A2-Level Complex Ion Chemistry
Ligands in a metal complex ion can be substituted.
Ligands that have a similar size can be substituted with no change to the co-ordination number of the complex ion.
If the ligands involved in substitution are different sizes, a change in co-ordination number can occur.
Chloride ions are larger ligands than water molecules so only four can fit around a metal ion – a change in co-ordination number occurs when chloride ions are substituted for water ligands (usually six to four).
Substitution occurs stepwise and the stability of the complex ion that would be formed from another substitution determines whether a further substitution will happen (this is why only partial substitution sometimes occurs, for example with copper and ammonia).
The ligands that co-ordinately bond and surround a metal ion can be swapped, this is a ligand substitution reaction.
For example, [Ni(H O) ] can react with ammonia to form [Ni(NH ) ] .Water and ammonia are similar sized molecules and therefore the same number of each can fit around the metal ion (six). Each ligand can form one co-ordinate bond, this means the co-ordination number of the complex remains the same (six).
Not all ligands are of a similar size, however. Chloride ions are much larger than water and ammonia ligands and six chloride ions simply cannot fit around the metal ion as easily. To not ‘bump’ into each other, only four chloride ions can get close enough to a metal ion to make a co-ordinate bond.
For example, [Co(H O) ] reacts with chloride ions to form [CoCl ] . Note that the number of co-ordinate bonds made with metal ion changed from six to four, meaning the co-ordination number of the complex has changed from six to four. The charge of the complex ion has also changed as each chloride ion has a charge of negative one (2+ from Co and 4 x 1- from each of the chloride ions gives an overall charge of 2-).
Why can ligand substitution reactions change the colour of a complex ion?
Colour arises in complex ions because of the d-orbtials in the metal ion becoming split into high and low energies. Electrons can become excited and move from the low to high energy orbitals by absorbing specific wavelengths of visible light. Changing the ligands in the complex ion changes the gap between 3-d orbitals and the wavelengths of light absorbed.