Coordination compounds are formulated and named according to the system set up by the Inorganic Nomenclature Committee of the International Union of Pure and Applied Chemistry (IUPAC). It is called the IUPAC System of Nomenclature.
The following rules are applied while writing the formulas:
(i) The formula of the cation whether simple or complex is written first followed by that of the anion.
(ii) The coordination entity is written in square brackets.
(iii) The sequence of symbols within the coordination entity is: first the symbol of the central metal atom followed by ligands in alphabetical order. According to the latest IUPAC recommendations, the placement of a ligand in the list does not depend on its charge. The ligands in the coordination entity are arranged as:
(a) The different ligands are arranged alphabetically according to the first symbol of their formulae. For example, H2O, NH3, NO3–, SO42– and OH–, etc. are cited at H, N, N, S, and O. The ligands containing carbon and hydrogen are cited only under C.
What is the IUPAC name of the complex compound \(K_3[Fe(CN)_6]\)?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Potassium hexacyanidoferrate(III)
In \( \ce{K3[Fe(CN)6]} \) the cation is \( \ce{K^+} \) and the complex ion \( \ce{[Fe(CN)6]^3-} \) is the anion. The name is built part by part:
Cation first: potassium.
Ligand: \( \ce{CN^-} \) is an anionic ligand, so its name ends in –ido: cyanido; six of them give hexacyanido.
Metal in a complex anion: the name ends in –ate, and the Latin name is used for iron: ferrate.
Oxidation state of Fe: x + 6(−1) = −3, so x = +3, written (III).
Name: potassium hexacyanidoferrate(III).
Why the other options are incorrect
Option (1) Potassium hexacyanatoferrate(III) — Incorrect. “Cyanato” is the name of the cyanate ligand, \( \ce{OCN^-} \); the ligand here is cyanide, \( \ce{CN^-} \).
Option (3) Potassium ferricyanide — Incorrect. This is an old common name; it does not follow the IUPAC rules (no ligand prefix, no oxidation state).
Option (4) Potassium cyanohexaferrate(III) — Incorrect. The multiplier hexa belongs before the ligand name, not before the metal, and the anionic ligand name should end in –ido.
NCERT (2026-27 edition):
5.3.2 Naming of Mononuclear Coordination Compounds – “(i) The cation is named first in both positively and negatively charged coordination entities.”
“Note: The 2004 IUPAC draft recommends that anionic ligands will end with–ido so that chloro would become chlorido, etc.”
“(vi) … If the complex ion is an anion, the name of the metal ends with the suffix – ate. For example, Co in a complex anion, Co(SCN)42− is called cobaltate. For some metals, the Latin names are used in the complex anions, e.g., ferrate for Fe.”