Pseudopotential SCF-MO studies of hypervalent

New evidence bearing upon the anomalous properties of xenon hexafluoride has been obtained via the ab initio molecular orbital approach applied successfully to the diand tetrafluorides in paper 1. Structures of both XeFt and XeF6 are governed by a stereochemically active lone pair. In the case of the square-pyramidal cation the F .. -Xe-Feq angle calculated for the bare ion is within 2° of the value observed in the crystalline complex. For the hexafluoride, however, the calculated deformation from Oh symmetry is appreciably greater than that deduceq from electron diffraction intensities. Nevertheless, the results of calculations are in sufficient conformity with the Bartell-Gavin, Pitzer-Bernstein interpretation and at variance with the "electronic-isomers" interpretation to leave little doubt about the answer. With increasing fluorination in the XeFn series the HOMO-LUMO energy difference decreases and the second-order Jahn-Teller effect is enhanced. Increasing fluorination (and increased positive charge on Xe) also shortens bond lengths; calculated shortenings parallel observed shortenings. The deformation of XeF6 from Oh is along tlu bend and stretch coordinates to a C3y structure with long bonds adjacent to the lone pair, as expected according to the valence-shell-electron-pair-repulsion model. Pure t2g deformations are destabilizing but anharmonic tlu -l2g coupling significantly stabilizes the deformation. Steric aspects of the structure and force field are diagnosed and found to be minor. Values for the force constants f44' Iss, 14444' 1444'4" and 1445 are derived and found to be of the magnitude forecast in the Bartell-Gavin and Pitzer-Bernstein treatments except that the calculations do not reproduce the delicate balances believed to lead to almost free pseudorotation in XeF 6'