Two examples of the Hückel and NBO smiles differing by 2 charge units are shown in Figure . For ACTBPD, the flexibility in charge assignment of the Hückel method means the carbon/sulphur coordinating ligand is assigned a charge of 1+ which results in a neutral Pd. The NBO on the other hand has found a ligand charge of 1-which results in a Pd 2+ . For AHUDIR we have the opposite where Hückel determines the ligand with the haptic ring to have a charge of -2 and NBO determines it to a charge of 0. Figure in the main text illustrated the fact that some SMILES had TMCs with extreme oxidation states and here these are analyzed further. Of the 59 878 huckel SMILES, 160 have oxidation states below 0 (0.2%) and 26 have oxidation states above 10 (0.06%). The fraction of these edge cases is therefore very low. Figure illustrates a case where Hg is assigned an oxidation state of 14+ with the Hückel method. It is clear from the molecular graph that the high oxidation state comes from the multiple O fragments. In contrast, the fully connected graph for the CSD SMILES on the right seems like the more accurate representation. In the Hückel method a chloride is only allowed to have a single bond. Since that is not the case here, the bonds to the TM is broken which results in multiple fragments containing Cl and O. It is therefore a result of a design choice that we get these edge cases for the Hückel SMILES. Almost all of the 26 cases with ¿ +10 TM oxidation state comes from this chloride constraint.