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.