Which is not a permissible resonating structure ?

Options

A.
B.
C.
D.

Explanation

To determine which of the given structures is not a permissible resonating structure, we need to analyze each option based on the rules of resonance and the octet rule. Here’s a step-by-step breakdown of the solution:

### Step 1: Analyze the first structure (Option A)
- The first structure is **CS3-NO**.
- Count the valence electrons:
- Carbon (C) has 4 valence electrons, and there are 3 carbon atoms (3 x 4 = 12).
- Nitrogen (N) has 5 valence electrons.
- Oxygen (O) has 6 valence electrons.
- Total valence electrons = 12 (from C) + 5 (from N) + 6 (from O) = 23.
- Check the octet rule:
- Each carbon has 4 bonds (octet satisfied).
- Nitrogen has 3 bonds and 1 lone pair (octet satisfied).
- Oxygen has 2 bonds and 2 lone pairs (octet satisfied).
- Conclusion: This structure is permissible.

### Step 2: Analyze the second structure (Option B)
- The second structure is **CS2=N=O**.
- Count the valence electrons:
- 3 Carbon atoms contribute 12 electrons.
- Nitrogen contributes 5 electrons.
- Oxygen contributes 6 electrons.
- Total valence electrons = 12 + 5 + 6 = 23.
- Check the octet rule:
- Each carbon has 4 bonds (octet satisfied).
- Nitrogen has 5 bonds (3 from double bonds with C and O), which exceeds the octet rule.
- Conclusion: This structure is **not permissible** because nitrogen has 5 bonds.

### Step 3: Analyze the third structure (Option C)
- The third structure is **CS2=N** with a negative charge.
- Count the valence electrons:
- 3 Carbon atoms contribute 12 electrons.
- Nitrogen contributes 5 electrons.
- Total valence electrons = 12 + 5 = 17.
- Check the octet rule:
- Each carbon has 4 bonds (octet satisfied).
- Nitrogen has 3 bonds and 1 lone pair (octet satisfied).
- Conclusion: This structure is permissible.

### Step 4: Analyze the fourth structure (Option D)
- The fourth structure is **CS3=N^+O**.
- Count the valence electrons:
- 3 Carbon atoms contribute 12 electrons.
- Nitrogen contributes 5 electrons.
- Oxygen contributes 6 electrons.
- Total valence electrons = 12 + 5 + 6 = 23.
- Check the octet rule:
- Each carbon has 4 bonds (octet satisfied).
- Nitrogen has 4 bonds (octet satisfied).
- Oxygen has 2 bonds and 2 lone pairs (octet satisfied).
- Conclusion: This structure is permissible.

### Final Conclusion
After analyzing all the options, we conclude that **Option B (CS2=N=O)** is not a permissible resonating structure because nitrogen exceeds the octet rule with 5 bonds.

**Answer: Option B is not a permissible resonating structure.**

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