Transition Elements - Origin Of Colour

ORIGIN OF COLOUR
IN
TRANSITION METAL COMPLEXES
FOR
CAPE UNIT 1 STUDENTS
INCLUDES THE 6 MAIN POINTS TO GET YOU FULL MARKS IN ANY CAPE QUESTION.
Compiled by Denison at Global in Cunupia. 739-2656.
http://www.facebook.com/CAPEChemistryLessons

YOU WILL LEARN
1. Crystal Field Theory and Ligand Field
Theory of colour.y
2. 6 main points to be used in answering a
CAPE question on the origin of colour in
transition element complexes.p
2Compiled by Denison at Global in Cunupia. 739-2656.

Origin Of Colour
Thi i l i d b th C t l Fi ld ThThis is explained by the Crystal Field Theory
and the Ligand Field Theory.
These theories regard the complex (ion) as an
agglomeration of a central ion surrounded byagglomeration of a central ion surrounded by
other ions or molecules with electrical fields.

Origin Of Colour
Th l t i l fi ld f th t l i ill ff tThe electrical field of the central ion will affect
the surrounding ligands, whilst the combined
field of the ligands will influence the
arrangement of electrons in the central ion.g

Origin Of Colour
Th litti f th fi d bit l f thThe splitting of the five d orbitals of the
central ion into two groups is one of the
fundamental ideas. In a free ion, as in the Fe3+
ion, the five d orbitals are degenerate, i.e. g
energetically alike.

Origin Of Colour
The five d orbitals are not, however, all alike in , ,
shape and that is why they split up into two
energetically different groups under theenergetically different groups under the
influence of the ligand field.

Origin Of Colour
The two groups consist of three orbitals (the g p (
t2g group) and of two orbitals (the eg group).
The extent and nature of the difference in
energy between the two groups depends onenergy between the two groups depends on
the field strength of the ligands and on their
geometrical arrangement around the central
ion.

Origin Of Colour
And the arrangement of electrons in the g
central ion is decided by the energy difference
between the t2 and e groups of orbitalsbetween the t2g and eg groups of orbitals.

Origin Of Colour
The colour of a particular transition metal ion p
depends upon the nature of the ligands
(either neutral molecules such as water which(either neutral molecules such as water which
contain lone pairs or negative ions such as the
chloride ion) bonded to the ionchloride ion) bonded to the ion.

Origin Of Colour
The pale blue hydrated copper(II) ion changes p y pp ( ) g
to dark blue in the presence of ammonia and
to green if sufficient chloride ions are added;to green if sufficient chloride ions are added;
copper(II) chloride solution is therefore either
blue or green depending upon the relativeblue or green, depending upon the relative
concentrations of water molecules and
chloride ions.

Origin Of Colour
The colour of a transition metal ion is
associated with:
(a) An incomplete d level (between 1 and 9
d electrons)d electrons).
(b) The nature of the ligands surrounding the
ion.

Origin Of Colour
A complete theory of colour is very complex
but, put simply, it is due to the movement of
electrons from one d level to another.

Origin Of Colour
Si th fi t 3d bit lSince the five separate 3d orbitals are
orientated differently in space an electron (or
electrons) which is close to a ligand will be
repelled and hence the energy of such orbitals p gy
will be raised relative to the others.

Origin Of Colour
Th d f th 3d l l i th fThe degeneracy of the 3d levels is therefore
destroyed; this is represented pictorially, for
the copper(II) ion, in the following diagram.

Origin Of Colour
The splitting of the 3d orbitals of the Cu2+ ion in an
octahedral environment of water ligands.
It will be seen from the above diagram that
two of the 3d levels are raised in energytwo of the 3d levels are raised in energy
relative to the other three, the energy
difference being ΔE
15
difference being ΔE .

Origin Of Colour
The above orbital diagram depicts the coloursThe above orbital diagram depicts the colours
absorbed in the excitation of the d electron to
the higher level
16
the higher level.

Origin Of Colour
Si th d f litti f th 3d l lSince the degree of splitting of the 3d levels
depends upon the particular ligands
themselves, the variation in colour of ions of a
particular transition metal is explained.p p

Origin Of Colour
This series of octahedral chromium(III) complexes illustrates how the
energy gap between the t2g and eg orbital groups affects colour.
18
2g g

Origin Of Colour
Ab ti t h th l thAbsorption spectra show the wavelengths
absorbed by a given metal ion with
different ligands and by different metal ions
with the same ligand. From suchg
data, we relate the energy of the absorbed
light to the Δ values, and two importantlight to the Δ values, and two important
observations emerge:

Origin Of Colour
1. For a given ligand, the colour depends
th id ti t t f th t l ion the oxidation state of the metal ion.
A solution of [V(H2O)6]2+ ion is violet, and
a solution of [V(H2O)6]3+ ion is yellow.
Solutions of [V(H2O)6]2+ (left) and[V(H2O)6]3+ (right) ions have different colours.
20
2 6 2 6

Origin Of Colour
2. For a given metal ion, the colour
d d th li d E i ldepends on the ligand. Even a single
ligand substitution can have a major
effect on the wavelengths absorbed and,
thus, the colour, as you can see for two y
Cr3+ complex ions below.
A change in even a single ligand can influence the colour. The [Cr(NH3)6]3+ ion is yellow‐
21
3 6
orange (left); the [Cr(NH3)5CI]2+ ion is purple (right).

CAPE has asked this
question only twice in q y
the last 10 years.y

CAPE has asked this
question only twice in q y
the last 10 years.y
(In fact they’ve only asked it twice
in the history of the exams.)

In 2009 it went like this:
“Use the distribution in the d‐orbitals to
account for colour in transition metal ions.f
[2 marks]”

In 2009 it went like this:
“Use the distribution in the d‐orbitals to
account for colour in transition metal ions.f
[2 marks]”
and in 2007 it went like this:and in 2007 it went like this:
“Account for the origin of colour in transitionAccount for the origin of colour in transition
metal complexes. [4 marks]”

NEVER MIND THE DIFFERENCE
IN MARKS ALLOCATEDIN MARKS ALLOCATED.

NEVER MIND THE DIFFERENCE
IN MARKS ALLOCATEDIN MARKS ALLOCATED.
Once you use the following 6 points in your
answer, even if the question counts for 6
marks, you’re getting all of them.y g g

Main Points For Answering A CAPE Question
1 If the ion has partially filled d orbitals it1. If the ion has partially filled d orbitals, it
will be coloured. Sc3+ and Ti4+ have d0
structures and Cu+ and Zn2+ have d10 andstructures, and Cu+ and Zn2+ have d10 and
so are not coloured.

2 d orbitals point in different directions in2. d orbitals point in different directions in
space, and so interact to different extents
with the electrons in the ligandswith the electrons in the ligands.

3. This causes a splitting of the d orbitals
into two groups of higher energy and
lower energy.gy

4. When white light is shone into the
substance, a d electron is moved from
the lower energy to the higher energy gy g gy
level.

5. The frequency of the light that causes
this jump is in the visible range, so that
colour at this frequency is removed from q y
the white light. The colour not removed
is seen as the colour of the complex (ion).is seen as the colour of the complex (ion).

6. The colour depends on the size of the
energy gap which varies with the metal
ion and with the type of ligand.yp g

END
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Compiled By Denison At Global In Cunupia. 739-2656.

Transition Elements - Origin Of Colour

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