Carbocations
Carbocation rearrangement reactions occur through three-center bond transition states. Because the three center bond structures have about the same energy as carbocations, there is generally virtually no activation energy for these rearrangements so they occur with extraordinarily high rates.
Carbonium ions have three-center two-electron bonds. Perhaps the best known and studied structure of this sort is the norbornyl cation.
Read more about this topic: Three-center Two-electron Bond