Multiple-scattering resonances and structural effects in the x-ray-absorption near-edge spectra of Fe II and Fe III hexacyanide complexes

The x-ray-absorption near-edge structure (XANES) at the Fe$K$ edge of ${\mathrm{K}}_3$Fe${\mathrm{}\left(\mathrm{CN}\right)\mathrm{}}_6$ and ${\mathrm{K}}_4$Fe${\mathrm{}\left(\mathrm{CN}\right)\mathrm{}}_6$ has been measured and interpreted by means of multiple-scattering calculations. The theory is able to account for the XANES over a 40-eV energy range. We demonstrate that the quasidiatomic "shape resonances" arising from multiple scattering within the CN groups play an important role in the XANES and suggest that this may be a common feature of XANES of metal atoms bound to molecular groups (e.g., CO, CN,...) containing multiple bonds. The effects on the XANES of bond-length changes and distortions of the coordination geometry of the Fe${\mathrm{}\left(\mathrm{CN}\right)\mathrm{}}_6$ cluster have been estimated. The different XANES spectra of ${\left[\mathrm{F}\mathrm{e}{\mathrm{}\left(\mathrm{CN}\right)\mathrm{}}_6\right]}^{2+}$ and of ${\left[\mathrm{F}\mathrm{e}{\mathrm{}\left(\mathrm{CN}\right)\mathrm{}}_6\right]}^{3+}$ have been interpreted in terms of charge-induced structural modifications. Comparison of XANES with extended x-ray-absorption fine-structure (EXAFS) and diffraction data shows that XANES can be used as a quantitative probe of local structure distortions which are not detected by EXAFS.