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Lead-207 n.m.r. chemical shift data have been recorded for the following bivalent lead compounds (solvent in parentheses) : Pb(N03)2 (water), Pb(C104)2*3H20 (water), Pb(02CCH3)2*3H20 (water), Pb(02CC6F5)2 (tetrahydrofuran, thf), Pb[S,P(OR)J, (R = C2H5 or i-C4H9) (thf and dichloromethane). The nature of the solution species is discussed. The very high-field chemical shifts exhibited by the hydrated lead (11) dication and the monoacetato-and mononitrato-lead (ti) monocations are ascribed to efficient nuclear shielding resulting from the confinement of the lead(ii) lone pair to the metal 6s orbital.

The local atomic structure surrounding uranium in borosilicate glass has been investigated by fluorescence X-ray absorption spectroscopy. By employing angles of incidence either side of the critical angle for total external reflection, surface and near-surface structure can be distinguished. The results of leaching in water at 100 "C have been examined in detail. Wet and dried glass surfaces can be differentiated. Analysis of glancing-angle spectra demonstrates that uranium occupies a uranyl-like environment that hydrates as leaching progresses. Uranyl complexing occurs in a planar geometry. The initial corrosion process can be readily understood by using the modified random network model for glass structure, which predicts diffusion of cations and water taking place via percolation pathways through the network. Following extended corrosion treatment, evidence for the formation of hydrated uranyl silicates at the surface has been found.

We have characterized at high temperature a model uniform heterogeneous catalyst for the oligomerization of hydrocarbons (a nickel-exchanged zeolite of initial composition Na59A159Si,330384 -xH,O treated with an aqueous solution of NiC1, so as to yield a homogeneous distribution of Ni, with Si/ Ni = 7) by recording the extended X-ray absorption fine structure (EXAFS) abov? the Ni edge and also its high-resolution diffraction pattern (at A = 1.5486 A). We have obtained unique insights into the microenvironment of the Ni'+ ions in the as-prepared and the dehydrated as well as the reduced state of the catalyst; in particular, values of atomic coordinates, site-occupancy and bond lengths, have been obtained. High-resolution X-ray diffraction, using a specially constructed environmental cell, has enabled the precise location of sorbed Xe atoms in a sodium-exchanged zeolite X adsorbent (1.5 atoms of Xe per unit cell) under a pressure of 1.75 bar at room temperature to be determined from a Rietveld refinement of the powder diffraction pattern. Direct proof of the role of the strongly polarizing Na+ ion located at the Sn site in firmly binding the Xe to the inner wall of the supercage is obtained.X-ray absorption near-edge structures (XANES) as well as parallel EXAFS studies above the Al, Si and Ga edges on a variety of other zeolites are also reported. Combined (Cia edge) XANES and EXAFS studies show that gallosilicate networks akin to those present in crystalline (gallo) zeolite X and Y are already formed in the precursor gel and are also present in the aqueous solution in contact with the colloidal-amorphous material, which together constitute the gel. We also report one of the first applications of A1 EXAFS for the semi-quantitative 3;Jdy of the dealumination of faujasitic zeolites.

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