“…For this complex, the reaction coordinate cannot be approximated by a simple average Fe-N bond elongation, e.g., it is anisotropic (Nance et al, 2015;Canton et al, 2014;2015;. As revealed by powerful studies combining ligand design, crystallography, and modeling tools, such anisotropy is the key to stabilizing the HS state (Steinert et al, 2016;Stock et al, 2016;2017a;2017b;Phan et al, 2017;and Kroll et al, 2019). A significant increase in the HS lifetime of mononuclear complexes occurs when radial and angular motions are coupled during the rearrangement of the ligand scaffold (Marchivie et al, 2005;Buhks et al, 1980;Hauser et al, 1991;1995;2004;and Gütlich et al, 1994), e.g., through a trigonal prismatic deformation known as the Bailar twist (Bailar, 1958), which was already proposed in the 1980s (Rodger and Johnson, 1988;Vanquickenborne and Pierloot, 1981;McCusker et al, 1992;and McCusker et al, 1993).…”