The cyanate ion (NCO–) has three possible Lewis structures. To determine which of these structures is the more preferred resonance structure, we can analyze the formal charges of each structure.
First, let’s calculate the formal charges for each possible Lewis structure:
- Structure 1: N has a formal charge of 0, C has a formal charge of 0, and O has a formal charge of -1.
- Structure 2: N has a formal charge of +1, C has a formal charge of -1, and O has a formal charge of 0.
- Structure 3: N has a formal charge of -1, C has a formal charge of 0, and O has a formal charge of +1.
Formal charge is calculated using the formula:
Formal Charge = Valence Electrons – (Non-Bonding Electrons + 1/2 Bonding Electrons)
In general, resonance structures with formal charges closest to zero are preferred. Additionally, any negative formal charges should preferably be on the more electronegative atoms, while positive charges should ideally be on less electronegative atoms.
Analyzing our calculations:
- In Structure 1, the formal charges are balanced, with the negative charge on oxygen, which is more electronegative than carbon and nitrogen.
- In Structure 2, the positive charge on nitrogen is less favorable due to nitrogen’s position in the periodic table, as it is less electronegative than carbon.
- Structure 3 places a negative charge on nitrogen, which is not ideal since nitrogen is less electronegative than oxygen.
Based on these observations, the first structure, where the negative charge is on the oxygen atom and both carbon and nitrogen have a formal charge of zero, is the most stable and favored resonance structure for the cyanate ion. This resonance structure minimizes formal charges and satisfies electronegativity considerations.