In the following brown complex, the oxidation state of iron is +_____________.
[Fe(H₂O)₆]²⁺ + NO → [Fe(H₂O)₅(NO)]²⁺_(Brown complex) + H₂O

Solution

Chemistry ma’am · 4 steps

The trick is the NO ligand: in the brown ring complex it sits as NO⁺, not as neutral NO.

1. What NO does. When NO replaces a water molecule, it gives one electron to the iron. NO becomes the nitrosonium ion NO⁺ and Fe²⁺ is reduced by one step.
2. Charge balance. Water is neutral, NO⁺ carries +1, and the whole complex carries +2. Let the oxidation state of Fe be x.

   x + 5(0) + (+1) = +2 ⇒ x = +1

3. A check. The measured magnetic moment of the brown complex is about 3.9 BM, which means 3 unpaired electrons. That matches Fe⁺ (3d⁷, high spin).
4. So. Iron is in the +1 oxidation state in [Fe(H₂O)₅(NO)]²⁺.

The common slip: 2. Treating NO as a neutral ligand, like water, gives Fe as +2, the oxidation state before NO came in.

The idea to keep. In the brown ring complex, always take nitrosyl as NO⁺. One electron moves from NO to iron, so Fe drops from +2 to +1.

Answer from the NTA final answer key. Solution by PrepWiser.