New Hydridic Rare Earth Compounds

Abstract

Modern textbooks dealing with the chemistry of the lanthanide elements state that the elemental lanthanides, with a few exceptions, dissolve in dilute aqueous acids with the formation of the trivalent lanthanide cation and hydrogen gas. One exception that has been reported is the red coloration imparted to dilute hydrochloric acid solutions containing a chip of dissolving samarium metal. This red coloration has been attributed to the divalent samarium cation. But careful observation of specimens of elemental lanthanides actually being dissolved in dilute acids may show unexpected results. For example, a faint brown coloration can sometimes be observed above a similarly dissolving chip of neodymium metal. There is nothing in the reported chemistry of neodymium that indicates a brown coloration under such circumstances, and the possibility of the formation of divalent neodymium cations is suggested by analogy. Also by analogy, the rapid disappearance of the brown coloration further suggests oxidation of the supposed divalent neodymium cations by hydrogen ions. To diminish the speed of the supposed oxidation reaction, the dissolution experiment may be repeated in a solution with a diminished concentration of hydrogen ions, as in a solution of a weak acid.