Aquanonadecatungstodiphosphate(14–) polyanion, [P₂W₁₉O₆₉(OH₂)]^14–: X-ray crystallographic structure of its potassium salt, chemical relationships in the tungstophosphate system, and conversion into the diaquaicosatungstodiphosphate [P₂W₂₀O₇₀(OH₂)₂]^10–

Abstract

The crystal structure of K14[P2W19O69(OH2)]·24H2O has been determined [space group Fddd, a= 28.800(3), b= 38.143(8), c= 32.005(7)Å, Z= 16, R= 0.048 for 3 108 reflections with I > 3σ(I)]. The anion has crystallographic symmetry 2, very close to a C2v point group symmetry. Two α-A-PW9O349– units are linked through one WO(OH2)4+ unit. Two interior and one exterior water molecules are hydrogen bonded to anionic oxygens; two K+ cations are complexed by the anion as a double cryptate to give the polyanion a compact close form. The behaviour of the salt in aqueous solution is discussed and studied by 183W and 31P n.m.r. spectroscopy which reveals a reversible transformation and the influence of K+ cations on its stability. Divalent polarizing cations such as Zn2+ transform it into [PZn2W10O38(OH2)2]7– as mixtures of isomers. Other chemical aspects on the polyanion are shown including its relationships with [PW9O34]9–[P2W17O61]10–, [KP2W20O72]13–, and the first reported quantitative preparation of the salt K10[P2W20O70(OH2)2]·22H2O, at pH 2.5–3.5. The last is obtained also by slight acidification of K7[PW11O39](aq).