A study of the photolysis of iodopentammine cobalt(III)

Abstract

The wavelength dependence of quantum yields of [Co(NH3)5I]2+, at 25° and pH of 4.73, was studied between 254 and 577nm. On irradiation of [Co(NH3)5I]2+ with monochromatic light (using interference filters) two products, cobalt(II) and iodine were formed. The amount of each product formed was determined; by extracting I2 in CCl4 to determine the amount of iodine present, and by using concentrated HCl to determine the amount of Co(II) present as CoCl2-. Quantum yields of iodine and cobalt(II) products were calculated from light intensity measurements, which were determined by using both potassium oxalate and potassium reinekate actinometers. Results showed that for all wavelengths studied (254 to 577nm) the quantum yield of formation of iodine was twice the quantum yield of formation of Co(II). A decrease in quantum yields with increase in wavelength was found (from ØI2 = 0.99 at 254 to ØI2 = 0.17 for 577nm). A mechanism was postulated to explain the results obtained. It was considered that the photolysis of [Co(NH3)5I]2+ followed the overall equation, The wavelength dependence of Øred for this system seems to provide evidence for internal conversion. At least two excited states were envisaged to be involved with interconversion between them possible.