Crystal field theory

1. 05.06.01 10:59 PM
CCrryyssttaal l FFiieelld d TThheeoorryy
The relationship The relationship
between colors and between colors and
complex metal ionscomplex metal ions
400 500 600 800

2. 05.06.01 10:59 PM
Transition Metal GemsTransition Metal Gems
Gemstone owe their color from trace transition-metal
ions
Corundum mineral, Al
2
O
3
: Colorless
Cr  Al : Ruby
Mn Mn  Al: Al:Amethyst
Fe Fe  Al: Al:Topaz
Ti &Co Ti &Co  Al: Al:SapphireSapphire
Beryl mineral, BeBeryl mineral, Be
3 3 AlAl
2 2SiSi
66OO
1818: Colorless: Colorless
Cr Cr  Al : Al : EmeraldEmerald
Fe  Al : Aquamarine

3. 05.06.01 10:59 PM
Crystal-Field TheoryCrystal-Field Theory
Model explaining bonding for transition metal complexesModel explaining bonding for transition metal complexes
• Originally developed to explain properties for crystalline material
• Basic idea:
Electrostatic interaction between lone-pair electrons result in coordination.

4. 05.06.01 10:59 PM
EnergeticsEnergetics
CFT - Electrostatic between metal ion CFT - Electrostatic between metal ion
and donor atomand donor atom
i) Separate metal and ligand
high energy
ii) Coordinated Metal - ligand
stabilized
iii) Destabilization due to
ligand -d electron repulsion
iv) Splitting due to octahedral
field.
i
ii
iii
iv

5. 05.06.01 10:59 PM
Ligand-Metal InteractionLigand-Metal Interaction
Crystal Field Theory - Describes bonding in Metal
Complexes
Basic Assumption in CFT:
Electrostatic interaction between ligand and metal
d-orbitals align along the octahedral d-orbitals align along the octahedral
axis will be affected the most.axis will be affected the most.
More directly the ligand attacks the More directly the ligand attacks the
metal orbital, the higher the the metal orbital, the higher the the
energy of the d-orbital.energy of the d-orbital.
In an octahedral field the degeneracy In an octahedral field the degeneracy
of the five d-orbitals is liftedof the five d-orbitals is lifted

6. 05.06.01 10:59 PM
d-Orbitals and Ligand Interactiond-Orbitals and Ligand Interaction
(Octahedral Field)(Octahedral Field)
Ligands
approach
metal
d-orbitals not pointing directly at axis are least
affected (stabilized) by electrostatic interaction
d-orbitals pointing directly at axis are
affected most by electrostatic interaction

7. 05.06.01 10:59 PM
Splitting of the d-OrbitalsSplitting of the d-Orbitals
Octahedral field Splitting Pattern:
The energy gap is The energy gap is
referred to as referred to as
D D (10 Dq) (10 Dq) , the , the
crystal field crystal field
splitting energy.splitting energy.
The dThe d
z2z2 and d and d
x2-y2 x2-y2 orbitals lie on the same axes as negative charges.orbitals lie on the same axes as negative charges.
Therefore, there is a large, unfavorable interaction between ligand (-) orbitals.Therefore, there is a large, unfavorable interaction between ligand (-) orbitals.
These orbitals form the degenerate high energy pair of energy levels.These orbitals form the degenerate high energy pair of energy levels.
The dThe d
xyxy , d , d
yxyx and d and d
xzxz orbitals bisect the negative charges. orbitals bisect the negative charges.
Therefore, there is a smaller repulsion between ligand & metal for these Therefore, there is a smaller repulsion between ligand & metal for these
orbitals.orbitals.
These orbitals form the degenerate low energy set of energy levels.These orbitals form the degenerate low energy set of energy levels.

8. 05.06.01 10:59 PM
Magnitude of CF Splitting (Magnitude of CF Splitting (DD or 10Dq) or 10Dq)
Color of the Complex depends on magnitude of Color of the Complex depends on magnitude of DD
1. Metal: Larger metal 1. Metal: Larger metal  larger larger DD
Higher Oxidation State Higher Oxidation State  larger larger DD
2. Ligand: Spectrochemical series2. Ligand: Spectrochemical series
ClCl
--
< F < F
--
< H < H
22O O < NH< NH
33 < en < NO < en < NO
22
--
< (N-bonded) < CN < (N-bonded) < CN
--
Weak field Ligand: Weak field Ligand: Low electrostatic interaction: small CF splitting.Low electrostatic interaction: small CF splitting.
High field LigandHigh field Ligand: High electrostatic interaction: large CF splitting.: High electrostatic interaction: large CF splitting.
Spectrochemical series: Increasing Spectrochemical series: Increasing DD

