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1 of 43 © Boardworks Ltd 2010 2 of 43 © Boardworks Ltd 2010 How do we see colour? Most transition metal compounds appear coloured. This is because they absorb energy corresponding to certain parts of the visible electromagnetic spectrum. The colour that is seen is made up of the parts of the visible spectrum that aren’t absorbed. For example, a red compound will absorb all frequencies of the spectrum apart from red light, which is transmitted. 3 of 43 © Boardworks Ltd 2010 What happens when light is absorbed? In transition metal ions, the d sub-level is only partially filled. This means that electrons can move between d orbitals. In a transition metal complex, the relative energies of the d orbitals change. Electrons can be promoted to higher energy orbitals. For electrons to be promoted, they need to absorb light energy of a particular frequency. This frequency depends on the precise difference in energy between the d orbitals. 4 of 43 © Boardworks Ltd 2010 Why do d orbitals change in energy? 5 of 43 © Boardworks Ltd 2010 Factors affecting colours The colour of a transition metal compound is determined by the difference in energy between its d orbitals. This can be affected by several factors: size and type of ligands coordination number strength of metal–ligand bonds oxidation state. complex shape [Cr(H2O)6]3+ [Ni(H2O)6]2+ [Fe(H2O)6]2+ 6 of 43 [Fe(H2O)6]3+ © Boardworks Ltd 2010 Colours of complexes A transition metal will appear different colours in complexes with different ligands. For example: [Cu(H2O)6]2+ 7 of 43 [CuCl4]2– © Boardworks Ltd 2010 Complex colours: true or false? 8 of 43 © Boardworks Ltd 2010 Determining concentration Ultraviolet–visible spectroscopy can be used to determine the concentration of a transition metal complex solution. A UV-Vis spectrometer passes different frequencies of light through a sample. Some of the light is absorbed while the rest passes through. A detector measures the absorbance of the sample. The amount of light absorbed is proportional to the concentration of the absorbing species. 9 of 43 © Boardworks Ltd 2010 UV–Vis spectroscopy 10 of 43 © Boardworks Ltd 2010 Improving UV–Vis spectroscopy 1 Some transition metal complex ions have a very pale colour. Accurate quantitative determination of the concentration of these solutions is difficult, because the difference in absorption between the different concentrations is too small. Replacing the ligands in a complex changes its colour. This can increase the absorption of the transition metal species, allowing lower concentrations to be analysed using UV–Vis spectroscopy. 11 of 43 © Boardworks Ltd 2010 Improving UV–Vis spectroscopy 2 [Fe(H2O)6]2+ is a pale green colour. If the water ligands are replaced by other types of ligands, a much stronger coloured solution is produced. The absorption values increase by 103, allowing analysis of lower concentrations. [Fe(H2O)6]2+ + 3bipy 12 of 43 [Fe(bipy)3]2+ + 6H2O © Boardworks Ltd 2010 13 of 43 © Boardworks Ltd 2010 Hydrolysis reactions Transition metal salts dissolve in water to form aqua ions – complexes with water molecule ligands. Some of these aqueous complexes are acidic, for example [Fe(H2O)6]3+. This is because the Fe3+ ion is strongly polarizing and weakens the O–H bonds in the water ligands. The complex ion releases an H+ ion, producing an acidic solution. The reaction is called hydrolysis because the water molecule is being split. The general equation for this reaction is: [M(H2O)6]3+ + H2O 14 of 43 [M(H2O)5OH]2+ + H3O+ © Boardworks Ltd 2010 Why are some aqua ions less acidic? A solution of [Fe(H2O)6]3+ is highly acidic, whilst a solution of [Fe(H2O)6]2+ is only very weakly acidic. This is because the polarizing power of the