Typical examples occur in the Keggin structure of the phosphomolybdate ion.
2.
The Keggin structure is the widely accepted structure for the ?-Keggin anions.
3.
The Keggin structure accounts for both the hydrated and dehydrated ?-Keggin anions without a need for significant structural change.
4.
The heteroatom may be located in the centre of the anion, as in the Keggin structure, or in the center of a structural fragment, such as the two phosphorus atoms in the Dawson ion, which are central to its two symmetric fragments.
5.
Reduction with uric acid or iron ( II ) sulfate produces a brown coloured compound . the related silicotungstic acid when reduced forms a similar brown compound where one of the four W 3 units in the Keggin structure becomes a metal-metal bonded cluster of three edge shared W ( IV ) octahedra.
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