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I have come across an example in my textbook, which shows resonance structures for the bromonium ion. However, I am confused as to why the sigma bond electrons "move" to get the tertiary carbocation and the secondary carbocation. Are sigma bond electrons not strictly localized and pi electrons delocalized?

enter image description here My question is: When are we allowed to move the sigma electrons when writing resonance forms?

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  • $\begingroup$ In the text, which talks about the diagram says this: "In resonance terms, we say that the weighting factor (coefficient) for the more-stable form is greater than that for the less-stable form (c1 > c2, Fig. 11.13)." Are you sure these are not resonance forms? $\endgroup$ – James Bond Dec 1 '18 at 6:14
  • $\begingroup$ Yes, I am sure these are rearrangement structures, see Wagner-Meerwein rearrangement in my answer. And yes the factor which decide the major product is the stability of intermediate (here carbocation). $\endgroup$ – Saurabh Singh Dec 1 '18 at 7:10
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    $\begingroup$ @Mithoron there are multiple descriptions possible, but they would involve sigma type orbitals on MO and VB theory. That doesn't make your point and more value. The bromonium is a very good example for nonclassical cations. // Very approximate rule of thumb: as long as you don't move the nuclei, you can use (for better or worse) the concept of resonance. (Here it's actually for better.) $\endgroup$ – Martin - マーチン Dec 1 '18 at 23:57
  • $\begingroup$ @Martin-マーチン I think it can be also viewed as an analogue of alkene - metal complex. $\endgroup$ – Mithoron Dec 2 '18 at 0:33
  • $\begingroup$ @Mithoron yes indeed. The pi orbital would still do a sigma type donation. There was a typo in my last comment (I'm on my phone), it should have been: that doesn't make your point any less valid. $\endgroup$ – Martin - マーチン Dec 2 '18 at 0:38

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