Is Chromium(VI) fluoride able to be synthesized properly?

Question

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:

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

Answer 1

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 $\ce{CrO4^{2-}}$.