Zinc phosphites ZnPHO3·2.5 H2O, Zn2H2P3H3O9·H2O, Zn3H4P5H5O15·1.5 H2O, ZnH2H2P2H2O6have been studied at higher temperatures and by X-rays and molecular spectroscopy. Hydrates ZnPHO3·2.5 H2O and Zn2H2P3H3O9·H2O, when heated, yield an anhydrous salt. Thermal decomposition of dihydrogen triorthophosphite and tetrahydrogen pentaorthophosphite leads, before oxidation of the anion, to a mixture of zinc phosphite ZnPHO3and dihydrogen diorthophosphite ZnH2P2H2O6and then after loss of water of constitution dihydrogen diorthophosphite converts to zinc diphosphite ZnP2H2O5. The results of the thermal decomposition study were confirmed by X-ray investigation. Anhydrous zinc dihydrogen triorthophosphite Zn2H2P3H3O9and zinc diphosphite ZnP2H2O5were hitherto unknown. Infrared spectra confirmed the existence of hydrogen bonding in all the phosphites studied and in the case of zinc phosphite ZnPHO3·2.5 H2O exhibited a symmetry decrease of the anion PHO32−from the point group C3vto Cs. In the crystal lattice of ZnPHO3·2.5 H2O hydrogen bonding by water molecules participates, with polyorthophosphites hydrogen bonding shares in the production of anions and in the case of their hydrates there is in addition hydrogen bonding by water m
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