On the oxidation states of metal elements in MO (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 MO 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 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) 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 ion, while GdO ion is in fact an OGd(O ) 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. [ABSTRACT FROM AUTHOR]