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}2μ–(ac(bipy)2)] (ac(bipy)2 = bridging ligand) are obtained by temperature-dependent far-infrared (FIR) spectroscopic measurements and assigned with the help of density functional theory (DFT) calculations. The experimental data confirm the high-spin (HS) state of the complexes at high temperature (≈ 200 K) and the low-spin (LS) state at 5 K. In the dimer, enhancement of otherwise absent vibrational modes at 335 cm–1 and 504 cm–1 around T1/2 reflects the presence of a mixed-spin HS-LS state during the course of the spin transition between HS-HS and LS-LS. The metastable HS state of the dinuclear complex resulting from light irradiation (532 nm) at 10 K results from a direct spin-state transition from LS-LS to HS-HS.
