MoF6 and WF6: Nonrigid Molecules?

Abstract
Calculations reveal that the octahedral–trigonal prismatic–octahedral rearrangement has particularly low‐energy barriers for MoF6, WF6, and (hypothetical) CrF6. Experimental evidence is obtained from the dynamic 19F NMR spectra of the derivatives CF3CH2OMoF5, CF3CH2OWF5, C6F5OMoF5, C6F5OWF5, and (CF3)3COWF5. The ground‐state structure of all these compounds is octahedral; at elevated temperatures the nonequivalent metal‐bound fluorine atoms undergo an intramolecular exchange. The exchange mechanism could be a 3+3 or a 2+4 twist; calculations favor the 3+3 twist.