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The IUPAC name of $[\mathrm{Co(NH_3)_6}]^{3+}$ is
Ahexammine cobaltate(III)
Bhexaamminecobalt(III) ion
Ccobaltammine hexa(III)
Dammoniumcobaltate(III)
Answer & Solution
Correct answer: B. hexaamminecobalt(III) ion
1. Naming a coordination complex (NCERT §5.3): ligands first (in alphabetical order), then central metal, oxidation state in Roman numerals.
2. Number of ligands: prefix is 'hexa' for six ammonia.
3. Ligand name: NH$_3$ as a ligand is called 'AMMINE' (note the double-m).
4. Metal name: since the complex is CATIONIC, the metal keeps its English name 'cobalt' (no '-ate' suffix). For anionic complexes, '-ate' would apply (e.g. cobaltate).
5. Oxidation state: as computed in earlier MCQs, Co is +3, written as (III).
6. Assemble: 'hexaamminecobalt(III) ion' — option B.
7. Option A uses '-ate' (only for anionic complexes — this is cationic). Option C reverses the order. Option D is gibberish.
_Source: NCERT Class 12 Chemistry Part 1, Ch 5, §5.3 (Nomenclature rules), p. 4–5._
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