Which of the following pairs of names and formulas of complexes is not correct?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Potassium hexacyanidoferrate(III) – K4[Fe(CN)6]
In the name of a complex, the oxidation state of the metal is given as a Roman numeral in parenthesis. It is found by charge balance on the coordination entity.
Option (1) Tetraamminecopper(II) sulphate – [Cu(NH3)4]SO4 — A correct pair. \( \ce{NH3} \) is neutral and the entity is \( \ce{[Cu(NH3)4]^2+} \) (balancing \( \ce{SO4^2-} \)), so copper is +2.
Option (2) Diamminesilver(I) chloride – [Ag(NH3)2]Cl — A correct pair. The entity is \( \ce{[Ag(NH3)2]^+} \), so silver is +1.
Option (3) Potassium hexacyanidoferrate(III) – K4[Fe(CN)6] — Not a correct pair. Four \( \ce{K^+} \) ions give the entity \( \ce{[Fe(CN)6]^4-} \): x + 6(−1) = −4, so x = +2. The correct name is potassium hexacyanidoferrate(II); hexacyanidoferrate(III) is \( \ce{K3[Fe(CN)6]} \).
Option (4) Potassium amminepentachloridoplatinate(IV) – K[Pt(NH3)Cl5] — A correct pair. The entity is \( \ce{[Pt(NH3)Cl5]^-} \): x + 0 + 5(−1) = −1, so platinum is +4; the ligands are named in alphabetical order (ammine before chlorido).
NCERT (2026-27 edition):
5.3.2 Naming of Mononuclear Coordination Compounds – “(v) Oxidation state of the metal in cation, anion or neutral coordination entity is indicated by Roman numeral in parenthesis.”