Applications of CFT for Oh Complexes 1. High- and low-spin

Applications of CFT for Oh Complexes
1.
High- and low-spin complexes
TM ions can contain varying numbers of unpaired electrons in
their d orbitals – how can we establish the number of unpaired e-?
magnetic measurements: not as straightforward as you might
think!
Oh complexes for d4 to d7 ions can display different numbers of
unpaired electrons, even for the same metal with different ligand
sets – depends on:
a) size of Δoct
b) magnitude of the crystal field stabilization energy (CFSE) for
a particular electronic configuration
d1
d2
+0.6 Δoct
Bary Centre
-0.4 Δoct
CFSE = -0.4 Δoct
= 2(-0.4) = -0.8 Δoct
d4 ion case – two possible arrangements: ‘high’ and ‘low’ spin
+0.6 Δoct
Bary Centre
-0.4 Δoct
= 4(-0.4 Δoct) + P
CFSE = 3(-0.4 Δoct) +
(+0.6 Δoct)
= -1.6 Δoct + P
= -0.6 Δoct
P = ‘pairing energy’ made up of:
a) exchange energy: electrons with parallel spins experience
less repulsion that electrons with anti-parallel spins even if
they are in different orbitals (Hund’s rule)
b) electron-electron repulsion greatest if spins are paired in the
same orbital
So, in this case, energies of the two configurations are equal if
P = Δoct
OR
High spin if Δoct < P
and Low spin if Δoct > P
The magnitude of Δoct and P are similar for 1st row TM so both
high spin and low spin complexes ARE actually observed:
eg. [Fe(H2O)6]3+ vs. [Fe(CN)6]3-
Δoct
[Fe(H2O)6]3+
Δoct
= 13,700 cm-1
= 35,000 cm-1
[Fe(CN)6]3-
H2O is a weak field ligand → high spin complex
CN- is a strong field ligand → low spin complex
2.
Jahn Teller distortions of d4 and d9 Oh systems
Tetragonal distortions (lengthening of either four ‘equatorial’ or
two ‘axial’ bonds) of an Oh complex are observed only with high
spin d4 or d9 electron counts. Why?
eg. [CrF6]4- high spin d4
axial
elongation
dx2-y2
dz2
dxy
dxz dyz
• distortion occurs when there is a partially filled degenerate
set of orbitals because this breaks the degeneracy allowing
more electrons to occupy the lower energy orbitals
• with axial elongation along z, orbitals pointed along z are
relatively stabilized compared to those in the xy plane so it is
better for electron to occupy the dz2 orbital of the (formerly)
eg set (if e- goes in dx2-y2, equatorial elongation will happen
instead)
Similarly for the d9 configuration:
axial
elongation
dx2-y2
dz2
dxy
dxz dyz
However, distortions are not usually observed in d5 system even
though there is a partially occupied t2g set – why not?
• t2g set do not point directly at a ligand so the gain in
stabilization caused by distortion is very small
CFT for 4-coordinate complexes
Square planar complexes – related to Jahn Teller distortions of
Oh complexes
d8 complexes
Oh
axial
elongation
Square
Planar
dx2-y2
dx2-y2
dz2
dxy
dxy
dxz dyz
dz2
dxz dyz
• Square planar complexes are always diamagnetic (spin
paired) because the difference in energy between dxy and
dx2-y2 is large
Tetrahedral (Td) crystal fields
How does a Td crystal field split the d orbitals? This is MUCH
harder to see visually but we again get two sets of orbitals:
dxz dyz dxy
t2
e
dx2-y2 dz2
• hard to see but dx2-y2 and dz2 point between the ligands more
than do dxz, dyz and dxy so they are relatively stabilized
• the tetrahedral crystal field (Δt) splitting is less than
octahedral splitting (Δoct) because no orbitals point directly
at the ligands:
Δt = 4/9 Δoct
CN4, d8 structural preferences: Td vs. SqP
Td
SqP
+1.2
+1.0
+0.8
E
N
E
R
G
Y
+0.6
+0.4
+0.2
0
-0.2
-0.4
-0.6
larger fields favour SqP over Td:
[Ni(CN)4]2- is SqP
BUT
[NiCl4]2- is Td
[PdCl4]2- is SqP
BUT
[NiCl4]2- is Td
2nd and 3rd row d8 complexes are virtually always SqP
easily distinguishable by magnetism because Td is paramagnetic
(unpaired electrons) while SqP is diamagnetic (paired electrons)
Crystal field splittings for other geometries (Fig. 21.11 H&S 3rd
Ed):
Which d count is most likely to favour a square pyramidal
structure over a trigonal bipyramid?
d4 would be a good bet and some Fe(IV) compounds do adopt this
geometry
What does the fact that the ferrate ion [FeO4]4- distorts from
tetrahedral tell you about the electron configuration?
If it distorts it is presumably doing so for electronic reasons and
that suggests a Jahn Teller distortion. For this to occur it must be
high spin because the low spin complex has a filled e set.