Spectroscopy and reactions of copper(II), nickel(II), and cobalt(III) compounds in molten nitrites

Abstract

Copper(II) sulphate and nickel(II) chloride formed green solutions in lithium nitrite–potassium nitrite eutectic at 120 °C. The electronic-absorption bands indicate bonding by both nitro- and nitrito-ligands {[Cu(NO₂)₄(ONO)₂]^4– and [Ni(NO₂)₃(ONO)₃]^4–}. The i.r. bands of the solidified melt also indicate both forms of nitrite ligand. At higher temperatures the cations are precipitated as the oxides by Lux–Flood acid-base reactions with the melt. Cobalt(III){as Na₃[Co(NO₂)₆] or [Co(NH₃)₆]Cl₃} is not soluble, but decomposes ultimately to form the oxide Co₃O₄via a series of reactions which include thermal decomposition, acid-base, cation exchange, and formation of the more stable complex K₃[Co(NO₂)₆]. The last reaction is facilitated by the presence of lower-melting nitrite eutectics.