The Photochemical and Thermal Reactions of p-Hydroxo-bis {pentaammine chromium(III)} Ion in Aqueous Acid Solution

The photochemistry of μ-hydroxo-bis {pentaamminechromium(III)} ion, [(NH3)6Cr-OH-Cr(NH35)5+], in aqueous acid solution (0.1 N HCl04, μ=0.2) has been studied using light which showed the absorption corresponding to the ligand field bands (18.5 X103 cm-1, 27.3x 10 cm-1). When [(NH8)5Cr-OH-Cr(NH3)5]5+ in aqueous acid solution was irradiated, absorption maxima of the complex (20.0x 10 cm-1, 27.0x 10 cm-1) shifted to longer wavelength and ammonium ion as well as pH of the solution increased with exposure time. So primary process of photochemical reaction was expressed as follows: [(NH8)5Cr-OH-(NH3)5]5+ + H20 ->[(NH3)5Cr-OH-Cr(NH3)4(H20)]5+ + NH3 Quantum yields for the primary process were obtained by the determination of NH4+ in the solution. The value was 0.30 and was independent of pH, reaction temperature, ionic strength of the solution and irradiating wavelength. Mechanism of the thermal reaction of the complex ion in aqueous acid solution has been studied in relation to photochemical reaction. The absorption maxima of the complex ion shifted to shorter wavelength as the thermal reaction proceeded. Thermal reaction was considered to be a rupture of Cr-0 bond as follows: [(NH8)5Cr-OH-(NH3)5]5+ +H20 ->2 [Cr (NH3)5(H2O)3+] The reaction was of first order and its rate was expressed as follows: v = kobs {(NH8)5Cr-OH-(NH8)5(H2O)5+} and kobs was dependent on reaction temperature (10.3X 10-3 min-1. at 45°C, 2.3x103 min-1 at 35°C and 0.4 X 10-3 min-1 at 25°C in 0.4 N HCl04) and on pH of the solution.