Rates of oxygen exchange between the [HxNb6O19](aq)8-x Lindqvist ion and aqueous solutions

Oxygen-isotope-exchange rates were measured between sites in the Lindqvist-type [HxNb6O19](8-x)((aq)) polyoxoanion and aqueous solution as a function of pH and temperature. The ion has a central mu(6)-O that is inert to exchange, 12 mu(2)-O(H), and 6 eta-O. The potassium salt of this ion is recrystallized in O-17-enriched water to O-17-label the anion, which is then redissolved into isotopically normal water so that the O-17 NMR signals from structural oxygens can be followed as a function of time. Because the central mu(6)-O retains its 17O signal throughout the experiments, it is clear that the polyoxoanion remains intact during isotopic equilibration of the other structural oxygens. At pH conditions where the [HNb6O19](7)-ion predominates, the mu(2)-O(H) sites isotopically exchange with solution about an order of magnitude more rapidly than the eta-O sites. Yet, we observe that the terminal and bridging oxo sites react at nearly the same rates when the ion is coordinated to 2-3 protons and possibly when it is unprotonated. On the basis of molecular models and experimental kinetic data, we propose metastable polymorphs of the hexaniobate structure where four of the mu(2)-O(H) and eta-O sites are temporarily equivalent and bonded to a coordinatively unsaturated Nb(V). This hypothesized intermediate allows facile access to bulk water molecules for exchange but cannot fully explain the kinetic results and additional experiments on other Lindvist ions are required.