Question
Determine the IUPAC name and the oxidation state of the central metal in . (NEET 2023)
Solution — Step by Step
Inside the brackets: five NH₃ (neutral) and one Cl⁻ (charge ). Outside the brackets: two Cl⁻ as counterions.
Let oxidation state of Co be . Total charge inside brackets must balance the counterions.
(The complex cation has charge because it pairs with outside.)
Apply the rules: ligands listed alphabetically, with prefixes (penta- for five NH₃), then metal with oxidation state in Roman numerals.
- Ligands: ammine (NH₃) and chlorido (Cl⁻). Alphabetically: ammine first, chlorido second.
- “pentaamminechlorido” + “cobalt(III)” + “chloride” (the counterion).
Pentaamminechloridocobalt(III) chloride.
Final answer: Oxidation state of Co is ; IUPAC name is pentaamminechloridocobalt(III) chloride.
Why This Works
Coordination naming has three layers: ligand prefixes, metal name with oxidation state, counterion. Alphabetical order ignores multiplying prefixes (di-, tri-, penta-) but counts the ligand stem.
The 2005 IUPAC recommendation changed “chloro” to “chlorido” — both forms appear in textbooks and JEE/NEET answer keys, but newer NCERT editions use “chlorido”.
Alternative Method
Determine oxidation state by recognising the complex charge first: the formula tells us has charge . Then balance ligand charges inside.
For NEET, memorise the names of common ligands: aqua (H₂O), ammine (NH₃, two m’s!), chlorido (Cl⁻), hydroxido (OH⁻), nitrito-N or nitrito-O (NO₂⁻), and cyanido (CN⁻).
Common Mistake
Spelling “amine” instead of “ammine” — this is the most repeated mistake in NEET 2022 answer scripts. “Amine” refers to organic R-NH₂ groups; “ammine” (double m) is the coordination ligand NH₃. One letter, full marks lost.