What is the IUPAC name of K4[Fe(CN)6]?

The correct answer is Option (2) → Potassium hexacyanidoferrate(II)

In naming a coordination compound the cation is named first, the ligands are named before the metal, the oxidation state of the metal is given in Roman numerals, and in a complex anion the metal name ends in -ate.

• Cation: four \( \ce{K^+} \) ions are outside the bracket, so the name starts with potassium and the complex ion is \( \ce{[Fe(CN)6]^4-} \), an anion.

• Ligands: six \( \ce{CN^-} \) ligands → hexacyanido (an anionic ligand name ends in -ido).

• Oxidation state of Fe: x + 6(−1) = −4, so x = +2 → (II).

• Metal in an anion: iron takes its Latin-based name with -ate → ferrate.

Name: potassium hexacyanidoferrate(II). “Iron” would be used only if the complex ion were a cation, and (III) would need \( \ce{Fe^3+} \), as in \( \ce{K3[Fe(CN)6]} \).

NCERT (2026-27 edition):
5.3.2 Naming of Mononuclear Coordination Compounds – “Note: The 2004 IUPAC draft recommends that anionic ligands will end with–ido so that chloro would become chlorido, etc.”

“(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. … If the complex ion is an anion, the name of the metal ends with the suffix – ate. … For some metals, the Latin names are used in the complex anions, e.g., ferrate for Fe.”