The Dawson-type anion [X(n)2M18O62]2(8 − n)−, is derived from the Keggin anion [X(n)M12O40](8 −n)− (X=Si, Ge, P, S) by fusing two trivalant shells, producing an oval shaped closed framework. The central oxoanions, contributing an overall charge to the clathratic cluster, retain tetrahedral coordination. Generally, the Dawson heteropolyoxometalates are more thermally stable and catalytically more active than the corresponding Keggin ions. In this work, the comparisons among the Keggin and Dawson ions are addressed in terms of relative stability, hardness and acidity. These predictions are based on the theoretical results obtained from the extended Hückel, EH calculations. This analysis of the Dawson structure presently represents the most significant modelling work on large oxyanions. Absolute comparisons between the EH and density functional are not available for Dawson structures. The correlations between these methods have been shown valid for the Keggin ions and are expected to be similar for the Dawson ions. The substitution chemistry of the central oxoanions is similar to that of the Keggin clusters in both the stability and acidity.