X-ray absorption fine structure spectroscopic study of (NH4)2ReF6

The (NH 4 ) 2 ReX 6 (X = F, Cl, Br, I) salts have been investigated by x-ray absorption fine structure spectroscopy. The Re-F distance determined by EXAFS in (NH 4 ) 2 ReF 6 (i.e., 1.95 Å) is in good agreement with the one determined by single crystal x-ray diffraction in A 2 ReF 6 salts (A = K, Rb, Cs). The XANES studies of (NH 4 ) 2 ReX 6 (X = F, Cl, Br, I) indicates that the positions of the absorption edge and of the white line are shifted to higher energy when moving from I to F. These shifts have been explained in terms of the crystal field splitting parameter and covalent charge carried by the Re atoms. Calculations of the XANES spectra of the ReX 6 2− (X = F, Cl, Br, I) anions at the Re-L 3 edge have been performed and the calculated shifts and intensity of the white lines reproduce well the experimental observations.