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

Explanation

In `BCl_(3)`,`B` has electronic configuration `2s^(2)2p^(1)`

An electron from `2s` is promoted to `2p`. The promotional energy required is more than compensated by the bond energy released in the formation of `BCl_(3)`,hence, `BCl_(3)` is a stable compound in which `B` shows an oxidation state of `+3`.
On the other hand, in `TICI_(3)`, fully filled lesser shielding `d`- an `f` electrons are present in-between the valence shell electron and noble gas core. Hence, much higher energy is required for electron transition from `6s^(2)`to`6p`, which is not compensated by the bond energy that will be released in the formation of `TICI_(3)`. Hence, only `6p^(1)` participate in the bond formation and lower oxidation state `(+1)` will be more stable in `TI` as compared to `+3`. Hence, `TICI_(3)` will be unstable.