3 Ligand substitution and precipitation reactions

Ligand substitution Reaction with excess NH3 With excess NH3 ligand substitution reactions occur with several transition aqueous ions. The ligands NH3 and H2O are similar in size and are uncharged. Ligand exchange occurs without change of co-ordination number for Co and Cr Note: This substitution in the case with Cu is incomplete as not all the water molecules are substituted. deep blue solution [Cu(H2O)6 ]2+ (aq) + 4NH3 (aq)  [Cu(NH3 )4 (H2O)2 ]2+ (aq) + 4H2O (l) blue solution [Co(H2O)6 ]2+ (aq) + 6NH3 (aq)  [Co(NH3 )6 ]2+ (aq) + 6H2O (l) Be able to write equations for other metal ions given information about the complex formed. No need to learn colours for these other ions. Reactions with Chloride ions Addition of a high concentration of chloride ions (from conc HCl or saturated NaCl) to an aqueous ion leads to a ligand substitution reaction. The Clligand is larger than the uncharged H2O and NH3 ligands so therefore ligand exchange can involve a change of co-ordination number. Addition of conc HCl to aqueous ions of Cu and Co lead to a change in coordination number from 6 to 4. [Cu(H2O)6 ]2+ + 4Cl-  [CuCl4 ]2- + 6H2O [Co(H2O)6 ]2+ + 4Cl-  [CoCl4 ]2- + 6H2O Be careful: If solid copper chloride (or any other metal )is dissolved in water it forms the aqueous [Cu(H2O)6 ]2+ complex and not the chloride [CuCl4 ]2- complex blue solution blue solution yellow/green solution pink solution. Biological Complexes Fe(II) in haemoglobin enables oxygen to be transported in the blood . Haem is an iron(II) complex with a multidentate ligand. O2 bonds to Fe2+ ions in the Haemoglobin and when required the O2 is released. CO is toxic to humans as CO can from a strong coordinate bond with haemoglobin. This is a stronger bond than that made with oxygen and so it prevents the oxygen attaching to the haemoglobin. With CO, the stability constant is greater than with complex in O2. Precipitation Reactions with sodium hydroxide and ammonia The bases OH- and ammonia when in limited amounts form the same hydroxide precipitates. [Cu(H2O)6 ]2+ (aq) + 2OH- (aq)  Cu(H2O)4 (OH)2 (s) + 2H2O (l) [Mn(H2O)6 ]2+ (aq) + 2OH- (aq)  Mn(H2O)4 (OH)2 (s) + 2H2O (l) [Fe(H2O)6 ]3+ (aq) + 3OH- (aq)  Fe(H2O)3 (OH)3 (s) + 3H2O (l) These reactions are classed as precipitation reactions [Fe(H2O)6 ]2+ (aq) + 2OH- (aq)  Fe(H2O)4 (OH)2 (s) + 2H2O (l) Pale brown ppt green ppt brown ppt Cu2+ (aq) + 2OH- (aq)  Cu(OH)2 (s) Mn2+ (aq) + 2OH- (aq)  Mn(OH)2 (s) Fe 2+ (aq) + 2OH- (aq)  Fe(OH)2 (s) Fe3+ (aq) + 3OH- (aq)  Fe(OH)3 (s) blue ppt blue ppt brown ppt Blue solution Blue solution Very pale pink solution green solution green solution Yellow/brown solution Yellow/brown solution Very pale pink solution Pale brown ppt green ppt Reaction with excess OH- With excess NaOH, the Cr hydroxide dissolves. Cr becomes [Cr(OH)6 ]3- (aq) green solution •This hydroxides is classed as amphoteric because it can react with alkali to give a solution and react with acid to form the aqueous salt Cr(H2O)3 (OH)3 (s) + 3OH- (aq ) [Cr(OH)6 ]3- (aq) + 3H2O(l) Cr(H2O)3 (OH)3 (s) + 3H+ (aq ) [Cr(H2O)6 ]3+ (aq) [Mn(H2O)6 ]2+ (aq) + 2NH3 (aq)  Mn(H2O)4 (OH)2 (s) + 2NH4+ (aq) [Fe(H2O)6 ]3+ (aq) + 3NH3 (aq)  Fe(H2O)3 (OH)3 (s) + 3NH4+ (aq) With ammonia when added in limited amounts the same hydroxide precipitates form. The ammonia acts as a base, removes a proton from the aqueous complex and becomes the ammonium ion. Reaction with excess NH3 With excess NH3 ligand exchange reactions occur with Cu and Cr, and their hydroxide precipitates dissolve in excess ammonia The ligands NH3 and H2O are similar in size and are uncharged. Ligand exchange occurs without change of co-ordination number for Cr Cr becomes [Cr(NH3 )6 ]3+ purple solution This substitution may, however, be incomplete as in the case with Cu Cu becomes [Cu(NH3 )4 (H2O)2 ]2+ deep blue solution Cr(OH)3 (H2O)3(s) + 6NH3 (aq)  [Cr(NH3 )6 ]3+ (aq) + 3H2O(l) + 3OH- (aq) Cu(OH)2 (H2O)4(s) + 4NH3 (aq)  [Cu(NH3 )4 (H2O)2 ]2+ (aq) + 2H2O (l) + 2OH- (aq)