Select the complexes in which iron is in `+2` oxidation state ?

Options

A. `K_(3)[Fe(CN)_(6))]`
B. `K_(4) [Fe(CN)_(6)]`
C. `Na_(2)[Fe(CN)_(5)NO]`
D. `[Fe(OH)_(2)]^(+)`

Answer

To determine which complexes have iron in the +2 oxidation state, we will analyze each complex step by step.

### Step 1: Analyze the first complex, K3[Fe(CN)6]
- Let the oxidation state of iron (Fe) be \( X \).
- Potassium (K) has an oxidation state of +1. Since there are 3 potassium ions, their total contribution is \( +3 \).
- Cyanide (CN) has an oxidation state of -1. Since there are 6 cyanide ions, their total contribution is \( -6 \).
- The overall charge of the complex is 0.

Setting up the equation:
\[ +3 + X - 6 = 0 \]
Solving for \( X \):
\[ X - 3 = 0 \implies X = +3 \]
**Conclusion:** Iron is in the +3 oxidation state in this complex.

### Step 2: Analyze the second complex, K4[Fe(CN)6]
- Let the oxidation state of iron (Fe) be \( X \).
- Potassium (K) has an oxidation state of +1. Since there are 4 potassium ions, their total contribution is \( +4 \).
- Cyanide (CN) has an oxidation state of -1. Since there are 6 cyanide ions, their total contribution is \( -6 \).
- The overall charge of the complex is 0.

Setting up the equation:
\[ +4 + X - 6 = 0 \]
Solving for \( X \):
\[ X - 2 = 0 \implies X = +2 \]
**Conclusion:** Iron is in the +2 oxidation state in this complex.

### Step 3: Analyze the third complex, Na2[Fe(CN)5NO]
- Let the oxidation state of iron (Fe) be \( X \).
- Sodium (Na) has an oxidation state of +1. Since there are 2 sodium ions, their total contribution is \( +2 \).
- Cyanide (CN) has an oxidation state of -1. Since there are 5 cyanide ions, their total contribution is \( -5 \).
- Nitrosyl (NO) has an oxidation state of 0.
- The overall charge of the complex is 0.

Setting up the equation:
\[ +2 + X - 5 + 0 = 0 \]
Solving for \( X \):
\[ X - 3 = 0 \implies X = +3 \]
**Conclusion:** Iron is in the +3 oxidation state in this complex.

### Step 4: Analyze the fourth complex, [Fe(OH)2]+
- Let the oxidation state of iron (Fe) be \( X \).
- Hydroxide (OH) has an oxidation state of -1. Since there are 2 hydroxide ions, their total contribution is \( -2 \).
- The overall charge of the complex is +1.

Setting up the equation:
\[ X - 2 = +1 \]
Solving for \( X \):
\[ X = +3 \]
**Conclusion:** Iron is in the +3 oxidation state in this complex.

### Final Answer:
The only complex where iron is in the +2 oxidation state is **K4[Fe(CN)6]**.

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