Formation of α- and β-Keggin-Type [PW12O40]3− Complexes in Aqueous Media

The formation process of the α- and β-Keggin-type [PW12O40]3− complexes was investigated in an aqueous 50 mM (M = mol dm−3) W(VI)–5.0 mM P(V) system. The 31P NMR study confirmed the formation of A-α-[PW9O34]9−, A-β-[PW9O34]9−, α-[PW11O39]7−, and β1-[PW11O39]7− on acidification of an alkaline solution of WO42− and HPO42− to pH 7.5 or below. An isomeric mixture of α- and β-[PW12O40]3− was obtained on further acidification of such solutions containing both α- and β-type lacunary complexes to pH 1 or below. On the other hand, heating of the W(VI)–P(V) system caused A-α-[PW9O34]9−, A-β-[PW9O34]9−, and β1-[PW11O39]7− to disappear completely, leaving α-[PW11O39]7−. Acidification of the α-[PW11O39]7− solution produced only α-[PW12O40]3−.