression effects are dictated by the amount of
overlap between the metal and ligand orbitals. Thus, this distortion
varies greatly depending on the type of metal and ligands. In general,
the stronger the metal-ligand orbital interactions are, the greater the
chance for a Jahn-Teller effect to be observed.

27. Elongation Jahn-Teller distortions occur when the
degeneracy is broken by the stabilization (lowering in energy)
of the d orbitals with a z component, while the orbitals
without a z component are destabilized (higher in energy) as
shown below
Compression Jahn-Teller distortions occur when the
degeneracy is broken by the stabilization (lowering in energy)
of the d orbitals without a z component, while the orbitals
with a z component are destabilized (higher in energy) as
shown below.

28. In a regular octahedral structure both the axial and equatorial bonds are at the
same length. During z-in distortion, the a<e and during z-out the a>e. this results
in to the interactions which lead to the increase/lowering of the energy.
This is due to the z-component d orbitals having greater overlap with the ligand
orbitals, resulting in the orbitals being higher in energy. Since the dz
2 orbital is
antibonding, it is expected to increase in energy due to compression. The dxz and
dyz orbitals are still nonbonding, but are destabilized due to the interactions.
As shown by the crystal field diagram below, during the splitting of the t2g and eg
orbitals when axial bonds elongate, the orbitals containing z-element will have
their energy lowered and stabilized and this phenomena is opposite for z-in when
the axial bonds shorten.

29. Crystal field diagram showing splitting of t2g and eg orbitals.

30. Splitting of the t2g and eg orbitals is due to tetragonal distortion. Lets
discuss the splitting of t2g and eg orbitals in tetragonal ligands.
Complexes with symmetrical t2g and eg orbitals DO NOT SHOW JTD
Complexes with unsymmetrical t2g orbitals show WEAK JTD
Complexes with unsymmetrical eg orbitals show STRONG JTD
Distortion lowers the energy of the metal complex which leads to
increased stability depending on the on which distortion.
Because of the Centre of gravity rule, orbitals without z-components
will have their energy raised due to interactions with the central
metal ion.

31. For Jahn-Teller effects to occur in transition metals there must be
degeneracy in either the t2g or eg orbitals. The electronic states of
octahedral complexes depend on the number of d-electrons and the
splitting energy, Δ.
When Δ is large and is greater than the energy required to pair electrons,
electrons pair in t2g before occupying eg.
On the other hand, when Delta small and is less than the pairing energy,
electrons will occupy eg before pairing in t2g.
The Δ of an octahedral complex is dictated by the chemical environment
(ligand identity), and the identity and charge of the metal ion. If their
electron configurations for any d-electron count is different depending on
Δ ,the configuration with more paired electrons is called low spin while
the one with more unpaired electrons is called high spin.

32. The electron configurations diagrams for d1 through d10 with
δ>p

33. Figure shows Electron configuration diagram of octahedral
complexes (red indicates no degeneracies possible, thus no
Jahn-Teller effects).
The electron configurations highlighted in red (d3, low spin d6,
d8, and d10) do not exhibit Jahn-Teller distortions. On the other
hand d1, d2, low spin d4, low spin d5, low spin d7, and d9, would
be expected to exhibit Jahn-Teller distortion. These electronic
configurations correspond to a variety of transition metals. Some

34. The electron configurations diagrams for d1 to d10 with δ<p are
illustrated in the figures below.
Notice that the electron configurations for d1, d2, d3, d8, d9, and d10 are the
same no matter what the magnitude of Δ
NB:Low spin and high spin configurations exist only for the electron

35. The electron configurations highlighted in red (d3, high spin d5, d8, and
d10) do not exhibit Jahn-Teller distortions. In general, degenerate
electronic states occupying the eg
orbital set tend to show stronger Jahn-Teller effects. This is primarily
caused by the occupation of these high energy orbitals. Since the system
is more stable with a lower energy configuration, the degeneracy of the eg

36. Spectroscopic Observation
Jahn-Teller distortions can be observed using a variety of
spectroscopic techniques. In UV-VIS absorption spectroscopy,
distortion causes splitting of bands in the spectrum due to a
reduction in symmetry (Oh to D4h). Consider a hypothetical
molecule with octahedral symmetry showing a single absorption
band. If the molecule were to undergo Jahn-Teller distortion, the
number of bands would increase as shown below.

