Match List I with List II LIST ICompoundLIST IIOxidation Number of Co…

The correct answer is Option (3) → A-IV, B-I, C-III, D-II

The oxidation state of the central metal is found from the charge balance: the charge on the complex ion equals the oxidation state of the metal plus the charges of the ligands. First get the charge of the complex ion from the counter ion; neutral ligands (\( \ce{NH3} \), CO) count as 0.

• (A) [Co(NH₃)₅(NCS)](SO₃) — This matches (IV) +3. Sulphite is \( \ce{SO3^2-} \), so the complex ion is +2; \( \ce{NCS^{-}} \) is −1:

      \( \displaystyle x + (-1) + 5(0) = +2 \Rightarrow x = +3 \)

• (B) [Co(NH₃)₄Cl₂]SO₄ — This matches (I) +4. Sulphate is \( \ce{SO4^2-} \), so the complex ion is +2; each \( \ce{Cl^{-}} \) is −1:

      \( \displaystyle x + 4(0) + 2(-1) = +2 \Rightarrow x = +4 \)

• (C) Na₄[Co(S₂O₃)₃] — This matches (III) +2. Four \( \ce{Na^{+}} \) make the complex ion −4; thiosulphate is \( \ce{S2O3^2-} \):

      \( \displaystyle x + 3(-2) = -4 \Rightarrow x = +2 \)

• (D) [Co₂(CO)₈] — This matches (II) 0. Carbon monoxide is neutral and the carbonyl has no charge, so each cobalt is 0:

      \( \displaystyle 2x + 8(0) = 0 \Rightarrow x = 0 \)

Shortcut: (B)-(I) appears only in Option (3).

Note: The same/similar question has been asked in different years by NTA as per the details given below: