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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?

Thanks for your help!

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  • $\begingroup$ I can't find the word "dative" in your linked article. Could you provide a quote? $\endgroup$ – DHMO Sep 4 '16 at 6:25
  • $\begingroup$ 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" $\endgroup$ – Rima Sep 4 '16 at 6:27
  • $\begingroup$ 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. $\endgroup$ – DHMO Sep 4 '16 at 7:50
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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 $\ce{F-}$ .

Hope this answers your question!

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  • $\begingroup$ About the first part I think dative bond is not the same as valence shell expansion. About the second part; you mean BF3 is more electron-deficient than oxygen? $\endgroup$ – Rima Sep 4 '16 at 9:21
  • $\begingroup$ Yes you're right - to clarify, F does not form dative bonds with O, or expand its octet by forming covalent bonds with O. $\endgroup$ – Kenneth Sep 4 '16 at 11:37
  • $\begingroup$ For your second question - yes, as B is bonded to 3 electronegative F atoms. But the point was that the octet structure of O and B affects the bonding. O has electrons available for covalent bonding unlike B in BF3 which has an empty orbital to accept a lone pair from F to form a dative bond :) $\endgroup$ – Kenneth Sep 4 '16 at 11:50
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There are two factors at play here:

  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.

  2. The fluorine atom is tiny, even by period 2 standards. There's simply not enough space around the proximity of the atom to fit three or more other atoms.

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