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All the trihalides of the Group 13 elements are known, but $\ce{TlI3}$ is not. Why is this so?

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This is due to the Inert Pair Effect. As we move down the 13th group, addition of d orbitals and f orbitals are taking place. Since the d orbitals and f orbitals are away from the nucleus, they have poor sheilding power or capability and thus the s orbitals of the outermost shell are attracted towards the nucleai, making it difficult for s orbital electrons to leave the orbitals and only pairing of the 1 electron in p orbital is possible. Thus +1 valency of Tl is more stable than +3 as it is not energetically favored. This,ofcourse, does not mean that formation TlI3 is not possible. It's just that it requires more energy.

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  • $\begingroup$ There's also relativistic effects. $\endgroup$ – gsurfer04 Oct 24 '15 at 1:34
  • $\begingroup$ You should also explain why only iodide is unable to bring out the +3 oxidation state. $\endgroup$ – orthocresol Oct 24 '15 at 8:25
  • $\begingroup$ @orthocresol Isnt that due to size of iodine? $\endgroup$ – Quark Oct 25 '15 at 9:09
  • $\begingroup$ Indirectly, yes, but just saying that doesn't explain anything $\endgroup$ – orthocresol Oct 25 '15 at 9:42

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