Crystal structure of Na-9[H3W12O42]center dot 24H(2)O, a compound containing the protonated paratungstate B anion ('acid paratungstate'), and cyclic voltammetry of acidified [H2W12O42](10-) solutions

Crystallization of a solid at pH 4.0 from an aqueous acidified Rh3+-[WO4](2 -) solution resulted in the isolation of Na-9[H3W12O42]. 24H(2)O, which con tains the protonated paratungstate B anion and which is likely the species identified previously as 'acid paratungstate'. The compound is triclinic, s pace group <P(1)over bar>, a 10.603(2), b 12.134(3), c 14.042(3) Angstrom, alpha 114.78(1), beta 101.84(1), gamma 108.34(1)degrees, V 1432.9(5) Angstr om(3), Z 1, and the structure was solved to an R-1 value of 0.0404 (wR(2) 0 .1108) for 5997 independent observed reflections. The anion exhibits essent ially the same isopolytungstate framework as paratungstate B, [H2W12O42](10 -), consisting of two W3O13 and two W3O14 structural subunits linked by sha red vertices. Bond valence arguments place two of the hydrogen atoms unequi vocally in the internal cavity of the anion, with the remaining hydrogen at om also likely located in this cavity, but disordered over several internal oxygen atoms. The protonation of [H2W12O42](10-) is shown to lead to speci es that are electrochemically reducible. Extended-Huckel molecular orbital calculations confirm that protonation of paratungstate B within the interna l cavity leads to a change in composition of the LUMO, now based mainly on electrochemically reducible W3O13 as opposed to (essentially) non-reducible W3O14 structural subunits. This results in species that are considerably m ore electrochemically active than the unprotonated anion. The role of [H3W1 2O42](9-) as an intermediate in the polymerization of [WO4](2-) to give the solution form of psi-metatungstate, [H7W11O40](7-), which crystallizes as [H4W11O38](6-), is also discussed.