Single crystals of the yellow form of [Pt(2,2'-bpy)(C1)2] have been prepared, and the crystal structure is compared to that of the Pd(I1) homologue reported earlier. The yellow and red modifications of the title compound are compared by FTIR, UV/vis, EXAFS, and hydration studies. The major differences in the two forms arise from differences in the stacking mode of the monomers in the solid-state lattice, with significantly different Pt-Pt distances being observed in the yellow (4.435( 1) A) and red (3.45 A) complexes. As expected from the similarity of the yellow forms of the Pt(I1) complex to the corresponding Pd(I1) complex, two inequivalent Pt-CI distances are noted, in contrast to the architecture of the red form. In the latter, a pseudo-2-fold symmetry axis is preserved in the stacked array, while this symmetry element is absent in the yellow solid owing to a non-2-fold rotation around the Pt-Pt chain axis in the yellow form. The differences in the Pt-Pt scattering maxima extracted from the EXAFS results are in consonance with the crystallographic data.

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Ni K-edge x-ray-absorption spectroscopy ͑XAS͒ measurements on a series of perovskite-layer-based nickelate compounds, with varying formal Ni valence, are presented. Spectral features that vary systematically with the Ni configuration are identified for these Ruddlesden-Popper phase and Sr substituted Ln 2Ϫx Sr x NiO 4 ͑LnϭLa and Nd͒ materials. Selected polarized XAS results on the La 2Ϫx Sr x NiO 4 system emphasize that a collapse of the Ni 4p -4p energy separation accompanies the substantial contraction of the apical Ni-O bond length in this series. In view of this, it is suggested that a displacement of the apical oxygen ͑O ab ͒ could play an important role in the small polarons previously proposed to form in this system. The polarized XAS results also provide evidence for in-plane character for the Ni-site component of the doped holes in this system. The XAS evidence supporting an increasing Ni d 7 ground-state admixture in these materials, with increasing formal Ni valence, is discussed. This discussion is couched in terms of simplified Ni d configuration modeling, which emphasizes the close energetic proximity of the Ni d 8 state to O p states in these systems.

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X-ray absorption near-edge structure ͑XANES͒ measurements were used to probe the H-charging-induced electronic structure changes of a 2400 Å Nb film capped with Pd. These results are discussed in terms of ab initio linear augmented plane-wave ͑LAPW͒ band-structure calculations for this material. The Pd-L 3 -edge XANES clearly manifested the spectral ͑Pd-d state related͒ changes expected for Pd-hydride formation, a white line feature degradation, and the appearance of a Pd-H antibonding feature at 6 eV above the threshold. The Nb-L 2,3 edge changes with H charging show a distinct enhancement of the white line strength; a feature 6 eV above the edges, associated with Nb-H antibonding states in analogy with the Pd results; the suppression of a threshold-onset feature of Nb metal; and a shift of the centrum of the white line feature towards the threshold. Comparison of the Nb sphere projection of the d 3/2 component of the LAPW density of states ͑DOS͒ to the Nb-L 2 -edge spectra yields good basic agreement with the observed spectral changes. In particular, the substantial theoretical reduction in the DOS at, and just above, the Fermi energy (E f ) is directly related to the near threshold Nb-L 2,3 spectral changes. The more modest white line enhancement in the theoretical DOS is noted and discussed. Nb-K-edge XANES are also discussed in terms of the Nb-site p-state projected LAPW DOS. This last comparison indicates a p-state reduction near E f upon H charging of the Nb.