9. 05.06.01 10:59 PM
Electron Configuration in Octahedral FieldElectron Configuration in Octahedral Field
Electron configuration of metal ion:Electron configuration of metal ion:
s-electrons are lost first. s-electrons are lost first.
TiTi
3+3+
is a d is a d
11
, V, V
3+3+
is d is d
22
, and Cr , and Cr
3+3+
is d is d
33

Hund's rule:Hund's rule:
First three electrons are in separate d First three electrons are in separate d
orbitals with their spins parallel.orbitals with their spins parallel.
Fourth e- has choice:Fourth e- has choice:
Higher orbital if Higher orbital if DD is small; High spin is small; High spin
Lower orbital if Lower orbital if DD is large: Low spin. is large: Low spin.
Weak field ligandsWeak field ligands
Small Small DD , High spin complex , High spin complex
Strong field LigandsStrong field Ligands
Large Large DD , Low spin complex , Low spin complex

10. 05.06.01 10:59 PM10
High Spin Vs. Low Spin (dHigh Spin Vs. Low Spin (d
11
to d to d
1010
))
Electron Configuration for Octahedral complexes of metal ion having dElectron Configuration for Octahedral complexes of metal ion having d
11
to to
dd
1010
configuration [M(H configuration [M(H
22O)O)
66]]
+n+n
. .
Only the dOnly the d
44
through d through d
77
cases have both high-spin and low spin configuration cases have both high-spin and low spin configuration..
Electron configurations
for octahedral
complexes of metal ions
having from d
1
to d
10

configurations. Only
the d
4
through d
7
cases
have both high-spin and
low-spin configurations.

11. 05.06.01 10:59 PM11
Color Absorption of CoColor Absorption of Co
3+3+
Complexes Complexes
The Colors of Some Complexes of the CoThe Colors of Some Complexes of the Co
3+ 3+
IonIon
The complex with fluoride ion, [CoFThe complex with fluoride ion, [CoF
66]]
3+3+
, is high spin and has one absorption band. , is high spin and has one absorption band.
The other complexes are low spin and have two absorption bands. In all but one The other complexes are low spin and have two absorption bands. In all but one
case, one of these absorptionsis in the visible region of the spectrum. The case, one of these absorptionsis in the visible region of the spectrum. The
wavelengths refer to the center of that absorption band.wavelengths refer to the center of that absorption band.
Complex IonComplex IonWavelength of Wavelength of Color of Light Color of Light Color of ComplexColor of Complex
light absorbed light absorbed Absorbed Absorbed
[CoF[CoF
66] ]
3+3+
700 (nm)700 (nm) RedRed GreenGreen
[Co(C[Co(C
22OO
44))
33] ]
3+3+
600, 420600, 420 Yellow, violetYellow, violetDark greenDark green
[Co(H[Co(H
22O)O)
66] ]
3+3+
600, 400600, 400 Yellow, violetYellow, violetBlue-greenBlue-green
[Co(NH[Co(NH
33))
66] ]
3+3+
475, 340475, 340 Blue, violetBlue, violetYellow-orangeYellow-orange
[Co(en)[Co(en)
33] ]
3+3+
470, 340470, 340 Blue, ultraviolet Blue, ultravioletYellow-orangeYellow-orange
[Co(CN)[Co(CN)
66] ]
3+3+
310310 Ultraviolet UltravioletPale YellowPale Yellow

12. 05.06.01 10:59 PM12
Colors & How We Perceive itColors & How We Perceive it
800
430
650 580
560
490
Artist color wheelArtist color wheel
showing the colors whichshowing the colors which
are complementary to oneare complementary to one
another and the wavelengthanother and the wavelength
range of each color.range of each color.
400

13. 05.06.01 10:59 PM13
Black Black & & WhiteWhite
If a sample absorbs all wavelength If a sample absorbs all wavelength
of visible light, none reaches our of visible light, none reaches our
eyes from that sample. eyes from that sample.
Consequently, it appears black.Consequently, it appears black.
When a sample absorbs light, what we see is the sum When a sample absorbs light, what we see is the sum
of the remaining colors that strikes our eyes.of the remaining colors that strikes our eyes.
If the sample absorbs noIf the sample absorbs no
visible light, it is white visible light, it is white
or colorless.or colorless.

