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Chromium(VI) fluoride, also known as chromium hexafluoride is a possible chemical compound with a chromium atom attached to 6 fluorine atoms. It has been unsuccessfully synthesized a number of times by heating up chromium to 400 °C in a fluorine atmosphere at 20 megapascals of pressure and freezing it as it was formed but this was still a failure as this instead produces chromium(V) fluoride or chromium pentafluoride. Is the following possible?

The predicted reaction was:

$$\ce{Cr + 3 F2 -> CrF6}$$

Instead, the following reaction was happening:

$$\ce{2 Cr + 5 F2 -> 2 CrF5}$$

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    $\begingroup$ I assume that CrF6 is such a strong oxidizer that it easily oxidizes CrF4 to give two CrF5. $\endgroup$
    – Andrew
    Apr 29, 2021 at 21:21
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    $\begingroup$ So what is the question? Your reference states clearly, "CrF6 has yet to be synthesized." It does not state it is impossible. Proving a negative is a bit difficult. $\endgroup$ Apr 29, 2021 at 21:21
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    $\begingroup$ Apparently, chromium favors oxide instead of fluoride ligands at such a high oxidation state. $\endgroup$ Apr 30, 2021 at 1:52
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    $\begingroup$ Also, some papers from the late 19th century reported that CrF6 is a deep-red vapor which condensed to a blood-red liquid at low temperature. It is made by reacting lead chromate, calcium fluoride and sulfuric acid($\ce{PbCrO4 + 3CaF2 + 4H2SO4 -> PbSO4 + 3CaSO4 + 4H2O + CrF6}$) It strongly fumes in air and decomposes on contact with water/moisture to form chromium trioxide and hydrofluoric acid. (But then again, this is from a very old source, so this info. maybe outdated). $\endgroup$ Apr 30, 2021 at 9:48

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Both steric and electronic factors inhibit the formation of chromium hexafluoride.

  • Steric factors. Chromium, like most transition metals, commonly forms octahedral complexes. But in the fourth period where chromium lies these octahedral complexes tend to occur with relatively low oxidation states like $+3$ for chromium. The metal center would be smaller in an oxidation state as high as $+6$, and possibly we would need a fifth or higher period metal to get the center large enough to continue to coordinate octahedrally. So we can get hexafluorides of molybdenum and tungsten but as yet not chromium.

  • Electronic factors. In this answer early transition metals are identified as strong pi acceptor and therefore prefer oxo complexes (with a better pi-donor ligand) over fluoro complexes. This does not prevent the synthesis of a hexafluoride (as we saw above with molybdenum and tungsten) or perhaps even a fluoro-cation (which would avoid the steric issues that a neutral hexafluoride would have with chromium). But it does imply that such compounds are likely to break down at the drop of an oxide-bearing hat. The respective Wikipedia pages for $\ce{MoF6}$ and $\ce{WF6}$ indicate that these hydrolyze in water, the tungsten compound specifically being reported to form oxyfluorides and ultimately the oxide. Presumably $\ce{CrF6}$ would react similarly with water, to form chromic acid and then, since that acid is strong, the anions $\ce{Cr2O7^{2-}}$ and $\ce{CrO4^{2-}}$.

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