How can you explain higher stability of `BCl_(3)` as compared to `TlCl_(3)` ?

Explanation

In the excited state, boron has three unpaired electrons in its valence shell which enter into covalent bond formation with three Cl atoms (e.g., Fig.) and form `BCl_3` molecule. In this compound, boron atom exhibits +3 oxidation state and is `sp^2` hybridised. The molecule is partly stabilised by `P pi-P pi` back bonding. On the other hand, due to inert pair effect, the `6s^2` electron pair in TI remains reluctant in bond formation. This is why the +1 oxidation state of TI is more stable than the +3 state. Hence, `TICI_3` formed in the +3 state is not much stable. Therefore, BCl3 is more stable as compared to `TICI_3`.