Matthias Bickelhaupt in PNAS with 8-Coordinate Fluoride
Theoretical Chemist Matthias Bickelhaupt, together with colleagues from Cornell University (USA) and TU Eindhoven, report in PNAS that a fluoride anion inside a silicate double 4-ring can form chemical bonds to the eight surrounding silicon atoms.
This finding, which was also highlighted in Science, constitutes hypervalent fluoride of exceptionally high coordination number.
Fluorine plays a supporting role in some of the best-known hypervalent compounds, such as PF5 and SF6. Goesten et al. now suggest in PNAS that the halogen can also play the lead part in constrained environments. Certain zeolites bind fluoride at the center of a box motif with silicons at each of eight vertices (see illustration). Using the Amsterdam Density Functional (ADF) program and Kohn-Sham MO theory, the authors report that all eight engage in stabilizing Si-F orbital interactions.
Whereas hypervalency is more often associated with third- and fourth-row elements, in this motif, sterics preclude analogous bonding to the heavier halides. The zeolite system constitutes hypervalent fluoride of exceptionally high coordination number.
This finding is nicely in line with, and augments the earlier report by Pierrefixe and Bickelhaupt of stable, electron-rich hypervalent carbon species that are normally transition states in SN2 reactions.