Main-Group Fluorides with Coordination Numbers Greater Than Six

Chapter 4 M a i n - G r o u p Fluorides with Coordination Numbers Greater T h a n Six Konrad Seppelt

Downloaded by MONASH UNIV on May 2, 2013 | http://pubs.acs.org Publication Date: April 29, 1994 | doi: 10.1021/bk-1994-0555.ch004

Institut für Anorganische und Analytische Chemie, Freie Universität Berlin, Fabeckstrasse 34-36, D-14195 Berlin, Germany

The principal geometries for neutral or anionic AF7 andAF8species are the pentagonal bipyramid and the square antiprism, respectively (A = heavy main group element). Examples of the first geometry include IF7, IOF6-, TeF7-, CH3OTeF6-, and (CH3O)2TeF5-. The ion IF8provides an example of the second geometry. The pentagonal bipyramid is not ideally realized: there are systematic deviations from planarity for the equatorial positions. The nonbonding electron pair, E, in species such asAF6EandAF8Eplays a dual role depending on the size of the central atom. For example, its stereochemical activity disappears if crowding of the structure becomes severe, as in BrF6and XeF82-. On the other hand, the stereochemical activity of the nonbonding pair is clearly apparent, in less crowded fluoride species such asIF6-as well as in the classic case of XeF6. The principal geometries for molecules or ions of the type AB2, AB3, AB4, AB5, and AB6 are well established. The most complicated case is clearly AB5: the trigonalbipyramidal and square-pyramidal geometries are close in energy. In most cases involving main group compounds, the trigonal bipyramid is the dominating structure, with the square pyramid proposed as a transition state for intramolecular ligand exchange via the Berry mechanism. Besides a few cases for which chelating ligands enforce a square-pyramidal geometry, there are only a handful of square-pyramidal main group compounds, namely Bi(C6H5)5 (i) and its many derivatives (7) and Sb(C6H5)5 (2). It has become clear very recendy that the highly symmetric octahedron is not always the principal geometry for AB^ compounds: an exception to the rule is W(CH3)6, which possesses a trigonal prismatic structure (3). Coordination Number 7 Keeping these facts in mind, it is immediately clear that structural preferences for compounds of the type AB7 (and ABs, see below) should not be easy to predict. 0097-6156/94/0555-0056$08.00/0 © 1994 American Chemical Society In Inorganic Fluorine Chemistry; Thrasher, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.


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Main-Group Fluorides with High Coordination Numbers 57

