Question

How can you explain higher stability of $BC{l}_3$ as compared to $TlC{l}_3$?

Answer 1

Boron and thallium belong to group 13 of the periodic table. In this group, the +1 oxidation state becomes more stable on moving down the group. $BC{l}_3$ is more stable than TlCl3 because the +3 oxidation state of B is more stable than the +3 oxidation state of Tl. In Tl, the +3 state is highly oxidising and it reverts back to the more stable +1 state.