What is the formula of tris(ethane-1,2-diamine)cobalt(III)sulphate?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → [Co(NH2CH2CH2NH2)3]2(SO4)3
To write the formula of a coordination compound from its name:
The complex ion is written in square brackets, with the metal first and then the ligands; the number prefix gives how many ligands there are (tris = 3 for a ligand whose name already contains a number).
The Roman numeral gives the oxidation state of the metal; with neutral ligands it is also the charge of the complex ion.
The cation and anion are then taken in the ratio that makes the compound electrically neutral.
Here, ethane-1,2-diamine (en, \( \ce{NH2CH2CH2NH2} \)) is a neutral bidentate ligand and cobalt is Co(III):
\( \displaystyle \text{cation: }\ce{[Co(en)3]^3+}\quad\quad\text{anion:}\ce{SO4^2-} \)
\( \displaystyle 2\times(+3)+3\times(-2)=0 \)
So two complex cations balance three sulphate ions: [Co(NH2CH2CH2NH2)3]2(SO4)3.
Why the other options are incorrect
Option (1) [Co(NH2CH2CH2NH2)3](SO4)3 — Incorrect. Total charge = +3 − 6 = −3; the compound would not be neutral.
Option (2) [Co(NH2CH2CH2NH2)3]3(SO4)2 — Incorrect. Total charge = +9 − 4 = +5; the ratio is inverted.
Option (4) [Co(NH2CH2CH2NH2)3](SO4)2 — Incorrect. Total charge = +3 − 4 = −1; this would need cobalt in the +4 state.
NCERT (2026-27 edition):
Naming of Mononuclear Coordination Compounds – “… The following examples illustrate the nomenclature for 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. …”