Turning now to model 2, the calculated isomer shifts without axial ligands do not match the experimental D1 doublet. Only a model with a OH -ligand comes close to explaining the D1 signal, where the slightly overestimated quadrupole splitting may be justified by an energetically low lying doublet state (see Table ). The models with one or two H 2 O molecules do not match any experimental data. D1b could be explained by one or two axial O 2 ligands. This is consistent with chemical expectations since oxygen could be bound at the non-activated catalyst during preparation. The D2 doublet could either be explained by the model with one O 2 ligand or by the free extended model, taking into account some additional uncertainty in its quadrupole splitting due to its planarity. The calculated Mössbauer parameters of 2-(OH) 2 fit well to the operando doublet D4, however it does not seem chemically likely that two OH -ligands would accumulate before the rate determining step, especially not in the acidic conditions under which the operando doublet was observed by Ni, Gallenkamp et al. Hence, only 2-(O 2 ) 2 gives an explanation for the D4 doublet. Overall, the explanation model 2 gives for the Mössbauer signals is not as satisfactory as with model 1. As the extended model 2 Large is also considerably less stable than 1 Large , it can be concluded that 1 is a better representation of the active site than 2.