Bending Frequencies and New Dimer Modes in the Far-Infrared Spectra of Transition-Metal Dihalides

This article describes measurements and analysis of the infrared absorption spectra of monomer and dimer species of the transition‐metal dihalides in the region of 520–40 cm⁻¹. Experiments have been carried out on MnCl₂, FeCl₂, CoCl₂, NiCl₂, and NiBr₂ isolated in argon matrices at 4°K. For the monomers, the following ${\nu }_2$ bending frequencies (cm⁻¹) have been observed: 83 (MnCl₂), 88 (FeCl₂), 94.5 (CoCl₂), 85 (NiCl₂), and 69 (NiBr₂). In addition to these, the ${\nu }_3$ frequencies have been observed. These data are compatible with a linear structure, but in connection with other information, suggest that a simple valence force field is an inadequate description of the normal vibrations. For the dimers, the following frequencies (cm⁻¹) have been observed and assigned on the basis of a bridge‐bond structure of $D_{\text{2h}}$ symmetry: ${\nu }_{12}\text{\hspace{0.17em}=\hspace{0.17em}436.2}$ (Fe₂Cl₄), 432.5 (Co₂Cl₄), 440 (Ni₂Cl₄); ${\nu }_{11}\text{\hspace{0.17em}=\hspace{0.17em}289}$ (Co₂Cl₄); ${\nu }_{10}\text{\hspace{0.17em}=\hspace{0.17em}110}$ (Fe₂Cl₄); ${\nu }_9\text{\hspace{0.17em}=\hspace{0.17em}323}$ (Co₂Cl₄).