The Dawson structure is a well-known structural motif for
heteropoly acid
The heteropolymetalates are a subset of the polyoxometalates, which consist of three or more transition metal oxyanions linked together by shared oxygen atoms to form a closed 3-dimensional molecular framework. In contrast to isopolymetalates, wh ...
s. The Dawson structure can be viewed as the fusion of two defect
Keggin structure
The Keggin structure is the best known structural form for heteropoly acids. It is the structural form of α-Keggin anions, having a general formula of , where X is the heteroatom (most commonly are pentavalent phosphorus PV, tetravalent silicon ...
, 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 , which can also be described as .
Commonly, Dawson structures feature
phosphate
Phosphates are the naturally occurring form of the element phosphorus.
In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthop ...
as the central
oxyanion An oxyanion, or oxoanion, is an ion with the generic formula (where A represents a chemical element and O represents an oxygen atom). Oxyanions are formed by a large majority of the chemical elements. The formulae of simple oxyanions are determine ...
s. When the Keggin anion is allowed to stand in aqueous solution, it converts to .
References
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Cluster chemistry
Heteropoly acids
Anions