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 P1-bar (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, α=81.044(2) deg. , β=72.248(2) deg. , γ=83.314(3) deg. , V=360.90(2) A3. 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>2σ(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