Study of thermal spin crossover in [Fe(II)(isoxazole)₆](BF₄)₂ with Mössbauer spectroscopy

⁵⁷Fe Mössbauer spectroscopy of the mononuclear [Fe(II)(isoxazole)₆](BF₄) ₂compound has been studied to reveal the thermal spin crossover of Fe(II) between low-spin (S = 0) and high-spin (S = 2) states. A temperature-dependent spin transition curve has been constructed with the least-square fitted data obtained from the Mössbauer spectra measured at various temperatures in the 240-60K range during the cooling and heating cycle. The compound exhibits a temperature-dependent two-step spin transition phenomenon with T_sco (step 1) = 92 and T_sco (step2) = 191K. The compound has three high-spin Fe(II) sites at the highest temperature of study; among them, two have slightly different coordination environments. These two Fe(II) sites are found to undergo a spin transition, while the third Fe(II) site retains the high-spin state over the whole temperature range. Possible reasons for the formation of the two steps in the spin transition curve are discussed. The observations made from the present study are in complete agreement with those envisaged from earlier magnetic and structural studies made on [Fe(II)(isoxazole)₆](BF₄)₂, but highlights the nature of the spin crossover mechanism.