Download Color and Bonding in Transition Metal Complexes

Color and Bonding in
Transition Metal Complexes
[Ni(NH3)6]2+
Cu3(CO3)2(OH)2
Malachite
Al2O3:Cr3+
Be2Al2Si6O18:Cr3+
Ruby
Emerald
Intra-atomic (d to d) excitations
MnSO4
FeSO4
CoSO4
NiSO4
CuSO4
ZnSO4
In these complexes the color comes from absorption of light that
leads to excitation of an electron from one d-orbital to a
different d-orbital on the same metal cation.
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[M(H2O)6]n+ Complex ions
M
H2O
MO6 Octahedron
Energy Levels of d-orbitals in an
octahedron
Energy
dz2
Δ
dxy
dyz
dxz
dz2
dx2x2-y2
Cr3+ ion with a
spherical crystal
field
dxy
dx2x2-y2
dyz
dxz
dz2
Free Cr3+ ion
dx2x2-y2
dxy
dyz dxz
Cr3+ ion in an
octahedral crystal
field
2
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. The repulsion is stronger for electrons in d-orbitals
pointing directly at the ligands.
Energy
Electronic Excitation
dx2-y2
dz2
dz2
dx2-y2
eg
Δ = Crystal Field
Splitting Energy
light
(photon)
t2g
dxy
dyz
dxz
dxy
dyz
dxz
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Absorption spectrum for
[Ti(H2O)6]3+
Absorbs:
Blue, Green
Partially Absorbs: Violet, Yellow
Transmits:
Orange, Red
Color = Reddish-Purple
UV-Visible Absorption Spectroscopy
Intra-atomic (d to d) excitations
MnSO4
d5
FeSO4
d6
CoSO4
d7
NiSO4
d8
CuSO4
d9
ZnSO4
d10
Let’s consider the colors of these ions more carefully.
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Cr3+
:NH3
5 dd-orbitals on Cr
(Cr3+ = d3 ion)
3 electrons in the
d-orbitals
: N
HH
6 Ligand Orbitals
Nitrogen lone pairs
(all containing 2 e-)
Only sigma interactions
are allowed
[Cr(NH3)6]3+
Octahedron
[Cr(NH3)6]3+
H
Antibonding (σ*)
Metal-Ligand MO’s
eg orbitals (dz2, dx2-y2)
Δ = Crystal Field Splitting Energy
t2g orbitals (dxz, dyz, dxy)
Energy
Metal (Cr) d-orbitals
Nonbonding
Metal d MO’s
Nonbonding
Ligand MO’s
Bonding (σ)
Metal-Ligand MO’s
Ligand (N) lone-pair
orbitals
Δ ~ 3.0 eV (~410 nm)
Absorption = Violet
Color = Yellow
5
Antibonding (σ*)
Metal-Ligand MO’s
[Cr(H2O)6]3+
eg orbitals (dz2, dx2-y2)
Δ = Crystal Field Splitting Energy
t2g orbitals (dxz, dyz, dxy)
Nonbonding
Metal d MO’s
Energy
Metal (Cr) d-orbitals
Nonbonding
Ligand MO’s
Ligand (O) lone-pair
orbitals
We simplify the MO diagram by
only considering one orbital
containing a nonbonding electron
pair per H2O ligand.
Upon changing the ligand
from NH3 to H2O the only
thing that changes are
subtle changes in the
energies of the orbitals.
Bonding (σ)
Metal-Ligand MO’s
[Cr3+(H2O)6]3+ Absorption Spectra
Antibonding (σ*)
Metal-Ligand MO’s
0.6
0.5
Absorbance
0.4
Δoct
0.3
0.2
Nonbonding
Metal d MO’s
0.1
0.0
250
300
350
400
450
500
550
600
650
700
750
Wavelength (nm)
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eg
eg
Δ
Δ
t2g
t2g
Cl–
Spectrochemical Series
< F- < H2O < NH3 < NO2- < CN-
Small Δ
Large Δ
Weak Field Ligand
Weak M-L interaction
Strong Field Ligand
Strong M-L interaction
Cu2+ complexes
2.0
1.8
1.6
absorbance
1.4
2+
[Cu(H2O)4]
1.2
2+
[Cu(NH3)4]
1.0
0.8
0.6
0.4
0.2
0.0
200
300
400
500
600
700
800
900
wavelength (nm)
NH3 is higher than H2O in the spectrochemical series. This increases the energy
splitting of the d-orbitals and shifts the absorption maximum to shorter wavelengths.
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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-)
Square Planar Coordination
Octahedron
Square Plane
(c)
Removing the two ligands from the z-axis reduces the overlap
with the dz2 orbital, thereby lowering its energy. The energy fo
the dxz and dyz are also lowered (by a lesser amount)
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Tetrahedral Coordination
Δtetr
Tetrahedron
Absorbance
The splitting of the d orbitals for a tetrahedron is
opposite that of an octahedron. In this case the higher
energy d-orbitals (dxy, dxz & dyz) don’t point exactly at
the ligands. So the crystal field splitting parameter,
Δtetr, is smaller than the crystal field splitting in an
octahedron, Δoct. Roughly speaking Δtetr ≈ (4/9) Δoct
Conduction
Band
Eg
Energy
400 nm
Eg
Valence
Band
Only visible light
with energy less
than Eg is reflected,
the remaining visible
light is absorbed
Wavelength
Energy
700 nm
UV
IR
Band Gap (eV
(eV)) Color
> 3.0
White
3.0-2.4
Yellow
2.3-2.4
Orange
1.8-2.3
Red
< 1.8
Black
Example
ZnO
CdS
GaP
HgS
CdSe
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