What students find difficult
Brackets
The use of square brackets in the formulae of complexes is a problem for many students.
Why is there a confusion about the use of square brackets in the formulae of complexes?
We normally use square brackets to mean concentration of a component in mol dm–3. In complexes, however, we use them to enclose the formula of metal ion and ligands. This is a particular problem in writing expressions for Kstab as we want to show the formula of the complex and to indicate that we're referring to its concentration. Sometimes two sets of square brackets are used but this can make the formula look quite messy.
Dative bonds
Some students find it difficult to grasp the bonding present in complexes because there is more than one type involved. They need to appreciate that ligands form dative covalent bonds with the central metal atom but the whole complex is usually part of a cation.
What simple ions could you remind students about that also involve dative covalent ions?
The H3O+ and NH4+ ions are examples that students will have probably met before.
Dynamic equilibria
There is a tendency for students to forget that ligand exchange reactions involve equilibrium systems. A good way of challenging this thinking is to add concentrated hydrochloric acid to a solution of copper(II) sulfate and to ask them what species are present at different stages when the colour of the mixture changes from blue through green to yellow.
What is the cause of the green colour that is seen?
The green colour is the result of a mixture containing the blue hexaaquacopper(II) complex, [Cu(H2O)6]2+ and the yellow tetrachlorocuprate(II) complex, [CuCl4]2–.
Cross-topic links
There is a great deal of similarity between ligand exchange reactions in inorganic chemistry and nucleophilic substitution reactions in organic chemistry. Most students find it very difficult to make such connections across different areas of their studies and you will need to encourage them to make such comparisons.
Why is it useful to point out similarities in behaviour across different topics in chemistry?
Pointing out similarities will help students focus on the principles involved rather than on trying to remember separate details. This helps them build up a unified and coherent view of chemistry.