Importance of multiple-scattering phenomena in XAS structural determinations of [Ni(CN)(4)](2-) in condensed phases

A quantitative analysis of the XAS spectra of the tetracyanonickelate compl ex [Ni(CN)(4)](2-) has been carried out. The simultaneous study of the EXAF S and XANES regions yielded complementary information regarding the geometr ic and electronic structures of the complex. XANES spectra were modeled by applying recently developed self-consistent. full multiple-scattering algor ithms in the FEFF8 code (version 8x34). XANES spectra for clusters of diffe rent sizes (from 9 to 125 atoms) were computed and compared with experiment al spectra. This region of the spectra was proportional to a broadened Ni p -density of states diagram above the Fermi Level. Although the main feature s of the XANES spectra were reasonably reproduced by computations, the weak dependence of the theoretical spectra on cluster size contrasts with the c lose similarity between the experimental spectra of the solid and solution systems. Because of the special geometry of the complex, calculations with polarized light parallel and perpendicular to the molecular plane were carr ied out, yielding a reasonable reproduction of the experimental data from a nother report for cluster sizes equal to or higher than 45 atoms. The highl y symmetric square planar structure of the complex was found to he responsi ble for the unusual amplitude of the multiple-scattering (MS) contributions to the EXAFS spectra. Spectra in this region were fitted using the FEFFIT EXAFS analysis program, taking into account only the MS paths that simultan eously have both a high amplitude, as calculated with the ab initio code FE FF, and a small Debye-Waller factor, as estimated by the independent-vibrat ion approximation model. Fitting results yielded very similar structures fo r the Ni2+ complex in the solid state and in solution, though the larger De bye-Waller factors found for the solid suggest higher static disorder in th is state.