KNOCKHARDY PUBLISHING AN INTRODUCTION TO TRANSITION METAL COMPLEXES KNOCKHARDY PUBLISHING

TRANSITION METALS CONTENTS Aqueous metal ions Acidity of hexaaqua ions Introduction to the reactions of complexes Reactions of cobalt Reactions of copper Reactions chromium Reactions on manganese Reactions of iron(II) Reactions of iron(III) Reactions of silver and vanadium Reactions of aluminium

THE AQUEOUS CHEMISTRY OF IONS Theory aqueous metal ions attract water molecules many have six water molecules surrounding them these are known as hexaaqua ions they are octahedral in shape water acts as a lone pair donor water forms a co-ordinate bond to the metal ion metal ions accept the lone pair

HYDROLYSIS OF HEXAAQUA IONS Lewis bases can attack the co-ordinated water molecules. Theoretically, a proton can be removed from each water molecule turning the water from a neutral molecule to a negatively charged hydroxide ion. This affects the overall charge on the complex ion. [M(H2O)6]2+(aq) [M(OH)(H2O)5]+(aq) [M(OH)2(H2O)4](s) [M(OH)2(H2O)4](s) [M(OH)3(H2O)3]¯(aq) [M(OH)4(H2O)2]2-(aq) [M(OH)4(H2O)2]2-(aq) [M(OH)5(H2O)]3-(aq) [M(OH)6]4-(aq) When sufficient protons have been removed the complex becomes neutral and precipitation of a hydroxide or carbonate occurs. e.g. M2+ ions [M(H2O)4(OH)2](s) or M(OH)2 M3+ ions [M(H2O)3(OH)3](s) or M(OH)3 OH¯ H+ OH¯ H+ OH¯ H+ OH¯ H+ OH¯ H+ OH¯ H+

REACTION TYPES • shape • stability to oxidation or reduction The examples aim to show typical properties of transition metals and their compounds. One typical properties of transition elements is their ability to form complex ions. Complex ions consist of a central metal ion surrounded by co-ordinated ions or molecules known as ligands. This can lead to changes in ... • colour • co-ordination number • shape • stability to oxidation or reduction Reaction types LIGAND SUBSTITUTION PRECIPITATION REDOX LS Ppt RED OX REDOX

REACTION TYPES LOOK FOR... The examples aim to show typical properties of transition metals and their compounds. LOOK FOR... substitution reactions of complex ions variation in oxidation state of transition metals the effect of ligands on co-ordination number and shape differences in reactivity of M3+ and M2+ ions with OH¯ and NH3 the effect a ligand has on the stability of a particular oxidation state

REACTIONS OF COPPER(II) Aqueous solutions of copper(II) contain the blue, octahedral hexaaquacopper(II) ion Most substitution reactions are similar to cobalt(II). OH¯ [Cu(H2O)6]2+(aq) + 2OH¯(aq) ——> [Cu(OH)2(H2O)4](s) + 2H2O(l) blue, octahedral pale blue ppt. insoluble in XS NaOH A-B

REACTIONS OF COPPER(II) Aqueous solutions of copper(II) contain the blue, octahedral hexaaquacopper(II) ion Most substitution reactions are similar to cobalt(II). OH¯ [Cu(H2O)6]2+(aq) + 2OH¯(aq) ——> [Cu(OH)2(H2O)4](s) + 2H2O(l) blue, octahedral pale blue ppt. insoluble in XS NaOH NH3 [Cu(H2O)6]2+(aq) + 2NH3(aq) ——> [Cu(OH)2(H2O)4](s) + 2NH4+(aq) blue ppt. soluble in excess NH3 then [Cu(OH)2(H2O)4](s) + 4NH3(aq) ——> [Cu(NH3)4(H2O)2]2+(aq) + 2H2O(l) + 2OH¯(aq) royal blue NOTE THE FORMULA A-B A-B LS

REACTIONS OF MANGANESE(VII) • in its highest oxidation state therefore Mn(VII) will be an oxidising agent • occurs in the purple, tetraoxomanganate(VII) (permanganate) ion (MnO4¯) • acts as an oxidising agent in acidic or alkaline solution acidic MnO4¯(aq) + 8H+(aq) + 5e¯ ——> Mn2+(aq) + 4H2O(l) E° = + 1.52 V N.B. Acidify with dilute H2SO4 NOT dilute HCl alkaline MnO4¯(aq) + 2H2O(l) + 3e¯ ——> MnO2(s) + 4OH¯(aq) E° = + 0.59 V

