Single-Crystal Neutron Structure Analysis of (NH4)21[H3Mo57V6(NO)6O183(H2O)18]·53 H2O

The crystal structure of (NH₄)₂₁[H₃Mo₅₇V₆(NO)₆O₁₈₃ (H₂O)₁₈]·53 H₂O a supramolecular heteropoly cluster compound (space group P6₃/mmcZ=2 final R1=0.1302 (I>2σ(I)) for 1745 unique reflections) was redetermined by single-crystal neutron diffraction studies at 20 K. The X-ray diffraction results reported in 1994 by Müller et al. (Z. Anorg. Allg. Chem. 620 599) are confirmed. Additionally we could localize many hydrogen positions not found so far and establish a phase transition near 240 K. Many of the ammonium ions the ligand and hydrate H₂O molecules and the hydroxy group are orientationally disordered even at 20 K. The central cavity of the structure is built up by two twelve-membered rings consisting of six O-H·sdot;·O hydrogen bonds each. These strong hydrogen bonds are obviously decisive for the stability of the cluster. The hydrate H₂O molecules are stronger-hydrogen-bond acceptor groups than the oxoligands of the cluster.