Select the correct formula for the coordination compound "Dichloridobis (ethane-1,2-diamine) cobalt(III) chloride"
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \([CoCl_2(en)_2]Cl\)
The formula of a complex is built from its name. Inside the square bracket go the metal and its ligands; the counter ion goes outside. The charge of the complex ion follows from the oxidation state of the metal and the charges of the ligands, and decides how many counter ions are needed.
Dichlorido: two \( \ce{Cl-} \) ligands, charge 2 × (−1) = −2.
bis(ethane-1,2-diamine): two neutral en ligands (\( \ce{H2NCH2CH2NH2} \)); “bis” is used because the ligand name already contains a number.
cobalt(III): Co is +3.
\( \displaystyle \text{charge of complex ion} = +3 + 2(-1) + 2(0) = +1 \)
A +1 complex ion needs one chloride ion outside the bracket, so the formula is [CoCl2(en)2]Cl. (Each en is didentate, so the coordination number is 2 + 2 × 2 = 6.)
Why the other options are incorrect
Option (2) \([CoCl_2(en)]Cl\) — Incorrect. It has only one en; “bis” means two.
Option (3) \([CoCl_2(en)_2]Cl_2\) — Incorrect. Two chloride ions outside would need a +2 complex ion, that is, cobalt in the +4 state.
Option (4) \([CoCl_2(en)_2]\) — Incorrect. A neutral complex would need cobalt(II); with Co(III) the complex carries a +1 charge, and the name ends in “chloride”.
NCERT (2026-27 edition):
Naming of Mononuclear Coordination Compounds – “… (iv) Prefixes mono, di, tri, etc., are used to indicate the number of the individual ligands in the coordination entity. When the names of the ligands include a numerical prefix, then the terms, bis, tris, tetrakis are used, the ligand to which they refer being placed in parentheses.”
Naming of Mononuclear Coordination Compounds – “Write the IUPAC names of the following coordination compounds: … (c) [CoCl2(en)2]Cl …
Solution: … (c) dichloridobis(ethane-1,2-diamine)cobalt(III) chloride”