Question
Find the oxidation state of iron and the IUPAC name of the complex . A student writes “potassium hexacyanoferrate(II).” Where is the error?
Solution — Step by Step
Total charge of the complex anion: (balanced by 3 K⁺).
CN⁻ has charge . Six CN⁻ contribute . So:
Iron is in the +3 oxidation state.
Rules: (1) cation first (potassium), (2) ligands alphabetical with prefixes (hexa for 6), (3) metal with oxidation state in Roman numerals, (4) suffix “-ate” for anionic complexes.
Ligand: 6 cyano (anionic ligand → “cyanido” in the latest IUPAC, “cyano” in older books — both accepted in CBSE).
Metal: iron → in anionic complex, use Latin name “ferrate.”
Oxidation state: III.
Full name: potassium hexacyanidoferrate(III) (or potassium hexacyanoferrate(III) in older notation).
The student wrote “(II)” instead of “(III).” This came from miscalculating the oxidation state — possibly by treating CN as neutral or by miscounting K. The correct answer is iron in +3.
Oxidation state: +3. IUPAC name: potassium hexacyanidoferrate(III).
Why This Works
For oxidation-state assignment: write down the overall charge of the complex ion, subtract the contributions from ligands (with their assigned charges), and what remains is the metal’s oxidation state. Always know your ligand charges by heart — CN⁻, NO₂⁻, OH⁻ are −1; H₂O, NH₃ are 0; CO is 0; ox²⁻ (oxalate), CO₃²⁻ are −2.
For naming, the order is: (cation prefix) (ligand alphabetical with multiplier prefix) (metal name) (oxidation state in Roman). Anionic complexes use Latin metal names (ferrate, cuprate, plumbate) — cationic and neutral complexes use English names (iron, copper, lead).
Alternative Method
Quick check via the related blood-yellow vs blood-red distinction. is the yellow ferrocyanide (Fe in +2). is the red ferricyanide (Fe in +3). Memorising this pair lets you cross-verify in seconds.
Common Mistake
Students forget that the number of K in the formula tells you the charge of the complex. → complex is , → complex is . Misreading as flips Fe from +3 to +2, which causes wrong colour, wrong name, wrong magnetic moment — wrong everything.