Chapter 4 - Jacobs University

Chapter 1
Introduction
α-[P 2 W 18 O 62 ] 6−
β-[P 2 W 18 O 62 ] 6−
Figure 1.5 Polyhedral representations of α- and β-[P 2 W 18 O 62 ] 6- . A 60º rotation of one of the
polar triads (shaded) about the vertical three-fold axis of symmetry of the alpha form leads to the
beta form.
1.4.1 The Mono- and tri-lacunary ligand
When a solution of tungstate is acidified below pH 2 in the presence of an excess of
phosphate, a mixture of α and β-isomers of the Wells-Dawson [P 2 W 18 O 62 ] 6- heteropolyanion is
obtained instead of the Keggin. The Wells-Dawson heteropolyanion shows a similar behaviour
to that of the Keggin, since basicification of a solution produces hydrolytic cleavage of the M—
O bonds to give rise to monovacant [α 1 -P 2 W 17 O 61 ] 10- and [α 2 -P 2 W 17 O 61 ] 10- lacunary species.
However if the final pH is between 4 and 6, the [α 1 -P 2 W 17 O 61 ] 10- anion is unstable and readily
undergoes isomerization to the [α 2 -P 2 W 17 O 61 ] 10- (Figure 1.6) anion. At about pH 10 the trilacunary
anion [P 2 W 15 O 56 ] 12- (Figure 1.6) is formed. Reaction of a stable, lacunary
polyoxometalate with transition metal ions usually leads to a product containing the unchanged
heteropolyanion framework. Depending upon the coordination requirement and the size of a
given transition metal ion, the geometry of the reaction product can therefore often be predicted.
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