Which of the following is the correct IUPAC name for the compound [Co(NH3)5Cl] Cl2?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Pentaamminechloridocobalt(III) chloride
IUPAC naming of coordination compounds follows these rules:
The cation is named first, then the anion, as in simple salts.
Inside the complex, ligands are named first, in alphabetical order, and then the metal. Prefixes like di-, tri-, penta- are ignored when alphabetising.
Neutral ligands keep their names (\( \ce{NH3} \) is ammine); anionic ligands end in -o (\( \ce{Cl^{-}} \) is chlorido).
The oxidation state of the metal follows in Roman numerals in brackets.
In \( \ce{[Co(NH3)5Cl]Cl2} \), the complex cation is \( \ce{[Co(NH3)5Cl]^{2+}} \) (balanced by two \( \ce{Cl^{-}} \)):
\( \displaystyle x+5(0)+(-1)=+2\ \Rightarrow\ x=+3 \)
The ligands are five ammine and one chlorido; “a”mmine comes before “c”hlorido. So the name is pentaamminechloridocobalt(III) chloride.
Note: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| 25-05 S1 | – | – | 01-06 S3 | – |