Blödite-type compounds Na 2Me(SO 4) 2·4H 2O (Me=Mg, Co, Ni, Zn): crystal structures and hydrogen bonding systems

Abstract

The crystal structures of synthetic blödite-type compounds Na 2Me 2+(SO 4) 2·4H 2O (Me=Mg, Co, Ni, Zn) were investigated using single crystal X-ray CCD diffraction data. They crystallize isotypic (space group P2 1/a ) and are built up from [Me(H 2O) 4(SO 4) 2] 2− clusters which are interlinked by NaO 6 polyhedra and hydrogen bonds of medium strength. While the octahedral mean Me–O bond lengths follow the trends expected from the cationic radii (Ni<Mg<Zn≤Co), several other structural features do not, e.g. the cell volumes (Ni<Zn<Co≤Mg) or the ‘cluster angles’ Me–O–S. Such deviations are attributed to a slightly different bonding character of the Mg–O bond compared to the 3d transition ions. The strengths of the hydrogen bonds formed in the title compounds as deduced from the infrared wavenumbers of the respective uncoupled OD stretching modes of matrix-isolated HDO molecules (isotopically dilute samples) are discussed in terms of hydrogen bond lengths O w⋯O and metal–water interactions (synergetic effect). The red-shift of the bands corresponding to the OD stretches on going from the magnesium to the zinc compound is caused by the increasing covalency of the respective Me–OH 2 bonds (Mg<Co<Ni<Zn). The intramolecular OH bond distances are derived from the novel ν OD vs. r OH correlation curves [J. Mol. Struct. 351 (1995) 205; J. Mol. Struct. 404 (1997) 63].