Spin Crossover in the Cis‐ [ Co II / III (bpym) ₂ (– NCSe ) ₂ ] 0/+ Complexes

Spin crossover (SCO) in cis‐ [Co II/III (bpym) 2 (–NCSe) 2 ] 0/+ complexes is studied using density functional theory UB3LYP/cc‐PVDZ[LANL2DZ] method. Analysis of normal vibrational modes and the corresponding frequencies and IR spectra calculated for these complexes in their more stable high‐spin (HS) and less stable low‐spin (LS) states shows that the HS ↔ LS spin transition has significant effects on the vibrational modes belonging to the regions with significant electron density redistribution. Analysis of the Duschinsky matrix calculated for the HS ↔ LS SCO in these complexes shows that most of mode mixing occur around the diagonal, and the extent of Duschinsky mixing during HS ↔ LS SCO is correlated with the corresponding atomic charge (electron density) redistribution. Analysis of the index, defined based on the Duschinsky matrix elements to quantify the overall vibrational mode mixing shows that the mode mixing due to HS ↔ LS SCO is larger for the [Co III (bpym) 2 (–NCSe) 2 ] + than that for the [Co II (bpym) 2 (–NCSe) 2 ] complex, which is compatible with the larger variations of the atomic charges of the former complex in its SCO conversion. The transition state of the HS ↔ LS SCO in the [Co III (bpym) 2 (–NCSe) 2 ] + complex is located using QST2 method, and characterized.