Relativistic quantum chemistry investigations are carried out to tackle the puzzling oxidation state problem in a series of MO_3~- 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 MO_3~- 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 PrO_3~- ion, while GdO_3~- ion is in fact an OGd~+(O_2~(2-)) complex with Gd(III). These results show that a na?ve assignment of oxidation state of a chemical element without electronic structure analysis can lead to erroneous conclusions. Relativistic quantum chemistry investigations are carried out to tackle the puzzling oxidation state problem in a series of MO_3 ~ - 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 MO_3 ~ - anions, the lanthanide elements Pr and Gd can not 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 PrO_3 ~ - ion, while GdO_3 ~ - ion is in fact an OGd ~ + (O_2 ~ (2-)) complex with Gd (III). These results show that a na? Ve assignment of oxidation state of a chemical element without electronic structure analysis can lead to erroneous conclusions.