Considering these factors, 1 and 1 Large may be consistent with the experimental doublets D1 or D1b, whose isomer shift is well reproduced. This is in agreement with the literature, where free sites without axial ligands are also assigned to D1. The predicted Mössbauer parameters for one OH - or H 2 O ligand do not fit to any experimental values. Since both axial coordination sites are equally accessible in 1 and water-derived ligands are ubiquitous in the experiment, having a single OH -or H 2 O ligand appears chemically less likely than having two axial ligands, which is also supported by experimental EXAFS data. For the models with two axial water or hydroxide ligands, 1-(H 2 O) 2 and 1-(OH) 2 , the calculated Mössbauer parameters match the experimental D2 doublets, making two OH -or two H 2 O ligands or perhaps a mixed aquo/hydroxide case an explanation for this type of doublet. The model with two O 2 ligands can explain the operando doublet D4, a chemically sensible suggestion, since weakening or splitting the O-O bond is likely associated with the ratedetermining step. As a trend, it can be noted that ligands that bind more strongly have smaller isomer shifts; this is partly true also for model 2. Overall, model 1 offers an explanation for all signals with low and intermediate isomer shift values (i. e. isomer shift values below 0.9 mm s -1 ).