14. 05.06.01 10:59 PM14
AbsorptionAbsorption and and ReflectionReflection
If the sample absorbsIf the sample absorbs
all but all but orangeorange, the, the
sample appears sample appears
orange.orange.
Further, we also perceive orange color Further, we also perceive orange color
when visible light of all colors except when visible light of all colors except blue blue
strikes our eyes. In a complementary strikes our eyes. In a complementary
fashion, if the sample absorbed only fashion, if the sample absorbed only
orange, it would appear blue; blue and orange, it would appear blue; blue and
orange are said to be complementary orange are said to be complementary
colors.colors.
750
430
650 580
560
490
400

15. 05.06.01 10:59 PM15
Light absorption Properties of Metal ComplexesLight absorption Properties of Metal Complexes
Recording the absorption Recording the absorption
SpectrumSpectrum

16. 05.06.01 10:59 PM16
Complex Influence on ColorComplex Influence on Color
Compounds of Transition metal complexes solution.Compounds of Transition metal complexes solution.
[Fe(H
2
O)
6
]
3+
[Co(H
2
O)
6
]
2+
[Ni(H
2
O)
6
]
2+
[Cu(H
2
O)
6
]
2+
[Zn(H
2
O)
6
]
2+
800
430
650 580
560
490
400

17. 05.06.01 10:59 PM17
Color Absorption of CoColor Absorption of Co
3+3+
Complexes Complexes
The Colors of Some Complexes of the Co
3+
Ion
Complex Ion Wavelength of
Light Absorbed
(nm)
Color of Light
Absorbed
Color of
Complex
[CoF6]
3+ 700 Red Green
[Co(C2O4)3]
3+
600, 420, Yellow, violet Dark green
[Co(H2O)6]
3+ 600, 400, Yellow, violet Blue-green
[Co(NH3)6]
3+ 475, 340 Blue,
ultraviolet
Yellow-orange
[Co(en)3]
3+ 470, 340 Blue, ultraviolet Yellow-orange
[Co(CN)6]
3+ 310 Ultraviolet Pale yellow
The complex with fluoride ion, [CoFThe complex with fluoride ion, [CoF
66]]
3+3+
, is high spin and has one absorption , is high spin and has one absorption
band. The other complexes are low spin and have two absorption bands. In band. The other complexes are low spin and have two absorption bands. In
all but one case, one of these absorptionsis in the visible region of the all but one case, one of these absorptionsis in the visible region of the
spectrum. The wavelengths refer to the center of that absorption band.spectrum. The wavelengths refer to the center of that absorption band.

18. 05.06.01 10:59 PM18
Octahedral, Tetrahedral & Square PlanarOctahedral, Tetrahedral & Square Planar
CF Splitting pattern for CF Splitting pattern for
various molecular geometryvarious molecular geometry
M
d
z2d
x2-y2
d
xzd
xy
d
yz
M
d
x2-y2
d
z2
d
xzd
xyd
yz
M
d
xz
d
z2
d
x2-y2
d
xy
d
yz
OctahedralOctahedral
TetrahedralTetrahedral
Square planarSquare planar
Pairing energy Vs. D
Weak field D < Pe
Strong field D > Pe
Small D  High Spin
Mostly d
8
(Majority Low spin)
Strong field ligands
i.e., Pd
2+
, Pt
2+
, Ir
+
, Au
3+

19. 05.06.01 10:59 PM19
SummarySummary
Crystal Field Theory provides a basis for explaining Crystal Field Theory provides a basis for explaining
many features of transition-metal complexes. many features of transition-metal complexes.
Examples include why transition metal complexes are Examples include why transition metal complexes are
highly colored, and why some are paramagnetic while highly colored, and why some are paramagnetic while
others are diamagnetic. The spectrochemical series for others are diamagnetic. The spectrochemical series for
ligands explains nicely the origin of color and ligands explains nicely the origin of color and
magnetism for these compounds. There is evidence to magnetism for these compounds. There is evidence to
suggest that the metal-ligand bond has covalent suggest that the metal-ligand bond has covalent
character which explains why these complexes are very character which explains why these complexes are very
stable. Molecular Orbital Theory can also be used to stable. Molecular Orbital Theory can also be used to
describe the bonding scheme in these complexes. A describe the bonding scheme in these complexes. A
more in depth analysis is required however.more in depth analysis is required however.