Single crystal growth, crystal structure and thermal behaviour of mercury(II) pyrophosphate dihydrate

Colourless single crystals of Hg2P2O7(H2O)(2) up to 0.4 mm in length were grown by a diffusion technique starting from aqueous solutions of Na4P2O7 and Hg(NO3)(2). The crystal structure is isotypic with that of Ca2P2O7(H2O)(2) and was determined from a four-circle diffractometer data set (space group P (1) over bar, Z=2, a=6.9374(7), b=7.4396(8), c=7.9863(7) Angstrom, alpha=84.685(8), beta=75.158(8), gamma=72.818(8)degrees, 2413 structure factors, 132 parameters, R[F-2 > 2sigma(F-2)]=0.0181, wR(F-2 all)= 0.0384). Hg2P2O7(H2O)(2) is composed of approximately eclipsed P2O74- anions and distorted [HgO6] octahedra and [HgO7] pentagonal bipyramids as the main building units. The structure is stabilized by inter-water hydrogen bonding and by hydrogen bonding between terminal pyrophosphate oxygen atoms and the water molecules. The P-O distances to the terminal oxygen atoms range from 1.501(4) to 1.536(3) Angstrom, with an average of 1.522 Angstrom; the mean distance of 1.615 Angstrom to the bridging O atom is considerably longer with an (O-P-O) bridging angle of 123.44(19)degrees. Both Hg atoms have two short Hg-O bonds around 2.17 Angstrom and additional bonds ranging from 2.381(3) to 2.708(4) Angstrom. Upon heating above 160degreesC, both crystal water molecules are released simultaneously and anhydrous Hg2P2O7 is formed which is stable up to ca. 660degreesC. Above this temperature the material decomposes completely.