Single-crystal neutron structure analysis of (NH4)21[H3Mo57V6(NO)6O183(H2O)18].53 H2O

Abstract The crystal structure of (NH 4 ) 21 [H 3 Mo 57 V 6 (NO) 6 O 183 (H 2 O) 18 ]·53 H 2 O a supramolecular heteropoly cluster compound (space group P 6 3 / mmc Z =2 final R 1=0.1302 ( I >2 σ ( 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 Muller 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 H 2 O 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 H 2 O molecules are stronger-hydrogen-bond acceptor groups than the oxoligands of the cluster.