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 (NH 3 ) 2 ] 2+ by acidic (pH 0.5−1.3) aqueous chlorine provides the nitrosyl complex cis -[Ru(bpy) 2 (NH 3 )(NO)] 3+ as the final product. The first step in this process involves the metal-centered oxidation of cis -[Ru(bpy) 2 (NH 3 ) 2 ] 2+ to cis -[Ru(bpy) 2 (NH 3 ) 2 ] 3+, and the rate law for this process is −d[Ru(II)]/d t = 2 k 1 [Ru(II)][Cl 2 ] with the second order rate constant, k 1, as (1.1 ± 0.1) × 10 3 M - 1 s - 1 . Independent studies conducted on the cis -[Ru(bpy) 2 (NH 3 ) 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 Cl 2 and HOCl react with deprotonated coordinated ammonia. The rate law for the k slow process has the form k slow = {( k 2 [H + ][Cl - ] + k 3 K Cl )/( K Cl + [H + ][Cl - ])}([Cl 2 ] tot /[H + ]), where K Cl corresponds to the equilibrium constant for the hydrolysis of Cl 2 . The rate constant k 2, corresponding to the Cl 2 term, is 6.5 ± 0.3 s - 1 while the rate constant k 3, corresponding to the HOCl reaction, is 2.0 ± 0.2 s - 1 . The k fast process involves further oxidation of intermediate B by Cl 2 . 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 Cl 2 . The rate law corresponding to the k fast process has the form k fast = [ k 4 K int [Cl 2 ] tot [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 k 4 is (7.8 ± 1.5) × 10 2 M - 1 s - 1 .