Na₈H₁₈[{Er₃O(OH)₃(H₂O)₃}₂Al₂-(Nb₆O₁₉)₅].40.5H₂O

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.

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