Unit 3 Summary

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Unit 3 Summary
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Crystal Field Theory
Which d-orbitals are effected the most?
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z
z
x
x
x
M
M
dz2 along z-axis
dx2- y2 along x-y axis
These two d-orbitals constitute the higher energy
eg set.
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dzx orbital has lobes between z-x axis
z
Less repulsion
x
M
Likewise interactions for dxy and dyz
These three d-orbitals constitute the lower
energy t2g set.
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Octahedral
Tetrahedral
t2
eg
?t
?o
e
t2g
Use diagram/spectrochemical series to
explain Absorption wavelengths due to electronic
excitation Paramagnetic properties (unpaired
electrons)
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Octahedral MO Diagram s-bonded complex
L
Can be pz, s, or a hybrid
Which d-orbitals form s-bonds with ligands at the
corners of the octahedron?
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Octahedral MO Diagram s-bonded complex
This results in the formation of 4 MOs (eg and
eg).
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Octahedral MO Diagram s-bonded complex
dzx orbital has lobes between z-x axis
NO OVERALP
L
Likewise interactions for dxy and dyz
These three d-orbitals are non-bonding
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The Molecular Orbital Diagram
t1u
a1g
?o
t2g
t1u
a1g
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M-L ?-Bonding Interactions
?-Donor Ligands (e.g. Cl-, Br-, I-)
?-Acceptor Ligands (e.g. CO, N2, C2H4)
z
z
x
M
X
L
x
L
M
L
Empty ? orbital
Occupied p-orbital
The t2g orbitals overlap with group L orbitals ?
3 Bonding MOs 3 Antibonding MOs
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?-Donor Ligands (e.g. Cl-, Br-, I-)
?-Acceptor Ligands (e.g. CO, N2, C2H4)
t2g
eg
?
?o
3d
L - AOs
t2g
eg
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Color in coordination complexes
The colors are determined by ?. 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 ? (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 ? emit light of longer
? and thus lower v. Similarly, stronger field
ligands with larger ? emit light of shorter ? and
thus higher v.
                                                  
                          ?
Absorbed Color observed400nm Violet
absorbed Green-yellow observed (? 560nm)450nm
Blue absorbed Yellow observed (? 600nm)490nm
Blue-green absorbed Red observed (? 620nm)570nm
Yellow-green absorbed Violet observed (?
410nm)580nm Yellow absorbed Dark blue observed
(? 430nm)600nm Orange absorbed Blue observed (?
450nm)650nm Red absorbed Green observed (?
520nm)
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Problems
1. (a) When water ligands in Ti(H2O)63 are
replaced by CN- ligands to give Ti(CN)63-, 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.
A(lmax) 500 nm Solution appears red
A(lmax) 450 nm Solution appears yellow
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(b) The Fe(H2O)63 ion has a pale purple color,
and the Fe(CN)63- 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.
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