# In what form does the respective halides of thallium exist as?

Thallium Halides like $$\ce{TlF}$$ and $$\ce{TlI}$$ exist. However, my text says that $$\ce{TlI}$$ exists as $$\ce{Tl+}$$ and $$\ce{I3-}$$. This is understandable with the help of the inert pair effect. Then why does the fluoride exist as $$\ce{Tl^{+3}}$$ and $$\ce{F-}$$?

• – Nilay Ghosh Feb 27 at 13:47
• – Nilay Ghosh Feb 27 at 13:48

First, at least in the sixth period the "inert pair" is not completely inert. It can be pulled into chemical bonding if we use an appropriately strong oxidizing agent. Halogens lighter than iodine are better oxidizing agents than iodine itself, so are more likely to draw off that inert pair. In the case of thallium, bromine and chlorine are capable of forming thallium(III) compounds, although these readily decompose upon warming. Wikipedia gives a brief summary of $$\ce{TlX3}$$ compounds, noting the contrast in structure between the (tri)iodide and the others.
So the combination of a less stable trihalide ion (in the concentrated salt environment) and greater oxidizing power favors lighter halogens forming $$\ce{TlX3}$$ as a largely covalent compound of thallium(III) rather than a trihalide-ion salt of thallium(I).