Q.49 Oxidation state of Fe in the complex K3 [Fe(CN)5NO] is (+) __________.

The oxidation state of Fe in K₃[Fe(CN)₅NO] is +2.

This complex, known as potassium pentacyanonitrosylferrate(II), features three K⁺ ions and the [Fe(CN)₅NO]³⁻ anion. Each CN⁻ ligand carries a -1 charge, while NO acts as a neutral nitrosyl ligand in standard coordination chemistry calculations for exam contexts like CSIR NET.

Calculation Method

Let the oxidation state of Fe be $x$. The three K⁺ ions contribute +3 charge, so the complex anion [Fe(CN)₅NO]³⁻ has -3 charge overall.

Set up the equation:
$x+5(-1)+0=-3$
$x-5=-3$
$x=+2$

Ligand Charges Explained

• CN⁻: Always -1 as a monodentate anionic ligand. Five CN⁻ groups total -5.

• NO: Treated as neutral (NO⁰) in IUPAC nomenclature for this complex, unlike NO⁺ in Na₂[Fe(CN)₅NO]²⁻ where Fe is also +2 but charge differs.

• Neutral NO aligns with the -3 anion charge, confirmed across educational sources.

Common Confusion Points

Advanced texts note NO’s “non-innocent” nature (possible NO⁺ with Fe³⁺ and charge redistribution), but exam standards (CSIR NET, JEE) use neutral NO for +2 Fe. Magnetic data (4 unpaired electrons, d⁶ low-spin) supports Fe²⁺.

The oxidation state of Fe in K3[Fe(CN)5NO] complex is a frequent query in coordination chemistry for competitive exams like CSIR NET Life Sciences. This potassium salt contains the pentacyanonitrosylferrate(II) anion, where precise ligand charge assignment reveals Fe in +2 state.

Step-by-Step Oxidation State Calculation

1. Identify counter ions: Three K⁺ contribute +3 charge.

2. Complex anion charge: [Fe(CN)₅NO]³⁻ totals -3.

3. Ligand contributions: 5 × (CN⁻ = -1) = -5; NO = 0 (neutral nitrosyl).

4. Equation: $x-5+0=-3$, so $x=+2$.

This matches IUPAC naming: potassium pentacyanidonitrosylferrate(II).

NO Ligand Charge Controversy

NO can bind as NO⁺ (+1, Fe³⁺ possible) or NO⁻ (-1), but in K3[Fe(CN)5NO]³⁻, neutral NO fits the -3 charge. Contrast with Na₂[Fe(CN)₅NO]²⁻ (NO⁺, Fe²⁺). Exam contexts prioritize charge balance over bonding nuance.

Exam Relevance for CSIR NET

CSIR NET questions test this via charge balance. Similar to K₃[Fe(CN)₆]³⁻ (Fe³⁺), but NO neutrality shifts Fe to +2. Practice verifies via magnetic moment (Fe²⁺: 4 unpaired e⁻).