HIGHLIGHTS Molecules and Ions with Heptacoordinated Central Atoms Rolf Minkwitz* Since the valence-shell electron-pair-repulsion (VSEPR) model was developed to predict molecular structures, it has proved to be a strong stimulus for advances in structural chemistry. Key concepts to illustrate this are “stereochemical activity of lone pairs” or “steric demand of single and multiple bonds”. Heptacoordination is currently the focus of investigations by K. 0. Christe et. al.,[’.’] K. Seppelt et. al..[3341and J. K. Cockcroft et. al.‘”‘] So far it is only known for IF,, ReF,, and OsF,, and the latter decomposes already at temperatures above 170K. Not until K. 0. Christe et. al. characterized the “naked fluoride”, the significance of which was previously highlighted”] in this journal. could the anions [XeF,]-[81. [OIFJ. [TeF,]-. [OTeFJ-.[91 [CH3TeII6-. and [(CH,),TeF,]- [31 be prepared. Monomeric and/or unchelated complex cations with the coordination number (CN) 7 around the central atom are unknown. For molecules and ions with coordination numbers 5 and 7 around their central atoms. we find many parallels with respect to their structural diversity and their intramolecular dynamics. For C N 5, calculations, which take into consideration that electrostatic repulsions occur between ligands, allow two polyhedra, the trigonal bipyramid and the square pyramid. Their difference in energy is small and they can be interconverted by a succession of angular shifts according to the Berry pseudorotation mechanism. For C N 7, three polyhedra with only small energy differences and minimum energies for the C,,, and C3rstructures“’] have been calculated under the same assumptions (Scheme 1). c2v c3v D5h Schcrne I The results of earlier IR spectroscopic investigations“] on IF, had been consistent with a pentagonal-bipyramidal stucture of the molecule. This assumption was confirmed by electron diffraction in the gas phase.“’] in spite of much uncertainty in the structural data. Crystal structure analyses performed by K. Seppelt et. al. on salts with the [TeF,]-, [CH,OTeFJ, [(CH,0),TeF,]-,[31 and [OIFJ anions give a more refined picture.”] Without exception, these anions are distorted pentagonal bipyramids with slightly shortened bonds to the axial ligands. The large methoxy ligands and the oxygen atom in [OIFJ also occupy axial positions. The distortion relates to the fluorine atoms in close contact’ around the pentagonal base. [*I Prol: Dr R. Minkwitz Fachbereich Cheinie der Universitiit D-44221 Dortmund (FRG) Telefax Int. code + (231)755-3771 They avoid steric crowding by arranging themselves above and below the theoretical plane of the pentagon. Details of this puckering can be studied in the crystal structure of ReF,. which has been determined at 1.5K by T. Vogt. A. N . Fitch, and J. K. Cockcroft, by using high-resolution powder neutron diffraction.“’ The mean displacement of the equatorial F atoms from the ideal ring plane is 0.17 8, and the mean deviation from perpendicular of the angle between axial and equatorial Re-F bonds is 6.2”. The average Re-F,, and Re-Fa, bonds are 1.851 and 1.823 8,, respectively, that is the axial bonds are 1.4% shorter. The F,,-Re-F,, angle is 174.6’ in the crystal and 172.5’ in the gas phase, as determined by electron diffraction analysis.[121The crystallographic point symmetry of the molecule is C , , but within experimental error it has a mirror plane, so that the molecular symmetry is C,. Because of strong deviations of the distorted pentagonal bipyramid from ideal D,, symmetry, the molecular structure at 1.5K is interpreted as a frozen-in state of the pseudorotational motion. In a remarkable paper by K. 0. Christe, E. C. Curtis. and D. A. Dixon, problems of heptacoordination are discussed for IF, as the most-studied prototype. On steric grounds. there is no high-symmetry arrangement of five ligands with normal bond lengths in a plane. Therefore, dynamic ring puckering occurs with large vibrational amplitudes, comparable to the vibrational ring motion in cyclopentane. This “ring puckering”, also termed “Bartell-type pseudorotation” by K . 