1. Transition metal complexes are colored due to d-d orbital splitting caused by ligand fields, which changes the energies of the d-orbitals. 2. Color is observed when electrons in partially filled d-orbitals are promoted to higher energy levels through absorption of visible light, causing d-d transitions. 3. Transition metals with completely filled (d10) or empty (d0) d-orbitals form colorless complexes since there are no electrons available for d-d transitions.

1. Dr. Narinderjit Kaur
Assistant Professor
PG Dept. Of Chemistry
Kanya Maha Vidyalaya, Jalandhar

2. DR. Narinderjit Kaur
2
Why are Transition
metal complexes
colored?
Why TM with d0
&
d10
form colorless
compounds?

3. DR. Narinderjit Kaur 3
Colors in TM complexes is due to their absorption spectrum.
A substance appears colored because it absorbs light at one or more wave-lengths
in the visible part of the electromagnetic spectrum (400 to 700 nm) and reflects
or transmits the others.
Each wavelength of light in this region appears as a different color. A
combination of all colors appears white.
We see the color which is reflected or transmitted

4. DR. Narinderjit Kaur
4
Observed color is due to transmitted or reflected light
that is complementary in color to light that is absorbed
Wavelenght absorbed, nm Color observed
400 (violet)
450 (blue)
490 (blue-green)
570 (yellow green)
580 (yellow)
600 (orange)
650 (red)
Greenish yellow
Orange-yellow
Red
Purple
Dark blue
Blue
Green
When we say that the hydrated cupric ion is blue, we mean that each ion
absorbs a photon a wavelength of about 600 nm (orange light), the
transmitted light appears blue to our eyes.

5. DR. Narinderjit Kaur
5
1. d-d orbital splitting
• When a metal ion forms a complex with ligands, the surrounding ligands interact
with the d-orbitals within the d-subshell to different extent.
• This results in a d-d orbital splitting where some of the d-orbitals have higher
energy level while others have lower energy level.
• The energy difference between the 2 energy levels corresponds to the energy
level of a particular color in the visible light region.
Criteria for transition metal complexes to be colored:
d-orbitals in
spherical
field
eg
t2g

6. DR. Narinderjit Kaur
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2. partially filled d-subshell
• When the d-subshell is partially filled (d1 to d9), the transition or promotion of
an electron from a lower energy state to a higher energy state is possible.
• When there are no electrons (d0), no d-d transition is possible since there are
no electrons.
• When the d-subshell is fully filled (d10), there is no available space in the
higher energy level for d-d transition to take place.
CuSO4.5H2O (d9) Anhydrous CuSO4 (d9)

7. DR. Narinderjit Kaur
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When visible light falls on TM complexes or ions, e’ gets excited from lower set of
orbitals to higher set of orbitals. E.g. in case of octahedral complex, e’ gets
excited from t2g to eg orbitals.
eg
t2g
eg
t2g
E = hν
Absorption of light is equal to the difference in energy between these two levels &
is E = hν

8. DR. Narinderjit Kaur
8
• When Ti3+
(d1
) is in the gaseous state, the d-subshell is partially filled but there is
no d-d orbital splitting. Therefore, no d-d transition is possible so Ti3+
(g) has no
color.
• When Ti3+ dissolves in water to form Ti3+ (aq) or a metal complex with water
ligands Ti[H2O]6
3+
, a d-d orbital splitting occurs with 2 orbitals (eg) at a higher
energy level and 3 orbitals (t2g) at the lower energy level.
Let's have an example of Ti[H2O]6
3+

9. DR. Narinderjit Kaur
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eg
t2g
eg
t2g
E = hν
When ion absorbs light in the visible region; the wavelength corresponding to maximum
absorption is 498 nm.

10. DR. Narinderjit Kaur
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ion absorbs light in the visible region; the wavelength corresponding to
maximum absorption is 498 nm.
Crystal field splitting is:
Δ = ℎ𝜈 =
ℎ𝑐
𝜆
=
(6.63 × 10−34Js)(3 × 108
m/s)
498 × 10−9m
= 3.99 × 10−19 J=240 kJ/mol
This is the energy required to excite one Ti[H2O]6
3+
ion

11. DR. Narinderjit Kaur
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The d-orbital splitting in this case is 240 kJ per mole which corresponds to light of green-
yellow color; absorption of this light promotes the electron to the upper set of d orbitals,
which represents the exited state of the complex. If we illuminate a solution of Ti[H2O]6
3+
with white light, the green-yellow light is absorbed, and the solution appears violet in color.
eg
t2g
eg
t2g
E = hν
Ground State Excited State
Energy difference
corresponds green
& yellow light
eg
t2g
Energy difference
corresponds blue
& red light

12. DR. Narinderjit Kaur
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13. DR. Narinderjit Kaur
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As CFS depends on following factors so is the color of complexes:
• Nature of ligands
• Transition metal ion
• Oxidation state of metal ion
Reason for the different colors of:
[Co(H2O)6]3+
[Co(CN)6]3-
[Co(NH3)6]3+
ligand Weak field Intermediate Strong field
Δo small Intermediate Large
Absorption λ large Intermediate small
Color absorbed Orange Blue Violet
Color transmitted Blue Orange yellow

14. DR. Narinderjit Kaur
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FeCl2 FeCl3
Let’s try to answer…..

15. DR. Narinderjit Kaur
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MnCl2 FeCl2
Let’s try this also…..

16. 16
Cu[H2O]6
2+
Hydrated cupric ion absorbs photon whose frequency is 5 × 1014 Hz (or wavelength 600 nm).
The energy change involved in the electron transition that occurs in the cupric ion is:
When we say that the hydrated cupric ion is blue, we mean that each ion absorbs a photon
wavelength of about 600 nm (orange light), the transmitted light appears blue to our eyes.
Δ𝐸 = ℎ𝜈 = (6.63 × 10−34
Js)(5 × 1014 s−1) = 3 10−19 J
Let’s take another example …..

17. DR. Narinderjit Kaur
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Color of CuSO4.5H2O
Cu2+
ion = d9
system.
Spectrum of d9
shows same behaviour in CFS as d1
But it is viewed as hole formulism means Cu2+
has
spherically symmetrical d10
system with a hole or missing
electron.
E = hν
eg
t2g
eg
t2g
Energy difference
corresponds orange
light
the transmitted light appears blue to our eyes

18. DR. Narinderjit Kaur
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Concept Check
Which of the following are expected to
form colorless octahedral compounds?
Ti4+ Cr3+ Mn2+
Fe2+ Fe3+ Co2+
Co3+
Ni2+ Cu+
Cu2+ Zn2+ Ag+

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