Transition Elements - Origin Of Colour

ORIGIN OF COLOUR

IN

TRANSITION METAL COMPLEXESFOR

CAPE UNIT 1 STUDENTS

INCLUDES THE 6 MAIN POINTS TO GET YOU FULL MARKS IN ANY CAPE QUESTION.

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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

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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. genergetically 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 gcentral 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 pdepends 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 ( ) gto 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 gywill 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

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Origin Of Colour

The above orbital diagram depicts the coloursThe above orbital diagram depicts the colours absorbed in the excitation of the d electron tothe higher level

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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.

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2g g


Origin Of Colour

Ab ti t h th l thAbsorption spectra show the wavelengths absorbed by a given metal ion withdifferent ligands and by different metal ions with the same ligand. From suchgdata, we relate the energy of the absorbed light to the Δ values, and two importantlight to the Δ values, and two importantobservations 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.

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2 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 yCr3+ complex ions below.

A change in even a single ligand can influence the colour. The [Cr(NH3)6]3+ ion is yellow‐

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3 6orange (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, itwill be coloured. Sc3+ and Ti4+ have d0structures 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 orbitalsinto two groups of higher energy andlower energy.gy



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



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 ythe 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


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Transition Elements - Origin Of Colour