Reactions of Metal Aqua Ions

Metal aqua ions are formed in aqueous solution:

[M(H₂O)₆]²⁺ and [M(H₂O)₆]³⁺

These aqua ions can also be present in the solid state.

For example copper sulfate is only blue as its aqua ion, in its anhydrous state it is white.

This is why copper (II) sulfate crystals lose water of crystallisation when they are heated.

Substitution Reactions

The ligands NH₃ and H₂O are similar in size and are uncharged, therefore ligand exchange between these two ligands occurs without change of co-ordination number.

The Cl^- ligand is larger than these uncharged ligands and that ligand exchange can involve a change of co-ordination number.

Substitution of unidentate ligands (ligands which forms one bond with the central metal) with a bidentate or a multidentate ligand leads to a more stable complex.

This is because there will be more ions in solution therefore higher entropy.

M(H₂O)₆²⁺  +  EDTA^4-  >  M(EDTA)^2-  +  6H₂O

Lewis Acid and Bases

Lewis acids are lone pair acceptors such as the central metal ion in transition metal ion.

Lewis bases are lone pair donors such as ligands.

These definitions of acids and bases extend acid -base chemistry to reactions which do not involve proton exchange.