Two synthetic routes generate two isopolyoxoniobates based on {Nb16} and {Nb20}.

Self-assembly reactions of the Lindqvist-ion salt K7HNb6O19·13H2O, SeO2, and [Cu(2,2'-bipy)]2+ give, depending on different routes, two isopolyoxoniobates based on {Nb16} and {Nb20}, respectively, which, as new members of the very limited isopolyoxoniobate family, are observed for the first time. Furthermore, the solution behavior was investigated to determine the stability of the {Nb16} cluster in water.