How can you explain higher stability of `BCl_3` as compared to `TlCl_3` ?

Explanation

Because of poor shielding effect on s-electrons of the valence shell by the inner d -and f -electrons (i.e., 3d, 4d , 5d, 4f -electrons), the inert pair effect is maximum in Tl. As a result, for Tl only the `6p^1` -electron becomes involved in bond formation. Hence the most stable oxidation state of Tl is + 1 and not +3 . Therefore, TICl is stable but `TICl_3` is unstable. On the other hand, due to absence of d -and f-electrons, B does not exhibit inert pair effect and all the three valence electrons become involved in bond formation. Hence, B exhibits an oxidation state of +3 and thus forms `BCl_3`. So, `BCl_3` is more stable than `TiCl_3`.