The local site symmetry of nanodispersed molybdenum oxide phases have been determined for the first time using X-ray absorption near edge spectroscopy (XANES) at the Mo L2,3-edges. Local site symmetries varying between pure tetrahedral coordination through mixed symmetries to pure octahedral coordination were determined for species present at concentrations above and below a submonolayer. X-ray absorption near-edge spectra at the Mo L2,3-edges were obtained for a series of supported molybdate catalysts and Mo6+ reference materials with a range of oxygen coordination types. Basic ligand field concepts can be used to interpret the spectra.Tetrahedral molybdates display d-orbital splitting in the 1.8-2.0-eV range, whereas octahedrally coordinated molybdates display split peaks in the 3.0-4.5-eV range. The direct comparison between a complete set of molybdenum reference compounds and dispersed magnesium oxide-supported molybdate catalysts has been used to determine the detailed symmetries of the dispersed molybdenum phases that are present as amorphous, crystalline, and mixed phases.

Although intensive efforts have been devoted during recent years to prepare and characterize metal-bound C02 complexes in conjunction with C02 fixation via reactions with transition-metal systems,2 only a few metal-coordinated C02 complexes have been isolated and crystallographically ascertained.3"6 These include a novel d6 Ir(III) IrCl(C204)(PMe3)3 product6 in which the C204 ligand has been formed from a head-to-tail 02C-0C0 dimerization of C02 by initial reaction with a d8 Ir(I) complex. On the other hand, the more reactive, related cumulene (or heteroallene) carbon disulfide has been shown from structural studies2,7,8 to

The crystal structure of a magnesian cronstedtite-2H2 from P~l'bram, Czechoslovakia, was refined in space group C 1 to a residual of 5.4% with 1832 independent reflections. Tetrahedral ordering between Fe a+ and Si is judged to be complete on the basis of electron density maps, the first confirmation of such ordering in a layer silicate. Octahedral cations are disordered on the M sites. Mean T-O bond lengths do not confirm the tetrahedral ordering, perhaps due to tetrahedral distortion in order to relieve the extreme corrugation of the sheet that would arise from ordering of such different size cations.In the initial stages of refinement the structure of the crystal under study could not be described adequately on the basis of a single 2H2 polytype. The structure contains two kinds of 2Hz domains that appear shifted by b/3 relative to one another due to a mistake in the interlayer stacking sequence. This mistake of zero shift (the normal 2H~ polytype consists of alternating -b/3 and +b/3 shifts) affects only the tetrahedral sheets. It creates adjacent enantiomorphic domains of 2H z packets such that domains with small Si tetrahedra sit above larger FeZ+-rich tetrahedra and vice versa. This mechanism to relieve strain may indeed be the cause of the stacking mistakes. A third kind of domain contains regions in which the sense of tetrahedral rotation is reversed, giving rise to split basal oxygens on electron density maps. This multiple domain model best explains extra atomic positions observed on Fourier maps, lack of streaking of k ~ 3n reflections, and possible nonintegral "satellitic" reflections observed on Weissenberg films.

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