The energy splittings that are of relevance to cobalt-based single-molecule magnets lie in the THz-to-Far-Infrared regime. Consequently, a range of experimental techniques has been used to experimentally determine these splittings, especially those related to ZFS. To study the lowest Kramers doublet and the ZFS for weakly anisotropic systems, high-frequency electron paramagnetic resonance (HFEPR) spectroscopy has been used. A much used and powerful method is variable-field far-infrared (FIR) spectroscopy, which allows direct determination of the energy gap between the two Kramers doublets. Furthermore, inelastic neutron scattering also allows access to this energy gap, and the q-dependence of the signal provides a means other than applying field to distinguish between phonon and magnetic transitions. Finally, also variable-field Raman spectroscopy has turned out to be a very powerful method in this regard. In some cases, clear signatures of spin-phonon coupling were observed, i.e., excitations having partial spin and partial vibrational character. Spin-phonon coupling is at the heart of the relaxation of the magnetic moment in single-molecule magnets. Similar studies have been carried out on lanthanide-based single-molecule magnets. Here we present a detailed experimental and theoretical study of the low-energy electronic structure of a previously reported mononuclear cobalt(II) single-ion magnet