Why does thallium form TlF and TlF3 but aluminium only forms AlF3?

Does this have something to do with the *inert pair effect*? 




Ahh, Thallium is a very nice element. *Because it's weird*.



> 
> The +3 oxidation states are favorable except for the heavier elements, such as Tl, which prefer the +1 oxidation state due to its stability; this is known as the inert pair effect. [Chemwiki.UCDavis](http://chemwiki.ucdavis.edu/Inorganic_Chemistry/Descriptive_Chemistry/p-Block_Elements/Group_13%3A_The_Boron_Family)
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> As an example in group 13 the +1 oxidation state of Tl is the most stable and Tl(III) compounds are comparatively rare. [**Inert pair effect**, Wikipedia](http://en.wikipedia.org/wiki/Inert_pair_effect)
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So, why?
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Inert pair effect was introduced to describe why the facts above are correct. However, the original explanation, provided by Sidgwick, relied on the fact that the $s$ subshell has an "inert" pair of electrons. This can't be correct, and [Wikipedia](http://en.wikipedia.org/wiki/Inert_pair_effect#Description) uses the trend in ionization potentials of "$s$" electrons to prove it wrong:
$$\ce{In < Al < Tl < Ga}$$


A better explanation lies within the lower bond enthalpies of $\ce{MX}$ where M is one of the p-block heavy elements.


To reach a high oxidation number, elements will need to have a strong bond. They wanna bond in order to lose energy (i.e. become more stable) after all! If the bond is weak, this energy isn't provided. Thus, a heavy element is better off with being less oxidised.


See [this](http://pubs.acs.org/doi/abs/10.1021/ja00045a027) and [this](http://pubs.acs.org/doi/abs/10.1021/j150561a027). (Sorry, they might be behind a paywall)


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