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

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 K_obs = 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.