In the purification of nickel, impure nickel is converted into a coordination compound X, and X is then decomposed to give pure nickel. Which of the following equations represents the formation of X?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \( \ce{Ni + 4CO -> [Ni(CO)4]} \)
A metal can be purified by first turning it into one of its coordination compounds and then decomposing that compound. Nickel combines with carbon monoxide to give tetracarbonylnickel(0), \( \ce{[Ni(CO)4]} \):
\( \displaystyle \ce{Ni + 4CO -> [Ni(CO)4]} \)
The compound X is then decomposed, which gives back the metal as pure nickel:
\( \displaystyle \ce{[Ni(CO)4] -> Ni + 4CO} \)
CO is a neutral ligand, so nickel stays in the zero oxidation state in both steps.
Why the other options are incorrect
Option (1) \( \ce{Ni + 6CO -> [Ni(CO)6]} \) — Incorrect. Six CO groups round one metal atom give an octahedral carbonyl, as in hexacarbonylchromium(0), \( \ce{[Cr(CO)6]} \). Nickel takes four CO groups and forms the tetrahedral \( \ce{[Ni(CO)4]} \).
Option (2) \( \ce{2Ni + 8CN^- + O2 + 2H2O -> 2[Ni(CN)4]^2- + 4OH^-} \) — Incorrect.
Cyanide with oxygen and water is used to bring gold into solution as \( \ce{[Au(CN)2]^-} \), from which gold is then set free by zinc.
The purification of nickel uses carbon monoxide, not cyanide.
Option (4) \( \ce{2Ni + 8CO -> [Ni2(CO)8]} \) — Incorrect.
A carbonyl with two metal atoms and eight CO groups is octacarbonyldicobalt(0), \( \ce{[Co2(CO)8]} \), a compound of cobalt with a Co–Co bond.
Nickel forms the carbonyl with one metal atom, \( \ce{[Ni(CO)4]} \).
NCERT (2026-27 edition):
5.7 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.”