ON THE PROBLEM OF HEPTACOORDINATION - VIBRATIONAL-SPECTRA, STRUCTURE, AND FLUXIONALITY OF IODINE HEPTAFLUORIDE

Iodine heptafluoride, the most studied prototype of a heptacoordinated molecule, had presented many mysteries concerning its spectroscopic and structural properties. It is shown by ab initio calculations and a reexamination of the vibrational spectra and their normal coordinate analysis that most of the previously implied abnormalities were due to incorrect assignments. All the available structural data for IF7 are consistent with a highly fluxional, dynamically distorted pentagonal-bipyramidal molecule possessing D5h Symmetry in the ground state. The fluxionality of IF7 can be attributed to (i) a rapid dynamic puckering of the highly congested pentagonal equatorial plane involving a very low frequency, large amplitude puckering mode which induces a small axial bend and (ii) a much slower intramolecular exchange of the axial and equatorial fluorines resulting in their equivalence on the NMR time scale. The high degree of ligand congestion in the equatorial plane of the pentagonal bipyramid, combined with a semi-ionic, 6-center 10-electron bonding scheme, results in the equatorial I-F bonds being significantly longer than the axial ones and the equatorial in-plane deformation force constants being much larger than the out-of-plane ones. It is shown that the VSEPR model of repelling points on a sphere cannot account for either the pentagonal-bipyramidal structure of heptacoordinated molecules or the planarity of their equatorial fluorine belts. These features can be explained, however, by a bonding scheme involving a planar, delocalized p(xy) hybrid on the central atom for the formation of five equatorial, semi-ionic, 6-center 10-electron bonds and an sp(z) hybrid for the formation of two mainly covalent axial bonds.