Inverse Bonding - Backbonding: Metal to Ligand sigma and Ligand to Metal pi Dative Interactions between Bismuth and Boron

There are many types of coordinating bond types, but an inverse bonding-backbonding (IBBB) with a metal to ligand sigma donation and a ligand to metal pi backdonation (a "sigma Z-pi L" interaction) is unheard of. Herein we computationally describe one such case based on a borabenzene ligand and a bismuth pincer complex. We believe that IBBB involving d orbitals in transition metals is impossible, but a pi charge transfer to a post-transition metal empty p orbital can do the trick.