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 FeK edge of K₃Fe(CN)₆ and K₄Fe(CN)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(CN)₆ cluster have been estimated. The different XANES spectra of [Fe(CN)₆]²⁺ and of [Fe(CN)₆]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.