Coordination chemistry of niobium and tantalum pentahalides: unusual activation reactions of simple organic molecules

Niobium and tantalum compounds lived for long time in the shadow of metal complexes of group 4, probably for the extremely interesting applications of the latter in alkene transformations. In this context, the chemistry of niobium and tantalum pentahalides was scarcely explored in the past. However, in the last decade, such commercial compounds have found increasing use in homogenous catalysis, encouraged by their easy availability and the non toxicity of the metal elements. The direct interaction of MX5 (M = Nb, Ta; X = F, Cl, Br) with limited amounts of simple organic molecules has been recently elucidated by a systematic investigation, aimed to contribute to the progress of the related catalytic processes. In some cases, the formation of coordination complexes, especially with O-donors, represents the first step of an activation process, which is usually driven by the nature of the halide. We report an overview of the main activation reactions, involving for instance carbonyl compounds, -aminoacids, arenes, etc.