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I learnt that the group 16 elements can form halides, but with the exception that polonium cannot form a tetrahalide with fluorine.

However, I thought that it was possible. This is because the electronegativity difference between polonium and fluorine is very high since polonium is a metal and fluorine is a highly electronegative atom and so should be able to form a salt.

Why is this wrong?

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    $\begingroup$ Polonium in it's +4 oxidation state would mainly form covalent compounds as removing the first 4 electrons (sum of first 4 ionization energies) would be very high. Thus, if $\ce{PoF4}$ were to exist, it would have to be a covalent halide. $\endgroup$ – Aniruddha Deb Jul 24 at 10:05
  • $\begingroup$ but isn't polonium a metal, also the electronegative difference between Po and Fl is very high, so ionic character has to be high right? $\endgroup$ – R. Anusha Jul 24 at 10:11
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    $\begingroup$ It is hard to characterize polonium compounds due of its radioactivity. $\endgroup$ – Nilay Ghosh Jul 24 at 10:27
  • $\begingroup$ @Anir is right - compounds on higher ox. states are molecular in spite of significant ionic component in bonding. $\endgroup$ – Mithoron Jul 24 at 16:58
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Before going into details, let me state Wikipedia:

$\ce{^{208}PoF6}$ was probably successfully synthesised via the same reaction (passing fluorine gas over $\ce{^{208}Po}$) in 1960 where a volatile polonium fluoride was produced, but it was not fully characterized before it underwent radiolysis and decomposed to polonium tetrafluoride.

Even attempts were made to dissolve polonium metal in hydrofluoric acid but no fluoride salts were isolated as a free salt as they radiolysed and decomposed. According to this 1961 paper:

Although no fluoride salts of $\ce{Po}$ have been isolated, $\ce{Po}$ is quite soluble in $\ce{HF}$. Solutions containing 2 curies of $\ce{Po}$ per ml of $\ce{1 N HF}$ are reported to be unsaturated. Attempts to prepare a volatile fluoride by reaction with $\ce{F2}$ gas or with liquid $\ce{BF3}$ have been unsuccessful. However recent work with $\pu{1 mg}$ of $\ce{^{208}Po}$ indicated the formation of a volatile $\ce{Po}$ compound, which decomposed before it could be isolated.

Polonium tetrafluoride is very difficult to isolate as it is seen to form either a complex or oxidize/decompose radiolytically. Also, bringing analytical data on such compounds is hard as they are produced in minute amounts possible in the range of micrograms. Below is a text from this e-book 1

A white solid possibly polonium tetrafluoride is obtained by treating polonium hydroxide/tetrachloride with aqueous dilute hydrofluoric acid. Treating this solid suspension in sulfur dioxide yields a bluish grey product (possibly $\ce{PoF2}$) which quickly reverts back to the solid on standing owing to radiolytic oxidation. The solubility of $\ce{Po(IV)}$ increases with the concentration of hydrofluoric acid indicating a complex formation

There has been unsuccessful attempts to make volatile hexafluoride by passing fluorine to $\ce{^{210}Po}$ but recently a fluoride has been made with $\ce{^{208}Po}$ plated on platinum. This product appers to be quite stable in vapor phase but on cooling, a nonvolatile compound is formed possibly polonium tetrafluoride resulting from radioactive decomposition. Analytical data are not recorded for any polonium fluorides owing to difficulty of determining fluoride ion at microgram level.

Reference

  1. Advances in Inorganic Chemistry and Radiochemistry, Academic Press, 1962
  2. https://www.osti.gov/servlets/purl/4367751/4367751.PDF
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  • $\begingroup$ Also note PoCl4 is known - en.wikipedia.org/wiki/Polonium_tetrachloride With intensely radioactive elements especially one needs to carefully separate "not known due to some chemical instability" with "not known because nobody has observed it yet" $\endgroup$ – Ian Bush Jul 24 at 11:02

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