metal ion depends on its size and charge. Smaller, more highlycharged metal ions exert a greater polarizing effect on the water ligands, so that more O–H bonds break, releasing H+ ions. As a general rule, M3+ ions are significantly more acidic than M2+ ions. 15 of 43 © Boardworks Ltd 2010 Order the complex ions by acidity 16 of 43 © Boardworks Ltd 2010 Hydrolysis of M2+ ions A series of hydrolysis reactions can occur until the overall charge on a complex is 0. For example, in transition metal 2+ ions, two H+ ions are removed. [M(H2O)6]2+(aq) + H2O(l) [M(H2O)5OH]+(s) + H3O+(aq) [M(H2O)5OH]+(s) + H2O(l) [M(H2O)4(OH)2](s) + H3O+(aq) 17 of 43 © Boardworks Ltd 2010 Hydrolysis of M3+ ions Three H+ ions are removed from transition metal 3+ ions. [M(H2O)6]3+(aq) + H2O(l) [M(H2O)5OH]2+(aq) + H3O+(aq) [M(H2O)5OH]2+(aq) + H2O(l) [M(H2O)4(OH)2]+(aq) + H3O+(aq) [M(H2O)4(OH)2]+(aq) + H2O(l) [M(H2O)3(OH)3](s) + H3O+(aq) 18 of 43 © Boardworks Ltd 2010 Reactions with bases If a base such as ammonia or hydroxide ions is added to a solution of transition metal aqua ions, H+ ions are removed from the water ligands until there is no overall charge on the complex. The final product is an uncharged, insoluble metal hydroxide that forms a precipitate. This reaction occurs in a series of steps, depending on whether the metal is a 2+ or 3+ ion. 19 of 43 © Boardworks Ltd 2010 Reactions with carbonate ions Sodium carbonate acts as a base with M3+ aqua ions to produce a hydrated metal hydroxide. 2[Cr(H2O)6 ]3+ (aq) + 3CO3 2– (aq) 2[Cr(H2O)3(OH)3](s) + 3CO2(g) + 3H2O(l) However, adding sodium carbonate to M2+ ions produces an insoluble metal carbonate. The M2+ ions are less acidic, and carbonate ions are unable to remove protons from the water ligands, so they displace the ligands instead. [Cr(H2O)6]2+(aq) + CO32–(aq) 20 of 43 CrCO3(s) + 6H2O(l) © Boardworks Ltd 2010 Adding bases to complexes 21 of 43 © Boardworks Ltd 2010 Balance the equations 22 of 43 © Boardworks Ltd 2010 Amphoteric character Some metal hydroxides can react as both an acid and as a base and are known as amphoteric. For example, chromium hydroxide will react in the following ways: With strong acids: Cr(H2O)3(OH)3 + 3H3O+ [Cr(H2O)6]3+ + 3H2O With strong, excess alkali: Cr(H2O)3(OH)3 + 3OH– 23 of 43 [Cr(OH)6]3– + 3H2O © Boardworks Ltd 2010 Reaction definitions 24 of 43 © Boardworks Ltd 2010 25 of 43 © Boardworks Ltd 2010 Ligand substitution A ligand substitution reaction occurs when a ligand in a complex ion is replaced by another type of ligand molecule. When concentrated hydrochloric acid is added to a solution of hexaaquacopper(II), chloride ions replace the water molecules as ligands. [Cu(H2O)6]2+ + 4Cl– [CuCl4]2– + 6H2O Chloride ions are much larger than water molecules so only four can fit around the copper ion. This means that the complex changes shape from octahedral to tetrahedral. The colour of the complex changes from blue to green. 26 of 43 © Boardworks Ltd 2010 Ligand substitution reactions 27 of 43 © Boardworks Ltd 2010 Reactants and conditions 28 of 43 © Boardworks Ltd 2010 Stability constants Hexaaquacopper(II) undergoes ligand substitution with ammonia in stages. At each stage, one water molecule is replaced by one ammonia molecule, until four molecules of water have been replaced by four molecules of ammonia. Each of these stages is an equilibrium reaction. [Cu(H2O)6]2+ + NH3 [Cu(NH3)(H2O)5]2+ + H2O [Cu(NH3)(H2O)5]2+ + NH3 [Cu(NH3)2(H2O)4]2+ + H2O [Cu(NH3)2(H2O)4]2+ + NH3 [Cu(NH3)3(H2O)3]2+ + H2O [Cu(NH3)3(H2O)3]2+ + NH3 [Cu(NH3)4(H2O)2]2+ + H2O 29 of 43 © Boardworks Ltd 2010 Stability constants For each stage of a reaction, an expression for the equilibrium constant can be written. This equilibrium constant for the overall reaction is called the stability constant: Kstab. [Cu(H2O)6]2+(aq) + 4NH3(aq) Kstab = [Cu(NH3)4(H2O)2]2+(aq) + 4H2O(l) [Cu(NH3)4(H2O)22+] [Cu(H2O)62+] + [NH3]4 Note that the square brackets now mean concentration in mol dm-3. The concentration of water is left out because it is in great excess and its concentration is almost constant. 