Expanding the +2 Oxidation State of the Rare-Earth Metals, Uranium, and Thorium in Molecular Complexes

Abstract

Abstract The review describes recent developments in the low oxidation state chemistry of the rare-earth elements since the publication in 2010 of Chapter 246 in this series on “The Molecular Chemistry of the Rare-Earth Elements in Uncommon Low-Valent States” by Nief. The landscape of low oxidation states in the rare-earth metal series has indeed changed dramatically between 2011 and 2016. The first isolable crystallographically characterizable molecular complexes of the + 2 rare-earth metal ions, La2 +, Ce2 +, Pr2 +, Gd2 +, Tb2 +, Ho2 +, Y2 +, Er2 +, and Lu2 + as well as the + 2 actinide ions, U2 + and Th2 +, have been discovered. Following a background section, these discoveries are presented chronologically to show the evolution of this new chemistry. A section describing previous literature that foreshadowed these discoveries is included to stimulate thinking about how current literature can be used to generate future discoveries. The importance of specific ligand sets which allowed isolation of ions with 4fn5d1, 5f36d1, and 6d2 electron configurations is described as well as the special conditions of low temperature and alkali metal chelate choice that provided isolable complexes. The identification of configurational crossover ions, whose electronic ground states can be either 4fn + 1 or 4fn5d1 depending on the ligand set, is also discussed.