Question

Write three resonance structures for (a) the cyanate ion \(\left(\mathrm{NCO}^{-}\right)\) and (b) the isocyanate ion \(\left(\mathrm{CNO}^{-}\right)\). In each case, rank the resonance structures in order of increasing importance.

Short answer

For both ions, draw three resonance structures and rank by stability based on formal charges and octet completion.

Step-by-step solution

Understand Resonance Structures

Resonance structures are multiple ways to draw a molecule that show different possible distributions of electrons. They help represent delocalization of electrons within the molecule.

Draw Lewis Structure for Cyanate Ion (NCO")

For the cyanate ion, draw the Lewis structure considering nitrogen (N), carbon (C), and oxygen (O). Ensure the total charge of -1 is accounted for. Possible arrangement: N=C=O - with formal charges placed correctly.

Create Resonance Structures for Cyanate Ion

Generate additional resonance structures by moving the electrons: (1) Move a lone pair from N to create an N-C triple bond and shift the pi bond between carbon and oxygen to form a double O-C bond. (2) Shift the triple bond to N-C and move the lone pair on oxygen to form a double bond with carbon. Adjust formal charges accordingly.

Rank Resonance Structures for Cyanate Ion

Rank these structures based on stability: structures with fewer formal charges and more complete octets are generally more stable. The most significant resonance form is usually the one with the lowest energy and most complete octet, generally the first structure you drew.

Draw Lewis Structure for Isocyanate Ion (CNO").

For the isocyanate ion NCO-, draw the Lewis structure considering carbon (C) first, followed by nitrogen (N) and oxygen (O). Possible arrangement: C=N=O - keeping in mind the -1 charge.

Create Resonance Structures for Isocyanate Ion

For CNO-, move pairs of electrons to generate three resonance structures: (1) Move a lone pair from N to form a triple bond C-N and a single bond O-N. (2) Shift double bond from C=O to form a bond C-N and a triple bond O-N. Adjust formal charges on all atoms accordingly.

Rank Resonance Structures for Isocyanate Ion

Rank the resonance structures by stability. More stable structures have complete octets and fewer formal charges. Evaluate and identify the most stable form as the lead structure.

Key concepts

Cyanate Ion

The cyanate ion is an interesting molecular ion that is represented as \(\mathrm{NCO}^{-}\). This ion consists of a nitrogen (N), a carbon (C), and an oxygen (O) atom held together in a linear arrangement. To understand its structure, one starts by drawing a simple Lewis structure.

### Drawing the Lewis Structure• Begin by considering the total number of valence electrons, which is 16 for the cyanate ion (5 from N, 4 from C, 6 from O, and 1 extra due to the -1 charge).
• When creating the Lewis structure, nitrogen usually forms a triple bond with carbon (N≡C) and a single bond with oxygen (O−). This results in a formal charge on each atom, needing careful arrangement to minimize these charges.

### Resonance StructuresCyanate ion can have multiple resonance structures. These are basically different "versions" of the molecule with electrons positioned differently:

• One common structure has a triple bond between nitrogen and carbon, and a single bond from carbon to oxygen, with appropriate formal charges.
• Shifting electrons can create a double bond from N-C and a double bond from C-O.
• Another arrangement includes a double bond from N-C and a triple bond from C-O.

Rank these structures by stability, prioritizing those where formal charges are minimized and octets are complete.

Isocyanate Ion

The isocyanate ion, represented as \(\mathrm{CNO}^{-}\), shares some similarities with the cyanate ion but has a different atomic arrangement. Here, carbon (C) is the central atom, bonded to a nitrogen (N) and an oxygen (O) atom.

### Drawing the Lewis Structure• Like the cyanate ion, isocyanate also has 16 valence electrons. The challenge comes from arranging these electrons to achieve a stable configuration.
• Typically, you place carbon in-between nitrogen and oxygen. A double bond is often drawn between carbon and nitrogen (C=N), and another between carbon and oxygen (C=O), maintaining the overall -1 charge of the ion.

### Resonance StructuresJust as with the cyanate ion, the isocyanate ion can be depicted with different resonance structures:

• A typical structure might involve a double bond from C to N and a double bond to O.
• By moving lone pairs and bonds, other structures can feature a triple bond between C and N with a single bond to O, adjusting charges as necessary.
• Another possible structure exchanges bonds such that N has a triple bond to O while bonding to C.

When ranking these structures, assess based on stability gained from fewer formal charges and more complete octets.

Lewis Structures

Lewis structures are a fundamental tool in chemistry for depicting molecules. They illustrate how atoms in a molecule are bonded together and show the placement of electrons around each atom. This portrayal helps in understanding a molecule’s geometry and reactivity.

### Constructing a Lewis Structure • Start by calculating the total number of valence electrons available from all the atoms in the molecule or ion.
• Arrange the atoms, selecting a central atom (usually the least electronegative).
• Use pairs of electrons to form bonds between atoms, typically represented by lines.
• Distribute the remaining electrons to satisfy the octet rule (8 electrons per atom) for main group elements.

### Importance of Lewis Structures These structures provide insight into the likely bond formation and electron allocation within molecules. In resonance situations, different feasible structures augment the understanding of possible electron delocalization leading to increased molecular stability.

Electron Delocalization

Electron delocalization is an essential concept related to resonance structures and is central to understanding molecular stability. It refers to the spreading of electron density across several atoms, rather than being localized between just two atoms. This phenomenon can enhance the stability of a molecule.

### How Delocalization Works• It occurs prominently in resonance structures where electrons can shift or "resonate" among different bonds.
• Delocalized electrons usually reside in \(\pi\)-bonds (as opposed to \(\sigma\)-bonds), which can spread over multiple adjacent atoms, offering greater stabilization.

### Importance in ChemistryThe concept of electron delocalization helps explain the unexpected stability of certain molecules that might appear unstable based on localized structures. This is because the energy of a delocalized structure is generally lower than that of any single resonance form.
A practical example is benzene, where delocalization accounts for its exceptional stability despite having alternating double bonds in its structure.