An aqueous route to [Ta6O19]8- and solid-state studies of isostructural niobium and tantalum oxide complexes

A new soft chemical route to [ Ta6O19](8-) has been developed by the dissolution of [Ta(O2)(4)](3-) in conditions alkaline enough to arrest formation of Ta2O5, followed by [VO4](3-)-catalyzed decomposition of the peroxide ligands and crystallization of the salt. An average of bond lengths and angles from isostructural salts of [ Ta6O19](8-) and [ Nb6O19](8-) indicate there is an increase in terminal M(eta = O) bond lengths and M-mu(2)-O-M angles and a decrease in bridging mu(2)-O-M bond lengths in [Ta6O19](8-), although the central mu(6)-O-M bond lengths are identical within experimental error. Two new structures of Na-7[ HNb6O19] center dot 15H(2)O ( 1) and Na-8[Ta6O19] center dot 15H(2)O ( 2) are exemplary of the fact that protonated mu(2)-OH are observed exclusively in the niobates. In these structures, the metal-oxide framework, seven sodium atoms, and all fifteen water molecules are located in identical unit cell positions, but in 2 an eighth charge-balancing sodium is located in close proximity to the protonated mu(2)-OH in 1. Differences in the basicity of Nb(V)- and Ta( V)-bound oxygen atoms are also manifested at the surfaces of O-17-enriched powders of Nb2O5 and Ta2O5. Oxygen exchange at the surface of these materials readily takes place at both terminal and bridging sites in Nb2O5 but only at terminal sites in Ta2O5.