inorganic compounds
The [{Er3O(OH)3(H2O)3}2Al2(Nb6O19)5]26- anion in the title compound, octadecahydrogen octasodium dialuminopentanonacontaoxobis(triaquatrihydroxooxotrierbio)triacontaniobate(26-)-water (1/40.5), is composed of a central [{Er3O(OH)3(H2O)3}2]8+ cluster surrounded by three equatorial [Nb6O19]8- anions and two axial Al3+ cations within [Al(Nb6O19)]5- moieties with approximate D3 symmetry. A comparison of the geometry of this anion with that of the anion in the isomorphous Eu complex reveals that the smaller ionic radius of the Er atom induces a contraction of the central {[Ln3O(OH)3(H2O)3]2}8+ cluster (Ln = Er, Eu) with a resultant attraction of the equatorial and axial [Nb6O19]8- anions toward the centre of the complex by 0.09 and 0.07 Å, respectively.