The cyanate ion (NCO-), C is the central atom, has three possible Lewis Structures. Draw the Lewis Structures. Calculate the formal charges to the atoms in each structure. Which of the structures would be preferred, explain, and does this molecule have resonance?
The cyanate ion (NCO-), C is the central atom, has three possible Lewis Structures. Draw the Lewis Structures. Calculate the formal charges to the atoms in each structure. Which of the structures would be preferred, explain, and does this molecule have resonance?
The Correct Answer and Explanation is:
Correct Answer:
The cyanate ion (NCO⁻), with carbon as the central atom, has three possible Lewis structures. Here’s a summary of each:
Structure 1:
:⁻N=C=O:
Formal Charges:
N: –1
C: 0
O: 0
Structure 2:
N≡C–O⁻
Formal Charges:
N: 0
C: 0
O: –1
Structure 3:
⁻N–C≡O
Formal Charges:
N: –1
C: 0
O: 0
Explanation n (C), and oxygen (O). Carbon is designated as the central atom, and the molecule has a total of 16 valence electrons:
- Nitrogen: 5
- Carbon: 4
- Oxygen: 6
- Extra electron for the negative charge: 1
Total = 16 electrons
Three plausible Lewis structures are possible, based on different bonding arrangements and lone pairs:
- Structure 1 has a double bond between C and both N and O (N=C=O), with the extra electron (formal charge –1) on nitrogen. This gives nitrogen a formal charge of –1 and neutral charges on carbon and oxygen.
- Structure 2 places a triple bond between N and C, and a single bond between C and O⁻. Here, oxygen carries the –1 formal charge, while nitrogen and carbon are neutral.
- Structure 3 has a single bond between N⁻ and C, and a triple bond between C and O. The formal charge is –1 on nitrogen, with neutral carbon and oxygen.
Among the three, Structure 2 is the most stable and preferred. This is because:
- Formal charges are minimized and placed on the most electronegative atom (oxygen).
- The octets are satisfied.
- The structure distributes the negative charge where it is best stabilized (on oxygen rather than nitrogen).
Thus, cyanate has resonance, since the actual electronic structure is a hybrid of these Lewis structures, especially between Structure 1 and Structure 2. The resonance stabilizes the molecule further by delocalizing the negative charge over oxygen and nitrogen.
