Metal Oxide Chemistry in Solution: The Early Transition Metal Polyoxoanions

Many of the early transition elements from large polynuclear metal-oxygen anions containing up to 200 atoms or more. Although these polyoxoanions have been investigated for more than a century, detailed studies of structure and reactivity were not possible until the development of modern x-ray crystallographic and nuclear magnetic resonance spectroscopic techniques. Systematic studies of small polyoxoanions in inert, aprotic solvents have clarified many of the principles governing their structure and reactivity, and also have made possible the preparation of entirely new types of covalent derivatives such as CH$_{2}$Mo$_{4}$O$_{15}$H$^{3-}$, C$_{5}$H$_{5}$TiMo$_{5}$O$_{18}{}^{3-}$, and (OC)$_{3}$Mn(Nb$_{2}$W$_{4}$O$_{19}$)$^{3-}$. Since most early transition metal polyoxoanions have structures based on close-packed oxygen arrays containing interstitial metal centers, their chemistry offers a rare opportunity to study chemical transformations in detail on well-defined metal oxide surfaces.