J Am Chem SOC 1990 112 28812886 - PDF document

1 J. Am. Chem. SOC. 1990, 112, 2881-2886
J. Am. Chem. SOC. 1990, 112, 2881-2886 288 1 reactions catalyzed have lead to the large perturbation to activate of giving an incoming ligand. the d metal orbital Mulliken population the fragment, quite characteristic a phosphine ligand which tends metal bonding and to the dlo intermediate situation. nitroxyl ligand achieves metal activation, the sp population from 0.36e 0.59e and these populations shows the d9si key point for nitroxyl ligand the reaction path energy between two fragments. not been point was beyond (25) (a) Backvall, J. E.; Bjorkman, E. E.; Pettersson, L.; Siegbahn, P. J. Am. Cfiem. SOC. 1984, 106,4369-4373. (b) BPckvall, J. E.; Bjorkman, E. E.; Pettersson, L.; Siegbahn, P. Ibid. 1985, 107, 7265-7267. (c) BPckvall, J. E.: Biorkman. E. E.: Pettersson. L.: Siegbahn. P.: Strich. 7408l7412. (d) Blomberg, M. R. A.; Siigbahn, P: E. M.;BPckvall, J. E:lbid: 1987, 109, 4450-4456. 7. Conclusion one hand two combinations other hand. interactions depends free metal. copper d levels yields for CuX, main halogen to the Recherche (Palaiseau, France) through a Ge-Ge Bonding High-pressure Modification Near-Zintl Phase Science Center, Baker 14853- 1301. Received Zintl-phase LiGe to undergo temperature and metastable modification, differs markedly from normal-pressure form the normal three-connected, but remainder have two (to the other atoms the same distorted square binary materials simplest theories covalent bonding molecules have large difference two component elements enables ionic model electrons between (usually alkali-metal alkaline-earth elements) their valence electrons to more elec- tronegative element (generally from one However, because resulting anions do not achieve 'Current address: SERC Daresbury Laboratory, Warrington, WA4 4AD, UK. 0002-7863/90/1512-288 1$02.50/0 distributed among these As noted Schafer and between phases

2 for Zintl concept works well, bonding b
for Zintl concept works well, bonding between two components, not distinct. A factors affect (1) Zintl, E. Angew. Cfiem. 1939, 52, I. (2) (a) Mooser, E.; Pearson, W. B. Pfiys. Reu. 1965, 101, 1608. (b) Klemm, W. Proc. Cfiem. SOC. 1958, 329. (c) Busmann, E. Z. Anor.?. Alla. -- Cfiem. 1961, 313, 90. (3) Schafer, H.; Eisenmann, B. Reu. fnorg. Cfiem., 1981, 3, 29. American Chemical 2882 J. Am. Chem. Soc., Vol. 112, No. 8, 1990 covalently bound solid. as either becomes more important, the Zintl electron-counting scheme fails, and the covalently bound network However, residual covalent bonding persists, often a result within a metallic also examples electron-counting rules, containing regions to the type metallic for which concept works Ge- ions form a three-connected net, the structure not shown here, all two fused has recently be transformed, rapid cooling into a metastable It also consists eight-membered rings, Sherwood and Hoffmann I this case such a as to share three atoms, rather than normal-pressure al- lotrope, has distinct1 layered Within a layer, four-coordinate (pseudotetrahedral) and Ge(2) thus formed two short layer, giving atoms distorted square-planar two crys- and alternately internuclear separations clearly suggest a covalently bonded does not fit interactions, each atoms (1 2 two-coordinate) atom, and isoelectronic with have closed-shell not expect any interlayer a two-di- (4) Volk, R.; Muller, W. Z. Narurforsch. 1978. 338, 593. (5) (a) Menges, E.; Hopf, V.; Schifer, H.; Weiss, A. Z. Naturforsch. 1969, 248, 1351. (b) Evers, J.; Oehlinger, G.: Sextl, G.; Becker, H.-0. Angew, Chem. 1987, 99, 69; Angew. Chem., Int. Ed. Engl. 1987, 26, 76. a b c Figure 1. Calculated DOS for the 2D slab 2. The Droiections (shaded (for labeling seems reasonable to assume weak interlayer and the deficiency” (in this paper examine this rela

3 tionship. valence electrons substructure
tionship. valence electrons substructure and not become in covalent bonding. atoms from all calculations, electron count to include their valence electrons. thus ignored, though it lie close a Ge-rich electronegativity difference small compared most systems behavior. Compression also favors begins with LiGe structure. We pendent on electron to planar Electronic Structure As expected, extended Huckel calculations described in the Computational a single two-di- a covalently bonded network. t Ge13) 2 3Ge, all bonding substantial band calculated density atomic orbitals and the the 16e/3Ge count, the isoelectronic relationship two nonbonding alternative representations 3 4 High- Pressure Modification of Lice J. Am. Chem. SOC., Vol. 112, No. 8, 1990 2883 1 I I Ge -mbonalq bonamq- a b C d e (shaded regions) indicate the contributions planar coordinated labeling scheme, Ge-Ge bond. that the highest occupied Ge(2), and Ge(3). [The net makes Ge(3)py, and for brevity from here Ge(2) and These orbitals with each similar orbitals neighboring unit a concentration nonbonding pair character (Figure IC). and the both composed largely overlap population this simple analysis. comes from p band, px-type lone Stacking of the Layers Before proceeding completely. Any different stacking the Li+ charge transfer from Li be very the same filling levels for 16 and that for a bonding p no longer a significant 16e filling, and the might expect some metallic projections with Figure 1 largest changes concern filling point. except for eV, considerably lower in antibonding band little effect 2e) is more interesting. the bonding/nonbonding p band -15 ' the bands character to interlayer contacts, to the 16e filling bonding between for lower electron counts, this net interlayer orbital terms plicated band structure as a its large unit this because communication bo

