Aqua ions

• Aqua ions form when a metal salt is dissolved in water

  CuSO4 + 6H2O --> [Cu(H2O)6]2+ + SO4 2-
• When there are 6 water molecules in the aqua ion we call it a hexaqua ion
• The bond between the metal ion and water molecules is so strong that the aqua ion is found in the solid hydrated salt. This gives the solid hydrated salt it's colour e.g. blue hydrated copper sulfate
• If the formula is given as [M(H2O)6]Cl2 then the Cl is not acting as a ligand
• If an exam question refers to a metal salt solution reacting it is referring to the aqua ion
• aqua ion colours

  • Cu2+ blue
  • Co2+ pink
  • Fe2+ pale green
  • Fe3+ purple
  • Al3+ colourless
• aqua ions are oxidised in the presence of KMnO4 in acidic condtions with dilute H2SO4
• when aqua ions are formed, it establishes a hydrolysis equilibrium. this is when the metal ion in the aqua ion polarises (distorts) the electron density of a coordinately bonded water ligand. The OH bond in water breaks, releasing a H+ ion. This free H+ reacts with a free water molecule to make H3O+ (acidic)
• hydrolysis reactions of aqua ions

  • [M(H2O)6]2+ + H2O --> [M(H2O)5 OH]+ + H3O+ (pH 5 weaker acid)
  • [M(H2O)6]3+ + H2O --> [M(H2O)5 OH]2+ + H3O+ (pH 3 weak acid)
  • aqua ion acts as a Bronsted-Lowry acid
  • water acts as a Bronsted-Lowry base
• why is a solution of Fe3+ aqua ions more acidic than a solution of Fe2+ aqua ions

  • Fe3+ has a higher charge density than Fe2+ so polarises the water ligand to a greater extent
  • releases more H+ ions and forms more H3O+ which makes the solution more acidic
  • the hydrolysis equilibrium of Fe3+ also lies more to the right so more H3O+ formed
• if we add aqueous NaOH dropwise to aqua ions, the OH- ion from NaOH reacts with H+ from the aqua ion. a second H+ ion is lost from the aqua ion to react with OH- to form a neutral hydroxide precipitate. this is the second step of hydrolysis

  [M(H2O)6]2+ + 2OH- --> M(H2O)4 (OH)3 + 3H2O
• if the metal aqua ion is +3 we need to do 3 steps of hydrolysis to form the neutral hydroxide precipitate

  [M(H2O)6]3+ + 3OH- --> M(H2O)3 (OH)3 + 3H2O
• if a complex has a positive or negative charge, it is a solution
• if a complex has a neutral charge, it is a precipitate
• neutral hydroxide precipitate colours

  • Cu(OH)2 (H2O)4 pale blue precipitate
  • Fe(OH)2 (H2O)4 green precipitate
  • Fe(OH)3 (H2O)3 brown precipitate
  • Al(OH)3 (H2O)3 white precipitate
• if we add aqueous ammonia dropwise to aqua ions , a hydroxide precipitate and NH4+ forms. similar to the reaction of aqua ions with NaOH but NH4+ formed instead of water
• if we add excess NH3 to aqua ion, NH3 acts as a ligand, replacing the other ligands in the complex. this is called ligand substitution. this only works with Cu or Co aqua ions
• overall reaction between aqueous dropwise ammonia (Bronsted-Lowry base) and a metal salt solution

  • [M(H2O)6]2+ + 2NH3 --> M(OH)2 (H2O)4 + 2NH4+
  • [M(H2O)6]3+ + 3NH3 --> M(OH)3 (H2O)3 + 3NH4+
• ligand substitution with excess NH3 - only with Copper or Cobalt

  • [Co(H2O)6]2+ + 6NH3 --> [Co(NH3)6]2+ + 6H2O forms a brown/yellow solution
  • [Cu(H2O)6]2+ + 4NH3 --> [Cu(H2O)2 (NH3)4]2+ + 4H2O partial ligand substitution forms a deep blue solution
• adding ammonia dropwise until excess to the copper aqua ion

