Color of Transition
Metal Ions in Water
Solution
Dr Dragica Minić
August 17, 2005
1
Position of Transition Metals in the Periodic Table
The elements in the periodic table are often divided into four categories:
The transition metals are the metallic elements that serve as a
bridge, or transition, between the two sides of the table.
2

Let me consider the first transition metal series, elements from
scandium (Z=21) to copper (Z=29). This metals either have
incompletely filled d subshells or readily give rise to cations that
have incompletely filled d subshells.

Along this series the added electrons are placed in the 3d orbitals
according to Hund's rule: [Ar]4s 2 3d1 to [Ar]4s1 3d9 .
1. irregulation:
electron configuration of chromium (Z=24) is [Ar] 4s1 3d5
and not [Ar] 4s2 3d 4 ;
2. irregulation:
electron configuration of copper (Z=29) is [Ar] 4s1 3d10
and not [Ar] 4s 2 3d9 .
3
 The reason
is that slightly greater stability
is associated with the half10
5
filled 3d and completely filled 3d
subshells.
 Electrons in the same subshells have equal energy but different
spatial distribution. Consequently, their shielding of one another is
relatively small and the electrons are more strongly attracted by the
nucleus when they have the 3d 5 configuration.
 Consequently the orbital diagram for Cr is
 and orbital diagram for Cu is
4
Transition metals have a distinct tendency to form complex ions
often with neutral molecules water. Many transition metal ions
and complex ions and anions containing transition metals are
distinctively colored. The origin of the color is electronic
transition involving d electrons.
In anhidrous form CoCl2 is blue
but in the hydrated form CoCl2 6H 2 O is pink.
5
Colors of some of the first-row transition metal ions in water
solution:
3+
3+
2+
2+
2+
2+
2+
Ti , Cr , Mn , Fe , Co , Ni , Cu .
6
Colors of aqueous solutions of compounds containing vanadium in
different oxidations states (V, IV, III and II).
7
 For explanation of the color transition metals ions in water
solution existing in complex form, we need to consider the
bonding in complex ions of transition metals;
There are three theories:
 The valence-bond theory
 The crystal field theory
 The ligand-field theory.
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 Crystal Field Theory
 This theory tried to describe the effect of the electrical field of
neighboring ions on the energies of the valence orbitals of an ion in a
crystal.
 Question:
 What effect will the surrounding ligands have on the energies five d
orbitals of metal’s atoms?
 Answer:
 d orbitals of ligands have various orientations and in absence
external disturbance have the same energy.
 When such metal ion is in the center of octahedron surrounding by
six lone pair of electrons of ligands, two types of electrostatic
interaction exist:
 The attraction between positive metal ion and negatively charged
ligand (this force holds the ligands to metal in complex).
 The repulsion between lone pairs on ligand and electron in d
orbitals metals.
9
 Electron in 3d x 2 and 3d x2  y2 orbitals lying along z, x, and y axes,
experience a greater repulsion from ligands than electrons in other
orbitals:
10
 and as result the equality energy of 5 d orbitals is nullified to give
two high-lying levels and three low-lying energy levels. The energy
difference between two sets d orbitals,
is called crystal field

splitting; its magnitude depends
on the metal and the nature of the
ligands.
11
 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, absence of lightwaves
appears black.
12
Relationship between wavelenght absorbed and color observed
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.
13
Quantum-mechanical
description
 Absorption of light may occur
when the frequency of the
incoming photon, multiplied by
the Plank constant, is equal to
the difference in energy between
these two levels.
14
Example:
Cu[H 2O]6
2+
 Hydrated cupric ion absorbs photon whose frequency is about 5 1014
Hz or 600 nm.
 The energy change involved in the electron transition that
occurs in the cupric ion is:
E  h  (6.63 1034 J s)(5 1014 s-1 )  3 10-19 J
 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.
15
Example
Ti[H 2O]3+
6
3+
 Ti has an outer configuration of 4s 2 3d 2 , so that Ti will be a
d1 ion. This means that in its ground state, one electron will
occupy the lower group of d orbitals, and the upper group will be
empty, after absorption of energy the lower groups d orbitals will be
empty.
16
Ti[H 2O]3+
6 ion absorbs light in the visible region; the wavelenght
corresponding to maximum absorption is 498 nm.
Crystal field splitting is:
(6.63 10-34 Js)(3 108 m/s)
-19
  h 


3.99
10
J=240 kJ/mol
-9

498 10 m
hc
This is the energy required to excite one
Ti[H 2O]3+
6
ion.
17
The d-orbital splitting in this case is 240 kJ per mole which
corresponds to light of blue-green 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[H 2O]3+
with white light, the blue-green light is
6
absorbed and the solution appears violet in color.
18
Position of Transition Metals in the Periodic Table
The elements in the periodic table are often divided into four categories:
The transition metals are the metallic elements that serve as a
bridge, or transition, between the two sides of the table.
19