Kepert has thoroughly analyzed the AB7 case under the assumption that only electrostatic forces determine the structure (4). In that case, there are three Hmiting geometries, the pentagonal bipyramid, the monocapped octahedron, and the monocapped trigonal prism, all three of which are shown in Figure 1. The monocapped octahedron is lowest in repulsive energy, followed very closely by the other two. Furthermore, relatively small angle deformations can interconvert all three geometries. Therefore, as a function of certain deformation angles, the repulsive energy hypersurface contains the monocapped octahedron as the global minimum, with the pentagonal bipyramid and the monocapped trigonal prism as saddle points only a little above the minimum. In other words, this model predicts that AB7 compounds should have low barriers for intramolecular ligand exchange. The compound IF7 is the only example of a binary main group molecule of the type AB7. It adopts a pentagonal-bipyramidal structure, but electron diffraction data indicate certain systematic deviations, in terms of pentagonal ring puckering, from the ideal structure in the gas phase (5). The solid state structure of IF7 remains uncertain (6). We have studied the question of the principal structure of AB7 species by looking at ionic relatives of IF7, namely the anions TeF7~, (CH30)TeF6~, (CH30)2TeF5~, and IOF^ . The influence of lattice energy on the structures was kept at a minimum because the anions are monovalent. Furthermore, the relatively large (CH3)4N counterion was chosen to further minimize cation-anion interactions that might influence the structure. X-ray crystallography showed that TeF7~, CH30TeF6~, (CH30)2TeF5~, and IOF6~ all exhibit slightly distorted pentagonal-bipyramidal geometries, as shown in Figure 2 (7,8). These results can be translated into rules for the geometry of seven-coordinate main group compounds: (1) The principal geometry is the pentagonal bipyramid. This is against the electrostatic model. This finding indicates that among main group elements the principal geometry is always the one with highest symmetry. The pentagonal bipyramid needs only two parameters (two bond lengths) to be completely described, in comparison to five parameters for the other two geometric alternatives. This seems to be a general principle, in that it also holds for the trigonal bipyramid vs. the square pyramid as well as for the square antiprism vs. the trigonal dodecahedron (see below). (2) The pentagonal-bipyramidal structure is never completely ideal. A l l structures show deviations in the location of the equatorial groups (i.e., pentagonal ring puckering). Most often the sequence up, down, up, down, in plane is observed. These distortions apparently diminish the considerable steric repulsions among the equatorial groups, which are a consequence of the very small B - A - B angles of -72°. (3) Oxygen ligands, considered to be larger ligands thanfluorineatoms, always occupy the sterically less crowded axial positions. (4) The pentagonal bipyramid undergoes intramolecular ligand exchange in solution. This is clearly proven by the F and T e NMR spectra of TeF7~ and CH30TeF6~, which show seven and six equivalent fluorine atoms on the NMR time scale, respectively (6). In case of (CH30)2TeF5~, the question of non-rigidity cannot be simply answered, since all five fluorine atoms are chemically equivalent in the observed solid-state structure. The anion IOF6~ is clearly an exception, however. The doublet-sextet pattern in its 1 F NMR spectrum indicates a rigid species. Most -

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In Inorganic Fluorine Chemistry; Thrasher, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1994.


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INORGANIC FLUORINE CHEMISTRY: TOWARD THE 21ST CENTURY

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Figure 1. The principal geometries of AB7 compounds: the pentagonal bipyramid, the monocapped octahedron, and the monocapped trigonal prism.


Main-Group Fluorides with High Coordination Numbers


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Figure 2. Crystal structures of TeF , CH30TeF6 , (Cr^O^TeFs , and I0F6 as examples of pentagonal bipyramids. Continued on next page. 7




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Figure 2.

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probably, the considerable double bond character of the 1-0 bond locks the oxygen atom into an axial position and thus prevents further intramolecular exchange (6). The Outlook for Transition Metal Compounds. Relatively little is known about this problem among seven-coordinate transition metal complexes like ReF7, WF7", NoF7~, and UF7". We have crystallized the salts Cs WF7~ and Cs MoF7~ from acetonitrile and have found that both anions exhibit a monocapped octahedral structure (9). Whether this is the result of cation-anion interactions in the cubic lattice, or whether there is a different rule governing the structures of sevencoordinate transition metal compounds, remains to be seen. Further work is in progress. +

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Coordination Number 8 The two principal geometries for A B compounds that result from D. L. Kepert's purely electrostatic model are the square (archimedean) antiprism and the trigonal dodecahedron, as shown in Figure 3. The cube, however, is quite unfavorable (4). Chemical realizations, however, are rare. No binary material A B exists for main group elements or for any transition metal; compounds such as XeF and OsF are unknown. We addressed the question of the principal structure for coordination number 8 by looking at anions. According to X-ray crystallographic results for NO IF ~2NOF and (CH3)4N IF ~, the IF ~ anion exhibits a very regular squareantiprismatic structure, as shown in Figure 4 (10). While it is not yet proper to generalize this finding, it is interesting that nature has once again chosen the more symmetric geometry. The square antiprism is defined by only two parameters, while the trigonal dodecahedron requires four. The two parameters for the square antiprism may be defined as one bond length and one angle that expresses the elongation or flatness of this geometry. In the case of IF ~ all of the I-F bond lengths are equal. All fluorine-fluorine distances are also quite similar, independent of whether the two fluorine atoms are within one hemisphere of the anion or belong to two hemispheres. Our finding indicates that, in this case, the electrostatic model governs the geometry. Whether or not this finding will be valid for transition metal compounds remains to be seen. 8

