The single-crystal X-ray structure of K4H2[P2W21O71(OH2)3]·28H2O [space group P63/mmc, a= 16.295(2), c= 19.771(5)Å, and Z= 2; Mo radiation, R= 0.037 for 1 042 unique data] has been determined and the 183W n.m.r. spectrum of the anion in H2O–D20 has been recorded. The anion has true point symmetry C2v but occupies a D3h site in the crystal with orientational disorder about a crystal three-fold axis. It contains two α-A-PW9O349– units, each with C3v symmetry, linked through three equatorial WO(OH2)4+ units. Two equatorial W atoms are closer to, and the third is further from, the anion (crystal C3) axis. The H2O ligand on the unique W is directed towards the interior of the anion and is hydrogen-bonded to the oxo Iigand, also directed internally, on each of the other equatorial W.
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TOURNE et al. (1986) studied this question.