Dawson structure

The Dawson structure is a well-known structural motif for heteropoly acids. The Dawson structure can be viewed as the fusion of two defect Keggin structure, fragments with three missing octahedra. As in Keggin structures, the Dawson structure has an oxyanion at its core. Unlike Keggin structures, there are two such anions, one at each side of the ellipsoidal anion. An example is [S₂W₁₈O₆₂]⁴⁻, which can also be described as [(SO₄)₂(WO₃)₁₈]⁴⁻.^[1]

Dawson structure

Commonly, Dawson structures feature phosphate as the central oxyanions. When the Keggin anion [PW₁₂O₄₀]³⁻ is allowed to stand in aqueous solution, it converts to [P₂W₁₈O₆₂]⁶⁻.^[2]