The Blue–Violet Color of Pentamethylbismuth: A Visible Spin‐Orbit Effect

Two‐component relativistic time‐dependent density functional theory calculations with spin‐orbit coupling predict yellow and orange–red absorption for BiPh(5) and BiMe(5), respectively, providing an excellent explanation for their respective violet and blue–violet colors. According to the calculations, the visible absorption is clearly attributable to a single transition from a ligand‐based HOMO to a low‐energy LUMO with a significant contribution from a relativistically stabilized Bi 6s orbital. Surprisingly, scalar releativistic calculations completely fail to reproduce the observed visible absorption and place it at the violet/near‐UV borderline instead.