The IUPAC name of the complex K_3[Fe(CN)_5NO] is

The correct answer is Option (2) → Potassium pentacyanonitrosylferrate(II)

The IUPAC name of a coordination compound follows fixed rules: the cation is named first; inside the complex the ligands are named in alphabetical order (prefixes such as penta- are ignored), then the metal and its oxidation state in Roman numerals. When the complex is an anion, the metal name ends in –ate, and for iron the Latin name is used: ferrate.

The oxidation state of iron: \( \ce{K+} \) is +1, \( \ce{CN-} \) is −1 and NO (nitrosyl) is neutral:

    \( \displaystyle 3(+1) + x + 5(-1) + 0 = 0 \Rightarrow x = +2 \)

“cyano” comes before “nitrosyl” in the alphabet, and the complex ion \( \ce{[Fe(CN)5NO]^2-} \) is an anion, so the name is potassium pentacyanonitrosylferrate(II).