We next assessed the metal-promiscuity of tetradentate ligands. Certain privileged ligands, such as tetraphenylporphyrin, have been characterized in complex with up to 15 distinct metals (Figure and Supporting Information Figure and Tables ). On average, we find that each unique tetradentate ligand from the set of octahedral complexes has been metalated with 1.3 metals (standard deviation: 1.0, median: 1.0), with the majority of ligands only appearing with a single metal (Figure and Supporting Information Table ). Ligands metalated with 3d transition metals are typically only found with other 3d transition metals, whereas complexes that can coordinate 5d metals frequently coordinate 3d metals as well (Figure and Supporting Information Figure ). In particular, macrocycles that have been characterized in complex with at least two different 5d metals have also been coordinated with 4d or 3d metals (Figure and Supporting Information Figure ). Because the majority of structurally characterized tetradentate ligands are only paired with a single metal, we next evaluated the likelihood of a ligand to coordinate a specific metal based on its relative binding pocket size with respect to that of the most promiscuous tetradentate ligand (i.e., tetraphenylporphyrin). This analysis could identify ligands that are compatible with other metals but for which crystallization of the resulting complexes has not been carried out. We evaluated the pocket sizes, which we define by the smallest and largest N-N distances of the metalcoordinating atoms in each instance (i.e., with each different metal) of tetraphenylporphyrin (Supporting Information Table ). The relatively small variation in pocket sizes in tetraphenylporphyrin but high promiscuity with metal identity suggests that a large number of existing tetradentate ligands may be able to coordinate a different metal. Indeed, we find 1,544 ligands that fall within the bond length ranges of the tetraphenylporphyrin cases with different metals, suggesting they can coordinate other metals they have not yet been crystallized in complex with (Supporting Information Table and a list of refcodes is provided in the Zenodo repository).