DFT computations successfully predict structural, energetic and spectral differences between the low- and high-spin forms of the Fe(II) spin crossover complexes with tris(pyrazol-1-yl) methane (HC(Pz)(3)). The spectral differences in the mid-IR range, where ligand vibrations are active, are caused by changes of the force constants of the [Fe{HC(Pz)(3)}(2)](2+) cation. Concomitant geometry changes and interionic interactions play only minor role in the spectral effects, which allows to use the combined quantum-chemical/spectroscopic approach for diagnostics of metal spin state in the complexes. To predict the energy gap between the low-and high-spin forms, the anions must be taken into account. (C) 2010 Elsevier B.V. All rights reserved.