+1 oxidation state of Tl is the most stable and TlIII compounds are comparatively rare. This is due to inert pair effect The inert pair effect is the tendency of the outermost s electrons to remain unionized or unshared in compounds of post-transition metals. The stability of the +1 oxidation state increases in the following sequence: Solution +1 oxidation state of Tl is the most stable and TlIII compounds are comparatively rare. This is due to inert pair effect The inert pair effect is the tendency of the outermost s electrons to remain unionized or unshared in compounds of post-transition metals. The stability of the +1 oxidation state increases in the following sequence:.