Spin-Phonon Coupling in compound 2 As for compound 1, the magnetic-field dependence of the FIR spectra and the low-wavenumber range of the Raman spectrum are likely due to perturba-tions of the phonon spectrum by spin-phonon coupling. In contrast to 1, compound 2 possesses an inversion center, and therefore vibrations cannot be both IR-and Raman-active. As a consequence, the peaks due to spin-perturbed phonon transitions are related to different types of phonons in both types of spectra, leading to different effective transition energies. For the theoretical modelling of spin-phonon coupling in 2, we have to extend the coupling term in the Hamiltonian to