| Abstract
| - XAFS spectra of palladium(II) in concentrated HNO3/HClO4 acid mixtures have been recorded and analyzed.Structural parameters of the Pd(H2O)42+ complex and the mixed nitric Pd(NO3)2(H2O)2 complex, for the firsttime, were determined by the XAFS method. For pure 5 M HClO4 and for mixtures (0−0.3 M HNO3), theXAFS spectra of the 0.02 M Pd solutions are indeed very similar and originated from four Pd−Ow equivalentdistances. For the Pd(H2O)42+ square-planar aqua ion in strong perchloric acid, the use of an FEFF6 theoreticalapproach led to a first-shell Pd−Ow distance of 2.00 (1) Å and a Debye−Waller (DW) factor of σ2 = 0.0030(3) Å2. Four water molecules are tightly bound to the Pd2+ ion in the equatorial plane, while two (or one)axial water molecules are weakly bound to the metal ion at 2.5 Å with a DW factor of 0.015 (5) Å2. Forhighly concentrated mixtures (4−6 M HNO3) and for pure concentrated (4−6 M) nitric acid as well as forcrystalline powder Pd(NO3)2(H2O)2, the XAFS spectra are very similar and are determined by the mixednitric complex Pd(NO3)2(H2O)2: four Pd−O near-equivalent distances of 2.01 (1) Å from two H2O and twoNO3 molecules with a total DW factor of σ2 = 0.0037 (3) Å2. Moreover, two Pd---N distances of 2.8−2.9 Åwere determined in the second coordination shell. Finally, for intermediate mixtures (1−3 M HNO3 in 5 MHClO4), the XAFS spectra are a superposition of the XAFS of Pd(H2O)42+ and Pd(NO3)2(H2O)2 complexes.The mean ligand number NO3- around Pd2+ has been calculated, and the XAFS results at pH close to zeroconfirm the spectrophotometric results previously published.
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