Draw all three resonance structures for the NCO- anion and determine which structure represents the BEST Lewis structure. Show your reasoning by assigning all non-zero formal charges NOTE: The atoms must be bonded in this exact order: NCO”
The Correct Answer and Explanation is:
Resonance Structures of NCO⁻ (isocyanate anion)
We are asked to draw all three resonance structures for the NCO⁻ anion with atoms bonded in the specific order N–C–O. Then we will determine which structure is the most stable (the best Lewis structure) using formal charge calculations.
Step 1: Count Valence Electrons
- Nitrogen (N): 5 electrons
- Carbon (C): 4 electrons
- Oxygen (O): 6 electrons
- Extra electron due to negative charge: +1 electron
Total = 5 + 4 + 6 + 1 = 16 valence electrons
Resonance Structures
Let’s draw all possible resonance structures with the N–C–O backbone.
Structure 1
N=C=O⁻
- N double bonded to C; C double bonded to O; O has 3 lone pairs.
- Formal Charges:
- N: 5 – (4 bonding + 0 lone pair) = +1
- C: 4 – (4 bonding) = 0
- O: 6 – (4 nonbonding + 2 bonding) = –1
✅ Total charge = –1 ✔
Structure 2
N≡C–O⁻
- N triple bonded to C; C single bonded to O; O has 3 lone pairs.
- Formal Charges:
- N: 5 – (6 bonding) = –1
- C: 4 – (4 bonding) = 0
- O: 6 – (6 nonbonding + 2 bonding) = –1
❌ Total charge = –2 ✖ (invalid – doesn’t match –1 charge)
Structure 3
N⁻–C≡O
- N single bonded to C; C triple bonded to O; O has 2 lone pairs.
- Formal Charges:
- N: 5 – (2 bonding + 6 nonbonding) = –1
- C: 4 – (4 bonding) = 0
- O: 6 – (4 bonding + 4 nonbonding) = 0
✅ Total charge = –1 ✔
Best Lewis Structure
Structure 3 is the most stable resonance form, and here’s why:
- It minimizes formal charges: only one atom (N) has a non-zero formal charge (–1).
- Negative formal charge is on the most electronegative atom (N over C).
- All atoms follow the octet rule.
Final Answer:
- The three resonance structures of NCO⁻ are:
- N=C=O⁻ (formal charges: N +1, O –1)
- N≡C–O⁻ (invalid: total charge –2)
- N⁻–C≡O (formal charges: N –1)
✅ Best structure: N⁻–C≡O due to correct formal charges and maximum stability.
