)(H2O)(3))AL-2(2)-(NB6O19)(5)]CENTER-DOT-40.5H(2)O

The [{Er3O(OH)(3)(H2O)(3)}Al-2(2)(Nb6O19)(5)](26-) anion in the title compound, octadecahydrogen octasodium dialuminopentanonacontaoxobis at rihydroxooxotrierbio)triacontaniobate(26-)-water (1/40.5), is composed of a central [{Er3O(OH)(3)(H2O)(3)}(2)](8+) cluster surrounded by thr ee equatorial [Nb6O19](8-) anions and two axial Al3+ cations within [A l(Nb6O19)](5-) moieties with approximate D-3 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 induce s a contraction of the central {[Ln(3)O(OH)(3)(H2O)(3)](2)}(8+) cluste r (Ln = Er, Eu) with a resultant attraction of the equatorial and axia l [Nb6O19](8-) anions toward the centre of the complex by 0.09 and 0.0 7 Angstrom, respectively.