HEPTACOORDINATION - PENTAGONAL BIPYRAMIDAL XEF(7)- IONS( AND TEF(7))

The TeF7- anion was studied experimentally by vibrational and F-19 and Te-125 NMR spectroscopy. Ab initio calculations employing effective c ore potentials and density functional theory calculations at the self- consistent nonlocal level with the nonlocal exchange potential of Beck e and the nonlocal correlation functional of Perdew were used for the analysis of the isoelectronic series TeF7-, IF7, XeF7+. It is shown th at XeF7+ is a stable structure, that all three members of this series possess a pentagonal bipyramidal equilibrium geometry, and that from t he two closest lying saddle point geometries only the monocapped trigo nal prism, but not the monocapped octahedron, is a transition state fo r the intramolecular axial-equatorial ligand exchange. The results fro m a normal coordinate analysis reveal the existence of an unusual new effect which counteracts the ligand-ligand repulsion effect and is cha racterized by axial bond stretching encouraging equatorial bond stretc hing. While in TeF7- the ligand-ligand repulsion effect dominates, in XeF7+ the new effect becomes preponderant.