The cyanate ion (NÇO-) has a central carbon atom and the fulminate ion (CNO-) a central nitrogen atom. Both share the same three atoms but cyanate ion (OCN-) and fulminate ion (CNO-) have vastly different properties. The cyanate ion is stable, while the fulminate ion is unstable and forms explosive compounds. The resonance structures of the cyanate ion and formal charges (in red) are shown in Example 9.8 in the textbook and discussed in lecture.
Draw all valid Lewis structures for the fulminate ion. Compare the structures for cyanate and fulminate ions to explain why fulminate ion is unstable and cyanate ion is stable.
The Correct Answer and Explanation is :
The fulminate ion (CNO-) has the same atoms as the cyanate ion (OCN-), but the resonance structures and the distribution of formal charges lead to drastically different stabilities. Here is how we can compare the two.
Lewis Structures for Fulminate Ion (CNO-):
- Structure 1: The nitrogen (N) is the central atom, bonded to carbon (C) and oxygen (O). A triple bond connects N and C, and a single bond connects N and O. Nitrogen has a lone pair, and carbon has a negative charge, while oxygen holds a positive charge.
- Structure 2: The nitrogen is still the central atom, but the bonding arrangement changes. In this case, a double bond connects nitrogen and carbon, and another double bond connects carbon and oxygen. Formal charges are adjusted with nitrogen carrying a positive charge and oxygen carrying a negative charge.
The second structure is less common but may arise depending on resonance shifts.
Comparison of Cyanate and Fulminate Ions:
- Resonance Structures: The cyanate ion (OCN-) has several resonance structures where the negative charge is delocalized between the oxygen and nitrogen atoms. In contrast, the fulminate ion (CNO-) has fewer resonance structures and a larger imbalance in formal charge distribution.
- Formal Charges: In the cyanate ion, the formal charges are spread more evenly, allowing for a more stable configuration. Oxygen in cyanate typically carries the negative charge, while the carbon and nitrogen are neutral or carry partial positive charges. In contrast, in the fulminate ion, the formal charges are more concentrated, especially on nitrogen, leading to instability.
- Electron Configuration and Bonding: In the fulminate ion, the nitrogen atom is more electronegative than carbon, but when nitrogen carries a positive charge, it creates an unstable configuration. The triple bond between nitrogen and carbon in the fulminate ion creates a strained system because nitrogen is very electronegative and doesn’t easily accommodate a positive charge.
Stability Explanation:
The cyanate ion is stable because its resonance structures distribute the charges more evenly and minimize high-energy charge imbalances. In contrast, the fulminate ion is unstable due to the highly polarized bonds and the central nitrogen atom carrying a positive charge, which destabilizes the molecule, making it more prone to decomposition or explosive reactions.
This distribution of formal charges in the cyanate and fulminate ions helps explain why cyanate is stable, while fulminate is not.