Vibrational study of the Fe(phen)2(NCS)2 spin-crossover complex by density-functional calculations.

The geometries and vibrational frequencies of an iron(II) spin-crossover complex Fe(phen)2(NCS)2 in the low- and high-spin states have been calculated using DFT (BP86 functional with 6-311G* + Wachters + f, 6-31G* and LANL2DZ basis sets). A redundancy-free set of internal coordinates has been defined and used to obtain a valence force field. The use of the symmetry coordinates of a perfect octahedron as internal coordinates for the FeN6 framework of the complex has enabled the description of the stretching-bending couplings. A comparison of the force constants of free ligands with the corresponding constants of the two spin states has provided a basis for the discussion of the changes in electronic charge distribution in the complex. By use of the generalized harmonic mode scrambling, it is shown that in the low-lying normal vibrations of the complex, the ligands participate as almost rigid units. Energies of these modes are close to the oscillators describing the interaction of the complex with the surrounding molecules in the solid phase.