Hydrogen bonding systems in metal(II) formate dihydrates, M(HCOO)2·2H2O (M = Mg, Mn, Co, Ni, Cu, and Zn). Double matrix infrared spectroscopy

The hydrogen bond strengths in the title isomorphous compounds as deduced from the infrared wavenumbers of the respective uncoupled OD stretches of matrix-isolated HDO molecules are discussed in terms of Ow⋯O hydrogen bond distances, metal–water interactions (synergetic effect) and unit-cell volumes (repulsion potential of the lattice). The νOD stretching modes caused by guest ions (Cu2+ and M2+ guest ions matrix-isolated in M(HCOO)2·2H2O and Cu(HCOO)2·2H2O, respectively) are shifted at higher frequencies as compared to those in the host and the parent compounds, respectively. The values of these shifts (Δν) for the MOH2⋯OCHOCu hydrogen bond systems formed when Cu2+ guest ions are included in the structures of M(HCOO)2·2H2O are considerably larger (i.e., weaker hydrogen bonds) as compared to those of the same type formed when M2+ guest ions are included in the structure of Cu(HCOO)2·2H2O. A variant of double matrix infrared spectroscopy (M(HCOO)2·2H2O, M = Mg, Co, and Ni) with incorporated Cu2+ ions up to 50 mol% additionally to matrix-isolated HDO molecules is proposed in order to deduce the cation distribution in mixed formates. The analysis of the infrared spectra reveals that the included Cu2+ ions are localized at the two available lattice positions in the M(HCOO)2·2H2O (M = Co and Ni) host lattices and preferentially at the M(1) sites (sites with a coordination through six formate oxygen atom) in the Mg(HCOO)2·2H2O host lattice.