37. Hypothetical absorption spectra of an octahedral molecule (left)
and the same molecule with Jahn-Teller elongation (right). The red
arrows indicate electronic transitions.
: Examples of Jahn-Teller distorted complexes
CuBr2 4 Br at 240 pm 2 Br at 318 pm
CuCl2 4 Cl at 230 pm 2 Cl at 295 pm
CuCl 2.2H 2O 2 O at 193 pm 2 Cl at 228 pm 2 Cl at 295 pm
CsCuCl3 4 Cl at 230 pm 2 Cl at 265 pm
CuF2 4 F at 193 pm 2 F at 227 pm
CuSO4.4NH3.H2O 4 N at 205 pm 1 O at 259 pm 1 O at 337 pm
K2CuF4 4 F at 191 pm 2 F at 237 pm
KCuAlF6 2 F at 188 pm 4 F at 220 pm
CrF2 4 F at 200 pm 2 F at 243 pm
KCrF3 4 F at 214 pm 2 F at 200 pm
MnF3 2 F at 209 pm 2 F at 191 pm 2 F at 179 pm

38. Problems
Qn: Predict the nature of Jahn-Teller distortion in;
a) [Ti(H2O)6]3+
b) [cu(H2O)6]2+
Qn: Explain reason for the appearance of the shoulder in the
electronic spectrum of [Ti(H2O)6]3+
Qn: Classify the compounds below as having no Jahn-Teller
distortion, weak or strong JTD. [co(en)3]2+,[Fe(CN)6]4-
,[Fe(CN)6]3+ and [Fe(CN)6]3+

39. Qn:Why do d3 complexes not show Jahn-Teller distortions?
Ans:Complexes with d3 electron configurations do not show Jahn-Teller distortions because
there is no ground state degeneracy.
Qn:Does the spin system (high spin v. low spin) of a molecule play a role in Jahn-Teller effects?
Ans:Yes. Examining the d5 electron configuration, one finds that the high spin scenario
contains all singly occupied d orbitals (no degeneracy). However, the low spin d5 electron
configuration shows degeneracy, which then leads to possible Jahn-Teller effects
Qn:What spectroscopic method would one utilize in order to observe Jahn-Teller distortions in a
diamagnetic molecule?
Ans:UV-VIS absorption spectroscopy is one of the most common techniques for observing these
effects. In general, it is independent of magnetism (diamagnetic v. paramagnetic). Thus, one
would see the effect in the spectrum of UV-VIS absorption analysis. Note that EPR requires at
least one unpaired electron, and therefore not EPR active
Qn:What spectroscopic method(s) would one utilize in order to observe Jahn-Teller distortions in a
paramagnetic molecule?
Ans:In addition to UV-VIS absorption, one can also employ EPR spectroscopy if a molecule
possesses and unpaired electron
Qn:Why are Jahn-Teller effects most prevalent in inorganic (transition metal) compounds?
Ans:Inorganic, specifically transition metal, complexes are most prevalent in showing Jahn-
Teller distortions due to the availability of d orbitals. The most common geometry that the

40. Qn:What spectroscopic method(s) would one utilize in order to observe Jahn-
Teller distortions in a paramagnetic molecule?
Ans:In addition to UV-VIS absorption, one can also employ EPR spectroscopy
if a molecule possesses and unpaired electron
Qn: Why are Jahn-Teller effects most prevalent in inorganic (transition metal)
compounds?
Ans:Inorganic, specifically transition metal, complexes are most prevalent in
showing Jahn-Teller distortions due to the availability of d orbitals. The most
common geometry that the Jahn-Teller effect is observed is in octahedral
complexes due to the splitting of d orbitals into two degenerate sets. Due to
stabilization, the degeneracies are removed, making a lower symmetry and
lower energy molecule.
Qn:What spectroscopic method(s) would one utilize in order to observe Jahn-
Teller distortions in a paramagnetic molecule?
Ans:In addition to UV-VIS absorption, one can also employ EPR spectroscopy if a
molecule possesses and unpaired electron
Qn:Why are Jahn-Teller effects most prevalent in inorganic (transition metal)
compounds?
Ans:Inorganic, specifically transition metal, complexes are most prevalent in

41. References
 Jahn, H. A.; Teller, E. Proc. R. Soc. London A, 1937, 161, 220-235. DOI: 10.1098/rspa.1937.0142
 Housecroft, C.; Sharpe, A. G. Inorganic Chemistry. Prentice Hall, 3rd Ed., 2008, p. 644. ISBN: 978-0-13-
175553-6
 Billy, C.; Haendler, H. A. J. Am. Chem. Soc., 1957, 79, 1049–1051. DOI: 10.1021/ja01562a011
 Moore, E. A. Metal-ligand bonding. The Open University, 2004, p. 23. ISBN 0-85404- 979-7
 P.T.Miller, P.G.Lenhert and M.D.Joesten, Inorg. Chem., 11, 2221, 1972.
 J.S.Wood, C.P.Keijzers and R.O.Day, Acta Crystallogr., Sect.C (Cr. Str. Comm.), 40, 404, 1984.
 M.D.Joesten, M.S.Hussain and P.G.Lenhert, Inorg. Chem., 9, 151, 1970