Structural and Electronic Properties of Fluoride Complexes of Nb-V, Ta-V, and Pa-V: The Influence of Relativistic Effects on Group V Elements

摘要

Aqueous fluorido complexes of niobium and tantalum were studied by using synchrotron-based extended X-ray absorption spectroscopy (EXAFS) as model systems for comparison of the structural and coordination chemistry of the group V metals Nb, Ta, and their pseudo-homologue Pa-V. The EXAFS measurements indicate differences in the coordination chemistry of Nb and Ta in these systems that were not intuitively based on their similar charge and ionic radii. The Ta speciation is dominated by six- and seven-coordinate fluorido complexes, while the chemical speciation of Nb under the same conditions is dominated by its hydrolyzed oxyfluoride complexes. Quantum chemical computations were performed to more fully describe these observations, and these results are compared to our earlier study on the fluorido complexes of protactinium. Computations performed with and without the inclusion of scalar relativistic effects reveal that the differences in chemistry among the group V elements and their pseudo-homologue protactinium are due to the increasing influence of relativistic effects as Z increases from niobium to protactinium.