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Transition metal complexes: colour 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 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- 2 Square planar Pt(NH3)2Cl2 Linear [Ag(NH3)2]+ Predict the shape of the following complexes – [Co(NH3)5Cl]2+ – [Cr(H2O)4Cl2]+ – [FeCl4]- 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]4K4[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 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? • A solution appears coloured because certain wavelengths of visible light are absorbed by the solution Colour 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 λ (nm) Red Orange Yellow Green 650 600 550 500 Green Blue Dark blue Red 500 450 430 650 Blue Violet 450 400 Yellow Green-yellow 600 560 COLOURED IONS a solution of copper(II)sulphate is blue because red and yellow wavelengths are absorbed COLOURED IONS a solution of copper(II)sulphate is blue because red and yellow wavelengths are absorbed COLOURED IONS a solution of nickel(II)sulphate is green because violet, blue and red wavelengths are absorbed Visible spectroscopy (Absorption spectroscopy) • A spectrophotometer measures the absorption of light at different wavelengths Quartz mirror splits light beam in two Light source M Sample PM Blank PM Analyser 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 Recorder What happens when light is absorbed? • Electrons are excited to a higher energy level What happens when light is absorbed? INCREASING ENERGY / DISTANCE FROM NUCLEUS 4f 4 IRON 4d 4p 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 What happens when light is absorbed? xy xz yz There are 5 different orbitals of the d variety x2-y2 z2 3d ORBITALS 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 OCTAHEDRAL TETRAHEDRAL 3d 3d 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. 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 or d10 (full) Cu+,Ag+ Zn2+ d0 (empty) Sc3+ configuration are colourless e.g. titanium(IV) oxide TiO2 is white colour depends on ... transition element oxidation state ligand coordination number 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?
