Color and Bonding in
Transition Metal Complexes
Chemistry 123
Dr. Patrick Woodward
Supplemental Lecture 3
Intra-atomic (localized) excitations
–
–
Transition metal ions, complex ions & compounds (d-orbitals)
Lanthanide ions and compounds (f-orbitals)
[Ni(NH3)6]2+
NiSO4
Cu3(CO3)2(OH)2
CuSO4
Malachite
In these complexes the color comes from absorption of light that leads to excition
of an electron from an occupied d-orbital to an empty or ½-filled d-orbital. The
energy separation between d-orbitals depends upon the interaction between the dorbitals and the ligands. There are two ways to rationalize the energy separation
Crystal Field Theory & Ligand Field Theory.
1
Energy
Crystal Field Splitting (Octahedron)
dz2 & dx2-y2 orbitals (e
(eg)
•Point directly at the ligands
•Stronger (repulsive) interaction
with the ligands
dxy, dyz & dxz orbitals (t2g)
dz2
dx2-y2
dyz
dxz
•Point between the ligands
•Weaker (repulsive) interaction
with the ligands
dxy
Electrons in d-orbitals are repelled from the electrons in the ligands, based
on electrostatic interactions. This causes two of the d-orbitals (dz2 & dx2-y2)
to be at higher energy than the other three (dxy, dyz, dxz)
Ligand Field Splitting (Octahedron)
dz2 & dx2-y2 orbitals (e
(eg)
•Point directly at the ligands
•Sigma antibonding interaction
with the ligands
dz2 σ*
dx2-y2 σ*
dxy, dyz & dxz orbitals (t2g)
•Point between the ligands
•Pi antibonding interaction
with the ligands
dxy π*
dyz π*
dxz π*
Ligand field theory is based on covalent interactions between the metal and
the surrounding ligands, we can use MO theory to understand it. The splitting
of orbitals into a lower energy t2g set of orbitals (non bonding, or piantibonding) & and a higher energy eg set of orbitals (sigma antibonding).
2
d-orbital Splitting (Octahedron)
dz2 dx2-y2
dz2 & dx2-y2 orbitals (e
(eg)
eg
•Point directly at the ligands
•Sigma antibonding interaction
with the ligands
Δ = Crystal Field
Splitting Energy
dxy, dyz & dxz orbitals (t2g)
•Point between the ligands
•Pi antibonding interaction with the
ligands
t2g
dxy
dyz
dxz
Cr3+
5 dd-orbitals on Cr
(Cr3+ = d3 ion)
3 electrons in the
d-orbitals
[Cr(NH3)6]3+
Octahedron
:NH3
: N
H
HH
6 Ligand Orbitals
Nitrogen lone pairs
(all containing 2 e-)
Only sigma interactions
are allowed
3
[Cr(NH3)6]3+
Antibonding (σ*)
Metal-Ligand MO’s
eg orbitals
Δ = Crystal Field Splitting Energy
t2g orbitals
Energy
Metal (Cr) d-orbitals
Nonbonding
Metal d MO’s
Nonbonding
Ligand MO’s
Ligand (N) lone-pair
orbitals
Δ ~ 3.0 eV (~410 nm)
Absorption = Violet
Color = Yellow
Bonding (σ)
Metal-Ligand MO’s
eg
eg
Δ
Δ
t2g
t2g
Cl–
Small Δ
Spectrochemical Series
< F- < H2O < NH3 < NO2- < CN-
Weak Field Ligand
Weak M-L interaction
Large Δ
Strong Field Ligand
Strong M-L interaction
4
High spin & low spin states
Large Δ
Small Δ
Low Spin
Configuration
High Spin
Configuration
The t2g set of
d-orbitals are
completely
filled before
electrons fill
the eg orbitals
All five dorbitals are
filled before
pairing up 2
electrons in
one orbital
Diamagnetism – All electrons are paired up, which leads to equal
numbers of spin up and spin down electrons (i.e. [Co(CN)6]3-)
Paramagnetism – Unpaired electrons, which leads to unequal
numbers of spin up and spin down electrons ((i.e. [CoF6]3-)
Cr3+ Gemstones
Corundum - Al2O3
Beryl - Be3Al2Si6O18
Ruby
Al2O3:Cr3+
In both gemstones Cr3+ substitutes
for Al3+, which is surrounded by 6
oxygen ions in an octahedron. The
color comes from a d-to-d excitation
on the Cr3+ center.
Emerald
Be2Al2Si6O18:Cr3+
5
Measurements show that the crystal
field splitting, Δ, of the Cr3+ ion in
ruby and emerald are:
1. Ruby = 2.3 eV (540 nm) & Emerald = 1.9 (650 nm)
2. Ruby = 1.9 eV (650 nm) & Emerald = 2.3 (540 nm)
50%
10
50%
Ruby = 2.3 eV (540 nm) & Emerald = 1.9 (650 nm)
Ruby = 1.9 eV (650 nm) & Emerald = 2.3 (540 nm)
Do you expect the Cr-O distances to
be shorter in ruby (Δ = 2.3 eV) or
emerald (Δ = 1.9 eV)
1. Shorter in Ruby
2. Shorter in Emerald
50%
50%
10
Shorter in Ruby
Shorter in Emerald
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Ligand Field Splitting (Tetrahedron)
dxy, dyz & dxz orbitals (t2)
•Stronger antibonding interaction
with the ligands
•Higher energy
dxy
dyz
dxz
dz2 & dx2-y2 orbitals (e)
•Weaker antibonding interaction
with the ligands
•Lower energy
dz2
dx2-y2
The crystal field splitting, Δ, for a tetrahedron is
considerably smaller than for an octahedron
Ligand Field Splitting (Tetrahedron)
dxy, dyz & dxz orbitals (t2)
dxy
dyz
dxz
•Stronger antibonding interaction
with the ligands
•Higher energy
t2
Δ = Crystal Field
Splitting Energy
dz2 & dx2-y2 orbitals (e)
•Weaker antibonding interaction
with the ligands
•Lower energy
e
dz2 dx2-y2
The crystal field splitting, Δ, for a tetrahedron is
considerably smaller than for an octahedron
7
5 dd-orbitals on Cr
(Cr6+ = d0 ion)
0 electrons in the dd-orbitals
Cr3+
O
4 Ligand Orbitals
Oxygen lone pairs
(all containing 2 e-)
CrO42-
Tetrahedron
[CrO4]2-
t2 orbitals
(antibonding)
e orbitals
(antibonding)
CT
Energy
Metal (Cr) d-orbitals
Nonbonding
Oxygen 2p MO’s
e orbitals
(bonding)
t2 orbitals
(bonding)
PbCrO4
12 Oxygen 2p orbitals
(4 oxygens x 3 p orbitals)
CT ~ 3.3 eV (~375 nm)
Absorption = Violet
Color = Yellow
8
Charge Transfer in Sapphire
• The deep blue color the gemstone sapphire is
also based on impurity doping into Al2O3. The
color arises from the following charge transfer
excitation:
Fe2+ + Ti4+ → Fe3+ + Ti3+
(λmax ~ 2.2 eV, 570 nm)
• The transition is facilitated by the geometry
of the corundum structure where the two ions
share an octahedral face, which allows for
favorable overlap of the dz2 orbitals.
Fe2+
Ti4+
• Unlike the d-d transition in Ruby, the chargetransfer excitation in sapphire is fully allowed.
Therefore, the color in sapphire requires only
~ 0.01% impurities, while ~ 1% impurity level is
needed in ruby.
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