What is the formula of the complex tris(ethane-1,2-diamine)chromium(III) sulphate?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → [Cr(en)3]2(SO4)3
The formula is built from the name, part by part:
Ligand: ethane-1,2-diamine (en), \( \ce{H2N-CH2-CH2-NH2} \), is a neutral molecule; “tris” means three of them.
Charge on the complex ion: Cr is +3 and the three en add no charge, so the complex cation is \( \ce{[Cr(en)3]^3+} \).
Counter ion: sulphate is \( \ce{SO4^2-} \), outside the square bracket.
Charge balance: 2 × (+3) = +6 is balanced by 3 × (−2) = −6, so two complex cations combine with three sulphate ions.
Formula: [Cr(en)3]2(SO4)3.
NCERT (2026-27 edition):
5.3.2 Naming of Mononuclear Coordination Compounds – “2. [Co(H2NCH2CH2NH2)3]2(SO4)3 is named as: tris(ethane-1,2–diamine)cobalt(III) sulphate
Explanation: The sulphate is the counter anion in this molecule. Since it takes 3 sulphates to bond with two complex cations, the charge on each complex cation must be +3. Further, ethane-1,2–diamine is a neutral molecule, so the oxidation number of cobalt in the complex ion must be +3.”