Aqueous Complexes and Reactions

Revision for aqueous complexes, transition metals, aqueous complexes and their reactions

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  • Created by: anna
  • Created on: 16-03-13 15:53

Acids and Bases

Bronsted-Lowry Acids and Bases:

- An acid is a proton donor

- A base is a proton acceptor

Lewis Acid and Bases:  

- An acid is an electron pair acceptor

- A base is an electron pair donor    

  • Ligands which form bonds to transition metal ions are acting as Lewis bases.
  • The metal ions are acting as Lewis acids.
  • All Bronsted-Lowry acids are Lewis bases aswell.
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Iron (II) and Iron (III) Aqua Complexes

Solutions of Fe 2+ are not noticably acidic but solutions of Fe3+ are, because...

  • Fe3+ ions are smaller and more highly charged so attract electrons more easily.
  • In [Fe(H20)6]3+ the Fe strongly attracts electrons from the oxygen in the water ligands, weakening the O-H bond in water.
  • This means it will readily release a H+ ion making the solution acidic.
  • [Fe(H2O)6]3+ -> [Fe(H2O)5(OH)]2+  +  H+

Generally aqua-ions of M3+ are more acidic than M2+.

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Reactions of M2+ ions and M3+ ions

If you add a base (such as OH-) it will remove protons from the aqueous complex. 


  • [M(H2O)6]3+(aq) + OH-  --> [M(H2O)5(OH)]2+(aq) + H2O(aq)
  • [M(H2O)5(OH)]2+(aq) + OH- --> [M(H2O)4(OH)2]+(aq) H2O(aq)
  • [M(H2O)4(OH)2]+(aq) + OH- --> M(H2O)3(OH)3 (s) + H2O(aq)


  • [M(H2O)6]2+(aq) + OH- --> [M(H2O)5(OH)]+ +H2O(aq)
  • [M(H2O)5(OH)]+(aq) + OH- --> M(H2O)4(OH)2 (s) + H2O(aq)

M(H2O)4(OH)2 is M(OH)2 and is uncharged, insoluble and a precipitate.

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Ligand Substitution Reactions

H2O ligands in metal aqua ions can be replaced by other ligands, this is either because...

  • the other ligands form stronger co-ordinate bonds (better Lewis bases)
  • they are present in higher concentration and an equilibrium is displaced


  • Water and ammonia are similar sized and uncharged so ligand exchange occurs without a change in co-ordination number or charge.
  • Ammonia is a better ligand as the lone pair electrons is more easily donated due to the nitrogen not being as electronegative as the oxygen.


  • Carbonates of 2+ transition metal ions exist while those of 3+ do not.
  • [Fe(H2O)6]2+(aq) + CO32-  --> FeCO3(aq) + 6H2O(aq)
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More things...

Test for Iron Ions:

  • In dilute solutions the 2+ and 3+ precipitates are hard to tell apart
  • Add alkali as the precipitate colours are more obvious

Amphoteric Hydroxides:

Amphoteric - shows both acidic and basic properties

Examples include aluminium hydroxide and chromium hydroxide:

  • Al(H2O)3(OH)3 + HCl -> [Al(H2O)6]3+ + 3Cl-
  • Al(H2O)3(OH)3 + OH-  -> [Al(OH)4]- + 3H2O
  • Cr(H2O)3(OH)3 + 3H3O+ -> [Cr(H2O)6]3+ + 3H2O
  • Cr(H2O)3(OH)3 + 3OH-  -> [Cr(OH)6]3- + 3H2O
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