Complexes and Coloured Ions Do Now: How do we see colour? What do we mean by complimentary colour? know that transition metal ions can be identified by their colour, limited to the complexes in this unit know that colour changes arise from changes in oxidation state, co-ordination number and ligand know that colour arises from electronic transitions from the ground state to excited states: ∆E = hν appreciate that this absorption of visible light is used in spectrometry to determine the concentration of coloured ions

Formation of Coloured ions Visible spectrum formed by EM radiation in a range of wavelengths between about 380nm (blue end) and 760nm (red end). White light – all colours of the spectrum Substance appears coloured if it ABSORBS some of the colours in white light. Colours that we see are COMPLIMENTARY colours (colours remaining after absorption of other colours by a substance). TM can be identified by their colour. You need to know: 1. How colour arises 2. Colours of the ions on the specification

What is white light? Traditional answer – made up of the 7 colours of the rainbow However… Seven is just an arbitrary number Made up of infinite number of colours!

Absorbs all light except red and purple How we see colours ... Appears purple Most TM compounds are coloured The colour cause by compounds ABSORBING energy that corresponds to light in visible part of the spectrum If a compound appears purple, it absorbs all of the light from a white beam of light shome on it EXCEPT red and blue Red and blue pass through Solution appears purple Absorbs all light except red and purple

Complimentary Colours Shown opposite each other When white light passes through a solution containing a transition metal complex, some of the wavelengths of light are absorbed by the complex. The light emerging will therefore contain proportionally more of the complimentary colour. E.g. if a complex absorbs red light, the light emerging will look blue or green.

Zinc Sulfate (aq) vs. Copper (II) Sulfate (aq)

3d energy levels

Why are Transition Metal Complexes Coloured? TM have part filled d-orbitals. So the e- can move from one d-orbital to another. In a TM ATOM on its own (isolated), all of the d-orbitals are of EXACTLY the same energy BUT in a COMPOUND, the presence of ligands makes the d-orbitals have slightly different energies. When an e- moves from one d-orbital to another of a higher energy level, they often ABSORB energy. This is known as ‘promotion’ or ‘excitation’ The amount of energy absorbed depends on the difference in the energy levels The bigger the energy, difference, the more energy the electron absorbs The amount of energy absorbed is directly proportional to the frequency of the absorbed light and inversely proportional to the wavelength of light For Copper 2+, the small difference means a low frequency and a high wavelength This corresponds to red on the visible light spectrum So blue light is transmitted Draw diagrams to support this explanation.

Electronic Transitions e- occupy particular energy levels in atoms When an atom ABSORBS energy, an e- can be promoted from its normal ground state to a higher energy level The e- is then in an excited state Such changes from one energy level to another = electronic transitions

Questions Look at the colour wheel to help answer the following questions: Fe3+ (aq) ions look yellow. What is the complimentary colour? How would you describe the difference in energy levels? Describe the frequency and wavelength of the radiation absorbed

Different types of reactions

What do we mean by the following? Redox reaction Acid-Base reaction Ligand exchange Coordination number change

Change in oxidation number

Formation of Coloured ions The size of the energy gap between the d-orbitals, and so the colour is affected by changes in: 1) the oxidation state [Fe(H2O)6]3+ [Fe(H2O)6]2+ Pale Green Yellow/Brown For a given metal, the higher the ox state of its ions, the greater the amount of d sub-shell splitting This in creases the energy level difference and so changes the colour of the light absorbed.

Acid-base and ligand exchange

Acid-Base Reaction [Cu(H2O)6]2+ + 2OH-  ? [Cu(H2O)6]2+ + 2NH3  ? [Cu(H2O)4(OH)2] + 4NH3 

Change In coordination number

Copper (II) sulfate solution and conc. Hydrochloric acid

Copper (II) sulfate solution and conc. Hydrochloric acid

Exam questions