this leads to AG* = 13.0 kcal mol-' and AC** = 7.4 (being w = 5.6 at zero ionic strength). The same authors reported that extrapolation using the Br$nsted-Bjerrum equation should give a lower value of rate constant; thus the difference with respect to the presently suggested value (AG** = 8.5 kcal mol-I) seems not very relevant.Conventional flash photolysis of the ruthenium(I1) complexes Ru(NH,),py-X*+ (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 p y x , Since only those species which undergo significant photoaquation (6, > 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 L F 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 K bonded to the Ru(I1) species Ru(NH,)~*+ 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.
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