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I would like to know if any of the tellurium fluorides or antimony fluorides are ionic. I am aware that the difference in electronegativity is greater than the 1.665 cutoff indicating ionic bonds. Melting point and reactivity seem to somewhat support that tellurium(iv) fluoride is ionic but this is not proof. Being an unusual case of an ionic bond between nonmetallic atoms only though, I would like to know if anything empirical (e.g. spectroscopic) has been observed to support this.

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  • $\begingroup$ Which ones? Electronegativity and more importantly degree of covalency depend on oxidation state $\endgroup$
    – Mithoron
    Feb 16 '16 at 20:49
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    $\begingroup$ en.wikipedia.org/wiki/Tellurium_tetrafluoride - it melts at about 130C and is coordination polymer. $\endgroup$
    – Mithoron
    Apr 21 '16 at 22:26
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TeF4 is polymeric and low-melting. Not ionic. Covalent.

In general, the elements that are metalloids (on the "border" of metal and non-metal) like Te and Sb, tend to behave chemically like nonmetals (looking at their compounds). But physically as metals (shiny, malleable, conductive). So that's another good clue that these elements would be form covalent compounds (molecules or polymers ["network solids"]) with fluorine.

Finally even for hard core transition or p group metals, you will find that the lower oxidation count species tend to be ionic. But as you get to tetravalent species a fair amount are molecular or polymeric (i.e. covalent) and few are ionic. No penta or higher flourides are ionic. And the few tetravalent ionic compounds tend to be pretty far to the left of the table (e.g. Zr)...pretty hard/electropositive atoms, not soft (i.e. polarizable) or less electropositive like those in the p group. (To which Te and Sb are even more marginal metals.)

The Wikipedia article on Compounds of Fluorine has some good details on the concepts and good references to review articles (and Greenwood/Earnshaw Chemistry of the Elements).

https://en.wikipedia.org/wiki/Compounds_of_fluorine#Nonmetal_fluorides

P.s. Note also that it is important to specify which species of fluoride you mean (for elements below the second row). Te forms a tetra and hexa fluoride (and some more complicated variants)--you did specific tetra, but that's not the only choice. Also for Sb, you didn't specify which compound and both the tri and penta fluoride are well known.

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