Double formates Ba2M(HCOO)6(H2O)4 (M ¼ Co, Ni, Cu, Zn): crystal structures and hydrogen bonding systems

The crystal structures of four members in the isomorphous series, Ba2M(HCOO)6(H2O)4 (M ¼ Co, Ni, Cu, Zn) are presented and thoroughly discussed. Discrepancies with a previous structural report on the Cu isolog [Z. Kristallogr. 110 (1958) 231] were cleared out through a re-refinement of the original data, the outcome of which definitely confirmed the present results. The strengths of the hydrogen bonds in the title compounds as deduced from the infrared wavenumbers of the uncoupled OD stretches of matrix-isolated HDO molecules are discussed in terms of the Ow· · ·O hydrogen bond lengths, the different hydrogen bond acceptor capabilities of the formate oxygen atoms and the weak Ba–OH2 interactions. The proton acceptor strength of the oxygen atoms is evaluated within the framework of the Brown’s bond-valence theory. The intramolecular OH bond lengths are derived from the novel nOD vs. rOH correlation curve [J. Mol. Struct. 404 (1997) 63]. q 2004 Elsevier B.V. All rights reserved.