On the oxidation states of metal elements in MO3- (M=V,Nb, Ta,Db, Pr,Gd, Pa) anions

Relativistic quantum chemistry investigations are carried out to tackle the puzzling oxidation state problem in a series of MO₃^- trioxide anions of all d- and f-block elements with five valence electrons. We have shown here that while the oxidation states of V, Nb, Ta, Db, Pa are, as usual, all +V with divalent oxygen O(-II) in MO₃^- anions, the lanthanide elements Pr and Gd cannot adopt such high +V oxidation state in similar trioxide anions. Instead, lanthanide element Gd retains its usual +III oxidation state, while Pr retains a +IV oxidation state, thus forcing oxygen into a non-innocent ligand with an uncommon monovalent radical (O^?) of oxidation state -I. A unique Pr^? - ^?(O)₃ biradical with highly delocalized unpairing electron density on Pr(IV) and three O atoms is found to be responsible for stabilizing the monovalent-oxygen species in PrO₃^- ion, while GdO₃^- ion is in fact an OGd⁺(O₂^2- ) complex with Gd(III). These results show that a naïve assignment of oxidation state of a chemical element without electronic structure analysis can lead to erroneous conclusions.