Highly resolved vibrational spectra of the mononuclear Fe(II) spin-crossover complex [Fe(bpz)2(bipy)];(bpz = dihydrobis(pyrazolyl)borate) and its dinuclear counterpart [{Fe(bpz)2}2 μ−(ac(bipy)2)](ac(bipy)2 = bridging ligand) are obtained by temperature-dependent far-infrared (FIR) and mid-infrared (MIR) spectroscopic measurements.