For model 3, the models with no axial ligand and with OH - can explain the D1 signal, though with a slightly lower isomer shift than typical D1 signals. The model with H 2 O can explain the D1b signal or the D2 signal and the model with O 2 the D4 signal, considering a possible multireference character due to the electronic complexity of O 2 , as is also discussed elsewhere. A model with two H 2 O ligands does not fit any experimental signals. However, given the concave shape of the model, binding the two axial ligands is not equally likely. As was shown by Ni, Gallenkamp et al., charged ligands can also explain the D3 doublets that have higher isomer shifts than shown in Figure .