1.
Are Jahn-Teller distortions possible in tetrahedral environments? Do you think such distortions are likely to occur? Explain your answer.
Jahn-Teller distortions are possible whenever there is a partially filled degenerate set of energy levels. This can of course occur here if either the e or t
2
levels are only partially filled. However, since neither set of orbitals is oriented directly towards the ligands, the relative amount of stabilization gained is smaller than when the e g
set is partially occupied in an octahedron so such distortions are much less likely to occur in a tetrahedral environment.
2.
Explain why a d 6 configuration favours a tetrahedral rather than a square planar environment in a 4 coordinate complex. Use the information given in Fig. 21.11 (p. 9 of
Section 8 in the class notes) and assume the pairing energy is approximately 1.5 times as large as ∆ for the particular ligand set involved.
From Fig. 21.11, the energies of the lowest two levels in a SqP ligand field are approximately -0.52
∆ (doubly degenerate) and -0.39
∆ (singly degenerate) while for T d these levels are at about -0.28
∆ (doubly degenerate e) and +0.19
∆ (triply degenerate t
2
).
It is clear that the next higher level in a SqP environment is much higher in energy so we can expect a low spin complex but for a T d
complex the gap between the e and t
2
levels is small so a high spin complex is more likely (you can calculate this for a low spin T d
as well and compare energies). From this we can calculate the relative energy of T d
vs. SqP as follows:
E (T d
-SqP) = [3(-0.28
∆ ) + 3(+0.19
∆ ) +P] – [(4)(-0.52
∆ ) + 2(-0.39
∆ ) + 3P] = 2.59
∆ -
2P but if we assume P = 1.5
∆ then
E
(T d
-SqP) = -0.41
∆ which means the T d
geometry is favoured electronically. Note, it is also favoured sterically and this is not taken into account by this calculation.
3.
Determine how the p orbitals would split in an a) octahedral and b) trigonal bipyramidal crystal field. a) They would NOT split as all three axes are equivalent in this crystal field b) The equatorial trigonal plane is distinct from the axial direction. If the axial direction is taken to be z, then the p x
and p y
orbitals in the equatorial plane will feel less repulsion because neither of these orbitals points directly at the ligands. In contrast,

the p z
orbital points along the z axis and right at the ligands so this orbital will be destabilized.
4.
The magnitude of  o
for CrL
6
(Cr 3+ ) complexes is 26,310 (L = CN ), 15,250 (L = H
2
O) and 9,620 (L = I) cm -1 . Explain the relative order in terms of the ligand bonding properties.
CN causes the largest  o
because it can interact with the t
2g
set of orbitals as a  acceptor. Water is a reasonably good σ -donor but it is also a weak  -donor since it has lone pairs on oxygen so this puts it in the weak to intermediate donor range. Iodine is a reasonable σ -donor by virtue of the fact that it is less electronegative (less able to hold its charge) and it can also act as a π -donor. Iodine’s σ -donor property should increase Δ o while its π -donor property should decrease Δ responsible for the very weak field strength of I o
, however, another factor may be
. The large size of iodine places the charge further away from the metal orbitals and therefore decreases the magnitude of the crystal field splitting effect.
5.
Is this complex likely to show a Jahn-Teller distortion?
No, an octahedral d 3 complex has one electron in each of the three t
2g
orbitals and will therefore not show a Jahn-Teller effect.
6.
The value of  o
in
[Mn(OH
2
)
6
]
3+ is 15,800 cm -1 while the mean pairing energy (P)
in this complex is 28,000 cm -1 . Do you expect this ion to be high or low spin?
This is the same example done in the class notes since this is a d4 ion. In this case, you should be able to show that when Δ o
> P, a low spin complex will result and when Δ o
< P a high spin complex will be observed. Since Δ o
= 15,800 < P = 28,000 cm -1 , a high spin complex will be observed.

7.
Determine whether either spin state for the complex in Question 6 is likely to be subject to a Jahn-Teller distortion. If it can occur, which spin state would display the greater effect?
High spin d 4 should have a strong Jahn-Teller effect as there is a single electron in the degenerate e g
level that is oriented directly at the ligands. A Jahn-Teller distortion is also possible for low spin d 4 as this places 4 electrons in the t
2g
set; however, since the t
2g
set does not point directly at the ligands, the magnitude of the electronic stabilization gained by doing a Jahn-Teller distortion may not be enough to offset unfavourable energy changes associated with longer bond lengths, more steric crowding etc. and the distortion may not actually be observed.