X-ray structure study of the light-induced metastable states of the spin-crossover compound [Fe(mtz)6](BF4)2

Iron(II) complexes exhibiting thermal spin-crossover may be converted from the1A1low-spin (LS) state to the5T2high-spin (HS) state by irradiation with green light (light-induced excited spin-state trapping, LIESST) and from the LS to the HS state by irradiation with red light (reverse LIESST). The lifetime of the metastable LIESST states may be sufficiently long to enable an X-ray diffraction study. The lattice parameters of a single crystal of [Fe(mtz)6](BF4)2(mtz = methyltetrazole) (space groupP21/n) were measured between 300 and 10 K. While one Fe lattice site (A) of the crystal changes from the HS to the LS state near 78 K, the other site (B) remains in the LS state. Using the green lig…