Axial group, VSEPR theory

A systematic study of the series (CH3) PFs n [ = 0,1,2,3] has been recently completed by Bartell and co-workers ° The observed variations in the bond distance of these trigonal bipyramidal compounds are well correlated with the number of methyl substituents. In all cases the least electronegative ligands (or CH3) occupy equatorii sites. The sterochemistry and trends in structure parameters are well accounted for by the VSEPR theory. Furthermore, the increase of the axial P-F bond lengths in this series correlates well with the increase in P—F amplitudes of vibration. The methyl groups essentially rotate freely. 

---