How does back bonding influence the stability of certain molecules?

Back bonding increases the stability of certain molecules by sharing electron density between atoms, reducing overall energy.

Back bonding is a type of covalent bonding that occurs when a filled orbital of one atom donates electron density to an empty or partially filled orbital of another atom. This process is particularly common in molecules where a central atom is surrounded by more electronegative atoms, such as in boron trifluoride (BF3) or carbon dioxide (CO2).

The concept of back bonding is rooted in molecular orbital theory, which describes the behaviour of electrons in molecules. According to this theory, when two atomic orbitals combine, they form two molecular orbitals: a lower-energy bonding orbital and a higher-energy antibonding orbital. Electrons will first fill the lower-energy bonding orbital, and only if there are additional electrons will they occupy the antibonding orbital.

In back bonding, the filled orbital of one atom (usually a lone pair of electrons) overlaps with an empty or partially filled orbital of another atom. This overlap allows the electrons to be shared between the two atoms, effectively creating a new bond. This additional bond can significantly reduce the overall energy of the molecule, making it more stable.

For example, in boron trifluoride (BF3), the boron atom has an empty p orbital, while the fluorine atoms each have a lone pair of electrons in their p orbitals. The fluorine atoms can donate some of their electron density to the boron atom, filling its empty p orbital and creating a new bond. This back bonding stabilises the BF3 molecule.

Similarly, in carbon dioxide (CO2), the oxygen atoms can donate some of their electron density to the carbon atom, stabilising the molecule. This is why CO2 is a linear molecule, despite the fact that the carbon atom is surrounded by more electronegative oxygen atoms.

In conclusion, back bonding is a crucial concept in understanding the stability of certain molecules. By sharing electron density between atoms, it reduces the overall energy of the molecule, making it more stable.