Synthesis, X-ray crystal structure and vibrational spectroscopy of the acidic pyrophosphate KMg0.5H2P2O7•H2O

The hydrated potassium hemimagnesium dihydrogen pyrophosphate (KMg0.5H2P2O7H2O)-H-. was synthesized. It crystallizes in the triclinic system, space group P (l) over bar (n. 2), Z = 2, with the following unit-cell parameters: a = 6.8565(2) Angstrom, b = 7.3621(3) Angstrom, c = 7.6202(3) Angstrom, alpha = 81.044(2)degrees, beta = 72.248(2)degrees, gamma = 83.314(3)degrees, V = 360.90(2) Angstrom(3). The structure was obtained by single-crystal X-ray diffractometry, and a full-matrix least-squares refinement based on F-2 gave a final R index of =0.0368 (wR = 0.0975), utilizing 1446 observed reflections with I > 2sigma(I). The crystal packing consists in a three-dimensional network made by layers parallel to ab plane Of PO4 double tetrahedra and MgO6 octahedra, linked by hydrogen bonds, while K atoms form complex coordination within cavities between tetrahedra and octahedra. The dihydro-pyrophosphate anion (H2P2O7)(2-) shows bent eclipsed conformation and the Mg2+ ion lies on inversion center. No coincidences observed between most of infrared and Raman spectral bands confirmed the centrosymmetric structure of the title compound; the vibrational spectra point to a bent POP bridge angle. (C) 2003 Elsevier Inc. All rights reserved.