FLASH-PHOTOLYSIS STUDIES OF RUTHENIUM(II)-AMMINE COMPLEXES .1. TRANSIENT INTERMEDIATES IN THE PHOTOLYSIS OF RU(NH3)5PY-X2+ AND THEIR RELATIONSHIP TO PHOTOSUBSTITUTION PATHWAYS

Conventional flash photolysis of the ruthenium(II) complexes Ru(NH3)5py-X2+ (where py-X is a substituted pyridine or related aromatic heterocycle) in aqueous solution leads to the formation of relatively long-lived transients for those species also showing net photoaquation of py-X. Since only those species which undergo significant photoaquation (φL>0.01 mol/einstein) display measurable transient formation and since these complexes are those previously concluded to have a ligand field (LF) state as the lowest energy excited state, it is concluded that this LF state is the immediate precursor of this transient. In this context, it is proposed that the transient is an intermediate (not an excited state) with the pyridine ring π bonded to the Ru(II) species Ru(NH3)52+ in a manner making the nitrogen lone pair free to be reversibly protonated. Rates of relaxation of the intermediate to starting material and to products are markedly pH dependent, indicating that the protonated intermediate is particularly long-lived but decays largely to form aquation products. A reaction scheme for formation and decay of these transients is proposed, and comparisons are made with relaxation rates and with quantum yields for net photoaquation. © 1979, American Chemical Society. All rights reserved.