Geometry of Pentaphenylantimony in Solution: Support for a Trigonal Bipyramidal Assignment from X-ray Absorption...

Pentaphenylantimony (SbPh₅) has been previously crystallized in either a square pyramidal or trigonal bipyramidal geometry. Investigation of the solution-state structure of SbPh₅ has been hampered by the extreme fluxionality of this compound, but previous vibrational spectroscopic studies concluded that it maintains a square pyramidal geometry in solution. This non-VSEPR-compliant geometry, which is also assumed by BiPh₅ in the solid state, stands in contrast to the trigonal bipyramidal geometries of PPh₅ and AsPh₅. A range of phenomena have been invoked to explain this discrepancy, most notably, the increased importance of relativistic effects as group 15 is descended. We present crystallographic, spectroscopic, and computational data revealing that SbPh₅ in fact assumes the VSEPR-compliant trigonal bipyramidal geometry in solution. In particular, Sb X-ray absorption spectroscopy (XAS) was used to obtain geometry-sensitive spectra that do not suffer from the slow spectroscopic time scale that has prevented NMR studies from elucidating the structure of this fluxional molecule. Sb K-edge and L_III-edge XAS spectra of crystalline solids featuring SbPh₅ in either a square pyramidal (nonsolvate) or trigonal bipyramidal (cyclohexane hemisolvate or THF hemisolvate) form were compared to spectra of SbPh₅ in solution. The solution-state spectra agree with those from solids containing trigonal bipyramidal SbPh₅. The most diagnostic spectroscopic feature was the distribution of intensity in the Sb L_III pre-edge features. These distributions were rationalized using time-dependent density functional theory calculations that take into account spin-orbit coupling. Our use of Sb XAS not only resolves a long-standing physical inorganic question but also demonstrates more widely the utility of XAS in establishing the structures of fluxional main-group compounds. This conclusion was further supported by solid- and solution-state Raman data. Finally, we note that the present high-resolution diffraction data allow τ for nonsolvated SbPh₅ to be revised to 0.216.