Title: Lesson 7 Colour Complexes and Catalysts

Learning Objectives:

• Understand the origin of colour in transition metal complexes• Understand the uses of transition metals as catalysts

TRANSITION METALS

Explaining colour

Using the key words: Absorbed, transmitted and reflected explain the colours in each of the following:

The Visible Spectrum

The colour of substance is determined by:

• Which colour(s) of light it absorbs• Which colour(s) it transmits or reflects (the complementary colours(s))

This Iron compound appears yellow because is absorbs the blue part of the spectrum. Yellow is opposite blue on the colour wheel above.

MnCl2 FeCl2

FeCl2 FeCl3

FeCl3 Fe(NO3)3

Use these 6 pictures to work out the three factors that affect the colour of a transition metal compound…

[Cu(H2O)6]2+(aq) [Cu(OH)2(H2O)4](s) [Cu(NH3)4(H2O)2]2+(aq)

Explaining the colour-ligand relationship

2NH3 4NH3

[Cu(H2O)6]2+(aq) [Cu(OH)2(H2O)4](s) [Cu(NH3)4(H2O)2]2+(aq)

Explaining the colour-ligand relationship

RBG

[Cu(H2O)6]2+(aq) [Cu(OH)2(H2O)4](s) [Cu(NH3)4(H2O)2]2+(aq)

Explaining the colour-ligand relationship

RBG BG

[Cu(H2O)6]2+(aq) [Cu(OH)2(H2O)4](s) [Cu(NH3)4(H2O)2]2+(aq)

Explaining the colour-ligand relationship

RBG RBG BG

[Cu(H2O)6]2+(aq) [Cu(OH)2(H2O)4](s) [Cu(NH3)4(H2O)2]2+(aq)

Explaining the colour-ligand relationship

RBG RBG BG B

[Cu(H2O)6]2+(aq) [Cu(OH)2(H2O)4](s) [Cu(NH3)4(H2O)2]2+(aq)

Explaining the colour-ligand relationship

RBG RBG BG B

[Cu(H2O)6]2+(aq) [Cu(OH)2(H2O)4](s) [Cu(NH3)4(H2O)2]2+(aq)

Explaining the colour-ligand relationship

RBG RBG BG B

[Cu(H2O)6]2+(aq) [Cu(OH)2(H2O)4](s) [Cu(NH3)4(H2O)2]2+(aq)

Explaining the colour-ligand relationship

RBG RBG BG B

[Cu(H2O)6]2+(aq) [Cu(OH)2(H2O)4](s) [Cu(NH3)4(H2O)2]2+(aq)

Explaining the colour-ligand relationship

RBG RBG BG B

Describe what happens to the average frequency of visible light absorbed as you increase the number of NH3 ligands…

[Cu(H2O)6]2+(aq) [Cu(OH)2(H2O)4](s) [Cu(NH3)4(H2O)2]2+(aq)

Explaining the colour-ligand relationship

RBG RBG BG B

Describe what happens to the average frequency of visible light absorbed as you increase the number of NH3 ligands…

[Cu(H2O)6]2+(aq) [Cu(OH)2(H2O)4](s) [Cu(NH3)4(H2O)2]2+(aq)

Explaining the colour-ligand relationship

RBG RBG BG B

Describe what happens to the average frequency of visible light absorbed as you increase the number of NH3 ligands…

The average frequency of visible light absorbed INCREASES when you substitute H2O ligands with NH3 ligands.

Ligand field theory

Ligand field theory

The negative charge due to the lone pair affects the orbitals energy differently

Ligand field theory

The negative charge due to the lone pair affects the orbitals energy differentlyWhen ligands approach orbitals that have lobes along the axes the energy is raised

Ligand field theory

The negative charge due to the lone pair affects the orbitals energy differentlyWhen ligands approach orbitals that have lobes along the axes the energy is raised

Ligand field theory

The negative charge due to the lone pair affects the orbitals energy differentlyWhen ligands approach orbitals that have lobes along the axes the energy is raised When ligands approach orbitals that have lobes between the axes the energy is lowered

Ligand field theory

The negative charge due to the lone pair affects the orbitals energy differentlyWhen ligands approach orbitals that have lobes along the axes the energy is raised When ligands approach orbitals that have lobes between the axes the energy is lowered

SUMMARY•When the 5 d-orbitals are free of ligands they are of equal energy (degenerate)•When the d-orbitals are surrounded by ligands the energy is split.•Two orbitals are higher in energy and three orbitals are lower.

Ligand field theory

The negative charge due to the lone pair affects the orbitals energy differentlyWhen ligands approach orbitals that have lobes along the axes the energy is raised When ligands approach orbitals that have lobes between the axes the energy is lowered

The average frequency of visible light absorbed INCREASES when you substitute H2O ligands with NH3 ligands.