VOLUMETRIC USE OF MANGANATE(VII) Potassium manganate(VII) in acidic (H2SO4) solution is extremely useful for carrying out redox volumetric analysis. MnO4¯(aq) + 8H+(aq) + 5e¯ ——> Mn2+(aq) + 4H2O(l) E° = + 1.52 V It must be acidified with dilute sulphuric acid as MnO4¯ is powerful enough to oxidise the chloride ions in hydrochloric acid. It is used to estimate iron(II), hydrogen peroxide, ethanedioic (oxalic) acid and ethanedioate (oxalate) ions. The last two titrations are carried out above 60°C due to the slow rate of reaction. No indicator is required; the end point being the first sign of a permanent pale pink colour. Iron(II) MnO4¯(aq) + 8H+(aq) + 5Fe2+(aq) ——> Mn2+(aq) + 5Fe3+(aq) + 4H2O(l) this means that moles of Fe2+ = 5 moles of MnO4¯ 1

REACTIONS OF IRON(II) When iron reacts with acids it gives rise to iron(II) (ferrous) salts. Aqueous solutions of such salts contain the pale green, octahedral hexaaquairon(II) ion OH¯ [Fe(H2O)6]2+(aq) + 2OH¯(aq) ——> [Fe(OH)2(H2O)4](s) + 2H2O(l) pale green dirty green ppt. it only re-dissolves in very conc. OH¯ but... it slowly turns a rusty brown colour due to oxidation by air to iron(III) increasing the pH renders iron(II) unstable. Fe(OH)2(s) + OH¯(aq) ——> Fe(OH)3(s) + e¯ dirty green rusty brown NH3 Iron(II) hydroxide precipitated, insoluble in excess ammonia A-B OX A-B

REACTIONS OF IRON(II) Volumetric Iron(II) can be analysed by titration with potassium manganate(VII) in acidic (H2SO4) solution. No indicator is required. MnO4¯(aq) + 8H+(aq) + 5Fe2+(aq) ——> Mn2+(aq) + 5Fe3+(aq) + 4H2O(l) this means that moles of Fe2+ = 5 moles of MnO4¯ 1 CONTENTS

REACTIONS OF IRON(III) Aqueous solutions contain the yellow-green, octahedral hexaaquairon(III) ion OH¯ [Fe(H2O)6]3+(aq) + 3OH¯(aq) ——> [Fe(OH)3(H2O)3](s) + 3H2O(l) yellow rusty-brown ppt. insoluble in XS NH3 [Fe(H2O)6]3+(aq) + 3NH3(aq) ——> [Fe(OH)3(H2O)3](s) + 3NH4+(aq) rusty-brown ppt. insoluble in XS A-B A-B

OXIDATION & REDUCTION - A SUMMARY • complex transition metal ions are stable in acid solution • complex ions tend to be less stable in alkaline solution • in alkaline conditions they form neutral hydroxides and/or anionic complexes • it is easier to remove electrons from neutral or negatively charged species • alkaline conditions are usually required e.g. Fe(OH)2(s) + OH¯(aq) ——> Fe(OH)3(s) + e¯ Co(OH)2(s) + OH¯(aq) ——> Co(OH)3(s) + e¯ 2Cr3+(aq) + 3H2O2(l) + 10OH¯(aq) ——> 2CrO42-(aq) + 8H2O(l) • Solutions of cobalt(II) can be oxidised by air under ammoniacal conditions [Co(NH3)6]2+(aq) ——> [Co(NH3)6]3+(aq) + e¯

OXIDATION & REDUCTION - A SUMMARY • complex transition metal ions are stable in acid solution • complex ions tend to be less stable in alkaline solution • in alkaline conditions they form neutral hydroxides and/or anionic complexes • it is easier to remove electrons from neutral or negatively charged species • alkaline conditions are usually required e.g. Fe(OH)2(s) + OH¯(aq) ——> Fe(OH)3(s) + e¯ Co(OH)2(s) + OH¯(aq) ——> Co(OH)3(s) + e¯ 2Cr3+(aq) + 3H2O2(l) + 10OH¯(aq) ——> 2CrO42-(aq) + 8H2O(l) • Solutions of cobalt(II) can be oxidised by air under ammoniacal conditions [Co(NH3)6]2+(aq) ——> [Co(NH3)6]3+(aq) + e¯ Reduction • Zinc metal is used to reduce transition metal ions to lower oxidation states • It acts in acid solution as follows... Zn(s) ——> Zn2+(aq) + 2e¯ e.g. it reduces iron(III) to iron(II) vanadium(V) to vanadium (IV) to vanadium(III)