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

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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.