30 of 43 © Boardworks Ltd 2010 Comparing stability constants The stability constant, Kstab, is the equilibrium constant for the formation of a complex ion from its constituent ions in solution. Kstab values show how stable a complex ion is. Complex ions with large Kstab values are easily formed. Whether a ligand substitution will occur can be predicted from Kstab values, by comparing that of the current complex ion with the value for the substituted complex ion. The most stable ion is most likely to occur. 31 of 43 © Boardworks Ltd 2010 The chelate effect Complex ions containing multidentate ligands such as EDTA are called chelates. They have much larger Kstab values and are far more stable than complex ions containing unidentate ligands. This is because of the effect of entropy. When a multidentate ligand replaces unidentate ligands in a complex, it releases many molecules, increasing the entropy. The reverse reaction involves a large decrease in entropy, which is why it is so unfavourable. [M(H2O)6]2+ + EDTA4– 2 species 32 of 43 [M(EDTA)]2– + 6H2O 7 species © Boardworks Ltd 2010 Predicting ligand substitution 33 of 43 © Boardworks Ltd 2010 Haemoglobin Haemoglobin is the molecule that causes blood to appear red. It carries oxygen from the lungs to cells in the body. Haemoglobin contains an Fe2+ ion which forms a haem complex with a tetradentate ligand called porphyrin. It also binds to a unidentate globin molecule. One coordination site is left that can bind loosely to an oxygen molecule. Oxygen is a poor ligand that is easily released to cells, where its concentration is low. Ligands that can form stronger bonds with the Fe2+ ion, such as carbon monoxide, bind irreversibly and destroy haemoglobin’s ability to carry oxygen. These substances are toxic. 34 of 43 © Boardworks Ltd 2010 35 of 43 © Boardworks Ltd 2010 Redox reactions Transition metals are able to exist in many different oxidation states, which is why they often undergo redox reactions. Oxidation of transition metals occurs most easily in alkaline solution. This is because negative ions tend to form in alkaline solution and it is easier to lose electrons from a negatively-charged species. Reduction of transition metals occurs most easily in acidic solution. 36 of 43 © Boardworks Ltd 2010 Oxidation of cobalt(II) In ammoniacal solution, Co2+ is oxidized to Co3+ by oxygen in the air. Several reactions occur because ammonia acts as both a base and a ligand: [Co(H2O)6]2+ + 2OH– [Co(H2O)4(OH)2] + 6NH3 4[Co(NH3)6]2+ + O2 + 2H2O [Co(H2O)4(OH)2] + 2H2O [Co(NH3)6]2+ + 2OH– + 4H2O 4[Co(NH3)6]3+ + 4OH– Co2+ can also be oxidized by hydrogen peroxide (H2O2) after adding an alkali such as sodium hydroxide. 2[Co(OH)6]4– + H2O2 37 of 43 2[Co(OH)6]3– + 2OH– © Boardworks Ltd 2010 Reduction of chromium(VI) In aqueous solution, chromium has an oxidation state of 6+. It exists in alkaline solution as CrO42– and as Cr2O72– in acidic solution. 2CrO42– + 2H+ Cr2O72– + H2O Chromium(VI) can be reduced to Cr3+ and Cr2+ by zinc in acid solution . Cr2+ is easily oxidized to Cr3+ in the presence of oxygen, but hydrogen is produced during the reduction, which excludes air. Cr2O72– + 14H+ + 3Zn Zn + 2Cr3+ 38 of 43 2Cr3+ + 7H2O + 3Zn2+ Zn2+ + 2Cr2+ © Boardworks Ltd 2010 Identify the reaction 39 of 43 © Boardworks Ltd 2010 40 of 43 © Boardworks Ltd 2010 Glossary 41 of 43 © Boardworks Ltd 2010 What’s the keyword? 42 of 43 © Boardworks Ltd 2010 Multiple-choice quiz 43 of 43 © Boardworks Ltd 2010