Polyoxoniobate chemistry in the 21st century

Polyoxometalate (POM) chemistry of W, Mo and V is rich and diverse; and new discoveries are frequent and abundant. The prolificacy of this POM chemistry is attributed to rich redox chemistry, an acidic nature that is compatible with addendum metal cations, and most importantly an understanding and ability to control solution behavior. In contrast, the POM chemistry of Nb and Ta (PONb, POTa) is hindered by minimal redox chemistry, alkalinity that is incompatible with the solubility of most metal cations, and a relatively poor understanding of the behavior in aqueous media. Despite these hurdles, considerable advancements in PONb chemistry (and to a much lesser extent POTa chemistry) have been made in the last decade. These include synthesis of the first heteropolyniobate Keggin derivatives, utilization of organic countercations to obtain unprecedented PONb geometries and compositions, and investigation of PONb solution behavior using advanced techniques such as nuclear magnetic resonance (NMR), electrospray ionization mass spectrometry (ESI MS) and small-angle X-ray scattering (SAXS). This Perspective article summarizes the recent successes, continued shortcomings, and some unique and potentially exploitable features of PONb chemistry. More importantly, this annotated compilation of recent PONb literature has revealed the most logical and promising directions for the continued growth of the most challenging of polyoxometalate chemistries.