Ultraviolet-visible spectrum, and kinetics of formation and decomposition, of penta-aquahydridochromium (III) and chromium(I) in aqueous perchlorate solutions

The reaction of hydrogen atoms with Cr2+(aq) has been studied and a specific rate constant of 1·5 × 109 l mol–1 s–1 determined. The product of this reaction in acid solution is [Cr(H2O)5H]2+. The absorption spectrum of this complex has been measured and found to be similar to that of complexes containing chromium–carbon bonds. The mechanism of decomposition of [Cr(H2O)5H]2+ obeys the rate equation Kobs=k17+k18[H3O+], where k17⩽ 1 s–1 and k18= 1·8 × 104 l mol–1 s–1 at 22 ± 2 °C. In neutral solutions Cr+(aq) is formed by reaction of e–(aq) with Cr2+(aq). The absorption spectrum of Cr+(aq) and the kinetics of its decomposition are reported and discussed. The implication of these results to the mechanism of dissolution of chromium metal and on the photochemistry of Cr2+(aq) are examined.