The oxidation number of Fe in [Fe(CN)$_6$]$^{3-}$ is

Analyze the question: We need the oxidation number of iron in the complex ion [Fe(CN)₆]³⁻.

Concept: In a complex ion, the sum of the oxidation numbers of the central metal and all the ligands equals the overall charge on the ion. Cyanide (CN⁻) is a ligand with a charge of -1.

Solution:

Let the oxidation number of Fe = x.

• There are 6 CN⁻ ligands, each with charge -1, giving a total of 6(-1) = -6
• The overall charge on the complex = -3

x + (-6) = -3 x = -3 + 6 x = +3

So iron is in the +3 state (this is the ferricyanide ion, and Fe³⁺ is the "ferric" state).

Answer: The correct option is A (+3).

Tip: For complex ions, always use: (oxidation number of metal) + (total charge of ligands) = charge on the complex. Memorize common ligand charges: CN⁻ = -1, Cl⁻ = -1, OH⁻ = -1, NH₃ = 0, H₂O = 0. Also, "ferricyanide" = Fe³⁺ and "ferrocyanide" [Fe(CN)₆]⁴⁻ = Fe²⁺, a useful naming link for exams.