Which of the following is the most stable resonance structure for the $CNO-$ ion when nitrogen is the central atom?

• A. $C=N=O-$ (carbon double bonded to nitrogen, nitrogen double bonded to oxygen, negative charge on oxygen)
• B. $C\equiv N--O$ (carbon triple bonded to nitrogen, nitrogen single bonded to oxygen, negative charge on oxygen)
• C. $C-N\equiv O$ (carbon single bonded to nitrogen, nitrogen triple bonded to oxygen, negative charge on carbon)
• D. $C=N-O$ (carbon double bonded to nitrogen, nitrogen single bonded to oxygen, negative charge on nitrogen)

Answer: (A)

Verified step by step guidance

1. Step 1: Draw all possible resonance structures for the CNO⁻ ion with nitrogen as the central atom, showing different bonding arrangements (single, double, triple bonds) between carbon, nitrogen, and oxygen, and placing the negative charge accordingly.
2. Step 2: Calculate the formal charges for each atom in every resonance structure using the formula: \(\text{Formal charge} = \text{Valence electrons} - (\text{Nonbonding electrons} + \frac{1}{2} \times \text{Bonding electrons})\) to identify the distribution of charges.
3. Step 3: Evaluate the stability of each resonance structure by considering the following criteria: (a) minimize formal charges on atoms, (b) place negative charges on the more electronegative atoms (oxygen > nitrogen > carbon), and (c) maximize the number of bonds (especially double bonds) to satisfy the octet rule.
4. Step 4: Compare the resonance structures based on the above criteria, noting which structure has the least formal charge separation, the negative charge on the most electronegative atom (oxygen), and full octets on all atoms.
5. Step 5: Conclude that the most stable resonance structure is the one with carbon double bonded to nitrogen, nitrogen double bonded to oxygen, and the negative charge localized on oxygen, as it best satisfies formal charge minimization, electronegativity considerations, and octet completion.