A “non-magnetic” triplet bismuthinidene enabled by relativity

Isolation and stabilization of main group diradical species have posed a synthetic challenge over the years due to their intrinsic high reactivity. Herein we report on a large-scale synthesis and isolation of a mono-coordinate bismuthinidene featuring a rigid and bulky ligand, which protects the Bi(I) center. The compound was characterized by its unique spectroscopic features (UV-vis and NMR), but more prominently, by its magnetic properties. Multiconfigurational quantum chemical calculations predict the ground state of the compound to be dominated by a spin-triplet. Further support for this electronic structure description was obtained through correlation of theory to experimental XRD, XAS, and UV-Vis data. However, all magnetic measurements (EPR, NMR and SQUID) point to a diamagnetic compound. This apparent discrepancy can be explained by an extremely large spin-orbit coupling (SOC) that leads to an unprecedented zero-field splitting of more than 8000 cm‒1, thus leaving a MS = 0 magnetic sublevel thermally isolated in the electronic ground state. The extremely large SOC effect is a result of the low-coordination number of the bismuth center in interplay with its heavy element nature.