Among the following, find out number of ions or molecules that can show back bonding.

Answer: The number of molecules or ions that can show back bonding is two. These are BF₃ and CF₃⁺.

What is Back Bonding?

Back bonding is a stabilizing interaction where an atom with an empty orbital accepts electron density from a neighboring atom’s filled orbital. It usually happens between electron-deficient atoms like Boron or Carbon and electronegative atoms like Fluorine or Oxygen that have lone pairs.

Analysis of Given Molecules

1. Cyclic Oxonium Ion (first)

The oxygen atom already carries a positive charge. Its lone pairs are destabilized and unavailable for donation. Hence, no back bonding occurs.

2. Cyclic Oxonium Ion (second)

Similar to the first oxonium ion, oxygen is positively charged. Lone pairs are not effectively available for donation. No back bonding occurs.

3. BF₃

Boron is electron-deficient with an empty p-orbital. Fluorine atoms donate electron density via their filled p-orbitals. This pπ–pπ back bonding stabilizes BF₃, reducing its Lewis acidity.

4. CF₃⁺

Carbon in CF₃⁺ is electron-deficient due to the positive charge. Fluorine atoms donate electron density through their lone pairs into carbon’s empty orbital. This back donation strongly stabilizes CF₃⁺.