Structural investigation of Pd(II) in concentrated nitric and perchloric acid solutions by XAFS

XAFS spectra of palladium(II) in concentrated HNO3/HCIO4 acid mixtures have been recorded and analyzed. Structural parameters of the Pd(H2O)(4)(2+) complex and the mixed nitric Pd(NO3)(2)(H2O)(2) complex, for the first time, were determined by the XAFS method. For pure 5 M HClO4 and for mixtures (0-0.3 M HNO3), the XAFS spectra of the 0.02 M Pd solutions are indeed very similar and originated from four Pd-O-w, equivalent distances. For the Pd(H2O)(4)(2+) square-planar aqua ion in strong perchloric acid, the use of an FEFF6 theoretical approach led to a first-shell Pd-Ow distance of 2.00 (1) angstrom and a Debye-Waller (DW) factor of sigma(2) = 0.0030 (3) angstrom(2). Four water molecules are tightly bound to the Pd 21 ion in the equatorial plane, while two (or one) axial water molecules are weakly bound to the metal ion at 2.5 angstrom with a DW factor of 0.015 (5) angstrom(2). For highly concentrated mixtures (4-6 M HNO3) and for pure concentrated (4-6 M) nitric acid as well as for crystalline powder Pd(NO3)(2)(H2O)(2), the XAFS spectra are very similar and are determined by the mixed nitric complex Pd(NO3)(2)(H2O)(2): four Pd-O near-equivalent distances of 2.01 (1) angstrom from two H2O and two NO3 molecules with a total DW factor of sigma(2) = 0.0037 (3) angstrom(2). Moreover, two Pd---N distances of 2.9-2.9 angstrom were determined in the second coordination shell. Finally, for intermediate mixtures (1-3 M HNO3 in 5 M HClO4), the XAFS spectra are a superposition of the XAFS of Pd(H2O)(4)(2+) and Pd(NO3)(2)(H2O)(2) complexes. The mean ligand number NO3 around Pd (2+) has been calculated, and the XAFS results at pH close to zero confirm the spectrophotometric results previously published.