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Big, bigger, biggest: Polyoxoniobate anions [HNb27O76]16− and [H10Nb31O92(CO3)]23− incorporate pentagonal Nb(Nb)5 building blocks; the central Nb ion is seven-coordinate within the clusters. The Nb27 species was observed using ESI-MS, thus demonstrating some solution stability; the Nb31 species is chiral and incorporates a carbonate ligand in the outer section of the cluster. The two species are the largest polyoxoniobates reported to date.
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Tsunashima et al. (2009) studied this question.
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