# Why do higher fluorine oxyanions not exist? [duplicate]

I asked this question from my teacher and he said that's because fluorine has a high electronegativity and does not form dative bond with oxygen. That's why only hypofluorite $\ce{FO-}$ exists and not $\ce{FO_n-}$ ($n = 2,3,4$).

But then I was wondering why $\ce{BF4-}$ exists. And according to this Wikipedia article, fluorine does form dative bond.

so why the anions in the title do not exist?

• I can't find the word "dative" in your linked article. Could you provide a quote? – DHMO Sep 4 '16 at 6:25
• I thought this reaction involves dative bonds: "Three equivalents of HF react to give the intermediate boron trifluoride and the fourth gives fluoroboric acid: B(OH)3 + 4 HF → H3O+ + BF− 4 + 2 H2O" – Rima Sep 4 '16 at 6:27
• I would say that it is because the electronegativity difference in $\ce{F-O}$ is too small, but I would wait for people much more knowledgeable than me. – DHMO Sep 4 '16 at 7:50

In the hypothetical fluorine- and oxygen- containing ions, F is the central atom. However, to have 2 or more O atoms bonded to it means that F will have more than 8 valence electrons. This is impossible as F is a period 2 element and has no energetically accessible orbitals for octet expansion.

This is because F is more electronegative than O - so you're right about F not donating electron pairs to O, hence F forms only covalent bonds here.

As for $\ce{BF_4^-}$ , B in $\ce{BF_3}$ is electron-deficient with 6 valence electrons and will gladly accept an electron pair from the nucleophile $\ce{F-}$ .

1. Fluorine is more electronegative than oxygen. In oxyanions, the central atom carries a substantial positive charge due to the oxygen atom pulling away electron density. In oxyfluorides, oxygen actually donates electron density to fluorine and oxygen would logically be the central atom. There are theoretical studies into $\ce{OF3+}$ but no successful syntheses yet.