Consider the NCO anion. the following three Lewis structures can be drawn for it: Structure 1 Structure 2 Structure 3 [N=C=0] [:N=C�Ö:]¯ � [:N�C=0:]¯ Based on the formal charges determined in the previous three questions, which of these three resonance structures would you predict to be the most stable? Structure 1 Structure 2 Structure 3 Structures 2 and 3 (equally stable) None
The Correct Answer and Explanation is:
The most stable resonance structure for the NCO⁻ (fulminate anion) is Structure 1: [N≡C–O]⁻.
Explanation
To determine the most stable resonance structure, we analyze formal charges, octet rule compliance, and electronegativity.
Structure Overview:
- Structure 1: [N≡C–O]⁻
- Nitrogen forms a triple bond with carbon.
- Carbon forms a single bond with oxygen.
- The negative charge is located on oxygen, the most electronegative atom.
- Structure 2: [:N=C–O:]⁻
- Nitrogen has a lone pair and is bonded to carbon with a double bond.
- Carbon is double-bonded to oxygen.
- The negative charge is on nitrogen, which is less electronegative than oxygen.
- Structure 3: [:N–C=O:]⁻
- Nitrogen is single-bonded to carbon.
- Carbon is double-bonded to oxygen.
- The negative charge is on nitrogen, and carbon has an incomplete octet or excessive formal charge.
Formal Charge Calculations:
Structure 1:
- Nitrogen (N): 5 valence electrons – (2 nonbonding + 3 bonds) = 0
- Carbon (C): 4 – (4 bonds) = 0
- Oxygen (O): 6 – (6 nonbonding + 1 bond) = –1
Structure 2:
- Nitrogen: 5 – (4 nonbonding + 2 bonds) = –1
- Carbon: 4 – (4 bonds) = 0
- Oxygen: 6 – (4 nonbonding + 2 bonds) = 0
Structure 3:
- Nitrogen: 5 – (6 nonbonding + 1 bond) = –2
- Carbon: 4 – (3 bonds) = +1
- Oxygen: 6 – (4 nonbonding + 2 bonds) = 0
Conclusion:
Structure 1 has formal charges closest to zero and places the negative charge on oxygen, the most electronegative atom. It also satisfies the octet rule for all atoms.
Thus, Structure 1 is the most stable resonance form of the NCO⁻ anion.
