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Unit 3 Summary
Crystal Field Theory
z
y
-
Mn+
-
x
-
-
Which d-orbitals are effected the most?
z
z
x-
M
x
dz2 along z-axis
-
-
M
x-
dx2- y2 along x-y axis
These two d-orbitals constitute the higher energy eg set.
dzx orbital has lobes
between z-x axis
z
Less repulsion
-
M
-
x
-
Likewise interactions for
dxy and dyz
These three d-orbitals constitute the lower energy t2g set.
Octahedral
Tetrahedral
t2
eg
Δt
Δo
t2g
e
Use diagram/spectrochemical series to explain:
Absorption wavelengths due to electronic excitation
Paramagnetic properties (unpaired electrons)
Octahedral MO Diagram – s-bonded complex
L
Can be pz, s, or a hybrid
L
L
z
y
Mn+
L
L
L
x
L
Which d-orbitals form s-bonds
with ligands at the corners of
the octahedron?
Octahedral MO Diagram – s-bonded complex
L
Can be pz, s, or a hybrid
L
z
z
y
M
x
L
M
x
dz2 along z-axis
L
dx2- y2 along x-y axis
L
This results in the formation of 4 MO’s (eg and eg*).
L
Octahedral MO Diagram – s-bonded complex
L
dzx orbital has lobes
between z-x axis
z
NO OVERALP
L
M
x
L
Likewise interactions for
dxy and dyz
L
These three d-orbitals are non-bonding
The Molecular Orbital Diagram
t1u*
4p
a1g*
4s
eg*
Δo
t2g
3d
eg
t1u
a1g
L - AO’s
M-L -Bonding Interactions
-Donor Ligands
(e.g. Cl-, Br-, I-)
-Acceptor Ligands
(e.g. CO, N2, C2H4)
z
L
M
z
x
L
M
x
L
X
Empty * orbital
Occupied p-orbital
The t2g orbitals overlap with group L orbitals  3 Bonding MOs + 3
Antibonding MOs
-Donor Ligands
(e.g. Cl-, Br-, I-)
eg*
-Acceptor Ligands
(e.g. CO, N2, C2H4)
t2g*
Δo
t2g*
eg*
p
*
Δo
3d
t2g
eg
L - AO’s
3d
t2g
eg
L - AO’s
Color in coordination complexes
The colors are determined by Δ. Different ligands generate crystal fields of different strength. When the molecules absorb
visible light, excited electrons jump from lower energy t2g to the higher energy eg orbital. The Δ (difference between energies
of the two orbitals) is equal to the energy of the absorbed photon, and related inversely to the wavelength of the light. Weaker
field ligands with smaller Δ emit light of longer λ and thus lower v. Similarly, stronger field ligands with larger Δ emit light of
shorter λ and thus higher v.
λ Absorbed
400nm Violet absorbed
450nm Blue absorbed
490nm Blue-green absorbed
570nm Yellow-green absorbed
580nm Yellow absorbed
600nm Orange absorbed
650nm Red absorbed
Color observed
Green-yellow observed (λ 560nm)
Yellow observed (λ 600nm)
Red observed (λ 620nm)
Violet observed (λ 410nm)
Dark blue observed (λ 430nm)
Blue observed (λ 450nm)
Green observed (λ 520nm)
Problems
1. (a) When water ligands in [Ti(H2O)6]3+ are replaced by CN- ligands to give [Ti(CN)6]3-, the maximum
absorption shifts from 500 nm to 450 nm. Is this shift in the expected direction? Explain. What color do you
expect to observe for this ion?
CN- is a stronger field ligand than H2O. Therefore the energy separation between the
t2g and eg levels is greater.
Ti3+ is a d1 metal ion. Do = hc/l. If D is larger, l is smaller.
[Ti(CN)6]3eg
[Ti(H2O)6]3+
eg
A(lmax) = 500 nm
Solution appears red
A(lmax) = 450 nm
Do
Do
Solution appears yellow
t2g
t2g
(b) The [Fe(H2O)6]3+ ion has a pale purple color, and the [Fe(CN)6]3- ion has a ruby red color.
What are the approximate wavelengths of the maximum absorption for each ion? Is the shift of
wavelength in the expected direction? Explain.
CN- is a stronger field ligand than H2O. Therefore the energy separation between the
t2g and eg levels is greater.
Fe3+ is a d5 metal ion. Do = hc/l. If D is larger, l is smaller.