Read the passage carefully and answer the questions.
The coordination compounds are of great importance. These compounds are widely present in the mineral, plants and animal world and are known to play many important functions in the area of analytical chemistry, metallurgy, biological systems, industry and medicine. For example, coordination compounds find use in many qualitative and quantitative chemical analysis. Some important metal extraction and purification make use of complex formation. Hardness of water can be estimated by stable complex formation of \( \text{Ca}^{2+} \) and \( \text{Mg}^{2+} \)
Impure nickel is converted to which of its complex to be decomposed to yield pure nickel?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → \( [\text{Ni}(\text{CO})_4] \)
A metal can be purified by turning it into a coordination compound that is easily separated from the impurities and then decomposing the complex back to the pure metal. For nickel this is done through its volatile carbonyl (the Mond process).
Impure nickel is heated in a stream of carbon monoxide. Nickel combines with CO to give tetracarbonylnickel(0), a volatile liquid; its vapour is carried away and the impurities are left behind:
\( \displaystyle \ce{Ni + 4CO} \xrightarrow{330-350\ \mathrm{K}} \ce{[Ni(CO)4]} \)
The vapour is then heated to a higher temperature, where the complex decomposes to pure nickel and the CO is set free again:
\( \displaystyle \ce{[Ni(CO)4]} \xrightarrow{450-470\ \mathrm{K}} \ce{Ni + 4CO} \)
CO is a neutral ligand, so nickel is in the zero oxidation state in the complex. So impure nickel is converted to \( \ce{[Ni(CO)4]} \).
NCERT (2026-27 edition):
Importance and Applications of Coordination Compounds – “… Similarly, purification of metals can be achieved through formation and subsequent decomposition of their coordination compounds. For example, impure nickel is converted to [Ni(CO)4], which is decomposed to yield pure nickel.”