Because octahedral geometries comprise a significant fraction of TMCs in the mononuclear CSD data set (35%, 85,575 out of 242,829), we carried out an analysis of the symmetry classes present in the octahedral mononuclear CSD data set (Supporting Information Text S1 and Figure ). We find that TMCs with two identical bidentate ligands in the equatorial plane and two identical axial monodentate ligands (i.e., |22||11|t where "||" groups identical ligands and t indicates ligands trans to each other, as detailed further in Supporting Information Text S1) are the most common symmetry class (Figure ). When preserving only unique cases, the |22|2 symmetry class, which contains three bidentate ligands with two unique ligand identities, is the most common (Figure ). Homoleptic monodentate compounds (i.e., |111111|) are the sixth-most common symmetry class, but the number of these shrinks dramatically when eliminating duplicates since the majority of such complexes are solvated metal ions (Figure ). Similarly, the homoleptic bidentate transition metal complexes (i.e., |222|) drop from the third-most frequent in absolute counts to the ninth-most common, due to multiple repeat cases of Fe(bpy) 3 and Ru(bpy) 3 (Figure ). Intriguingly, we find that many of the most common symmetry classes (e.g., |22||11|t, |111111|, |1111||11|t, and 4|11|t) contain monodentate ligands that may likely be removable to leave an open site in the complex amenable for catalysis. TMCs with tetradentate ligands account for 12% (8,520 out of 70,080) of unique octahedral complexes. The 4|11|t symmetry class with a planar tetradentate ligand and identical monodentate axial ligands is the sixth-most frequent configuration when accounting for unique compounds (Figure ). Overall, we find that tetradentate macrocycles are evenly divided between planar (49%) and seesaw (51%) configurations (Supporting Information Table ). We find that 7% (143 out of 2,091) of the unique tetradentate macrocycles that appear in planar configurations (i.e., the 4|11|t, 411t, or 4planar2 symmetry classes) also appear in configurations in which the macrocycle is bent (i.e., the 4|11|c, 411c, or 42 symmetry classes, Supporting Information Table ). Macrocycles that appear most frequently in both seesaw and planar configurations for the same metal center tend to be ligands containing saturated elements with rotatable bonds (Figure ). For instance, we find that two complexes containing the same cyclam ligand coordinated to Mn exist in different symmetry classes (refcode SIQMUA: 411c, refcode AFEJOL: 4|11|t, Figure ). More surprisingly, we find exceptions to this general rule, with some rigid, planar ligands (e.g. tetraphenylporphyrin)