The IUPAC nomenclature of the complex K$_4$Fe(CN)$_6$ is

To determine the IUPAC name of the complex K$_4$[Fe(CN)$_6$], we need to follow the rules for naming coordination compounds.

1. Analyze the components

• Cation: Potassium (K$^+$)
• Anion (Coordination Sphere): [Fe(CN)$_6$]$^{4-}$
• Ligand: Cyanide (CN$^-$), which is named "cyano" in a complex. Since there are six, it is "hexacyano".
• Central Metal: Iron (Fe). Because the complex ion is an anion (negative charge), we use the Latin name "ferrate".

2. Determine the Oxidation State of the Metal

To find the oxidation state of Iron (Fe), we set the sum of the charges in the complex to equal the overall charge of the coordination sphere:

• Let $x$ be the oxidation state of Fe.
• The charge of a cyanide ligand (CN$^-$) is $-1$.
• The total charge of the complex [Fe(CN)$_6$]$^{4-}$ is $-4$.

The equation is: $x + 6(-1) = -4$ $x - 6 = -4$ $x = -4 + 6$ $x = +2$

The oxidation state of iron is +2, which is represented in Roman numerals as (II).

3. Assemble the IUPAC Name

• Name the cation first: Potassium
• Name the ligands: hexacyano
• Name the metal with its oxidation state: ferrate(II)

Combining these, we get: Potassium hexacyanoferrate(II).

4. Conclusion

Comparing this to your options:

• A. potassium hexacyanoferrate(II) — Matches our calculation.
• B. potassium hexacyanoferrate(III) — Incorrect oxidation state.
• C. potassium hexacyanoferrate(IV) — Incorrect oxidation state.
• D. potassium hexacyanoferrate(VI) — Incorrect oxidation state.

Exam Tip: When naming coordination complexes, always remember that if the complex ion is negative (anionic), the metal name must end in "-ate". If it were a positive complex ion, you would simply use the English name (e.g., "iron" instead of "ferrate").