Base hydrolysis of aqueous chromium(III) solutions: on the existence of [Cr(OH)4]–, and speculation regarding a new chromium polyoxocation

Abstract

The base hydrolysis of chromium(III) aqueous solutions was investigated in an attempt to determine whether a [CrO₄Cr₁₂(OH)₂₄(H₂O)₁₂]⁷⁺ polyoxocation (i.e. Cr₁₃) analogous to the tridecameric aluminium and gallium ions (Al₁₃ and Ga₁₃) is formed. Clay mineral intercalation studies showed that a species formed during base hydrolysis of aqueous solutions of Cr³⁺ has approximately the same height, relativer to the phyllosilicate sheets, as that of the Ga₁₃ polyoxocations. Gel permeation chromatography of the hydrolysed solution revealed that a chromium species was eluted at exactly the same volume as were the other tridecameric ions, under analoguous conditions. These results indicate that a chromium ion similar in size to that of the know [GaO₄Ga₁₂(OH)₂₄(H₂O)₁₂]⁷⁺ does form during base hydrolysis. Quantitative pillaring studies also suggested that the chromium species has a similar charge. Infrared studies of sulfate salts formed from the chromium-containing cations present in the hydrolysed solutions indicated that no tetrahedrally coordinated chromium was present; this implies that the chromium species cannot be analoguous to the Al₁₃ and Ga₁₃ polyoxocations as they both contain a central tetrahedrally co-ordinated metal atom. Since this tetrahedral central atom in the tridecamers has been postulated to arise because of the existence of [M(OH)₄]^x– ions in solution at high pH, its apparent absence in the case of chromium led to an investigation of whether or not [Cr(OH)₄]^– ions, the formation of which has generally been assumed, really exist. Through ultraviolet-visible and near-infrared spectroscopic studes, as well as the analysis of titration curves, it is concluded that no [Cr(OH)₄]^– ions are formated upon hydrolysis of solutions of Cr³⁺ to OH^–:Cr³⁺ mole ratios of 4:1. For these reasons it is speculated that the large species present in hydrolysed chromium solutions may be [Cr₁₂(OH)₂₈(H₂O)₁₂]⁸⁺ ions in which the central tetrahedral position is vacant.