  • [Cu(H2O)6]2+ + 2NH3 --> Cu(OH)2 (H2O)4 + 2NH4+
  • Cu(OH)2 (H2O)4 + 4NH3 --> [Cu(H2O)2 (NH3)4]2+ + 2H2O + 2OH-
• 2+ aqua ions are not acidic enough (doesn't release enough H+) to form carbon dioxide and water with carbonate ions. no effervescence and forms metal carbonate precipitates

  • [M(H2O)6]2+ + CO3 2- --> MCO3 + 6H2O
• metal carbonate precipitates form when 2+ aqua ions react with carbonate ions

  • FeCO3 - green precipitate
  • CuCO3 - green/blue precipitate
• 3+ aqua ions are acidic enough (releases enough H+) to react with carbonate ions to form carbon dioxide and water. forms neutral hydroxide precipitates and observe effervescence.

  • 2[M(H2O)6]3+ + 3CO3 2- --> 2M(H2O)3 (OH)3 + 3CO2 + 3H2O
• All hydroxide precipitates will dissolve in acid (H+) to reform their aqua ion

  • M(OH)2 (H2O)4 + 2H+ --> [M(H2O)6]2+
  • M(OH)3 (H2O)3 + 3H+ --> [M(H2O)6]3+
• only the aluminium hydroxide precipitate will dissolve in base as aluminium is amphoteric

  • Al(OH)3 (H2O)3 + OH- --> [Al(OH)4 (H2O)2]- + H2O
  • forms a colourless solution
• we can replace the water ligands on an aqua ion using the following ligands

  • ammonia (only with Cu or Co)
  • Chloride
  • Bidentate and Multidentate ligands
• Ligand substitution with chloride ions by adding excess concentrated HCl to the aqua ion

  • [Co(H2O)6]2+ + 4Cl- --> [CoCl4]2- + 6H2O blue solution
  • [Cu(H2O)6]2+ + 4Cl - --> [CuCl4]2- + 6H2O yellowy green solution
  • [Fe(H2O)6]3+ + 4Cl- --> [FeCl4]- + 6H2O
• when we replace water ligands with bidentate or multidentate ligands we form very stable complexes
• Bidentate and multidentate ligands are known as Chelating ligands. they can surround the metal ion with lots of strong coordinate bonds which prevent other ligands being able to substitute further
• Chelate effect - occures when we substitute a unidentate ligand with a bidentate or mulitdentate ligand, forming a very stable complex. the reaction involves an increase in entropy
• ligand substitution with bidentate ligands and multidentate ligands results in an increase in disorder

  • [Co(H2O)6]2+ + 3H2NCH2CH2NH2 --> [Co(H2NCH2CH2NH2)3]2+ 6H2O
  • increase in disorder as we go from 4 moles on the left to 7 moles on the right
  • [Co(H2O)6]2+ + EDTA 4- --> [Co(EDTA)]2- + 6H2O
  • increase in disorder as we go from 2 moles on the left to 7 moles on the right
• uses of transition metal complexes

  • tollens reagents is made using silver nitrate solution
  • [Ag(H2O)2]+ + 2NH3 --> [Ag(NH3)2]+ + 2H2O
• the haem group in haemoglobin is a multidentate complex. central Fe2+ ion surrounded by a stable porphyrin ring (octahedral structure) which is a multidentate ligand. made up of nitrogens and bonds to the globin group and an oxygen, allowing it to carry oxygen around the body
• if carbon monoxide gets into the blood it bonds to the Fe2+ irreversibly which prevents O2 from bonding to the haem group. this is carbon monoxide poisoning
• cis-platin is used as a cancer therapy drug. it bonds to the guanine base in DNA and prevents the DNA doulbe helix from unwinding. stops transcription of DNA and cancer cells can't replicate. cannot differentiate between healthy cells and cancer cells so causes side effects such as hair loss