Antibonding molecular orbitals

Definition

Antibonding molecular orbitals are specific types of molecular orbitals formed when atomic orbitals combine in such a way that they create a region of high electron density away from the bond axis, which results in destabilization of the molecule. These orbitals are denoted with a star (*) and typically have higher energy than the corresponding bonding molecular orbitals. The presence of electrons in these orbitals can weaken or even prevent the formation of a stable bond between atoms.

5 Must Know Facts For Your Next Test

1. Antibonding molecular orbitals have a node between the two nuclei, indicating a region where electron density is low, which contrasts with bonding orbitals that have electron density in this area.
2. The energy level of antibonding molecular orbitals is always higher than that of bonding molecular orbitals due to the destabilizing effects of electron-electron repulsion.
3. When filling molecular orbitals with electrons, according to the Aufbau principle, antibonding orbitals are filled only after all bonding orbitals have been occupied.
4. The occupation of antibonding molecular orbitals can result in a decrease in bond order, which is calculated as (number of electrons in bonding orbitals - number of electrons in antibonding orbitals) / 2.
5. In coordination compounds, antibonding molecular orbitals play a crucial role in determining the electronic properties and color of metal complexes due to their interaction with ligand field splitting.

Review Questions

• How do antibonding molecular orbitals affect the stability of a molecule compared to bonding molecular orbitals?
  • Antibonding molecular orbitals decrease the stability of a molecule because they create regions where there is low electron density between the nuclei, leading to destabilization. While bonding molecular orbitals increase stability by concentrating electron density between the nuclei, the presence of electrons in antibonding orbitals counters this effect. Consequently, if there are more electrons occupying antibonding orbitals than those in bonding ones, it can lead to weaker bonds or even no bond formation at all.
• Discuss how the concept of bond order relates to antibonding molecular orbitals and what it indicates about a molecule's stability.
  • Bond order is determined by the formula (number of electrons in bonding molecular orbitals - number of electrons in antibonding molecular orbitals) / 2. A higher bond order generally indicates greater stability and stronger bonds within a molecule. If antibonding molecular orbitals are populated, they reduce the effective bond order by subtracting from the number of stabilizing bonding electrons. Therefore, analyzing bond order provides insight into how strongly atoms are bonded together and how stable or unstable a molecule may be.
• Evaluate how antibonding molecular orbitals influence the electronic properties and reactivity of coordination compounds.
  • Antibonding molecular orbitals significantly influence the electronic properties and reactivity of coordination compounds by participating in ligand field splitting. When ligands approach a metal center, they interact with its d-orbitals and can lead to the filling of both bonding and antibonding molecular orbitals. The presence of filled antibonding levels can affect the absorption of light and consequently the color observed in these complexes. Additionally, if certain antibonding states are energetically favorable for excitation, they can influence reactivity patterns during chemical processes involving coordination compounds.

Related terms

bonding molecular orbitals: Bonding molecular orbitals are formed when atomic orbitals combine constructively, resulting in a region of increased electron density between two nuclei, which stabilizes the bond.

Molecular Orbital Theory: A theoretical framework that describes the electronic structure of molecules by considering the combination of atomic orbitals to form molecular orbitals, which can be either bonding or antibonding.

Ligand Field Theory: An extension of Molecular Orbital Theory that focuses on the interactions between metal ions and ligands in coordination compounds, emphasizing the role of d-orbitals.