In general, the number of SMILES that are not accepted by the different toolkits as valid SMILES are quite different (see "Invalid" entries in Table ) and also depend on whether or not normalization is used. CDK seems to be more "forgiving" than RDKit and OCL, but only if normalization is used. ), however this "correct" SMILES does not truly reflect the aromatic structure with a distributed negative charge and its coordinative bonding nature. This problem will be seen for all coordinative compounds, as the current SMILES syntax does not allow for coordinative or hydrogen bonds like they are available in the MDL MOL file version V3000 definitions . This is a serious deficiency of the current SMILES notation, excluding most metal complexes from the universe of SMILES and InChI descriptions, and is a topic under discussion within the InChI committee and IUPAC. The 7 th invalid SMILES was generated by OSRA, with the hypervalent carbon atoms as shown and discussed in Figure above (OCID190014261931).