Repeat Problem 16-10 for the cyclopentadienyl ions.
Draw one all-bonding MO, then a pair of degenerate MOs,
and then a final pair of degenerate MOs.
Draw the energy diagram, fill in the electrons,
and confirm the electronic configurations of
the cyclopentadienyl cation and anion.

Verified step by step guidance

1. Identify the cyclopentadienyl ion: The cyclopentadienyl ion can exist as a cation (C5H5+) or an anion (C5H5-). Both ions are derived from the cyclopentadiene molecule (C5H6) by either losing or gaining an electron.
2. Draw the molecular orbitals (MOs): Start by drawing the cyclic structure of cyclopentadiene and then construct the molecular orbitals. For a five-membered ring, there will be five MOs. Begin with the lowest energy all-bonding MO, followed by a pair of degenerate non-bonding MOs, and finally a pair of degenerate anti-bonding MOs.
3. Construct the energy diagram: Arrange the MOs vertically according to their energy levels. The all-bonding MO is at the lowest energy, followed by the degenerate non-bonding MOs, and the degenerate anti-bonding MOs at the highest energy.
4. Fill in the electrons: For the cyclopentadienyl cation (C5H5+), remove one electron from the neutral cyclopentadiene, resulting in 5 π-electrons. For the cyclopentadienyl anion (C5H5-), add one electron, resulting in 7 π-electrons. Distribute these electrons in the MOs according to Hund's rule and the Pauli exclusion principle.
5. Confirm the electronic configurations: For the cation, the configuration will be such that the 5 π-electrons fill the MOs starting from the lowest energy. For the anion, the 7 π-electrons will fill the MOs similarly. Check for aromaticity using Hückel's rule (4n+2 π-electrons) to confirm stability.

Key Concepts

Molecular Orbitals (MOs)

Molecular orbitals are formed by the linear combination of atomic orbitals when atoms bond together. In the context of cyclopentadienyl ions, understanding how these orbitals combine to form bonding and antibonding MOs is crucial. The all-bonding MO represents a state where electrons are stabilized, while degenerate MOs indicate equal energy levels for certain configurations, which is important for predicting the stability and reactivity of the ions.

Cyclopentadienyl Ions

Cyclopentadienyl ions refer to the cation (C5H5+) and anion (C5H5-) derived from cyclopentadiene. The cation has a deficiency of electrons, leading to a higher energy state, while the anion has an extra electron, resulting in a lower energy state. Understanding the electronic configurations of these ions is essential for predicting their chemical behavior and stability, particularly in reactions involving electron transfer.

Energy Diagrams

Energy diagrams visually represent the relative energy levels of molecular orbitals and the distribution of electrons within them. For cyclopentadienyl ions, constructing an energy diagram involves placing the MOs in order of increasing energy and filling them according to the Aufbau principle, Hund's rule, and the Pauli exclusion principle. This helps in confirming the electronic configurations and understanding the stability of the ions based on their filled and unfilled orbitals.