Which of the following correctly represents a valid resonance structure for the cyanate ion $C_N{-}^1$ ($\mathrm{CNO}-$)?

• A. A structure with a single bond between $C$ and $N$, a triple bond between $C$ and $O$, and a negative charge on $N$
• B. A structure with a double bond between $C$ and $N$, a double bond between $C$ and $O$, and a negative charge on $C$
• C. A structure with a single bond between $N$ and $O$, a triple bond between $C$ and $N$, and a negative charge on $C$
• D. A structure with a triple bond between $C$ and $N$, a single bond between $C$ and $O$, and a negative charge on $O$

Answer: (D)

Verified step by step guidance

1. Step 1: Draw the Lewis structure of the cyanate ion (CNO⁻) by counting the total valence electrons. Carbon has 4, nitrogen has 5, oxygen has 6, and there is one extra electron due to the negative charge, giving a total of 16 valence electrons.
2. Step 2: Arrange the atoms linearly as N–C–O or C–N–O and connect them with single bonds initially. Then distribute the remaining electrons to satisfy the octet rule for each atom, keeping in mind the total number of electrons.
3. Step 3: Identify possible resonance structures by moving electrons (pi bonds and lone pairs) without changing the positions of the atoms. For example, shift double or triple bonds between C–N and C–O, and adjust formal charges accordingly.
4. Step 4: Calculate the formal charges for each resonance structure using the formula: \(\text{Formal charge} = \text{Valence electrons} - (\text{Nonbonding electrons} + \frac{1}{2} \times \text{Bonding electrons})\). The most stable resonance structures have formal charges closest to zero and negative charges on the more electronegative atoms.
5. Step 5: Compare the resonance structures and select the valid ones where the octet rule is satisfied, formal charges are minimized, and the negative charge is placed on the most electronegative atom (oxygen in this case). This will help identify the correct resonance structure with a triple bond between C and N, a single bond between C and O, and a negative charge on oxygen.