Theoretical study of pseudorotation of pentacoordinated silicon anions: the prototypical SiH5-

Content Provider	Semantic Scholar
Author	Gordon, Mark S. Windus, Theresa L. Burggraf, Larry W. Davis, Larry P.
Copyright Year	1990
Abstract	Ab initio and semiempirical calculations are used to analyze the minimum energy path for the pseudorotation of SiH5-. Both AMI and MP2/6-31++G(d,p) predict pseudorotation barriers of 2.4 kcal/mol. A decomposition of the projected vibrational frequencies along the path is used to assist in the interpretation of the process. Introduction Pentacoordinated silicon compounds preferentially bond in trigonal-bipyramidal (tbp) shapes rather than square-pyramidal (spy) or other geometries. 1 In the tbp geometry, the substituents can assume either one of the two axial positions or one of the three equatorial positions. Depending on the nature of the substituents, any or all of the possible permutations of the ligands may or may not be stable structures. For example, in the model compound SiX4 v-, two distinguishable isomers are predicted, one with Y axial and the other with Y equatorial. With a larger variety of substituents, there are a proportionately larger number of possible isomers of the pentacoordinated structure. These stereoisomers of simple pentacoordinated silicon compounds are not experimentally separable at room temperature; rapid ligand exchange occurs between the axial and equatorial positions.2 There has been a large body of work devoted to understanding these processes in the analogous pentacoordinated phosphorus compounds,l-7 and studies of pentacoordinated silicon make use of this body of work as a base. Differences between the two systems will be strongly dictated by the more electropositive nature of the silicon atom as compared with phosphorus8 and to the presence of a formal negative charge on silicon. One possible mode for rapid ligand exchange is the process of pseudorotation. Strauss defines pseudorotation as an intramolecular motion of nuclei in a molecule in which conformers interchange to equivalent structures differing only by the number of the atoms.9 In a broader sense, pseudorotation can also include the exchange of nonequivalent nuclei to produce a trigonal-bipyramidal stereoisomer of the original structure. Berry proposed a specific type of pseudorotation, now widely known as Berry pseudorotation, to explain fluxional behavior of phosphoranes. 10 This Berry pseudorotation process is now widely used to explain isomerization phenomena in 10-electron systems. 11 In this mechanism, shown in Figure I, a single equatorial substituent (the pivot group) is held stationary, while the two axial ligands become equatorial and the two equatorial ligands become axial. At some intermediate point in the process, a square-pyramidal structure is formed with the four interconverting ligands forming basal t North Dakota State University. I Air Force Office of Scientific Research. 0002-7863/90/1512-7167$02.50/0 positions in the pyramid and the pivot ligand occupying an apical position in the pyramid (see Figure I). If all ligands are equivalent, the trigonal-bipyramidal structures have D3h symmetry while the square-pyramidal structure has C4v symmetry. In this case, the path joining the C4v and D3n structures will have C2v symmetry. There are a number of other types of pseudorotations that are possible: see either Musher12 or Gillespie et a1. 13 for discussion of all possible rearrangements of these systems. The prototypical pentacoordinated silicon compound, SiH5-, anion has recently been observed in the gas phase.14 A number of calculations have been done on this and related systems at both the semiempirical and ab initio levels of theory, as recently reviewed by Burggraf, Davis, and Gordon. 15 Predictions at all levels of theory confirm that the D3h pentacoordinated trigonal-bipyramidal structure is a minimum on the potential surface and the C4v tetragonal pyramid is higher in energy, but only a few studies have addressed the nature of the tetragonal structure as a transition state for Berry pseudorotation. Reed and Schleyer have done the most extensive characterization of the SiH5system to date, (l) Burdett, J. K. Struct. Bonding (Berlin) 1976,31,67. (2) Carre, F. H.; Corriu, R. J. P.; Guerin, C.; Henner, B. J. L.; Wong Chi Man, W. W. C. J. Organomet. Chern. 1988, 347, Cl-c4. (3) Rauk, A.; Allen, L. C.; Mislow, K. J. Am. Chern. Soc. 1972, 94, 3035-3040. (4) Musher, J. I. Angew. Chern., Int. Ed. Engl. 1969, 8, 54. (5) Hoffman, R.; Howell, J. M.; Muetterties, E. L. J. Am. Chern. Soc. 1972, 94, 3047. (6) Florey, J. B.; Cusachs, L. C. J. Am. Chern. Soc. 1972, 94, 3040. (7) (a) Pentacoordinated Phosphorus-Structure and Spectroscopy; ACS Monograph 175; American Chemical Society: Washington, DC, 1980; Vols. I and II. (b) Deiters, J. A.; Holmes, R. R. J. Am. Chern. Soc. 1987, 109, 1686-1692, 1692-1696. (8) Muetterties, E. L. Ace. Chern. Res. 1970, 3, 226. (9) Strauss, H. L. Annu. Rev. Phys. Chern. 1983, 34, 301-328. (10) Berrry, R. S. J. Chern. Phys. 1960, 32, 933-938. (II) Mislow, K. Ace. Chern. Res. 1970, 3, 321. (12) Musher, J. I. J. Chern. Educ. 1974,51,94-97. (13) Gillespie, P.; Hoffman, P.; Klusacek, H.; Marquarding, D.; Pfohl, S.; Ramirez, F.; Tsolis, E. A.; Ugi, I. Angew. Chern., Int. Ed. Engl. 1971, 10, 687-715. (14) Hajdasz, D. J.; Squires, R. R. J. Am. Soc. 1986, 108, 3139. (15) Burggraf, L. W.; Davis, L. P.; Gordon, M.S. Topics Phys. Organamet. Chern., 1989, 3, 75. © 1990 American Chemical Society 7168 J. Am. Chern. Soc., Vol. 112, No. 20, 1990
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DOI	10.1021/ja00176a014
Alternate Webpage(s)	https://works.bepress.com/mark_gordon/90/download/
Alternate Webpage(s)	https://doi.org/10.1021/ja00176a014
Volume Number	112
Language	English
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