PENTACOORDINATED MOLECULES .14. MOLECULAR VIBRATIONS AND STEREOCHEMICAL NONRIGIDITY OF TRIGONAL-BIPYRAMIDAL MODEL MX3Y2

Based on complete infrared and Raman vibrational spectra, a normal mode analysis of the series of trigonal-bipyramidal molecules PF5, AsF5, VF5, PCl3F2, PCl5, SbCl5, Sb(CH3)5, and NbCl5 has led to a description of the ground-state motions. Barrier energies for intramolecular exchange are compared for a square-pyramidal and tetragonal-pyramidal intermediate. The conclusion shows that either process gives a similar ordering and magnitude of barrier energies for these molecules. Examination of appropriate combinations of bending coordinates modified in most cases by ligand reorganization leads to a classification of model processes for intramolecular exchange mechanisms in terms of relative strengths of equatorial and axial bonds. Application is seen in transition metal chemistry. Further, transferability of data allows estimation of an exchange barrier for the hypothetical PH5 molecule. The latter is considered the only case where a tunneling mechanism for exchange might be significant. © 1969, American Chemical Society. All rights reserved.