The chemical formula for potassiumhexacyanoferrate(II) is

Analyze the question: You need the formula of potassium hexacyanoferrate(II). The name tells you everything you need.

Explain the concept (naming coordination compounds):

• Potassium is the cation outside the complex ion.
• Hexa means 6, and cyano means CN⁻ ligands, so there are 6 CN⁻.
• Ferrate(II) means the complex ion is anionic and the iron has oxidation state +2.

Solution:

1. Charge on the complex ion: Fe²⁺ + 6(CN⁻) = (+2) + (−6) = −4 So the complex ion is [Fe(CN)₆]⁴⁻.

2. The complex carries a 4− charge, so it needs 4 K⁺ ions to balance it: 4(+1) + (−4) = 0

3. The formula is therefore K₄[Fe(CN)₆].

Checking the options:

• A. [Fe(CN)₆]⁴⁻ is only the complex ion, not the full compound, because the potassium is missing.
• B. K₃Fe(CN)₆ is potassium hexacyanoferrate(III) (iron is +3), also called potassium ferricyanide.
• C. Fe(CN)₆ is not a charge-balanced species.
• D. K₄Fe(CN)₆ is correct. It is potassium ferrocyanide.

Tip: The Roman numeral gives the oxidation state of the metal. Use it to find the charge of the complex, then add as many K⁺ ions as needed to balance it.

• Ferrate(II) → 4− → K₄
• Ferrate(III) → 3− → K₃

Also remember that "-ate" in the name means the complex is an anion, so a cation like K⁺ must be present.