3 Substitution reactions

H2O, NH3 and Cl− can act as monodentate ligands. The ligands NH3 and H2O are similar in size and are uncharged. Exchange of the ligands NH3 and H2O occurs without change of co-ordination number (eg Co2+ and Cu2+). [Co(H2O)6 ]2+ (aq) + 6NH3 (aq)  [Co(NH3 )6 ]2+ (aq) + 6H2O (l) This substitution may, however, be incomplete as in the case with Cu. Cu becomes [Cu(NH3 )4 (H2O)2 ]2+ deep blue solution [Cu(H2O)6 ]2+ (aq) + 4NH3 (aq)  [Cu(NH3 )4 (H2O)2 ]2+ (aq) + 4H2O (l) 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. [CuCl4 ]2- yellow/green solution [CoCl4 ]2- blue solution [Cu(H2O)6 ]2+ + 4Cl-  [CuCl4 ]2- + 6H2O These are tetrahedral in shape [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. [Fe(H2O)6 ]3+ + 4Cl-  [FeCl4 ] – + 6H2O