%0 Journal Article %A Reger, Daniel L. %A Elgin, J. Derek %A Foley, Elizabeth A. %A Smith, Mark D. %A Grandjean, Fernande %A Long, Gary J. %D 2009 %T Structural, Magnetic, and Mössbauer Spectral Study of the Electronic Spin-State Transition in [Fe{HC(3-Mepz)2(5-Mepz)}2](BF4)2 %U https://acs.figshare.com/articles/journal_contribution/Structural_Magnetic_and_Mo_ssbauer_Spectral_Study_of_the_Electronic_Spin_State_Transition_in_Fe_HC_3_Mepz_sub_2_sub_5_Mepz_sub_2_sub_BF_sub_4_sub_sub_2_sub_/2823763 %R 10.1021/ic901259e.s002 %2 https://ndownloader.figshare.com/files/4521448 %K bond %K 85 K %K relaxation rate yields %K 294 K %K Fe %K HC %K 150 K show %K C 3v symmetry averages %K Mo %X The complex [Fe{HC(3-Mepz)2(5-Mepz)}2](BF4)2 (pz = pyrazolyl ring) has been prepared by the reaction of HC(3-Mepz)2(5-Mepz) with Fe(BF4)2·6H2O. The solid state structures obtained at 294 and 150 K show a distorted iron(II) octahedral N6 coordination environment with the largest deviations arising from the restrictions imposed by the chelate rings. At 294 K the complex is predominately high-spin with Fe−N bond distances averaging 2.14 Å, distances that are somewhat shorter than expected for a purely high-spin iron(II) complex because of the presence of an admixture of about 80% high-spin and 20% low-spin iron(II). At 294 K the twisting of the pyrazolyl rings from the ideal C3v symmetry averages only 2.2°, a much smaller twist than has been observed previously in similar complexes. At 150 K the Fe−N bond distances average 1.99 Å, indicative of an almost fully low-spin iron(II) complex; the twist angle is only 1.3°, as expected for a complex with these Fe−N bond distances. The magnetic properties show that the complex undergoes a gradual change from low-spin iron(II) below 85 K to high-spin iron(II) at 400 K. The 4.2 to 60 K Mössbauer spectra correspond to a fully low-spin iron(II) complex but, upon further warming above 85 K, the iron(II) begins to undergo spin-state relaxation between the low- and high-spin forms on the Mössbauer time scale. At 155 and 315 K the complex exhibits spin-state relaxation rates of 0.36 and 7.38 MHz, respectively, and an Arrhenius plot of the logarithm of the relaxation rate yields an activation energy of 670 ± 40 cm−1 for the spin-state relaxation. %I ACS Publications