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In one of the questions in a qualifying exam it said that $\ce{UF6}$ is a "covalent compound". This fits the physical properties of the compound well, e.g. low boiling point, existence as molecules, etc. However, it still seemed a bit counter-intuitive to me as I was taught that covalent character is attributed to (1) the difference in electronegativity of the two atoms and (2) the oxidation state of the central atom.

A Wikipedia search gave a $\Delta E.N.$ of 2.60 between $\ce{U}$ and $\ce{F}$. In comparison, $\ce{MnO4^-}$, a compound with 4 electronegative atoms as ligands, a high central atom oxidation state, and covalent character, has a $\Delta E.N.$ of just 1.89. It seems that the $\ce{U-F}$ bond should possess ionic character.

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    $\begingroup$ U(VI) can be considered much more electron withdrawing than, say, U(III). $\endgroup$
    – Mithoron
    Commented Sep 30, 2022 at 17:49

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Probably the bonding in $\ce{UF6}$ has significant ionic character; as with many compounds neither the extreme of "all covalent" nor "all ionic" would really be accurate.

Ionic character in $\ce{UF6}$ need not imply the high melting and boiling points we commonly associate with ionic compounds. We have to reckon with the 6:1 stoichiometric ratio of proposed anions to cations -- such an extreme deviation from equality discourages forming a strongly stable lattice (whether ionic or covalent with delocalized bonds). An ionic model of $\ce{UF6}$ could be an ambient gas of neutral clusters similar to sodium chloride or lithium fluoride at higher temperatures.

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