Question
How can you explain the higher stability of BCl3 as compared to TlCl3?
Solution
This is because inert pair effect is maximum in thallium in which poor shielding of the s-electrons of the valence shell (6s2) by the 3d, -4d, -5d and 4f-electron occurs. As a result, only 6p1 electron participates in bond formation and thus the most stable oxidation state of Tl is +1 and not +3. Therefore, TlCl is stable while TlCl3 is unstable.
On the other hand, all the three valence electrons (2s22p1) of boron take part in the bond formation due to the absence of d–and f-electrons in B(no inert pair effects). Hence B shows an oxidation state of +3 and thus forms BCl3 easily. Thus BCl3 is more stable than TlCl3.