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High Coordination Number Species with One Nonbonding Electron Pair. The Valence Shell Electron Pair Repulsion model correctly predicts the structures of main group compounds and cfi transition metal compounds in most cases. The structures of less symmetric molecules can be predicted semi-quantitatively. The model is particularly successful if the coordination number (including the count of nonbonding electron pairs) does not exceed six. A peculiarity of the model is the stereochemical activity of the free electron pair. Whether the VSEPR model can be successfully extended to coordination numbers larger than 6 is the topic of the following discussion.




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Figure 4. Crystal structure of IFs as an example of a square antiprism.


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Main-Group Fluorides with High Coordination Numbers

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A case of long dispute is the structure of XeF^ It is now clear that XoFe is not octahedral in the gas phase, having a monocapped octahedral structure in which the cap is the free electron pair. This structure undergoes, of course, rapid intramolecular ligand exchange (77). To complicate matters further, XeF6 is tetrameric and hexameric in solid phases (72). These oligomers are composed of square pyramidal XeF5 units, that are bridged by F~ ions. In solution, however, the tetramers exist as isolated nonrigid species with four magnetically equivalent xenon atoms and 24 magnetically equivalent fluorine atoms, indicating that the bridging fluorine atoms in the solid state structure cannot be left out when computing the coordination number count (73). The question is, will this complicated structural problem be found in the structures of JF^~ and BrFs"? In the crystal structure of (CH3)4N IF6~, the IF6~ units appear as dimer dianions (14). Each IF6~ anion has a distorted octahedral structure; the seventh coordination site, the cap, can be thought to be occupied by the nonbonding electron pair, as shown in Figure 5. If dimer formation by two bridging fluorine atoms is overlooked, this structure is identical to the gas phase structure of XeF6. The bridging fluorine atoms exchange with the residual IF5 units upon warming to room temperature. This underscores the need for low temperature crystallography as a routine method. Interestingly, by changing the cation to NO , the IF6~ anion appears as a tetramer (14). This arrangement is nearly identical to the XeFo tetramer, except that the bridging fluorine atoms are more regularly spaced in ( I F ^ than in (XeFô)4. All in all, it can be said that IF6~ is a true structural relative to XeF6It is possible that with a cation larger than (CH3)4N , a non-bridged IF6~ monomer will be found. Note that the iodine analogue to the non-rigid ( X e F ^ tetramer in solution has not been established. The BrF6~ anion, on the other hand, has a regular octahedral structure. This conclusion was reached based on X-ray crystallographic results for Cs BrF6~ as well as on vibrational spectroscopic results for (CH3)4N BrF6~ in the solid state and in solution (15,16). The surprising disappearance of the stereochemical activity of the nonbonding electron pair may be explained in two ways. First of all, crowding of the smaller bromine atoms (i.e., smaller relative to iodine or xenon atoms) by six fluorine atoms leaves no room for the free electron pair. Alternatively, the electron pair in BrF6~ may occupy a centrosymmetric (but not necessarily spherically symmetric) orbital. The nonbonding electron pair has no other effect on the structure other than causing a considerable lengthening of the Br-F bonds in BrF6~ relative to the bond lengths in BrFg . The preference for an a\g orbital in BrF6~ may be the result of the transition metal contraction: the imperfect shielding of the positive charge on the bromine nucleus by electrons in the first d-subshell increases the effective nuclear charge for centrosymmetric and inner orbitals (see Figure 6). There is a second case in which the stereochemical activity of a nonbonding electron pair disappears, and not surprisingly, it also involves a species with a high coordination number, XeFg (77). The structure, determined long ago, was found to be a regular square antiprism. Interestingly, the Xe-F bond distances are longer than the I-F distances in IF ~, although a xenon atom is smaller than an iodine atom. +


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Therefore, in two separate cases the formal redox reactions BrF6 and XeF

