Deeper experimental insight into the spin structure of the two compounds 1 and 2 can be obtained from spectroscopy. To this end, electron paramagnetic resonance (EPR) spectroscopy can be expected to be an excellent tool. Unfortunately, the two Co(II) compounds presented in this work are EPR silent, because transitions within the lowest Kramers doublet (KD) are formally forbidden, while the allowed inter-KD transitions are in the far-infrared (FIR) regime and hence at much higher energies (|2D| ≈ 250 cm -1 ) than conventional microwave sources cover. Therefore, variable-field far-infrared (FIR) and Raman spectra were recorded for both 1 and 2, instead. Both techniques are complementary to each other since Raman and FIR follow different selection rules, and also because, as it will turn out (see below), the Raman spectra can be recorded up to higher excitation energies. Signal 1A is overlaid with the energies of the calculated spin-phonon coupled transitions. Mononuclear compound 1. FIR spectra were recorded on a pressed pellet of 1 at 3 and 5 K in the range between 30 cm -1 and 680 cm -1 and at fields from 0 to 30 T (Fig. , 5, Table ; for a complete field overview see supporting information Fig. ). In order to bring field-dependent features to the fore, the in-field FIR spectra were normalized with respect to the zero-field spectrum by division of the transmission signal of the zero field spectrum by that of the in field spectra (for a detailed description see supporting information section S2). This means that positive features indicate increased absorption in field compared to zero field. In these spectra, a series of peaks (denoted 1A) is observed in the region around 230-240 cm -1 , where, in view of the fits of the magnetometry data, only the excitation from the ground to excited KD, with energy 2D is expected. The presence of several peaks in this region had been noted before for this, and related complexes, and this finding was attributed to spin-phonon coupling, but not investigated in detail.