Crystallographic Evidence\nfor Bi(I) as the Heaviest\nHalogen Bond Acceptor

Complexation of the green bismuthinidene (RBi) with two\nequivalents\nof a highly fluorinated aryl iodide at low temperature allows the\ncrystallographic identification of an unstable red species that can\nbe regarded as an intermediate in an overall Bi(I) → Bi(III)\noxidation process. Both C–I bonds are orientated toward the\nfilled 6p orbital of bismuth (Bi–I distances 3.44–3.52\nÅ), leading to an elongation of the C–I bonds by 0.05\nand 0.07 Å. Density functional theory (DFT) calculations confirm\nthat the Bi(I) center is indeed acting as an electron donor, establishing\ntwo strong and directional halogen bonds. The color change from green\nto red upon halogen bond formation is a consequence of the energetic\nstabilization of a Bi(I) lone pair by interactions with the sigma-holes\nof the halogen bond donors. Overall, this study presents the first\nstructural proof of bismuth, and more generally of heavy organopnictogen(I)\ncompounds, acting as halogen bond acceptors.