TRANSITION METAL COMPLEXES

THE AQUEOUS CHEMISTRY OF IONS - HYDROLYSIS when salts dissolve in water the ions are stabilised this is because water molecules are polar hydrolysis can occur and the resulting solution can become acidic the greater charge density of the cation... the greater the polarising power and the more acidic the solution

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 Lewis Base – a lone pair donor water forms a co-ordinate bond to the metal ion metal ions accept the lone pair - Lewis Acids

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 Lewis Base – a lone pair donor water forms a co-ordinate bond to the metal ion metal ions accept the lone pair - Lewis Acids Acidity as charge density increases, the cation has a greater attraction for water and it weakens the O-H bond, this makes the H more acidic (more d+) it can then be removed by solvent water molecules to form H3O+(aq).

HYDROLYSIS - EQUATIONS M2+ ions [M(H2O)6]2+(aq) + H2O(l) [M(H2O)5(OH)]+(aq) + H3O+(aq) the resulting solution will now be acidic as there are more protons in the water this reaction is known as hydrolysis - the water causes the substance to split up Stronger bases (e.g. CO32- , NH3 and OH¯ ) can remove further protons...

HYDROLYSIS - EQUATIONS M3+ ions [M(H2O)6]3+(aq) + H2O(l) [M(H2O)5(OH)]2+(aq) + H3O+(aq) the resulting solution will also be acidic as there are more protons in the water this SOLUTION IS MORE ACIDIC due to the greater charge density of 3+ ions Stronger bases (e.g. CO32- , NH3 and OH¯ ) can remove further protons...

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) In some cases, if the base is strong, further protons are removed and the precipitate dissolves as soluble anionic complexes such as [M(OH)6]3- are formed. Very weak bases H2O remove few protons Weak bases NH3, CO32- remove protons until precipitation Strong bases OH¯ can remove all the protons OH¯ H+ OH¯ H+ Precipitated OH¯ H+ OH¯ H+ OH¯ H+ OH¯ H+

HYDROLYSIS OF HEXAAQUA IONS AMPHOTERIC CHARACTER Metal ions of 3+ charge have a high charge density and their hydroxides can dissolve in both acid and alkali. Aluminium hydroxides show this property. [M(H2O)6]3+(aq) [M(OH)3(H2O)3](s) [M(OH)6]3-(aq) H+ OH¯ Soluble Insoluble Soluble

REACTION TYPES • shape • stability to oxidation or reduction One typical property 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 ACID-BASE LIGAND SUBSTITUTION PRECIPITATION A-B LS Ppt

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 CO32- [Cu(H2O)6]2+(aq) + CO32-(aq) ——> CuCO3(s) + 6H2O(l) blue ppt. A-B A-B LS Ppt

REACTIONS OF COPPER(II) Cl¯ [Cu(H2O)6]2+(aq) + 4Cl¯(aq) ——> [CuCl4]2-(aq) + 6H2O(l) yellow, tetrahedral • Cl¯ ligands are larger than H2O and are charged • the complex is more stable if the shape changes to tetrahedral • adding excess water reverses the reaction LS

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 CO32- Off-white coloured iron(II) carbonate, FeCO3, precipitated A-B OX A-B Ppt

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 CO32- 2[Fe(H2O)6]3+(aq) + 3CO32-(aq) ——> 2[Fe(OH)3(H2O)3](s) + 3H2O(l) + 3CO2(g) rusty-brown ppt. The carbonate is not precipitated but the hydroxide is; the high charge density of M3+ makes the solution too acidic to form a carbonate CARBON DIOXIDE EVOLVED. NH3 [Fe(H2O)6]3+(aq) + 3NH3(aq) ——> [Fe(OH)3(H2O)3](s) + 3NH4+(aq) rusty-brown ppt. insoluble in XS SCN¯ [Fe(H2O)6]3+(aq) + SCN¯(aq) ——> [Fe(SCN)(H2O)5]2+(aq) + H2O(l) blood-red colour Very sensitive; BLOOD RED COLOUR confirms Fe(III). No reaction with Fe(II) A-B A-B A-B LS

REACTIONS OF SILVER(I) • aqueous solutions contains the colourless, linear, diammine silver(I) ion • formed when silver halides dissolve in ammonia eg AgCl(s) + 2NH3(aq) ——> [Ag(NH3)2]+(aq) + Cl¯(aq) [Ag(NH3)2]+ Used in Tollen’s reagent (SILVER MIRROR TEST) Tollen’s reagent is used to differentiate between aldehydes and ketones. Aldehydes produce a silver mirror on the inside of the test tube Formed when silver halides dissolve in ammonia - TEST FOR HALIDES

REACTIONS OF ALUMINIUM • aluminium is not a transition metal as it doesn’t make use of d orbitals • BUT, due to a high charge density, aluminium ions behave as typical M3+ ions • aqueous solutions contain the colourless, octahedral hexaaquaaluminium(III) ion OH¯ [Al(H2O)6]3+(aq) + 3OH¯(aq) ——> [Al(OH)3(H2O3](s) + 3H2O(l) colourless, octahedral white ppt. soluble in XS NaOH As with all hydroxides the precipitate reacts with acid [Al(OH)3(H2O)3](s) + 3H+ (aq) ——> [Al(H2O)6]3+(aq) being a 3+ hydroxide it is AMPHOTERIC and dissolves in excess alkali [Al(OH)3(H2O)3](s) + 3OH¯(aq) ——> [Al(OH)6]3-(aq) + 3H2O(l) colourless, octahedral CO32- 2 [Al(H2O)6]3+(aq) + 3CO32-(aq) ——> 2[Al(OH)3(H2O)3](s) + 3H2O(l) + 3CO2(g) As with 3+ ions, the carbonate is not precipitated but the hydroxide is. NH3 [Al(H2O)6]3+(aq) + 3NH3(aq) ——> [Al(OH)3(H2O)3](s) + 3NH4+(aq) white ppt. insoluble in XS NH3 A-B A-B A-B A-B A-B