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Advanced Inorganic Chemistry: A Comprehensive Text on Pg. 228, Chapter 8 titled 'Boron' talks about three factors which appear to be responsible for the shortness of $\ce{B-X}$ bonds(where $\ce{X}$ is a halogen). The second point was stated as:

  1. Strengthening and hence shortening of the $\ce{B-X}$ bonds by ionic-covalent resonance, especially for the $\ce{B-F}$ and $\ce{B-O}$ bonds because of the large electronegativity differences. Evidence that this is important, in addition to the dative pπ-pπ bonding, is afforded by the fact that even in $\ce{BF3}$ complexes such as $\ce{(CH3)3N^+B^-F3}$ and $\ce{BF4^-}$, where the pπ-pπ bond must be largely or totally absent, the $\ce{B-F}$ bonds are still apparently shortened

What is this "ionic-covalent" resonance exactly? I tried looking it up,but found some unsatisfactory and vague answers here where it seems like they are presenting regular resonance as ionic-covalent. This answer on the other hand seems to be talking about compounds possessing both ionic and covalent factor although which is correct,does not really address the issue at hand. So is this term really just refering to regular π-bond resonance, or is it an entirely different phenomenon?

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  • $\begingroup$ I recommend checking rather earlier posts here like chemistry.stackexchange.com/questions/27535/… chemistry.stackexchange.com/questions/32306/… $\endgroup$ – Mithoron Jun 2 at 18:49
  • $\begingroup$ @Mithoron I understand the concept being talked about in these posts: no bond is completely ionic or completely covalent. But my question is regarding what ionic-covalent resonance is, and how is it related to this concept $\endgroup$ – Yusuf Hasan Jun 2 at 18:57
  • $\begingroup$ But you didn't bother to read what Dave cited? $\endgroup$ – Mithoron Jun 2 at 19:06
  • $\begingroup$ Oh sorry,got what you meant. So in the examples written in the question above, they will also form structures like B+ F- and B- F+ with B-F being a minor contributor(if we talk about BF4-)? @Mithoron $\endgroup$ – Yusuf Hasan Jun 2 at 19:20
  • $\begingroup$ I guess covalent component still dominates, but structures with "broken" bond would be also important in depicting it. $\endgroup$ – Mithoron Jun 2 at 20:29

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