Chemistry of non-traditional oxidation states of rare earth metals

The chemistry of rare-earth metals (Sc, Y and lanthanides) has been dominated by trivalent oxidation states, except for divalent europium, ytterbium, and samarium and tetravalent cerium. The lack of readily accessible oxidation states limits the participation of rare earth metals in redox chemistry. Therefore, expanding the oxidation states of rare earth metals and synthesizing molecular rare earth metal complexes of non-traditional oxidation states are not only the long-term goal sought by synthetic chemists but also the pre-requisite for developing redox chemistry mediated by rare earth metals. Recently, major breakthroughs have been achieved in this field. All rare earth metals, except radioactive promethium, have been shown to exist in divalent states in molecular complexes, while molecular complexes of tetravalent terbium and praseodymium have been synthesized for the first time. Judicious ligand design is key to stabilizing these non-traditional oxidation states of rare earth metals. Moreover, upon studying the electronic structures and bonding interactions of these complexes, it has been shown that the coordination environment has a non-negligible influence on the electron configuration of rare earth metals. In this review, we will first analyze the electron configuration of rare earth metals and introduce the history of pursuing oxidation states other than +3 for rare earth metals. Then we will show the recent advancement in expanding the non-traditional oxidation states of rare earth metals with a focus on molecular complexes of both high and low valent rare earth metals. At the end, we will provide perspectives and outlook for the future development of this field.