Figure shows an example of two SMILES where the TM has negative oxidation states. The overall charge of both complexes is -1. For ALOBIM the naive approach where we get the charge of the TM atom will give you an oxidation state of -1. However, due to the nature of the hydride ligand that is not correct. In fact, the negative charge belongs to the hydride and the TM has an oxidation state of 0. RDKit will in some cases remove a datively bound hydride and instead put it as an implicit hydrogen on the TM. Therefore, this implicit hydrogen with a negative charge has to be taken into account when calculating the TM oxidation state. An example of this is given in a notebook in the repo : .