How can you explain higher stability …

Both boron and thallium belong to group 13 of the periodic table. Boron does not show inert pair effect as it does not have d or f electrons. As we go down the group, the shielding effect of the electrons also decrease therefore the increased nuclear charge down the group is not neutralized by the poor shielding effect of the electrons in the inner orbitals.Due to this the inert pair effect becomes more and more predominant and the s electrons of the valency shell experiences strong attraction .This effect is maximum in thallium and therefore only the 6p1 electrons take part in the bond formation. This makes +1 oxidation state most stable in case of thallium. Therefore TlCl is stable and TlCl3 is unstable. In case of BCl3 , boron does not have d or f orbital , so all the valence electron in the 2s and 2p orbital takes part in the bond formation making BCl3 stable.