The inverted bond in [1.1.1]propellane is a charge-shift bond.

Bonded or not bonded? An ab initio valence bond study of [1.1.1]propellane shows that the two bridgehead carbons are linked by a strong and direct sigma bond that is neither classically covalent nor classically ionic, but rather a charge-shift bond, in which the covalent-ionic resonance energy plays the major role. As such, the central bond of [1.1.1]propellane closely resembles the single bond of difluorine.