3.2.5.2 Substitution Reactions

Complex transition metal ions bonded with water They are formed when simple ions of transition metals dissolve in water They are called hexaaqua cations because they have six water ligands bonded to the metal ion e.g. [ Cu (H2O)6 ]2+

[ Ni (H2O)6 ]2+ is formed Sulfate is present in the solution

Mg2+, Zn2+ and Al3+ as they have empty orbitals

Square brackets The charge is written as a power

The water is not charged so the charge of the hexaaqua ion equals the charge of the transition metal ion

The number of co-ordinate bonds between a metal ion and ligands Some have more than one lone pair

Unidentate or monodentate Bidentate Multidentate

Ligands which form one co-ordinate bond e.g. water, ammonia, chloride, cyanide

Ligands which form two co-ordinate bonds The lone pairs must be on different atoms to allow the ligand to form more than one co-ordinate bond Water has two lone pairs but these are both on the oxygen atom so it is monodentate

Ligands which form more than two co-ordinate bonds per ligand EDTA derivatives form 6 co-ordinate bonds All multidentate ligands form complexes which are called chelates

Multidentate ligand Charge of 4+ It can act as a hexadentate ligand using lone pairs on four oxygen and both nitrogen atoms

Complex ions with polydentate ligands

They can be used to effectively remove d-block metal ions from solution

An iron(II) complex with a multidentate ligand Oxygen forms a co-ordinate bond to Fe(II) in haemoglobin, enabling oxygen to be transported in the blood. Carbon monoxide is toxic because it replaces oxygen co-ordinately bonded to Fe(II) in haemoglobin

If you add a hexadentate ligand to a solution of a transition metal salt, the ligand will replace all six water ligands in the hexaaqua ion This creates more species due to the number of water molecules removed This increase causes a significant increase in entropy which forces the reaction to the right so that more hexadentate ligands react For this reason, chelate complexes with polydentate ligands are favoured over complexes with monodentate ligands This is called the chelate effect