Dihalo bismuth cations: unusual coordination properties and inverse solvent effects in Lewis acidity.

A series of well-defined cationic hepta-coordinate bismuth halides [BiX 2 (py) 5 ][B(3,5-(CF 3 ) 2 -C 6 H 3 ) 4 ] (X = Cl, Br, I), stabilized only by substitutionally labile solvent molecules, were synthesized and fully characterized. Their apparent D 5h symmetry with a lone pair at the central atom is unprecedented for main group compounds. The potential of BiX 3 to show unexpectedly high Lewis acidities in moderately polar solvents is likely due to the formation of [BiX 2 (solv) 5 ] + and related ionic species.