1. Transition metal complexes: colour

2. Transition metal complexes
Objectives:
Describe common shapes of transition metal complexes
Explain why transition metal complexes are coloured
Outcomes:
Revise understanding of the terms complex and ligand
Describe the shape of some specific transition metal complexes
Explain why solutions appear coloured
Use a visible spectrum to deduce the colour of a complex ion in solution

3. CO-ORDINATION NUMBER & SHAPE
the shape of a complex is governed by the number of ligands around the central ion
the co-ordination number gives the number of ligands around the central ion
a change of ligand can affect the co-ordination number
Co-ordination No. Shape Example(s)
6 Octahedral [Cu(H2O)6]2+
4 Tetrahedral [CuCl4]2-
Square planar Pt(NH3)2Cl2
2 Linear [Ag(NH3)2]+

4. Predict the shape of the following complexes
[Co(NH3)5Cl]2+
[Cr(H2O)4Cl2]+
[FeCl4]-

5. Check your understanding
For each of the following complexes, give the charge on the central metal ion and its coordination number and its name
[Co(NH3)6]3+
[Cu(NH3)4(H2O)2]2+
CuCl42-
[Fe(CN)6]4-
K4[Fe(CN)6]
Na2CoCl4
[Co(NH3)6]Cl3
[Cu(CN)2]-
What shape is each of the complex ions likely to be?
Define each of the following terms, using the complex [Ni(CN)4]2- to illustrate your answer:
Ligand
Coordination number
Shape

6. Complexes and colour
Complex formation is often accompanied by a change in colour.
Example: Adding ammonia to aqueous copper (II) sulphate: Light blue to deep blue colour change
Q: How does colour arise?

7. Colour
A solution appears coloured because certain wavelengths of visible light are absorbed by the solution
blue and green not absorbed
white light
a solution of copper(II)sulphate is blue because
red and yellow wavelengths are absorbed
Absorbed colour
λ (nm)
Observed colour
Red
650
Green
500
Orange
600
Blue
450
Yellow
550
Dark blue
430
Violet
400
Green-yellow
560

8. COLOURED IONS a solution of copper(II)sulphate is blue because
red and yellow wavelengths are absorbed

9. COLOURED IONS a solution of copper(II)sulphate is blue because
red and yellow wavelengths are absorbed

10. COLOURED IONS a solution of nickel(II)sulphate is green because
violet, blue and red wavelengths are absorbed

11. Visible spectroscopy (Absorption spectroscopy)
A spectrophotometer measures the absorption of light at different wavelengths
Blank
Analyser M
Sample PM
Recorder
Quartz mirror splits light beam in two
Light source
M = monochromator: selects wavelength
PM = photomultiplier: converts light into electric current
Analyser: Compares two electric currents. Any difference is dependent on the absorption of light by the sample
Recorder: pen traces absorption spectrum

12. What happens when light is absorbed?
Electrons are excited to a higher energy level

13. INCREASING ENERGY / DISTANCE FROM NUCLEUS
What happens when light is absorbed? 4 4p 4d 4f
IRON
INCREASING ENERGY / DISTANCE FROM NUCLEUS 3d 4s 3 3p
1s2 2s2 2p6 3s2 3p6 4s2 3d6
In an isolated atom or ion, the isolated d orbitals have the same energy

14. There are 5 different orbitals of the d variety
What happens when light is absorbed? xy xz yz
There are 5 different orbitals of the d variety
x2-y2 z2
3d ORBITALS

15. SPLITTING OF 3d ORBITALS
Placing ligands around a central ion causes the energies of the d orbitals to change
Some of the d orbitals gain energy and some lose energy
In an octahedral complex, two (z2 and x2-y2) go higher and three go lower
In a tetrahedral complex, three (xy, xz and yz) go higher and two go lower
Degree of splitting depends on the CENTRAL ION and the LIGAND
The energy difference between the d orbitals corresponds to the frequency of visible light.
The energy difference between the levels affects how much energy is absorbed when an electron is promoted. The amount of energy governs the colour of light absorbed.
OCTAHEDRAL
TETRAHEDRAL 3d 3d

16. What happens when light is absorbed?
Theory
ions with a d10 (full) or d0 (empty) configuration are colourless
(there needs to be at least 1 electron which can be excited AND there needs to be an empty d orbital which can be occupied when the electron is excited)
ions with partially filled d-orbitals tend to be coloured
it is caused by the ease of transition of electrons between energy levels
energy is absorbed when an electron is promoted to a higher level
the frequency of light is proportional to the energy difference
ions with d10 (full) Cu+,Ag+ Zn2+
or d0 (empty) Sc3+ configuration are colourless
e.g. titanium(IV) oxide TiO2 is white
colour depends on ... transition element
oxidation state
ligand
coordination number

17. Plenary Describe two common shapes of transition metal complexes
Explain why solutions are coloured in terms electrons
What does the colour of a transition metal complex depend upon?