Soluble heteropolyniobates from the bottom of Group IA.

Surprising solubility: While it is already well known that [Nb(6)O(19)](8-) salts exhibit an unusual solubility trend, that is, Cs>Rb>K>Na>Li, the heteropolyniobates of Cs and Rb had not yet been crystallized. These very soluble entities have now been obtained from solution by a simple and universal process. New polyoxoniobate geometries are thus unveiled, and the [SiNb(12)O(40)](16-) Keggin ion is characterized in solution for the first time.