Molecular metal-oxo clusters are important intermediates between hydrated metal ions and metal oxides. They help elucidate the formation processes of metal oxides in nature and in synthesis; and they have numerous applications in catalysis, materials science, microbiology, separations and environmental science. Considerable focus is placed on determining the solid-state structures of these clusters. However, of equal and perhaps greater importance is the solution speciation of clusters. Small-angle X-ray scattering (SAXS) is a very powerful but underutilized technique to obtain speciation information of cluster solutions. Here I highlight the type of information that can be obtained concerning the size and shape of clusters, interactions between clusters and evolution of cluster solutions with reactivity or aging. The highlighted X-ray studies include both simulated and experimental X-ray data of transition metal and actinide polyoxometalates, recently reported Hf-oxo polyanions and bismuth iron oxo clusters. Importantly and intentionally, all of the data presented herein was collected on a lab instrument, of which are becoming more commonly employed in all solution phase disciplines. This review is intended to inspire other researchers to determine how they could employ X-ray scattering in their own research in the wide range of disciplines that metal-oxo clusters impact.
Nyman, M. Small-angle X-ray scattering to determine solution speciation of metal-oxo clusters. Coord. Chem. Rev. 2016