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

XAFS spectra of palladium(II) in concentrated HNO 3 /HClO 4 acid mixtures have been recorded and analyzed. Structural parameters of the Pd(H 2 O) 4 2 + complex and the mixed nitric Pd(NO 3 ) 2 (H 2 O) 2 complex, for the first time, were determined by the XAFS method. For pure 5 M HClO 4 and for mixtures (0-0.3 M HNO 3 ), 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(H 2 O) 4 2 + square-planar aqua ion in strong perchloric acid, the use of an FEFF6 theoretical approach led to a first-shell Pd-O w distance of 2.00 (1) A and a Debye-Waller (DW) factor of σ 2 = 0.0030 (3) A 2 . Four water molecules are tightly bound to the Pd 2 + ion in the equatorial plane, while two (or one) axial water molecules are weakly bound to the metal ion at 2.5 A with a DW factor of 0.015 (5) A 2 . For highly concentrated mixtures (4-6 M HNO 3 ) and for pure concentrated (4-6 M) nitric acid as well as for crystalline powder Pd(NO 3 ) 2 (H 2 O) 2 , the XAFS spectra are very similar and are determined by the mixed nitric complex Pd(NO 3 ) 2 (H 2 O) 2 : four Pd-O near-equivalent distances of 2.01 (1) A from two H 2 O and two NO 3 molecules with a total DW factor of σ 2 = 0.0037 (3) A 2 . Moreover, two Pd---N distances of 2.8-2.9 A were determined in the second coordination shell. Finally, for intermediate mixtures (1-3 M HNO 3 in 5 M HClO 4 ), the XAFS spectra are a superposition of the XAFS of Pd(H 2 O) 4 2 + and Pd(NO 3 ) 2 (H 2 O) 2 complexes. The mean ligand number NO 3 - around Pd 2 + has been calculated, and the XAFS results at pH close to zero confirm the spectrophotometric results previously published.