| Summary: | The new acidic sodium polytungstates, Na5[H7W12O42]·20H2O ( 1) and Na2[H10W12O42]·20H2O ( 2), have been synthesised under hydrothermal conditions where variations of pH and temperature allow control of the protonation of the [W12O42]12- cluster in the products. The polytungstate cluster, [HnW12O42](12-n)-, in each compound consists of the well known polyhedral unit based on 12 edge- and corner-sharing WO6 octahedra, though the decoration of this unit with varying levels of protons results in significant changes in the W-O distances. The polytungstate clusters exist in arrays separated by sodium ions and water molecules forming well separated, (polytungstate centre to centre distance of 10.87 Å), rhombohedrally stacked units in 1 and strongly hydrogen bonded and more tightly packed units, (centre to centre distance of 9.34 Å), due to the high H : Na ratio in 2
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