Chromium (III)hydroxide hydrate - a new hydroxide structure type with cross-linked hydrogen bonding

Abstract

Precipitation of hexaquo chromium(III) solutions with OH^- ions yields a hydroxide with analytical composition Cr(OH)₃・3 H₂O of the anti-bayerite type. The lattice constants of the pseudo-hexagonal unit cell are a₀ = 5,047 and c₀ = 4,73 Å. This lattice has vacancies where bayerite would have occupied octahedral sites, and vice versa, in a hexagonal closest arrangement of OH^- and H₂O. It consists of layers of isolated [Cr(OH₂)₃(OH)₃] units. These are held together by a network of two kinds of hydrogen bonds. IR evidence gives O . . . HO distances of 2,7 and 2,9 Å, respectively. Precipitation experiments with H₂¹⁸O enriched solvent could be explained by a simple and fast deprotonation mechanism where the inner coordination sphere of the hexaquo chromium (III) ion remains unaltered. Upon ageing an amorphous phase of undefined composition is formed, where corner and/or edge sharing of hexaquo chromium (III) octahedra occurs. The solubility rate of this aged product in acids is distinctly lower, as the inner coordination sphere of the Cr³⁺ has to be attacked.