Since preparation methods can differ significantly, it is not expected that identical FeNC sites are formed in the experimental studies. This observation is well reflected in the broad range of isomer shift and quadrupole splitting values reported for the most common doublets used in fitting the experimental Mössbauer parameters, D1 and D2. Future experimental studies focusing on a combination of techniques for structural and electronic elucidation of the active sites under different reaction conditions would provide additional constraints for the construction of computational models.