The average frequency of visible light absorbed INCREASES when you substitute H2O ligands with NH3 ligands.

The average frequency of visible light absorbed INCREASES when you substitute H2O ligands with NH3 ligands.

What is the electron configuration of a Cu2+

ion? (spdf notation)

The average frequency of visible light absorbed INCREASES when you substitute H2O ligands with NH3 ligands.

The average frequency of visible light absorbed INCREASES when you substitute H2O ligands with NH3 ligands.

The average frequency of visible light absorbed INCREASES when you substitute H2O ligands with NH3 ligands.

The average frequency of visible light absorbed INCREASES when you substitute H2O ligands with NH3 ligands.

The average frequency of visible light absorbed INCREASES when you substitute H2O ligands with NH3 ligands.

Ammonia is a stronger base that water. Predict the effect that this will have on the energy difference between the split orbitals…

The average frequency of visible light absorbed INCREASES when you substitute H2O ligands with NH3 ligands.

The average frequency of visible light absorbed INCREASES when you substitute H2O ligands with NH3 ligands.

The average frequency of visible light absorbed INCREASES when you substitute H2O ligands with NH3 ligands.

The average frequency of visible light absorbed INCREASES when you substitute H2O ligands with NH3 ligands.

What happens next? Many sources explain that the electron de-excites and re-emits light. The problem with this is that the same frequency of light would be emitted as was absorbed in the first place and no net absorption would take place so the compound would be colourless. Other mechanisms of de-excitation are being investigated such as collisional de excitation

The average frequency of visible light absorbed INCREASES when you substitute H2O ligands with NH3 ligands.

The average frequency of visible light absorbed INCREASES when you substitute H2O ligands with NH3 ligands.

Ener

gy

The average frequency of visible light absorbed INCREASES when you substitute H2O ligands with NH3 ligands.

The average frequency of visible light absorbed INCREASES when you substitute H2O ligands with NH3 ligands.

The average frequency of visible light absorbed INCREASES when you substitute H2O ligands with NH3 ligands.

The average frequency of visible light absorbed INCREASES when you substitute H2O ligands with NH3 ligands.

Ener

gy

The average frequency of visible light absorbed INCREASES when you substitute H2O ligands with NH3 ligands.

The average frequency of visible light absorbed INCREASES when you substitute H2O ligands with NH3 ligands.

Absorbed low frequency Absorbed high frequency

The average frequency of visible light absorbed INCREASES when you substitute H2O ligands with NH3 ligands.

Absorbed low frequency Absorbed high frequency

General rule of colour of aqueous octahedral complexes

The more ligand molecules that are stronger lewis bases means...The colour shifts towards the high frequency / high energy end of the spectrum

The energy separation between orbitals is ΔE so the colour of the complex depends on the following factors:

• Nuclear charge and identity of the central metal ion• Charge density of the ligand• Geometry of the complex ion (this effects the electric field)• Number of d electrons present and hence the oxidation number of the central ion

Colour depends on nuclear charge and identity of the central metal ion

• Strength of the coordinate bond depends on the attraction of the lone pair of electrons and the nuclear charge of the central ion. (More effective with ions of a higher nuclear charge)

E.g. [Mn(H2O)6]2+ and [Fe(H2O)6]3+ = Same electron configuration but Iron has a higher nuclear charge Water ligand bonds stronger.

Charge density of ligand• Greater charge density will cause a larger split in the d orbitals. (Look back the diagram with additional Ammonia ligands (higher charge density)

The spectrochemical series arranges ligands according to the energy separation, ΔE, between the two sets of d orbitals.

• Wavelength at which maximum energy absorbance occurs, , decreases with charge density of the ligand

• Lowest charge density is I-, so repels the d electrons the least = small d orbital splitting.

•Electrons in p orbitals on carbon atoms can interact with d orbitals of the transition metals.

• The spectrochemical series can be found in the data booklet section 15.

Geometry of the complex

• Co-ordination number and geometry can affect colour of the complex.

• E.g. The Cobalt complex below goes from pink [Co(H2O)6]2+ to blue [CoCl4]2- when HCl is added. The chloride ions displace the water forming a new complex ion. It can be reversed by adding water.

Number of d electrons and oxidation state of the central metal ion

Number of d electrons and oxidation state of the metal determines:

•The strength of the interaction between the ligand and the central metal ion•The amount of electron repulsion between the ligand and the d electrons

Example question

Key Points

• The formation of complexes causes d-orbitals to split into two energy levels– Electron transitions between these energy levels

give rise to their colour

• Transition metals are hugely important for their catalytic properties