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IF leaves the structure unchanged. Addition or subtraction 8





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of the two nonbonding electrons only results in a symmetric "breathing" of the entire anion. + 2 — The series XéFs » XeF6> XeF serves as a perfect example of the gradual disappearance of the stereochemical activity of a nonbonding electron pair with increasing crowding: in XeF5 , the electron pair serves as a ligand larger than fluorine, since the cation has a square-pyramidal umbrella structure; in XeFô (or in IFg"), the electron pair is stereochemically active enough to strongly disturb the octahedral geometry; in XeF ~, however, the stereochemical activity of the electron pair is negligible. An interesting, but as yet unknown, case would be the structure of XeF ". Literature Cited 8

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(a) Schmuck, Α.; Buschmann, J.; Fuchs, J.; Seppelt, K. Angew. Chem., Int. Ed. Engl. 1987, 26, 1180. (b) Schmuck, Α.; Leopold, D.; Wallenhauer, S.; Seppelt, K. Chem. Ber. 1990, 123, 761. 2. (a) Wheatley, P. J. J. Chem. Soc. A 1964, 3718. (b) Beauchamp, A. L.; Bennett, M. J.; Cotton, F. A. J.Am.Chem.Soc. 1968, 90, 6675. 3. Haaland, Α.; Hammel, Α.; Rypdal, K.; Volden, Η. V. J. Am. Chem. Soc. 1990,112,4547. 4. Kepert, D. L. Inorganic Stereochemistry; Springer: Berlin, 1982. 5. Adams, W. J.; Thompson, H. B.; Bartell, L. S. J. Chem. Phys. 1970, 53, 4040. 6. (a) Burbank, R. D.; Bensey, F. N. J. Chem. Phys. 1957, 981. (b) Burbank, R. D. Acta Crystallogr. 1962,15,1207. (c) Burbank, R. D. J. Chem. Phys. 1963, 16, 700. (d) Donehue, J. J. Chem. Phys. 1959, 30, 1618. 7. Mahjoub, A.-R.; Seppelt, K. J. Chem Soc. Chem. Commun. 1991, 840. 8. Mahjoub, A.-R.; Drews, T; Seppelt, K. Angew. Chem., Int.Ed.Engl. 1992, 31, 1036. 9. Geise, S.; Seppelt, K. Manuscript in preparation. 10. Mahjoub, A.-R.; Seppelt, K. Angew. Chem., Int. Ed. Engl. 1991, 30, 876. 11. (a) Gavin, R. M.; Bartell, L. S. J. Chem. Phys. 1968, 48, 2460. (b) Bartell, L. S.; Gavin, R. M. ibid. 1968, 48, 2466. 12. Burbank, R. D.; Jones, G. R. J. Am. Chem. Soc. 1974, 96, 43. 13. Rupp, H. H.; Seppelt, K. Angew. Chem., Int. Ed. Engl. 1974, 13, 612. (b) Seppelt, K.; Rupp, Η. Η. Ζ. Anorg. Allg. Chem. 1974, 409, 331. (b) Schrobilgen, G. J.; Holloway, J. H.; Granger, P.; Brevard, C. Inorg. Chem. 1977,17,980. 14. Mahjoub, A.-R.; Seppelt, K. Angew. Chem., Int. Ed. Engl. 1991, 30, 323. 15. Mahjoub, A.-R.; Hoser, Α.; Fuchs, J.; Seppelt, K. Angew. Chem., Int. Ed. Engl. 1989, 28, 1526. 16. Christe, K. O.; Wilson, W. W. Inorg. Chem. 1989, 28, 3275. (b) Wilson, W. W.; Christe, K. O. ibid. 1989, 28, 4172. 17. Peterson, S. W.; Holloway, J. H.; Coyle, Β. Α.; Williams, J. M. Science 1971, 173, 1238. RECEIVED May 4, 1993


Main-Group Fluorides with Coordination Numbers Greater Than Six

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