0. Christe, has an unexpectedly low vibrational frequency (59 cin(0.1 7 kcalmol- I ) ) , which means that there are thermally activated overtones even at very low temperatures. Unfortunately, these vibrations (E; in D5,,) are neither IR nor Raman active and can be detected only indirectly in the form of very lowintensity combination bands. All F atoms in neutral molecular fluorides and fluoro anions with CN 7 are equivalent by N M R spectroscopy. Aside from the very fast ring-puckering mechanism, this equivalence is attributed to a much slower fluxionality between the axial and the equatorial ligands according to the Berry mechanism. The lifetime of a single configuration for IF, is estimated to be 2.7 x s. The exchange starts from a deformation mode at 265 cm- and the activation energy corresponds to a multiple of this frequency. The ligand exchange motion as such is best described as a combination of the out-of-phase axial and equatorial deformation modes (described by the symmetry coordinates S , and F r S7)* accompanied by an out-of-plane twisting >F F C I I mode of the remaining three equatorial F atoms (Scheme 2). In the [OIF,]- ion, the 0 atom avoids the Fequatorial position because of a partial Scheme 2, double bond and thus inhibits axial-equatorial fluxionality. Additionally, in crystalline [(CH,),N][OIF,], the puckering of the equatorial F atom plane is static rather than fluxional, since interionic H . . F contacts hinder the vibrations. Questions about the true structure of molecules with strong ’(K TF D-6945/ Weinltrinr, 1994 OS70-0X33:94:191Y-1941S /O.OO+ .25:0 1941 HIGHLIGHTS fluxionality can only be answered with respect to the method of measurement and its corresponding time scale. According to ab initio calculations at different theoretical levels. IF, in its minimum potential"] has an undistorted D,, symmetry. Indicative of this is the absence of microwave transitions and of a permanent dipole moment. Also, temperature-dependent crystal structure analyses of [(CH,),N][OI F,] show that the ring puckering diminishes with falling temperature. that is the arrangement of the equatorial F atoms approaches the arrangement in a plane. Normal coordinate analyses of IF,. [OIFJ, and [XeFJ give a picture of the vibrating molecule or ion. They are dynamically rather than statically distorted, and on average also have D,,symmetry. According to force-field calculations, the equatorial (in-plane) deformation force constant is in good approximation a measure of the steric hindrance of the ligands in the pentagonal plane. Crowding increases with decreasing bond length and decreasing size of the central atom. It also increases with increasing size of the ligand and increasing temperature. Because in electron diffraction in the gas phase the time scale is very short, such investigations register only average structures with an equilibrium symmetry between C , and C,. Not only is the equatorial ring puckering evident. but also an inclination of the axial F atoms away from the ideal 180' angle towards approximately 171 for IF,"] and 172.5" for ReF, . [ I 2 ] Through the dynamic ring puckering in the pentagonal plane, I these axial F atoms experience a non-uniform repulsion in such a way that they are each dynami& F cally forced aside by the closest lying ligand Scheme 3. (Scheme 3). In other words, the dynamic equatorial puckering gives rise to a coupled in-phase precession of the axial F atoms, which had earlier been interpreted by Bartell as a static inclination. We now know that it is a consequence of the dynamic ring puckering. q: Molecules and ions with a heptacoordinated central atom do not adopt the minimum-energy structures of a capped prism (C3csymmetry) or a capped octahedron symmetry) as predicted by VSEPR theory. They rather reside on a slightly raised saddle point of the energy hypersurface, where they adopt a pentagonal-bipyramidal structure with D,, symmetry. K. Seppelt has pointed outf3]that of the possible structures for compounds with coordination numbers 5 , 8 , and 7, the arrangement with the highest symmetry is also the most common one, and not the minimum-energy structure. This seems to be a general principle for the for the formation of such structures. As regards chemical bonding, K. 0 . Christe et. al. have proposed a model,[" in which the five pentagonal ligands are bound through p,, hybrid orbitals from the central atom in a semiionic six-center, ten-electron bond. The axial ligands are mainly covalently bonded through two sp, hybrid orbitals. German version: Angew. Chem. 1994, 106, 2017 [l] K . 0. Christe, E. C. Curtis, D. A Dixoii. 1. A m . Cheni Soc. 1993, 115. 15201526.  K . 0. Christr, D. A Dixon. A,-R. Mahjaub. H. P. A. Mercier, J. 0. P. Sanders. K . Seppelt. G. J. Schrobilgen, W. W. Wilson, J A m . C h m . Sot.. 1993, 115. 2696 -2706. [ 3 ] A.-R. Mahjauh, R. Drews. K. Seppelt. Angew. Chern. 1992. 104. 1047-1050: Al'l~f'll'. Chl'J?l./ l l / . Ed. €Jig/. 1992, 31, 1036-1039.  A.-R. Mahjaub. K. Seppelt. L. Chem. Soc. Cheni. C'ommun. 1991, 840-841. 151 T. Vogt. A . N. Fitch, J. K . Cockcroft. 1 Solid Sture Cheni. 1993. 103.275-279.  T. Vogt. A. N . Fitch. J. K . Cockcroft. Science. 1994, 243, 1265 -1267.  K . Seppelt. dngeu,. Cheni. 1992, 104. 299-300, A n p i . 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