We curated a data set of Fe(II) SCO complexes from the CSD 123 following a protocol used in prior work 122 . We performed an initial search of the CSD targeted at octahedral Fe(II) SCO systems via the Conquest graphical interface to the CSD, applied to the v5.41 data set with complexes from November 2019, March 2020, and May 2020 updates. We completed a Conquest query to identify structures containing hexa-coordinate Fe with no restrictions on connecting atom elements or bond orders. We applied filters to the molecular formula to ensure that only one Fe atom was present in a selected complex and the chemical name did not contain Fe(0), Fe(III), or Fe(IV) to select for cases with Fe(II). We excluded polymeric molecules and limited the search to those complexes with 3-dimensional (3D) coordinates determined. This initial search produced 13,028 compounds, each with a single unique refcode, i.e., a six-letter code which in some cases is followed by two more digits identifying additional structure determinations (Figure and Supporting Information Table ). This new set contained all refcodes reported in the SCO data set from prior work 122 . From this set of 13,028 refcodes, we retained 2,494 refcodes that corresponded to multiple (≥ 2) cases of matching six-letter patterns at the beginning of their refcodes (i.e., 828 sets with unique base refcodes), indicating that they were likely captured in multiple spin states (Figure and Supporting Information Table ). Using the CSD Python application programming interface (API) along with molSimplify 125 , we checked the geometries of these complexes to ensure that they were octahedral, which resulted in the elimination of 671 refcodes, i.e., 284 sets with unique base refcodes (Supporting Information Tables ). We then reviewed these sets of refcodes to confirm that at least one component in the crystal had a chemical name containing "iron(ii)" to further ensure Fe(II) complexes were selected, leading to removal of 273 refcodes, i.e., 90 sets with unique base refcodes (Supporting Information Table ). We further examined all the retained refcodes to ensure that structures with identical base refcodes correspond to complexes with identical connectivity based on the determinant of the atomic-number-weighted connectivity matrix 122 . For each base refcode, complexes corresponding to the most frequently observed molecular graph determinant were selected, and refcodes corresponding to these complexes along with their reported temperatures in the CSD were saved (Figure and Supporting Information Table ). Consequently, all refcodes corresponding to a given base refcode that exhibit entirely different connectivities are filtered out, resulting in the elimination of 9 refcodes, i.e., 4 sets of unique base refcodes (Supporting Information Table ). were identified. The data set was narrowed down based on the presence of Fe(II) and octahedral geometries, followed by abstract mining. Lastly, complexes were checked for charge assignment, bond length requirements, and presence of trustworthy H atoms. Representative complexes along with their refcodes are shown after each curation step as insets. Atoms in the insets are colored as: H in white, C in gray, N in blue, O in red, S in yellow, and Fe in brown. When two refcodes are discussed, their structures are overlaid and distinguished by coloring the carbon atoms of one structure in gray and the other in cyan.