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 C_2v point group symmetry. Two α-A-PW₉O₃₄^9– 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).