The Methylbismuth Dication: Pentagonal Pyramidal Coordination and Ligand-Induced Lewis Superacidity

A profound understanding of the naturally preferred coordination geometry of molecular complexes is the basis for scientists to rationalize, predict, and design their physico-chemical properties and reactivity. Gaining access to compounds with an unusual coordination chemistry and developing a fundamental knowledge about their properties represent key challenges in the field. Tackling these questions promises an entry to unexplored chemical space and reactivity patterns that are inaccessible for compounds found in more traditional coordination geometries. Here, we present the synthesis, isolation, and characterization of the simplest organobismuth dication, the methylbismuth dication [BiMe(thf)₅][SbF₆]₂ (1), stabilized only by five substitutionally labile THF ligands. The hexa-coordinate compound shows a rare pentagonal pyramidal coordination geometry around the central atom, which is extremely unusual given the fact that this is an organometallic species with only simple monodentate monoanionic and/or neutral ligands without considerable steric bulk. The detailed investigation of this compound with experimental and theoretical approaches reveals the cause of the unusual coordination chemistry and uncovers multiple Lewis acidic binding sites and ligand-induced Lewis superacidity.