Draw a BH3 MO diagram, and describe the group orbitals (3F) and atomic orbitals (B) that overlap to make each MO shown BH3 MO molecule
The Correct Answer and Explanation is:
The Molecular Orbital (MO) diagram for BH3 can be constructed based on its molecular symmetry and the overlap of the group orbitals (from 3F — representing the fluorine group orbitals) and atomic orbitals of boron. Here’s how the MOs are formed:
1. Atomic Orbitals (Boron)
Boron (B) has an atomic number of 5, meaning it has an electron configuration of 1s² 2s² 2p¹. In BH3, the boron atom uses its 2s and 2p orbitals for bonding with the hydrogen atoms.
2. Group Orbitals (Hydrogen)
Each hydrogen atom contributes a 1s atomic orbital. Since BH3 has three hydrogen atoms, this results in three 1s orbitals coming into play. These hydrogen orbitals overlap with the boron orbitals to form bonding and antibonding molecular orbitals.
3. Molecular Orbitals (MO)
- Bonding MOs: When the boron 2p orbitals and hydrogen 1s orbitals combine, they form bonding molecular orbitals (MO), where the electron density is concentrated between the nuclei of boron and hydrogen. These orbitals are lower in energy and promote bonding in the molecule.
- Antibonding MOs: On the other hand, when the orbitals combine in a way that their electron densities cancel out, antibonding molecular orbitals (denoted by σ or π depending on their symmetry) are formed. These are higher in energy and do not favor bonding.
4. Symmetry Considerations
The Molecular Orbitals formed in BH3 follow the symmetry of the molecule. BH3 has a trigonal planar geometry with D3h symmetry. The group orbitals from the hydrogens will combine with the Boron’s orbitals in specific symmetries, and these combinations define how the molecular orbitals are placed in the energy diagram.
- Symmetry of LGO (Linear Group Orbitals): LGO represents the combination of orbitals with same symmetry. In BH3, the three hydrogen orbitals will combine in a symmetric manner with the boron orbitals, resulting in bonding molecular orbitals for the lower energy states.
Explanation of the Overlaps:
Each boron 2p orbital interacts with the hydrogen 1s orbitals to form bonding and antibonding MOs, which are then arranged by energy level. The bond formation is facilitated by these interactions, allowing the BH3 molecule to stabilize in its trigonal planar structure.
