Temperature dependent nuclear inelastic-scattering (NIS) of synchrotron rad
iation was applied to investigate both spin states of the spin-crossover co
mplex [Fe(tpa)(NCS)(2)] (tpa = tris(2-pyridylmethyl)amine). A remarkable in
crease of the iron-ligand bond stretching upon spin crossover has unambiguo
usly been identified by comparing the measured NIS spectra with theoretical
simulations based on density-functional calculations.