Question

Write the formula of the following compounds:
(i) Potassium trioxalatoaluminate (III)
(ii) Hexaaquairon (II) sulphate

Answer 1

Hint: Naming any coordination compound involves a set of rules which includes oxidation number of the central metal ion, the anionic and cationic properties of that molecule and the chemical composition of its ligands.

Complete answer: Coordination compounds are composed of a metal atom and a surrounding array of bound molecules known as ligands. Coordination refers to the coordinate covalent bonds between the ligands and the central atom.
Let us first write the rules of naming any coordination compound before naming the compounds in the question.
- While naming a complex ion, the ligands are named before the metal ion.
- Names of the ligands are written in the order: neutral, negative, positive. If there are multiple ligands of the same charge, then they are named in alphabetical order.
- Neutral ligands are written with their respective name with some exceptions; $\text{N}{{\text{H}}_{3}}$becomes ammine,${{\text{H}}_{2}}\text{O}$ becomes aqua and CO becomes carbonyl.
- If monodentate ligands receive a prefix when occurs multiple times as per occurrence number: di-, tri-, etc. Polydentate ligands receive bis-, tris-, etc. as prefix.
- The final ‘e’ in anions, ‘-ate’ and ‘-ide’ is replaced by ‘-ido’ and ‘ato’ in the end. Like, (cyanide becomes cyanido).
- If the complex is an anion, the name of central metal will end with -ate and its Latin name will be used.
- If the central atom’s oxidation state needs to be mentioned; write it as a Roman numeral in parentheses.
(i) Potassium trioxalatoaluminate (III): Symbol for potassium, oxalate and Aluminium are $\text{K,ox and Al}$. There are three oxalate groups, the oxidation state of $\text{K}$ is +3. The charge on $\text{ox}$ is -2. Formula of Potassium trioxalatoaluminate (III) is ${{\text{K}}_{3}}\left[ \text{Al}{{\left( \text{ox} \right)}_{3}} \right]$.
(ii) Hexaaquairon (II) sulphate: Symbol for aqua, iron and sulphate are ${{\text{H}}_{2}}\text{O,Fe and SO}_{4}^{2-}$. There are six water groups, the oxidation state of $\text{Fe}$is +2. The charge on ${{\text{H}}_{2}}\text{O}$ is 0. Formula of Hexaaquairon (II) sulphate is $\left[ \text{Fe}{{\left( {{\text{H}}_{2}}\text{O} \right)}_{6}} \right]\text{S}{{\text{O}}_{4}}$.

Note: While writing the names of coordination compounds, the net charge and the oxidation state of the compounds should be taken care of. The metals exist in various oxidation states like $\text{Fe}$ in +2 and +3. So, the charge on the central metal atom should be found out with care.