Rates of oxygen-isotope exchange between sites in the [HxTa6O19](8-x)-(aq) Lindqvist ion and aqueous solutions: comparisons to [HxNb6O19](8-x)-(aq).
Rates of steady oxygen-isotope exchange differ in interesting ways for two sets of structural oxygens in the [HxTa6O19](8-x)-(aq) Lindqvist ion when compared to published data on the [HxNb6O19]8-x(aq) version. Because of the lanthanide contraction, the [HxTa6O19](8-x)-(aq) and [HxNb6O19](8-x)-(aq) ions are virtually isostructural and differ primarily in a full core (Kr vs Xe) and the 4f14 electrons in the [HxTa6O19](8-x)-(aq) ion. For both molecules, both pH-dependent and -independent pathways are evident in isotopic exchange of the 12 mu2-O(H) and 6 eta=O sites. Rate parameters for eta=O exchange at conditions where there is no pH dependence are, for the Ta(V) and Nb(V) versions respectively, K(298)(0) = 2.72 x 10(-5) s(-1) and 9.7 x 10(-6) s(-1), DeltaH = 83.6 +/- 3.2 and 89.4 kJ.mol(-1), and DeltaS = -51.0 +/- 10.6 and -42.9 J.mol(-1).K-1. For the mu2-O sites, K(298)(0) = 1.23 x 10(-6) s(-1), DeltaH = 70.3 +/- 9.7 and 88.0 kJ.mol(-1), and DeltaS = -116.1 +/- 32.7 and -29.4 J.mol(-1).K-1. Protonation of the 6 eta=O sites is energetically unfavored relative to the 12 mu2-O bridges in both molecules, although not equally so. Experimentally, protonation labilizes both the mu2-O(H) and eta=O sites to isotopic exchange in both molecules. Density-functional electronic-structure calculations indicate that proton affinities of structural oxygens in the two molecules differ with the [HxTa6O19](8-x)-(aq) anion having a smaller affinity to protonate than the [HxNb6O19]8-x(aq) ion. This difference in proton affinities is evident in the solution chemistry as pKa = 11.5 for the [HTa6O19]7-(aq) ion and pKa = 13.6 for the [HNb6O19]7-(aq) ion. Most striking is the observation that eta=O sites isotopically equilibrate faster than the mu2-O sites for the [HxTa6O19](8-x)-(aq) Lindqvist ion but slower for the [HxNb6O19](8-x)-(aq) ion, indicating that predictions about site reactivities in complicated structures, such as the interface of aqueous solutions and oxide solids, should be approached with great caution.
Departments of Chemistry and Geology, University of California, Davis, CA 95616, USA.
Edina
E
Balogh
Travis M
TM
Anderson
James R
JR
Rustad
May
M
Nyman
William H
WH
Casey