Synthesis, Structure and Catalytic Activity of ... - Jacobs University

Ch.1 Introduction
_____________________________________________________________
1.4 The Wells-Dawson ion
The Wells-Dawson structure is only known for the heteroatoms P(V) and
As(V). It is a dimeric heteropolyanion of the formula [X2M18O62] x- where M=
W(VI) or Mo(VI). The anion is viewed as a fusion between two A-α-XW9 units
through all six oxygens of the lacuna resulting in a virtual D3h symmetry
cluster of the α isomer type (Fig. 1.4). Two sets of tungsten form this
polyanion, six “polar” and twelve “equatorial” constituting the two caps and
the two belts of the anion respectively. The β isomer forms when one of the
caps is rotated by 60°. Similar to the β Keggin, the β Wells-Dawson is more
easily reduced.
Fig. 1.4 Polyhedral representation of the Wells-Dawson structure, WO6 octahedra (teal) and
XO4 tetrahedra (yellow)
Stable lacunary species are also derived from α-[P2M18O62] x- plenary anion
upon alkalination. Two isomers of the monolacunary species exist, these are
the α1 and the α2-[P2W17O61] 10- , where the one addenda is lost from the belt in
the first and it is lost from the cap in the second upon reaction with base,
typically potassium bicarbonate. 9
7