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

The hydrated potassium hemimagnesium dihydrogen pyrophosphate KMg0.5H2P2O7 H2O was synthesized. It crystallizes in the triclinic system, space group (n. 2), Z=2, with the following unit-cell parameters: a=6.8565(2) A°, b=7.3621(3) A°, c=7.6202(3) A°, alfa =81.044(2)°, beta=72.248(2)°, gamma=83.314(3)°, V=360.90(2) A°3. The structure was obtained by single-crystal X-ray diffractometry, and a full-matrix least-squares refinement based on F2 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.