Draw Lewis structures for the cyanate ion (OCN-) including possible resonance forms (at least three). Use formal charges on atoms to determine the most stable resonance structure of the cyanate ion
The Correct Answer and Explanation is:
Resonance Structures:
Structure 1:
mathematicaCopyEdit :Ö: .. ..
|| :C≡N: :C≡N:
⁻ [:O≡C–N:] [:O=C=N:] [:O–C≡N:]
FC: O = 0 FC: O = -1
C = 0 C = 0
N = -1 N = 0
Structure 2:
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:Ö: :Ö:
||| ||
[:O–C≡N:] [:O≡C–N:]
FC: O = 0 FC: O = -1
C = +1 C = +1
N = -2 N = 0
Structure 3:
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[:O=C=N:]
FC: O = 0
C = 0
N = -1
Explanation (Textbook-Style):
The cyanate ion (OCN⁻) is a linear polyatomic ion composed of one oxygen, one carbon, and one nitrogen atom, carrying an overall -1 charge. To understand its bonding and stability, we draw its resonance structures, which are multiple Lewis structures representing delocalized electron configurations.
To begin, we calculate the total valence electrons:
- Oxygen has 6 valence electrons
- Carbon has 4
- Nitrogen has 5
- Plus 1 additional electron for the negative charge
Total = 6 + 4 + 5 + 1 = 16 electrons
Structure 1 shows a double bond between oxygen and carbon and a triple bond between carbon and nitrogen. The formal charges are: O = 0, C = 0, N = -1. This is the most stable resonance structure because all atoms have formal charges closest to zero, and the negative charge resides on nitrogen, the most electronegative atom that can best accommodate it.
Structure 2 places the negative charge on oxygen and gives carbon a formal charge of +1 and nitrogen a charge of -2. This distribution is less favorable because the negative charge is split unevenly and the formal charges are higher in magnitude.
Structure 3, which places the negative charge on oxygen with a triple bond between C and N, is also less favorable due to higher formal charges.
Conclusion:
The most stable resonance form of OCN⁻ is Structure 1, where the negative charge is on nitrogen and all atoms have minimal formal charges. This form contributes the most to the resonance hybrid.
