Oxidation of coordinated ammonia to nitrosyl in the reaction of aqueous chlorine with cis-[Ru(bpy)2(NH3)2]2+

Rapid-scan stopped-flow kinetic studies show that the multielectron oxidation of coordinated ammonia in cis-[Ru(bpy)2(NH3)2]2+ by acidic (pH 0.5-1.3) aqueous chlorine provides the nitrosyl complex cis-[Ru(bpy)2(NH3)(NO)]3+ as the final product. The first step in this process involves the metal-centered oxidation of cis-[Ru(bpy)2(NH3)2]2+ to cis-[Ru(bpy)2(NH3)2]3+, and the rate law for this process is -d[Ru(II)]/dt = 2k1[Ru(II)][Cl2] with the second order rate constant, k1, as (1.1 ± 0.1) x 103 M-1 s-1. Independent studies conducted on the cis-[Ru(bpy)2(NH3)2]3+ complex show that conversion to the nitrosyl complex follows an A → B → C type consecutive pathway with k(slow) and k(fast) components, respectively. In the pH range of 0.5-2.8, the k(slow) process follows a competitive pathway where both Cl2 and HOCl react with deprotonated coordinated ammonia. The rate law for the k(slow) process has the form k(slow) = {(k2[H+][Cl-] + k3K(Cl))/(K(Cl) + [H+][Cl-])}([Cl2](tot)/[H+]), where K(Cl) corresponds to the equilibrium constant for the hydrolysis of Cl2. The rate constant k2, corresponding to the Cl2 term, is 6.5 ± 0.3 s-1 while the rate constant k3, corresponding to the HOCl reaction, is 2.0 ± 0.2 s-1. The k(fast) process involves further oxidation of intermediate B by Cl2. The intermediate of this reaction is speculated as either a nitrene, nitrido, or chloroamine complex. The kinetic studies indicate that the conversion of this unidentified intermediate to the final nitrosyl complex proceeds through a fast preequilibrium, involving deprotonation of the intermediate, followed by a direct attack by Cl2. The rate law corresponding to the k(fast) process has the form k(fast) = [k4K(int)[Cl2][H+][Cl-]]/[{K(int) + [H+]}{[H+][Cl-] + K(Cl)}], the equilibrium constant K(int) for the deprotonation process is 0.11 ± 0.04 M, and the rate constant k4 is (7.8 ± 1.5) x 102 M-1 s-1.