Photolysis of bromopentammine cobalt (III) perchlorate

Abstract

The wavelength dependence of the cobaltous, bromine and bromide quantum yields of Co(NH3)5Br2+ photolysis, were studied for the range 254nm to 436nm, at a pH of 1 and a temperature of 25 C. For the lower energy charge transfer region, equivalent yields of Co2+ and Br2 were obtained. Yields for πBr → Co excitation (313nm peak) were higher than for σ Br → Co excitation (253nm peak), although this may be due to an intensity effect. Aquation yields were much higher than those for pure ligand field excitation, but aquation was only 0.3 times the redox yield. Total yield reached a maximum of 0.5 at 334nm Rapid decrease in yield occurred as wavelength increased beyond 334nm. The primary quantum yield was considered to arise by two mechanisms, immediate dissociation upon excitation, Φ (r), and thermal dissociation of a long lived excited state Φ (t). Quantitative predictions of ligand field photoaquation yields, in good agrrement with experiment, could be obtained by using Φ (r). Redox and aquation yields in the charge transfer state were considered to arise from a complex summation of Φ (r), Φ (t) and secondary reactions. A series of experiments were suggested as important for future work.