What is the IUPAC name of [Co(NH3)6]Cl3?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Hexaamminecobalt(III) chloride
The complex cation is named first: its ligands, then the metal with its oxidation state in Roman numerals; the anion follows.
Complex ion: three \( \ce{Cl^-} \) are outside the bracket, so \( \ce{[Co(NH3)6]^3+} \) is a cation.
Oxidation state of Co: \( \ce{NH3} \) is neutral, so x + 6(0) = +3 and x = +3, written (III).
Ligand name: \( \ce{NH3} \) as a ligand is ammine; six of them give hexaammine.
Metal in a cation: named as the element, cobalt.
Why the other options are incorrect
Option (1) Hexaammoniacobalt(III) chloride — Incorrect. “Ammonia” is not the ligand name; \( \ce{NH3} \) bound to a metal is named ammine.
Option (3) Hexaamminecobaltate(III) chloride — Incorrect. The ending -ate is used only for a complex anion; here the complex ion is a cation.
Option (4) Hexaamminecobalt(II) chloride — Incorrect. Three chloride ions balance a +3 complex ion, so cobalt is +3, not +2.
NCERT (2026-27 edition):
5.3.2 Naming of Mononuclear Coordination Compounds – “(iii) Names of the anionic ligands end in –o, those of neutral and cationic ligands are the same except aqua for H2O, ammine for NH3, carbonyl for CO and nitrosyl for NO. …”
“(v) Oxidation state of the metal in cation, anion or neutral coordination entity is indicated by Roman numeral in parenthesis.
(vi) If the complex ion is a cation, the metal is named same as the element. For example, Co in a complex cation is called cobalt and Pt is called platinum. …”