Structure of the SO2F- anion, a problem case

Recently, room-temperature crystal structures of SO2F- in its K+ and Rb+ sa lts were published in Z. Anorg. Allg. Chem. 1999, 625, 385 and claimed to r epresent the first reliable geometries for SO2F-. However, their almost ide ntical S-O and S-F bond lengths and O-S-O and O-S-F bond angles are in shar p contrast to the results from theoretical calculations. To clarify this di screpancy, the new [(CH3)(2)N](3)SO+ and the known [N(CH3)(4)(+)], [(CH3)(2 )N](3)S+, and K+ salts of SO2F- were prepared and their crystal structures studied at low temperatures. Furthermore, the results from previous RHF and MP2 calculations were confirmed at the RHF, B3LYP, and CCSD(T) levels of t heory using different basis sets. It is shown that all the SO2F- salts stud ied so far exhibit varying degrees of oxygen/fluorine and, in some cases, o xygen-site disorders, with [(CH3)(2)N](3)SO+SO2F- at 113 K showing the leas t disorder with r(S-F) - r(S-O) = 17 pm and angle (O-S-O) angle (F-S-O) = 6 degrees. Refinement of the disorder occupancy factors and extrapolation of the observed bond distances for zero disorder resulted in a geometry very close to that predicted by theory. The correctness of the theoretical predi ctions for SO2F- is further supported by the good agreement between the cal culated and the experimentally observed vibrational frequencies and their c omparison with those of isoelectronic ClO2F. A normal coordinate analysis o f SO2F- confirms the weakness of the S-F bond with a stretching force const ant of only 1.63 mdyn/Angstrom and shows that there is no highly characteri stic S-F stretching mode. The S-F stretch strongly couples with the SO2 def ormation modes and is concentrated in the two lowest a' frequencies.