How can you explain higher stability of B$C{{l}_{3}}$ as compared to Tl$C{{l}_{3}}$?



B$C{{l}{3}}$ is more stable than Tl$C{{l}{3}}$ because inert pair effect is missing in B$C{{l}{3}}$. Due to inert pair effect thallium will prefer to form TlCl rather Bran Tl$C{{l}{3}}$.
It prefers to show an oxidation state of + 1.