Composition and Structure of Polynuclear Chromium(III) Hydroxo Complexes

The structures of the polynuclear hydrolysis complexes of chromium(III) formed in acidic and alkaline aqueous solution have been determined by means of extended X-ray absorption fine structure and large angle X-ray scattering. Chromium(III) is present as mononuclear hydrated ions below pH 3-4, with the upper limit depending on concentration, ionic strength, and temperature. Chromium(III) hydrolyzes in aqueous solution above this pH in concentrated aged solutions to mainly cationic tetrameric complexes containing both single and double hydroxo bridges between the chromium(III) ions. Above pH 5: the solubility of chromium(III) decreases sharply to form solid hydrated chromium(III) hydroxide. The solubility of chromium(III) increases sharply at elevated pH, and fairly concentrated solutions can be prepared in concentrated aqueous solutions of sodium hydroxide. According to the literature, mononuclear tetrahydroxochromate(III) complexes, Cr(OH)(4)(-) are formed in alkaline aqueous solution. However, this study shows polymeric chains of six-coordinated chromium(III) ions connected through double hydroxo bridges and with two terminal hydroxo groups giving an overall composition of (Cr(OH)(4))(n)(n-) This polymer is stable for a long time in solution at pH=15. However, at pH < 15, this polymer precipitates slowly with time, over months, to an amorphous phase with a structure around chromium similar to that in alpha-chromium(III) oxide hydroxide.