The low frequency motions of solvated Mn(ii) and Ni(ii) ions and their halide complexes

Abstract

We have investigated the low frequency (30–350 cm⁻¹) spectra of solvated MnCl₂, MnBr₂, NiCl₂, and NiBr₂. Using a chemical equilibrium model in combination with principal component analysis, we were able to dissect the spectra into molar extinction coefficients due to the solvated ions and – for MnCl₂, MnBr₂, and NiCl₂ – to extract information on the ion pair spectra. The deduced anion spectra (calculated as MnCl₂–MnBr₂ and NiCl₂–NiBr₂) are very similar and nearly identical to the anion spectra observed for LaCl₃–LaBr₃. The differences between the cationic contributions MnCl₂–NiCl₂ and MnBr₂–NiBr₂ indicate that the solvated cation spectra can be understood in terms of distinct resonances of the octahedrally solvated cation complex that are red-shifted for Mn²⁺ compared to Ni²⁺. The description of the full extinction spectra requires the introduction of an additional resonance at a center frequency of around 130 cm⁻¹ that we tentatively assign to hydration water. Cooperative effects are small and are reflected in a change in the band intensity. However, the center frequencies of the observed modes remain unchanged when exchanging the counter ion. Analysis of the ion pair extinction spectra supports contact ion pair formation for MnBr₂ and NiCl₂ and MnCl₂.