Complexation 

Main terms:

• Complex - formed when one or more small molecules or ions attach themselves to a central cation - usually a transition metal. The surrounding molecules have to be ligands --- see below:

• Ligand - A molecule that acts as an electron pair donor ---- see below:

• Coordinate covalent bond - formed when one atom that does not have a full valence shell bonds with an atom that does have a full shell, but there are un-bonded electrons. For this example, DRAW a Bohr model of H2O. Circle the lone electrons. It is here that Hydrogen (with one electron in its outer shell) will bond to the H2O creating a coordinate bond H3O+. Here water is a ligand! 

Others include NH3 (Try this example with H+)

A lot of the time, when it comes to precipitation reactions, a hydrated molecule will undergo a complexation reaction where one or more water molecules are swapped out for another molecule - NH3 or HCL for example.