4 nding zone next (Le., is likely Differen
nding zone next (Le., is likely Different two-coordinate appear to single bonds. One-Dimensional Model have chosen a ribbon shown in missing bonds 5 off with cell of indicated above, contains four a charge of -3 to produce nonbonding electrons each pair atoms there be seven nonbonding electrons. in px to the ribbon plane, t, 6 7 Brillouin zone shown in 2884 J. Am. Chem. SOC., Vol. 112, No. 8, 1990 Sherwood and Hoffmann r- k-X a b “unfolded” about figure. Note that there filled, low-lying not shown 3, which have nonbonding band remaining crystal orbitals the same avoided crossing between being present the center brought together with resulting band many bands symmetry element; to this element also. It can gives rise with respect to the This symmetry the appearance Figure 4a structure about all bands to the this point, Figure 4b. ribbon-type systems the fate a narrow very similar corresponding band Figure 3, indicating these orbitals flat band, due to the avoided crossing. In form a band considerable dis- the orbitals (Ge,H2), chains, the top an electron this band not give rise to any bonding. However, the top bonding between orbital (3u, energy (by than the lowest Ge-Ge relative phases Figure 3 possible with similar band on D chain, gives rise Figure 4b. effect offers an explanation for the appearance antibonding bands the DOS emphasized, without consideration cannot approach (6) Hoffmann, R.; Minot, C.; Gray, H. B. J. Am. Chem. Soc. 1984, 106, 200l. worth considering, one alternative which all This stacking pattern is shown 8 reader may its topology; brought together to the and the symmetry, this fully bonded a much performed a calculation to compare this “expanded electron count appropriate to LiGe formu- lation. Although populations for interlayer bonding higher total energy. to the filling levels fully occupy to the

5 overlap populations, (analogous Figures
overlap populations, (analogous Figures 3 remaining electrons 2u band Figure 4b. In form bands nonbonding electrons here), and resulting bands be filled lower lying crystal orbitals antibonding effects the higher put it titions half into strongly bonding ones, half then ideal for filling half bonding ones, is destabilizing as one center count High-pressure Modification of Lice 2u band the language extended systems, it When the is asso- than eight clear from electron counts than eight J. Am. Chem. SOC., Vol. 112, No. 8, 1990 2885 stacking the layers is shown in 11. This differs from that in the +- y yAz 9 the 2b2 electron count bonding most analogous to that solid, since the latter both ends long Ge-Ge bonds. However, model band a Mulliken popu- lation analysis, 0.86 for Hypothetical Structures Based structure and the structure cubic diamond. It consider a LiGe and some interactions type, while some A large number large unit generated this way, which observed for systems have electron framework atoms. more intriguing IO ordination for atoms on the surface of one of the slabs, and distorted square planar for those on the other. Another way of (7) Albright, T. A.; Burdett, J. K.; Whangbo, M. H. Orbital Inreracrions in Chemistry; Wiley-Interscience: New York, 1985. a relative one sheet a unit with respect dination for the surface between px-type neighboring slabs the LiGe “-type overlap, expect such electron count similar to that the in which this might 12 trigonal coordination, character to the the additional four-fold coor- can generate structures the atoms 14 coordination, whereas illustrate one sharing surface atoms, which consequently have a planar the shared atoms 2886 J. Am. Chem. SOC. 1990, 112, 2886-2891 IS the cubic diamond to the and the distorted square-planar geometry the structure. and the probably largely such b

6 onding structure than have distorted hed
onding structure than have distorted hedral geometries, adopt planar structures. Computational Appendix extended systems. were taken from properties averaged DOS and curves) special k-point Ramirez and BohmIo were sets assumed tetragonal symmetry, L.; Hoffmann, R. (9) Hoffmann, R. (10) Ramirez, R.; Nuclear Magnetic Resonance Spectrometry L. White, Station, Texas 77843. May 30, to deactivation pore blockage to oligomerize primarily 1,4-addition reaction its melting point Secondary reactions strongly dependent linear product undergoes cyclization to form fused rings but isolated the smaller channels enchainment result an appreciable group content oligomers formed insight into the mechanisms which oxide pore blockage. reactive intermediates were observed spectroscopically, presumably because such intermediates reacted either free butadiene detectable steady state concentration. situ MAS studies of chemical reactions described. This procedure insufficiently volatile recently reported other adsorbates such reactions inferences based sample treatment.I high resolution magic-angle spinning such reactions a realistic goal, vestigation, a sample preparation (cryogenic adsorption enabling rotor nestling) olefin on were then Author to correspondence should Permanent address: Department tingdon, PA 16652. 0002-7863/90/ 15 12-2886$02.50/0 were induced a stepwise different isotopomers propene) showed scrambling did reaction mechanism simple olefins particularly challenging S. (1) Anderson, M. W.; Klinowski. J. Nature 1989, 339, 200. (2) For reviews of the CP/MAS NMR technique, see: (a) Yannoni, C. Ace. Chem. Res. 1982, 15, 210. (b) Maciel, G. E. Science (Washington, (3) Haw, J. F. Anal. Chem. 1988, 60, 559A. (4) Haw, J. F.; Richardson, B. R.; Oshiro, I. S.; Lazo, N. D.; Speed, J. A. D. C.) 1984, 226, 282. J. Am. Chem. SOC. 1989, Ill, 2052. American Che