Fluoride-Ion Donor Properties of AsF 5 .

Arsenic pentafluoride undergoes ligand-induced autoionization in the presence of 1,10-phenanthroline (phen) in a SO 2 ClF solution to form the donor-stabilized [AsF 4 (phen)][AsF 6 ] salt. Reacting [AsF 4 (phen)][AsF 6 ] with the strong Lewis acid SbF 5 ·SO 2 yields the mixed arsenic-antimony salt [AsF 4 (phen)][Sb 2 F 11 ]. These salts are the first examples of crystallographically characterized donor-stabilized [AsF 4 ] + cations. The analogous reaction of AsF 5 and 2,2'-bipyridine (bipy) does not result in autoionization but leads to the formation of the neutral 2:1 adduct (AsF 5 ) 2 ·bipy. The gas-phase and solution fluoride-ion affinities of [AsF 4 ] + and [SbF 4 ] + were calculated, revealing them to be incredibly strong Lewis acids. Density functional theory calculations and natural bond orbital analysis show that significant electron-pair donation from phen to the As center in [AsF 4 (phen)] + occurs and quenches the extreme electrophilicity of the [AsF 4 ] + cation.