Abstract
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 naive assignment of oxidation state of a chemical element without electronic structure analysis can lead to erroneous conclusions.
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