The IOF6- anion: the first example of a pentagonal bipyramidal AX5YZ species

• Published in: Journal of the American Chemical Society Vol. 115; no. 7; pp. 2696 - 2706
• Main Authors: Christe, K. O, Dixon, D. A, Mahjoub, A. R, Mercier, H. P. A, Sanders, J. C. P, Seppelt, K, Schrobilgen, G. J, Wilson, W. W
• Format: Journal Article
• Language: English
• Published: WASHINGTON American Chemical Society 01.04.1993; Amer Chemical Soc
• Subjects: Atomic and molecular physics; Chemistry; Chemistry, Multidisciplinary; Exact sciences and technology; Interatomic distances and angles; Molecular properties and interactions with photons; Physical Sciences; Physics; Properties of molecules and molecular ions; Science & Technology; VIBRATIONAL SPECTRA; EFFECTIVE CORE POTENTIALS; MICROWAVE; ELECTRON DIFFRACTION; OSOF5; MILLIMETER-WAVE SPECTRUM; IF7; TRANSITIONS; STATE SPECTRUM; FLUORIDE; Chemical solution; Temperature; Bond length; Molecular structure; Ab initio method; Bond angle; Force constant; Theoretical study; Effective potential; Electron charge distribution; Experimental study; X ray diffraction; Inorganic compound; Solid state; Inorganic anion; Infrared spectrum; Vibrational mode; NMR spectrum; Raman spectrum; Local density approximation; Crystalline structure
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja00060a019

The IOF6- anion, which is the first example of a pentagonal bipyramidal AX5YZ type species, was prepared in the form of its stable N(CH3)4+ salt. Its X-ray crystal structure was determined at -93-degrees and -155-degrees-C (tetragonal, space group P4/nmm, Z = 2, a = 8.8590 (10) and 8.8151 (10) angstrom, c = 6.3690 (10) and 6.3213 (10) angstrom, R = 0.0373 and 0.0291 for 876 [I > 3sigma(I)] and 885 [I > 3sigma(I)] reflections, respectively). In addition to two perfectly ordered N(CH3)4+ cations, the structure contains two IOF6- anions of approximate C5v symmetry which are subject to a positional 4-fold disorder for the equatorial plane. The O-I-F(ax) angle is constrained by symmetry to be 180-degrees, whereas there are no constraints on the positions of the equatorial fluorines. The I-O bond length indicates substantial double bond character, and the axial I-F bond length is significantly shorter than the five equatorial I-F bond lengths. The mean O-I-F(eq) bond angle is slightly larger than 90-degrees, due to the doubly bonded oxygen atom being more repulsive than the singly bonded axial fluorine ligand. The equatorial IF5 plane is puckered to alleviate its congestion. In contrast to the highly fluxional, free IF7 molecule, in which the equatorial fluorines undergo a very rapid, dynamic, pseudorotational ring puckering and a slower intramolecular equatorial-axial ligand exchange, the puckering of the IOF6- anion in its N(CH3)4+ salt is frozen out due to anion-cation interactions, and the equatorial-axial ligand exchange is precluded by the more repulsive oxygen ligand which occupies exclusively axial positions. Therefore, the IOF6- anion is ideally suited for studying the nature of the equatorial puckering in species of 5-fold symmetry. The puckering in IOF6- is of the C(s) symmetry type, and the deviations from the ideal equatorial plane are relatively small and decrease with decreasing temperature. Furthermore, the axial I-O bond length decreases and the mean O-I-F(eq) bond angle increases with decreasing temperature. These findings demonstrate that in agreement with our results from ab initio calculations and contrary to the VSEPR concept of repelling points on a sphere, the minimum energy structures of these main group heptacoordinated fluorides or oxyfluorides are those of pentagonal bipyramids with an unpuckered equatorial plane and not those of either monocapped octahedra or monocapped trigonal prisms. Whereas in solid N(CH3)4+IOF6- the equatorial ring puckering of IOF6- is frozen out, in the dissolved free ion this puckering becomes dynamic, as demonstrated by F-19 NMR spectroscopy which shows rive equivalent equatorial fluorine ligands. Contrary to IF7 and TeF7-, the IOF6-, anion does not undergo an intramolecular equatorial-axial ligand exchange on the NMR time scale because of the more repulsive, doubly bonded oxygen. The vibrational spectra of N(CH3)4+IOF6- in both the solid state and CH3CN solution were recorded and assigned with the help of ab initio calculations on IOF6 using effective Core potentials and local density functional theory. Normal coordinate analyses were carried out for the pentagonal bipyramidal series IF7, IOF6-, XeF5- which show that the equatorial, in-plane deformation force constants (f(alpha)) are a good measure for the degree of congestion in the equatorial plane. Puckering increases with decreasing bond lengths, increasing ligand and decreasing central atom sizes, and increasing temperature. The pentagonal bipyramidal structures of these molecules and the coplanarity of their equatorial ligands, which are found for their minimum energy structures, are explained by a bonding scheme involving delocalized p(xy) hybrid orbitals of the central atom for the formation of a coplanar, semi-ionic, 6-center 10-electron bond system for the five equatorial bonds and of an sp(z) hybrid orbital for the formation of two, more covalent, colinear, axial bonds. This bonding scheme can account for all the observed structural features and also the bond length differences.